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Biology 30
Module 2:
Lesson 5
Body Actions and Systems:
Maintaining Life
Body (Cell) Requirements
Copyright: Ministry of Education, Saskatchewan
May be reproduced for educational purposes
Biology 30
1
Lesson 5
Biology 30
2
Lesson 5
Lesson 5
Body (Cell) Requirements
Directions for completing the lesson:
Text References for suggested reading:

Read BSCS Biology 8th edition
Pages 373-398
OR
Read Nelson Biology
Pages 122-135

Study the instructional portion of the lesson.

Review the vocabulary list.

Do Assignment 5.
Biology 30
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Lesson 5
Vocabulary
alveoli
autotrophs
bronchioles
bulk feeders
catalysts
chemical digestion
chloroplasts
cloaca
closed digestive tube
coelom
diaphragm
diaphragm
diffusion
digestion
enzyme
epiglottis
esophagus
extracellular digestion
Biology 30
fluid feeders
gastrovascular cavity
glottis
guard cells
heterotrophs
ingestion
intracellular digestion
minerals
peristalsis
phloem
physical digestion
stomata
stomates
vascular bundle
villi, microvilli
vitamins
xylem
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Lesson 5
Body (Cell) Requirements
Introduction
Observations of the different kinds of activities involving organisms – whether they
are microscopic and unicellular or large and multicellular, would reveal that a great
deal of time is spent in obtaining materials for bodies or cells. How materials are
obtained and the manner in which they are processed by living things vary
considerably; however, their essential purpose of maintaining life is the same for all.
The action of photosynthesis is important in changing the energy of sunlight into the
chemical energy of bonds in organic compounds. This process requires particular
kinds of matter and also particular movements of gases. Once organic matter is
formed, it can then be used in various ways by the photosynthetic organisms
themselves or by heterotrophic organisms once they have acquired it.
Some of the uses of organic matter can include:


Direct energy sources for cell or body activities;
Building materials for cells or bodies; or, formation of substances which either
maintain the two previous actions or maintain suitable internal body conditions.
Cellular respiration is an important process that is part of all of these actions. As
with photosynthesis, certain kinds of gas movements are associated with cellular
respiration as well.
This lesson will show how matter or nutrients are obtained and processed by
organisms in different kingdoms. It will also trace how gas exchanges, associated
with these actions, are accomplished.
Biology 30
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Lesson 5
After completing this lesson, you should be able to:
Biology 30
•
name the basic purposes served by material taken into
living bodies.
•
state how the nutritional requirements of autotrophs and
heterotrophs differ.
•
list and indicate the importance of some nutrients
important in sustaining life.
•
explain the purpose and value of digestion.
•
state the two general phases in the digestive process.
•
explain the general role of enzymes and how they
function.
•
distinguish between intracellular and extracellular
digestion.
•
describe some of the methods of obtaining and
processing nutrients by organisms in various kingdoms.
•
distinguish between cellular respiration and respiration.
•
explain the need for carbon-dioxide or oxygen in plant or
animal cells.
•
discuss the photosynthesis-respiration relationship in
plants.
•
explain the significance of diffusion in respiration.
•
describe how gases move in and out of various plant cells
or organs and the structures that may be involved with
these movements.
•
indicate the major difference between plant and
(multicellular) animal respiration.
•
distinguish between respiration and breathing.
•
trace the processes, along with possible specializations,
related to respiration in “animal” kingdoms.
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Lesson 5
Foods and Nutrients
Every organism must continually take in matter if it is to sustain a living condition.
Any such matter or material taken into cells or bodies to sustain life is given the term
food or nutrient. Some references use the terms interchangeably. Others make a
distinction by saying that food is all matter which goes into a cell or body while
nutrients (or nutriments) make up that part of food which is actually used or is of
value.
This course will use the two terms, food and nutrient,
interchangeably to refer to any substances taken into cells or
bodies for purposes of work, growth or maintenance.
Autotrophs – Heterotrophs
The general definition given for food or nutrient means that autotrophs such as green
plants or heterotrophs such as ourselves all require foods. Previous definitions of
autotrophs may have stated that this general grouping could make its own food. In
describing the general relationships of autotrophs and their requirements, a major
difference will be emphasized as to the nature of plant requirements and those of
animals.
Autotrophic plants are plants that contain chlorophyll and can photosynthesize.
They take in inorganic foods or nutrients and convert them into organic compounds
by utilizing the energy of sunlight.
There are certain varieties of bacteria that are autotrophic.
 Photosynthetic bacteria possess their own special type of chlorophyll in order to
use the energy of sunlight.
 Chemosynthetic bacteria have enzymes capable of splitting some inorganic
compounds (containing iron, sulphur and other matter) and obtaining energy from
them.
While autotrophs may obtain their initial energies from different sources, their foods
or nutrients are still inorganic before being processed into organic compounds.
Therefore, it would be better to say that autotrophs are
organisms capable of converting inorganic matter into
organic compounds.
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Lesson 5
Heterotrophic organisms are either directly or indirectly dependent on autotrophs.
Their foods or nutrients are largely made up of pre-formed organic matter. These
organic nutrients come from plant or animal sources. Heterotrophs may be classified
differently depending on the ways in which they obtain their nutrients.
 Herbivores include those organisms whose main food sources are plant matter.
 Decomposers, saprophytes and scavengers rely on the bodies of plants and
animals having died of natural causes or having being killed by other organisms.
 Carnivores and parasites either obtain their nutrients from living bodies or ones
which they have killed.
Processes Involving Foods
Some of the values of matter and nutrients were mentioned as being energy sources
and also sources for building and maintaining an organism's own cells or body. The
processes of food breakdown to release energy (oxidation) and the formation of an
organism's own cells and tissues (assimilation and synthesis) are really the final
actions involving nutrients before they are used up or become part of the organism
itself. Some important actions must occur before this.
Getting nutrients into a cell (for a unicellular or plant organism) or into body cavities
(of multicellular animals) is one of the first steps. For most plants, which are not
mobile like animals, nutrients must come from the air or soil in contact with plant
parts. Such plant parts often play passive roles themselves as the actions of
diffusion or osmosis result in molecules moving through membranes and into cells.
In some instances, nutrient molecules are actively transported through plant
membranes against diffusion gradients. Such active transport requires some
expenditure of plant energy.
For mobile organisms, the actual capture of nutrients may require much more effort
than is required to get those nutrients into cells or into body cavities. Many
predators spend much time making unsuccessful stalks and rushes in capture
attempts.
The term ingestion is usually applied to the action of taking
nutrients into a body cavity, like a stomach. Though such
nutrients are within a body space, they are still outside of
cells.
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Lesson 5
Enclosing nutrients within cells or within body cavities does not automatically make
them available for use. Many are simply too large to be able to pass through a
membrane and into the cytosol or the main part of a cell where they can be utilized.
Even if some could pass through, they may still be in a form that can’t be used.
Another action must occur. The process of digestion becomes very important at this
point.
Digestion involves actions that convert nutrients into smaller, soluble forms that
could pass through membranes, be transported throughout plant or animal
bodies and finally, be used by cells. Two phases of digestion can be involved:
Phase One: Physical or Mechanical
Foods are broken down into smaller amounts making them easier to ingest. Chewing
or tearing with teeth and churning or grinding actions of stomachs or gizzards are
some of the more common physical actions of digestion.
Phase Two: Chemical
In this phase, enzymes accomplish the final breakdown. Nutrients are usually
converted to smaller molecules having different characteristics from their previous
forms.
Let’s review the role and actions of enzymes:
 Many chemical actions require higher temperatures to initiate them. Inside plant
or animal bodies these higher temperatures are not possible. Instead, proteinbased enzymes are able to join or split molecules.
 The enzymes assume the role of catalysts in that they initiate and control rates of
reactions but do not change the final nature of the products themselves.
 The enzymes are not used up either; after reactions are over, they become free to
work on other molecules.
 Enzymes are specific, apparently operating on a "lock and key" principle. Each
enzyme is constructed in such a way that only one particular molecule or set of
molecules can fit onto it. This means that thousands of enzymes are required in
digestive and metabolic processes.
 Many of the enzymes today are named by taking parts of the names of the
substances upon which they act and adding "ase" to them. Thus an enzyme that
acts upon a lipid (fat) is called lipase; one that acts upon sugar is sucrase.
Closely associated with enzymes, especially in vertebrate animals, are hormones.
These chemical secretions are produced by ductless glands and are released directly
into circulatory systems. The importance of hormones to digestion relates to the ability
that some of them have in stimulating the enzyme-secreting glands. Hormones are
examined more closely in another section.
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Lesson 5
Even though plants, animals and other organisms differ widely in the kinds of
nutrients they take in or in the actual manners of intake of these nutrients, some of
the processes of digestion are very similar.
Frequently, enzymes are similar and even the same in plants and animals. Some
insectivorous plants have "digestive systems" where modified leaves are used to trap
organisms (insects) and direct them into special cavities. Cells surrounding these
cavities secrete enzymes which digest the bodies of the trapped victims. Once
nutrients have been digested or converted into soluble forms, they may pass into and
be utilized by adjacent cells.
In larger, multicellular organisms, much of the digested matter must be transported
in order that cells in all body areas satisfy their nutrient requirements. This
transport is just as necessary and important in larger plants as it is in animals.
Extracelluar – Intracellular Digestion
Mechanical or chemical digestion of foods can occur in various places in organisms'
bodies. Where the process does take place greatly depends on whether an organism
is unicellular or multicellular and, if multicellular, how complex the body or digestive
system is.
Locations of digestion are generally classified as being either:
Extracellular
Extracelluar digestion is accomplished when enzymes leave the cells and carry out
their actions outside of the cells. This type of digestion could occur completely
outside an organism or in cavities or spaces within an organism. A cell or group of
cells then absorbs the digested matter.
Extracelluar digestion increases the possible food supplies
for organisms, as they are no longer limited to very small
foods. Large amounts of organic matter could be ingested
and then partially digested in order to be able to enter
individual cells. Some examples of organisms that show
extracellular digestion are:




bracket fungi
rusts
bread molds
yeasts
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Lesson 5
Intracellular.
Intracellular digestion takes place within the surrounding or boundary membranes
of a cell. Food particles themselves may or may not be enclosed in smaller cavities or
vacuoles enclosed by membranes. Some examples of organisms that show
intracellular digestion are: Amoeba and Paramecium.
Some Major Nutrients and Their Values
Totally depriving any organism of all foods or nutrients will bring about its death
when its body reserves are used up. For a human cut off from all food and water this
may take about two weeks. Reducing or eliminating only certain kinds of foods may
sustain lives for much longer periods of time. However, organisms lacking certain
kinds or levels of nutrients could show poor growth, poor body functioning or actual
deterioration in general body conditions. Such consequences as eventual death or
poor body functioning indicate the general importance of nutrients to organisms.
Nutrient values could be divided into more specific body uses:
1.
Much of the food taken in by organisms is used as a source of energy.
Energy is used for such actions as:




Cell divisions
Moving substances within bodies,
Moving parts of bodies themselves (muscle contractions)
Maintaining body temperatures.
2.
Matter is also used for cell formation and cell and body maintenance.
3.
Nutrients are re-organized to form an organism's own unique kinds of cells.
Proteins and other substances are formed to keep the cells or body functioning
properly.
Some of the major organic nutrients taken in by organisms were examined earlier.
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Lesson 5
Carbohydrates, which include sugars and starches, are primarily fuel or energy
sources. Glucose is the most immediate energy source in most cells. Excess
amounts of this are converted to starches, which could be glycogen (animal "starch"
in the liver).
Another carbohydrate, cellulose, helps to form the
cell walls of plants. Herbivorous animals (like
cattle) can utilize the help of bacteria and protists
in their digestive systems to break down cellulose.
In other animals, including ourselves, cellulose is
largely indigestible and passes out of the system.
It is still valuable as roughage or bulk which helps
matter to move along the digestive systems, partly
by stimulating muscle contractions.
Fats or vegetable oils (lipids) are also energy sources, providing about twice the
energy levels supplied by comparable amounts of carbohydrates. In animals, excess
amounts are frequently stored under the skin or in tissue spaces such as those
around organs.
Proteins are important for cell formation, growth and repair. They are also used in
the production of enzymes that are so important in many metabolic activities. Excess
amounts of protein or shortages of the other organic compounds will result in the
breakdown of this organic nutrient.
Vitamins are an organic group that has not been
previously described. They are just as important.
Vitamins are not like any of the other organic
compounds since they are not broken down to
provide energy. They are necessary for normal body
growth and action. Their functions may not be
particularly noticeable unless a shortage or
deficiency exists. When this happens any number
of conditions could develop. Two kinds of vitamins
are:


Fat soluble
Water soluble
Some vitamins, their sources and deficiency diseases are indicated in the following
table.
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Lesson 5
Nutrients
Some Functions
Food Sources
Deficiency Symptoms
Fat Soluble Vitamins
Vitamin A*
• aids in the ability to see in
dim light
• keeps skin healthy
• forms bones and teeth
• needed for reproduction
Dark green and yellow
vegetables, yellow fruits, egg
yolks, liver, milk and milk
products, fortified margarine,
fish liver oils
Nightblindness; rough skin and
mucous membranes; no bone
growth; teeth cracked or
decayed; dry eyes.
Vitamin D*
• builds strong bones and teeth
Vitamin D fortified milks and
margarines, fish or fish liver oils
Rickets; bowed legs; soft bones
prone to fracture; muscle
spasms and twitching.
Vitamin E*
• protects vitamins A and C
• formation of red blood cells,
muscles and other tissues
Vegetable oils, wheat germ,
whole grain products
Damage to blood cells, cell
membranes.
Vitamin K*
• normal clotting of blood
Dark green, leafy vegetables
Hemorrhaging
Water Soluble Vitamins
Vitamin C*
• maintains healthy bones,
teeth and blood vessel walls
• helps body use iron, calcium
and folacin
Citrus fruits and their juices,
vitaminized apple juice,
tomatoes, melons, berries,
broccoli, brussel sprouts,
cauliflower, cabbage, turnips,
potatoes
Scurvy; bleeding gums; dry and
rough skin; loose teeth;
degeneration of muscles.
Thiamin
(Vitamin B1)
• releases energy from
carbohydrates
• normal functioning of nervous
system
Whole grain and enriched
products, pork, organ meats,
legumes (peas, beans, lentils),
potatoes
Berberi: muscular weakness;
swelling of heart; leg cramps;
mental confusion.
Riboflavin
(Vitamin B2)
• releases energy from
carbohydrates and fats
• forms red blood cells
• normal growth and
development
Milk, meat, fish, poultry, eggs,
whole grain and enriched
products
Skin disorders - most noticeable
around nose and lips; eyes more
sensitive to and irritated by
light.
Niacin*
(Vitamin B3)
• releases energy from fat,
protein and carbohydrates
• helps to build body protein
Meat, liver, poultry, fish, peanut
butter, legumes, whole grain and
enriched products
Pellagra: skin disorders particularly where exposed to
sun; diarrhea; irritability; mental
confusion.
Pyridoxine*
(Vitamin B6)
• helps the body use protein
and fat
• resists infection
Meat, liver, fish, whole grain
products, eggs, legumes,
bananas
Skin disorders; cracked mouth
corners; dizziness; anemia;
convulsions.
Folacin*
(Folic Acid)
• forms genetic material
• forms red blood cells
Most fruits and vegetables,
meat, fish, legumes, whole grain
products
Anemia; diarrhea.
Cobalamin
(Vitamin
B12)
• helps functioning of the
nervous system
• forms red blood cells
Meat, liver, kidney, eggs, milk,
cheese
Anemia; degeneration of
peripheral nerves.
Biotin
• forms fatty acids and protein
• releases energy from
carbohydrates
Liver, kidney, milk, egg yolk,
nuts, legumes
Fatigue; depression; nausea;
pains; appetite loss.
Pantothenic
Acid
• releases energy from protein,
fat and carbohydrate
• aids absorption
Liver, kidney, whole grain
cereals, milk, mushrooms, dark
green vegetables, citrus fruits,
bananas, berries, cantaloupe,
legumes
Abdominal pain; vomiting;
fatigue; sleep problems.
Biology 30
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Lesson 5
Most of the vitamins required by animals must come from their in their food sources.
Only a few can be produced within bodies, such as vitamin D produced in the skin
subjected to sunlight; or, certain bacteria producing vitamin K in the intestines of
animals.
Humans may also take in vitamins in the form of supplements. Animals can be given
supplements as well. Excess amounts of water-soluble vitamins are usually excreted
from animal bodies in the urine. Fat-soluble vitamins are stored in the liver and fatty
tissue and are eliminated much more slowly. Because they are stored for long periods
of time there is a greater risk of toxicity. Eating well-balanced meals is the best way
to supply your body with vitamins.
Consultations with dieticians or medical professionals would be wise courses of action
before attempting supplemental programs.
For both autotrophs and heterotrophs, inorganic substances can form varying
amounts of the matter taken into bodies.
Minerals, in the form of dissolved salts in soils or incorporated in other plant and
animal tissues, can be used in diverse ways.
 Some, such as calcium and phosphorous, are important building components of
animal body parts like bones and teeth.
 Other minerals are important in the proper functioning of muscles, nerves and
plasma membranes.
As with vitamins, the values of some minerals are not realized or appreciated until
specific deficiency diseases develop.
In humans, these can include:
 Anemia - deficiency of iron.
 Thyroid disorders - deficiency of iodine.
 Rickets - deficiency of calcium.
Other abnormalities develop as a result of shortages in combinations of minerals or
minerals and vitamins. In plants, mineral deficiencies are frequently noticed by poor
growth performances of specific plant parts or entire plants.
The following chart shows the mineral, its function, its food source, and the
deficiency disease it can cause :
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Lesson 5
Nutrients
Some Functions
Food Sources
Deficiency Symptoms
Calcium
• builds strong bones
and teeth
• helps blood clot
• aids in muscle
contraction
Milk and milk products,
canned salmon or sardines
with bones, sunflower seeds,
soybeans and tofu (soybean
curd)
Rickets; poor bone growth;
osteoporosis; convulsion.
Chromium
• releases energy from
carbohydrates
Vegetables, whole grains,
fruit, meat, cheese
Reduction in ability to
metabolize glucose.
Copper*
• part of many enzymes
• aids in iron absorption
Liver, shellfish, nuts,
mushrooms, whole grain
cereals
Anemia; bone disorders.
Fluoride*
• forms strong bones and
teeth
Fluoridated water, fish, tea,
wine
Increased tooth decay.
Iron*
• forms hemoglobin,
which supplies oxygen
to cells
• part of several enzymes
and proteins
• helps resist infection
Kidney, liver, meat, legumes,
whole grain or enriched
products, eggs, dried fruits
Anemia; shortness of
breath; weakness.
Magnesium*
• involved in almost all
reactions involving
carbohydrate, protein
and fat
• conduction of nerve
impulses to muscles
• builds bones and teeth
• helps regulate body
temperature
Vegetables, legumes,
seafood, nuts, whole grain
products, milk products
Poor body growth;
weakness; nerve and
behavioral disturbances;
muscle spasms.
Selenium*
• needed for growth
• prevents breakdown of
fats and other body
chemicals
Meat, dairy products, fruits
and vegetables
No known defects.
Zinc*
• part of over 40 enzymes
which function to:
maintain acid-base
balance, digest protein,
make genetic material
Meat, seafood, whole grain
cereals, legumes, eggs
Growth failure; slow or
incomplete sexual
maturation; poor appetite;
glucose intolerance.
Potassium
Acid-base balance; body
water balance; nerve
function
Fruits, vegetables, legumes,
cereals; meat; dairy
products
Muscle weakness;
paralysis; body water
retention; possible high
blood pressure.
Minerals
* Large amounts of these nutrients have toxic effects in humans.
Biology 30
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Lesson 5
Water – A Food or Nutrient?
Although many may find it hard to regard water as a food or nutrient, it easily fits
into the general definition of being essential for the cells or bodies of organisms.
Water makes up anywhere from sixty to over ninety percent
of the composition of certain tissues or organisms. (The
mass of our own bodies is between 60 and 70 percent water.)
Functions of Water
 Water is a means of diluting waste substances (such as urea) and transporting
them out of bodies.
 It is important as a dissolving agent or solvent, so that substances can be
transported within bodies and through cell membranes.
 It functions as a lubricant (saliva, mucus...).
 Water acts as a temperature regulator (as perspiration evaporates from skin)
and as a regulator of turgor in plant and animal cells.
 Other functions may not be as obvious, as water may have indirect
involvements – as with the formation of blood, which is essential for oxygen
transport, or as an activator of enzymes.
Water can enter a body by:




Absorbing it.
Drinking it directly
Entering bodies from the foods of a plant or animal.
Some animals in desert regions manage to exist without ever drinking water.
Dehydration synthesis and oxidation of certain foods meet their requirements.
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Lesson 5
How Organisms Obtain and Process Food
Comparison of Food Ingestion and Processing Food
The next section (pages 17-36) will look at how organisms ingest their food and how it
is processed. The material will take a quick look at these processes in the fungus
and plants and then move on to animals. The following chart shows the progression
through the animals.
It is important that you are able to understand:
 How food is ingested at each level.
 The key differences between each group.
 Use the process of digestion as it occurs in humans.
Plants
Inorganic nutrients required by plants could come
from the atmosphere, soil or water. Gases such as
carbon dioxide and oxygen in dissolved form pass
through cell membranes or through openings such
as stomates and lenticels. Dissolved minerals
commonly diffuse into plant cells from the soil.
Some minerals are actively transported through
such membranes. Movement of water into plant
cells or organs such as roots takes place by
osmosis. Aquatic plants or even those grown
hydroponically (in liquid nutrients rather than soils)
derive much of their inorganic matter in dissolved
form from the water.
Insectivorous plants (such as Pitcher plants, Venus
flytraps or Sundews) are unusual in their abilities
to utilize the organic compounds in the bodies of
small organisms such as insects. Quite often, such
plants are able to function quite well on inorganic
nutrients, just as most other plants do.
Biology 30
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Lesson 5
In other instances, the ability to digest organic matter is helpful to individuals,
especially if they are growing in low-lying, boggy areas. The acid nature of the water
and the soils in such areas makes them nitrogen deficient. Insectivorous plants are
able to increase their supply of this valuable element by digesting trapped organisms.
Nitrogen is an important building component of proteins and the highly protein
bodies of insects are therefore able to yield this element to the plants.
The process of photosynthesis, which takes place in chlorophyll bearing cells exposed
to light, transforms inorganic nutrients into organic compounds. One of the principal
endproducts, glucose (sugar), may be transported for use or storage in other cells.
When stored, glucose is generally converted into longer, polysaccharide (starch)
chains. If cells need to use some of this starch, it must be digested to return it to a
smaller, more soluble sugar – for use within cells or for transporting elsewhere.
Fungi and Heterotrophic Bacteria
Whether these organisms are unicellular or multicellular, their nutrient requirements
are satisfied by extracellular digestion. Enzymes produced within cells or hyphae
are released into a surrounding substrate (possibly a suitable food supply) to begin
breaking down the material. Digested matter can then be absorbed into the cells.
The secretion and release of enzymes into substrates is typical of all fungi and most
bacteria. Puffballs, mushrooms, bracket fungi, rusts, bread molds and yeasts are
just some of the common fungal examples. Many of these are saprophytes in that
they feed on dead bodies. The term "saprophyte" is the plant equivalent of the term
"scavenger" applied to some animals.
Other fungi such as the grain rusts and athlete's foot, or bacteria such as Salmonella,
are parasitic. Their enzymes are released into the cells of living hosts, often resulting
in harmful effects to the hosts as cell contents are digested or cell functions are
disrupted. Some of the enzymes could even be poisonous in animals, resulting in
muscle spasms or paralysis and nerve damage.
Breaking down nonliving organic matter, such as plant or animal bodies or
excretions from them, reduces the substances to simpler forms. The fungi and
bacteria involved in such actions are really decomposers. A decomposing action may
be considered harmful if it involves matter that is still useful to humans or to other
organisms.
The role of decomposers is very valuable in breaking down
dead matter and returning it to the many cycles in the
biosphere.
Biology 30
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Lesson 5
Protists (or Protozoans)
These microscopic, unicellular organisms may fit into a number of different types of
nutritional or feeding relationships. The different relationships include:


Some are free-living, while "capturing" other small organisms or taking in
bits of organic matter.
Others are restricted to living in or on particular kinds of hosts, where they
may be parasitic or mutualistic in their relationships.
Protists such as Amoebae and Paramecia form
vacuoles around food particles. An Amoeba
flows around the food and encloses it in some
membrane that breaks off from the external
membrane.
food vacuole
food particle
about to be enclosed
In a Paramecium, hair-like cilia sweep food
particles into an oral groove along one side
of the organism. A food vacuole forms at
the end of the groove and then breaks away
to move about in the cytoplasm.

In both Amoebae and Paramecia, digestive enzymes move into the vacuoles to
break down the food and allow digested matter to be absorbed into the circulating
cytoplasm. Even though membranes separate the vacuoles from the rest of the
cytoplasm, the characteristic of being enclosed within the main unicellular
organism makes the digestion intracellular. Undigested particles in an Amoeba
remain enclosed in the vacuole and are simply expelled through the outer
membrane at any point. Solid, undigested particles in a Paramecium are expelled
at a point not far from the oral groove called the anal pore.

Some protists, such as the flagellated Euglena, possess chlorophyll. This enables
them to photosynthesize, in addition to capturing organic particles. Inorganic
nutrients are absorbed through the external membrane at any point while organic
matter may enter at localized points where vacuoles are formed.
Biology 30
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Lesson 5

Many other protists utilize extracellular digestion. Members of the sporozoan
phylum are all parasitic. Living in the cells, bloodstreams or cavities of host
organisms, they either release enzymes to break down material outside their own
cells and then absorb it or they absorb already soluble nutrients. Many of these
are disease-causing, such as the malaria-causing Plasmodium.

Protists utilizing extracellular digestion may also be involved in mutualistic roles.
There are large protist numbers in the digestive tracts of ruminant herbivores
such as cattle or deer. These protists are able to release enzymes capable of
breaking down cellulose, which their hosts cannot do. Both protists and hosts are
then able to absorb the digested nutrients into their cells.
Cnidarians
By-passing sponges, which show many traits of colonial cells including intracellular
digestion, we come to this multicellular animal group. Cnidarians are the first to
show something like a body cavity. Cnidarians such as Hydras, jellyfish, sea fans,
sea anemones that are found in corals, have a digestive cavity or pouch with one
external opening. The characteristic of having only one opening leads to this cavity
sometimes being called a "closed tube".
Tentacles with stinging, paralysing cells capture other small organisms and push
them through an opening or mouth and into a gastrovascular cavity. Some of the
inner or endodermal cells release enzymes to begin digesting the food inside the
cavity. This extracelluar digestion helps individual endodermal cells absorb food
particles through their membranes, enclose them in food vacuoles and continue with
intracellular digestion.
Biology 30
20
Lesson 5
Worms
Worm phyla show diversity in the manner of obtaining and digesting food – if it needs
to be digested.



Free-living flatworms have a closed
tube with a branching
posterior gut
gastrovascular cavity. "Closed"
means that there is only one opening
to the cavity. A muscular pharynx
draws in soft food particles that are
pharynx
frequently from decaying matter.
Inside the branching cavity or "gut",
anterior gut
enzymes released from surrounding
cells carry out some extracellular
digestion. Nutrients are then
absorbed and used by cells or
enclosed in food vacuoles and
subjected to further intracellular
digestion.
Parasitic flatworms tend to show a degeneration of systems since they appear to
have taken backward steps in body developments. They have poorly developed
nervous and muscular systems and generally lack digestive systems. Most could
be considered as specialized fluid "feeders".
Tapeworms, which may be found in many animals including ourselves, have
heads with special hooks enabling them to fasten to intestinal walls. The
remaining segments of the worms float free, absorbing the already digested matter
from their hosts' intestines. These parasites have thick cuticles, or nonliving,
outer layers of cells, to prevent the parasites from being digested by their own
hosts.
Biology 30
21
Lesson 5

The roundworm group shows two significant advances over previous groups. One
advance is the appearance of a digestive tube with two openings – a mouth and an
anus. This tube within a tube type of structure is more efficient as soft or fluid
foods move slowly from one end to the other while being digested extracellularly.
The efficiency of an open tube largely comes about with the varying degrees of
specialization along its length. In roundworms, some specialization of this "open"
tube includes a muscular section near the mouth called the pharynx, for
generating suction, and then a thinner-walled intestine behind this. Both
free-living and parasitic roundworms possess this type of system. The second
important improvement that appears in roundworms is a small fluid-filled cavity
or pseudocoelom between the digestive tube and the mesoderm. Circulation of
fluids in this cavity around the digestive tube assists in food and oxygen
circulation (and waste removal).

The most advanced of the worm phyla are the annelids or segmented worms.
Most are marine, although our familiar earthworm can serve as an example for the
group. Body segmentation in any group of organisms is important as it enables
certain areas of a body or certain systems within that body to become more
specialized for particular functions. This can be seen in the earthworm's open
tube digestive system with the development from mouth to anus of: a short,
muscular pharynx for generating suction; an esophagus leading to a crop where
the ingested matter is temporarily stored and moistened; a muscular gizzard for
grinding the material; and, an intestine where the processes of extracellular
digestion and absorption take place.
Biology 30
22
Lesson 5
Annelids have also progressed to having a coelom or cavity within the mesoderm
rather than between the endoderm and the mesoderm as in roundworms.
The advantages to having a coelom are:





A portion of the mesoderm has surrounded the digestive tube to become a layer of
muscle independent from the rest of the body. The digestive tract has a muscle
layer of its own to push food along independently of the outer body muscles.
The fluid-filled coelomic cavity with its suspended digestive tube has even more
space than a pseudocoelom.
The intestine begins to twist and loop within this cavity, presenting a much
greater area where digestion can occur.
More nutrients and oxygen can circulate
Wastes can be more efficiently removed.
Mollusks


Mollusks continue with an open (mouth-anus) tube and specialized areas along
that tube.
Some, like clams, are filter feeders. Cilia lining the gills sweep food particles
entrapped in mucus towards a mouth at the edge of the gills. A snail or slug uses
a rasping, tongue-like structure called a radula to scrap plant matter off surfaces
and into its mouth.
Biology 30
23
Lesson 5


Cephalopods, which include squids and octopuses, catch or move prey with their
tentacles.
Crustaceans, fish and shellfish (other mollusks) are brought to the mouth where
beak-like teeth crush and tear the food so that it may be ingested.
Arthropods
The tube-like digestive tract, with some specialized areas along its length, continues
in arthropods.
Organisms in some classes (such as crayfish or grasshoppers) have hard tooth-like
structures made of chitin in their stomachs or gizzards. Chitin is a hard, protein
substance which is also used in the formation of the exoskeleton.



These stomach or gizzard "teeth" assist in the mechanical digestion or breakdown
of food substances.
Digestive glands, which are also present in mollusks, are quite common in many
members of a number of arthropod classes. It appears that foods move into these
glands for the final stages of chemical digestion before nutrients are absorbed.
Some arthropods have finger-like projections extending from areas near the
stomach. Opinions vary on what these caeca actually do.
Biology 30
24
Lesson 5
Caeca may increase the area where digestion may occur or they may be places where
(mutualistic) bacteria are concentrated.

Another structure in some arthropods is a pouch-like enlargement in the intestine
just before the anal opening. This is called a rectum and it serves as a temporary
storage area for solid wastes before they are expelled.
Types of Feeders:

Many of the arthropods are classified as bulk feeders where parts of, or whole,
organisms are eaten. Mouth parts are adapted mainly for biting and chewing in
this group.

Other arthropods are fluid feeders, sucking out plant or animal liquids or soft
tissues.
Examples:
 Spiders inject enzymes into their prey and then suck the partly broken
down, soft tissues into their digestive systems.
 Bees and mosquitoes have tubes for penetrating flowers or other plant parts
and sucking out nectar or other plant juices.
 female mosquitoes will pierce and penetrate the skins of animals to remove
blood as a necessary protein for egg development.
Biology 30
25
Lesson 5
Vertebrates
The varieties of food and the manners in which they are gathered or caught and then
ingested by vertebrates seem almost endless. Sight, smell and sound are probably
the most prominent senses which most organisms use to detect or locate their foods.
Another variation of sound involving echo location is used by some organisms such
as bats. Body temperatures or heat given off by some warm-blooded animals can
help to guide some snakes (pit vipers) to their locations. Detection of vibrations in
water or soil can be used to reveal the presence of prey for other predators.
Locating Food
After locating food, the major difficulty that may face herbivorous animals is that the
food could be out of reach. In most instances, the herbivores simply move on
searching for something more accessible. More determined or desperate animals will
sometimes use physical skills or force that they normally wouldn't use – such as
knocking or pushing down shrubs and small trees to get at leaves.
Animals that prey upon other animals are usually faced with more challenges in
obtaining food. Stealth and short bursts of speeds, as with cats; sustained chases by
dogs or wolves; the swooping actions of hawks and owls; and, the "herding" of
minnows into shallower, shorewaters by feeding walleye pike ("pickerel"), are some
techniques more often rewarded by failure than success.
Ingesting the Food
Once food is within reach or captured, vertebrates
demonstrate a variety of techniques in ingesting it.


Long tongues may be used to reach plant nectars
or insect prey and pull them into mouths; tongues
can also wrap around and pull at grass or twigs.
Many animals are bulk feeders in the sense of
taking in food whole or in large chunks. Birds are
good examples of this as they swallow food such as
seeds, fish or animals whole or use their beaks to
break or tear apart the food into smaller pieces.
Amphibians and snakes also commonly swallow
their prey whole. Snakes are especially well suited
for this as many have lower jaws which can
"unlock" from the top jaws. As well, their lower
jaws are flexible enough to allow the two halves to
move independently of each other. This enables
snakes to slowly pull entire prey into their mouths and bodies.
Biology 30
26
Lesson 5

Teeth are common to the digestive systems of many vertebrates, with the
exception of the present day birds. The greatest variations appear in mammals,
with teeth adapted for gnawing, chewing, crushing or tearing. Some organisms
possess combinations of these as our own teeth illustrate, with biting and gnawing
incisors at the front, two canines in each jaw and the crushing and grinding
molars farther back. Although teeth shapes, sizes and numbers could be different
between species, general structures are similar as shown by the illustration.
molar
premolars
canine
incisor
premolars
molars
s
In a few species, certain teeth experience constant growth. This is characteristic of
some rodents such as the beaver and the Norway rat. Observations have indicated
that a rat's incisors can grow approximately 15 cm in length in one year.
Biology 30
27
Lesson 5
Variations In Digestive Systems
Although the varieties of food and the methods of
gathering or catching it are quite different, digestive
systems and processes are fairly similar among all
vertebrate classes. Some minor variations do occur and
these seem to be adaptations to food or feeding
techniques.

A bird has a crop that enables it to hurriedly eat larger
amounts of food in a short time and temporarily store
and moisten it. A powerfully muscled stomach or
gizzard grinds and crushes the food into smaller
pieces. Some birds ingest gravel or small stones to assist in the grinding process.

Birds, amphibians and reptiles have an enlargement in the digestive tract just
before the anal opening called a cloaca. This chamber receives and temporarily
stores both solid and liquid wastes and also eggs and sperm during reproductive
periods.


Stomach adaptations may include extra
projections from the stomach, as with
the caeca in some fish, or more
stomach chambers, as in many
ruminants. These provide more surface
area along which digestion and
absorption can occur.
Ruminants (eg cows, giraffes, sheep, goats) can ingest larger amounts of grasses
or twigs in a relatively short period of time. Later, they can regurgitate the food, in
smaller amounts, for a more thorough chewing in a less hurried fashion.
Biology 30
28
Lesson 5
The stomach chambers of some herbivores,
particularly ruminants, are homes for many
bacteria and protists. These microorganisms
play an important role in releasing enzymes
that can break down cellulose. Both host and
microorganisms can then absorb and digest
the nutrients. (Some ruminants have bacteria
and protists which can digest only certain
kinds of vegetable matter and this is why
some animals, like deer or moose, can eat only
certain kinds of plant matter.)
Inside a Vertebrate Digestive System
With vertebrate digestive systems being fairly similar, the human example will serve
as a common representative for the actions happening to food once it enters the
mouth.
Biology 30
29
Lesson 5
What happens in the Mouth




The tongue and teeth can begin mechanical digestion by breaking up the food into
smaller pieces.
Three pairs (in humans) of salivary glands and numerous mucous glands along
the cheeks supply saliva and mucus to help moisten and break up the food.
Saliva also has the enzyme amylase, which begins the chemical digestion of
starches.
After some chewing a swallowing action is initiated. The tongue moves backward
toward the throat or pharynx region pushing food ahead of it to where it can be
taken over by muscular contractions of the esophagus. Although we have some
control of swallowing, there is little control of the muscular contraction, or
peristalsis, once it begins. During the swallow, the trachea and larynx are pulled
upwards against the epiglottis to close the tracheal opening and prevent food from
going the wrong way (into the lung passages).
Ever heard of food or water going down your Sunday
throat? Your drink of water beat the epiglottis covering
your trachea—so you cough to try to get the water out
of your trachea!
Peristalsis, a contraction of longitudinal and circular muscles, then pushes food
along the esophagus and into the stomach.

One of several valves along the digestive tract exists
at the juncture of the esophagus and stomach. This
thickened, circular muscle, called the lower
esophageal sphincter, normally prevents partly
digested food from moving back along the
esophagus. Accidental relaxations or a disorder in
the muscle could result in the acidic contents of the
stomach being released into the esophagus, causing
a burning sensation. This burning sensation is often
referred to as heartburn.
This sphincter and others in different locations serve
more importantly in keeping foods in particular sections
of the digestive tract for longer periods of time. This
allows more efficient digestion and absorption, rather
than pushing food too quickly through the system.
Biology 30
30
Lesson 5
What happens in the Stomach?
In the stomach both physical and chemical digestions occur.
Physical Digestion
Three muscle layers make up the stomach walls. Circular, longitudinal and oblique
muscles cause rhythmic contractions along the stomach's length resulting in the
mixing of food, mucus, acid and enzymes.
Chemical Digestion
Glands throughout the stomach wall secrete chemicals called gastric juices. These
are:
1.
Hydrochloric acid (HCl) – This acid helps the enzymes to function, dissolves
minerals, kills bacteria and regulates the pyloric valve. It also stops the action
of the starch and glycogen-digesting amylase that began in the mouth.
Very little digestion of carbohydrates occurs in the stomach.
2.
The enzymes: pepsin and gastric lipase. The enzyme pepsin begins the
chemical breakdown of proteins into short-chain amino acids. Lipase acts
upon fats and oils.
3.
Mucus: Mucus, or mucin, is important as a lubricant for the food and also as a
protector of the stomach wall against the actions of the acid and enzymes.
Despite this, mucus itself is continually broken down along with cells lining the
stomach. When the protection by the mucus is not complete and when the
lining cells are not replaced fast enough, ulcers could result.
Food may remain in the 1.5 to 2 liter capacity stomach for two to three hours. It
eventually reaches a consistency and an acid level, that cause a pyloric sphincter
between the stomach and the small intestine to open. A quantity of food, or chyme
as it is called at this point, passes into the small intestine.
Biology 30
31
Lesson 5
The Work of the Small Intestine
The Influence of the Pancreas
The acid nature of the chyme stimulates the secretion
of two hormones by glands in the first part of the
small intestine or duodenum.
bile duct
body of pancreas
gall bladder
head of pancreas
junction of bile and pancreatic duct
tail of
pancrea
s
main
pancreatic
duct
duodenum
One of these hormones acts upon the adjacent pancreas causing it to
produce pancreatic juice. Pancreatic juice makes its way from the
pancreas through a duct that joins with a bile duct from the liver. The two
ducts form one common duct that enters the duodenum a short distance
below the pyloric sphincter or valve.
Pancreatic juice is made up of:


A number of enzymes - Pancreatic amylase continues the digestion of
carbohydrates that began in the mouth and pancreatic lipase breaks down
fats. A number of protein-digesting enzymes, including trypsin and
chymotrypsin, are included in the pancreatic juice.
A mixture of sodium bicarbonate - The carbonates convert the nature of the
food from acid (pH 2-3) to alkaline (pH 9), enabling the pancreatic enzymes
to function. As this happens, the stomach enzymes cease functioning.
The pancreas is significant in another way besides secreting enzymes. Within the
pancreas are also cells that release hormones.
These hormones: insulin and glucagon, regulate the amount of sugar in the blood.
The significance of maintaining certain sugar levels in the blood and the importance of
these hormones will be looked at in another lesson.
Biology 30
32
Lesson 5
The Influence of the Liver
The other hormone produced in the duodenum affects the
liver. This large organ continually produces bile that, in
some organisms, is temporarily stored in a pouch-like cavity
called a gall bladder. (Some animals such as horses, deer
and rats do not have a gall bladder, but release bile
continuously in small amounts.)
Bile is made up of:


Bile salts in the alkaline bile cause fats to emulsify or to break up into very
fine droplets. This permits the enzyme lipase to continue with its digestion
of the fats. Some of the fats may be emulsified so that they may be able to
pass into the lymphatic circulatory system, where they recombine into
larger molecules again.
Bile also consists of bile pigments. These are largely made up of broken
down red blood corpuscles and hemoglobin which the liver removes from the
circulatory system. It is the brown color of these remains which results in
the characteristic brown color of the solid wastes or feces which are
eliminated from the large intestine.
Apart from its involvement in digestion, the liver is also an important homeostatic
organ. Blood collected from the small intestines makes its way by means of the
hepatic portal vein into the liver. Before allowing the blood to continue into the
general circulatory system, the liver carries out actions that affect the concentration
levels of certain substances.




Excess monosaccharides are converted to glucose and if glucose
itself is above a certain concentration, it is changed to glycogen.
(A lower amount of glucose will result in the opposite action.)
Since amino acids cannot be stored, the liver breaks down
excess amounts. The liver removes the nitrogen portion of the
molecule and converts it to waste urea. The remaining portion
can be oxidized to release energy.
The liver also controls excess amounts of other chemicals,
including vitamins and minerals.
Drugs, including alcohol, are filtered through the liver and are
neutralized or converted into other forms by special enzymes.
Biology 30
33
Lesson 5
Absorption in the Small Intestine.
In the small intestine, the work of the pancreatic enzymes and additional enzymes
produced by intestinal glands, largely complete chemical digestion. What were
once originally large molecules or macromolecules of carbohydrates, fats and proteins
have been reduced to small molecules of glucose or monosaccharides, fatty acids and
amino acids. In these small forms, such molecules are capable of being absorbed
into the lymph and blood systems for transport to other body areas.
Although some substances (such as alcohol and sugar) begin to be absorbed in the
stomach, by far the greatest amount of absorption occurs through the walls of the small
intestine.
Examination of the inside of the small intestine of many animals would show the
presence of many folds that increase the total absorptive area. The total absorptive
area is increased by many finger-like projections called villi (sing. villus) as well as
smaller projections called microvilli coming from the outer cell membrane that line
the small intestine. Compared to an intestine that was completely smooth, the folds,
villi and microvilli are estimated to increase the surface area by 500 to 700 times.
A Cut Away Section of the Wall of the Small Intestine
blood vessel
microvilli
capillary
Individual Villus
The small intestine would be up
to 70 m long if villi did not exist!
Biology 30
34
Lesson 5
Glucose and other monosaccharides, amino acids, vitamins and minerals pass
through the epithelial cells of the villi and into the blood capillaries. While much of
this movement is by diffusion, some does occur by active transport. In this last
process, cells use up some energy in moving certain molecules towards a higher
concentration in the body transport systems or cells. Droplets of (emulsified) fat and
fatty acids, along with some fat-soluble vitamins such as A, D and K, make their way
into the villi as well. However, rather than entering the (blood) capillaries, these
larger molecules enter lymph ducts or lacteals which are more porous than the
capillaries. The lymphatic system eventually joins the regular circulatory system in
the shoulder areas where lymph enters veins.
The Large Intestine
The final stages in the digestive process occur in
the large intestine. Relaxations of a sphincter at
the junction of the small and large intestine
permit chyme to leave the small intestine. This
begins to occur about four to six hours after it
had entered. In some herbivorous animals
(horse, rabbit...) there is an enlargement of the
intestine at this point called the caecum. Home
to many bacteria, it may be an important area for
the final breakdown of the hard-to-digest
cellulose.
caecum
where small
intestine joins
large intestine
appendix
In humans, only a small, blind pocket exists of what may have been a larger caecum
in early ancestors. It plays no major role now except perhaps in a negative way.
Another vestigial structure, a finger-like appendix, is attached to the caecum and
could become infected. This could require surgical removal.
Small amounts of digestion and absorption of nutrients continue in the large
intestine.
The more important function of this part of the digestive tract appears to be reabsorption
of water.
A large amount of water has entered the system up to
this point in saliva, mucus and as water. Reabsorption
of it from the large intestine prevents rapid body
dehydration. During the ten to fourteen hours in which
chyme normally remains in the large intestine, it is
converted from a watery consistency to the normally
solid fecal matter.
Biology 30
35
Disorders can shorten
the reabsorption time,
resulting in diarrhea or
can increase the time,
to bring about
constipation.
Lesson 5
The large intestine, unlike the stomach or the small intestine, has neither a strongly
acid nor a strong alkaline environment. This, along with the still remaining nutrients
in the chyme, makes it an ideal home for countless numbers of coliform bacteria. A
member of this group, Escherichia coli, is a common inhabitant of humans and other
warm-blooded animals. Its relationship could be considered as commensalism and
even one of mutualism, as it could be involved in several useful functions for its host.
Some of these may include:
 inhibiting the presence of harmful bacteria
 some breakdown of cellulose
 the production of K and B vitamins which could be absorbed by the host.
Numbers and rates of reproduction of these bacteria are so high that ten to fifty
percent of the dry fecal matter could consist of bacterial remains. Even if no food is
eaten, solid fecal matter is still produced from dead body cells and dead bacteria.
Solid wastes accumulate in the final portion of the large intestine. Two sphincters,
one at the beginning and one at the end of a slightly enlarged portion called the
rectum are involved in:


The final elimination of the wastes.
The last of these, the anal sphincter allows wastes to pass out of the body through
the anal opening.
Extracellular – Intracellular Digestion
Much of the digestion that occurs in vertebrates is extracelluar. Digestive tracts,
despite the different specialized "compartments" or areas among different species, are
still really extensions of the "outside" into organisms' bodies. Nutrients begin to be
incorporated into bodies when they are absorbed through cells lining the digestive
tracts and into circulatory systems and finally into individual cells themselves. Some
of the same enzymes secreted into the digestive tract, as well as others, continue the
final nutrient breakdown inside the cells during intracellular digestion.
Biology 30
36
Lesson 5
Comparison of Respiratory Systems
The next section (pages 37-59) will look at how gas exchange occurs in organisms
and the mechanisms the each organism uses for gas exchange to occur. The
material will take a quick look at these processes in fungi and plants and then move
on to animals. The following flow chart shows the progression from unicellular
organisms to the more complex systems of vertebrates.
It is important that you are able to understand:
 The progression in development of gas exchange and the development of new
structures as the organisms get larger.
 The key differences between each group.
 How breathing occurs in humans.
Gas Exchanges (Respiration)
Photosynthesis and cellular respiration are both actions associated with particular
movements of gases. Photosynthesis requires the carbon from carbon-dioxide gas to
build an organic compound. In the process, oxygen is released. Cellular respiration
is the breaking down of organic compounds into simpler forms to release energy in
the form of ATP. This activity requires oxygen and releases carbon dioxide. The
oxygen is needed to continually remove electrons during aerobic respiration.
Both processes are distinct and unique in what they produce
but there is a relationship that they both share. This
relationship is gas exchange or respiration.
Organisms must maintain a proper balance of required gases, depending on the
dominant action(s) occurring in their cells or bodies. In some organisms, such as
green plants, both photosynthesis and cellular respiration can be going on at the
same time at different rates. Both shortages and excesses of certain gases can have
damaging or lethal effects on cells and organisms.
Biology 30
37
Lesson 5
Organisms must maintain a proper balance of required gases, depending on the
dominant action(s) occurring in their cells or bodies. In some organisms, such as
green plants, both photosynthesis and cellular respiration can be going on at the
same time at different rates. Both shortages and excesses of certain gases can have
damaging or lethal effects on cells and organisms.
Diffusion
Respiration, or the actual exchange of oxygen and carbon
dioxide at the cellular or cell membrane level for all organisms,
takes place by diffusion.
Gas molecules, which are in constant motion and colliding with other molecules,
move from areas of high concentration to areas of low concentration. Inside
organisms, membranes form boundaries around cells and gases must be dissolved in
order to pass through these membranes. Therefore, cell membranes must be moist
for diffusion to occur. Should the membranes dry out, respiratory failure could
occur.
Respiration (gas exchange – oxygen moves in and carbon dioxide moves out) across
cell membranes has other factors that affect the process. Diffusion and diffusion
rates through membranes are affected by molecule concentrations on both sides of
the membranes.
Consequently, some of the actions of bringing or removing molecules near the
membranes are almost as important as diffusion itself. The means by which
organisms allow, regulate or are actively involved with gas movements vary greatly
and are all part of the overall process of respiration.
The next section will deal with respiration (gas exchange)
and the importance of it, starting with the plants, then
moving through the consumers and ending with gas
exchange in humans.
Biology 30
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Lesson 5
Respiration in Plants and other Producers
To look at some plants, plant parts or plant cells, one may find it difficult at first
observation to see any evidence of either cellular respiration or of gas exchange which
should accompany it. However, cells must continually release energy if they are to
remain alive. Even seemingly dormant vegetables, potato tubers, fruits and seeds
have enzyme systems which continue to function slowly. Actual evidences of cellular
respiration may be noted from such observations as:



Higher temperatures towards the middle of seed or fruit storage containers;
Positive limewater tests by gases released from seeds
Gradual weight losses over time.
The fact that enzyme actions continue in some plants even after harvesting is evident
by many tissues or seeds changing from being sweet and tender right after picking to
less sweet and more "woody" as quickly as a day afterwards. Corn and peas, in
particular, have their sugars quickly converted to starches. "New" potatoes also lose
their sweetness and their tender, outer skins become thicker and more rigid over
time.
The rate of cellular respiration in plant cells can be affected by a number of external
factors:
 Temperature - Lowering temperatures by refrigeration or having cool storage
areas can decrease rates greatly. (Lowering the temperature around
strawberries from 24C to a little above freezing for instance, can reduce their
respiration rate by ten times.)
 Moisture - Surrounding humidity and internal moisture can also extend or
shorten plant or seed life. Prairie grain producers are very aware of the
difficulties of storing seeds above a certain moisture content. In many cereal
seed varieties, once the moisture content is above 18%, the respiration rate
doubles for every 1% increase in moisture above that. A high respiration rate
could quickly lead to "heating" of grain and a loss in quality and germinating
ability (or life) of the seeds.
 The amount of oxygen or carbon dioxide surrounding plant parts is another
determining external factor.
 Internal conditions relating to injury or to age could also affect plant
respiration and the need for gas exchange.
Biology 30
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Lesson 5
Respiration in Plant Organs
Photosynthesis and Cellular Respiration
In photosynthesis, carbon dioxide is taken in while oxygen is released. Cellular
respiration reverses these movements by needing oxygen and releasing carbon
dioxide.
Cellular respiration in green plants could be going on at the same time as
photosynthesis but they are occurring at different rates. (Check back to the
conclusions formed from the experiments in assignment 4)
Under conditions where there is light, the two processes can go on at the same time.
Gases released by one process are raw materials for the other process. Daylight
conditions generally result in photosynthetic rates being anywhere from five to ten
times faster than the other process. At this time, a plant would be releasing oxygen
to its surroundings.
In darkness, photosynthesis slows and may even stop while cellular respiration goes
on. The cells or plants would now take in oxygen while releasing carbon dioxide. At
certain low light intensities (considered by some to be about 2% of full sunlight), the
rates of photosynthesis and cellular respiration balance each other. At this time, gas
requirements between the two processes balance each other and there may be no net
movement of gases into or out of certain plant parts.
Normally, over a longer period of time, green plants consume more carbon dioxide and
release more oxygen, than the other way around.
Biology 30
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Lesson 5
Respiration in Plant Organs
Leaves
Leaves are common areas where the two processes of photosynthesis and cellular
respiration could be going on at the same time. Most of the cells between the upper
and lower epidermis are collectively called mesophyll. Some of the mesophyll
consists of palisade mesophyll or palisade cells. The cells making up this layer are
mainly photosynthetic in function. The remaining mesophyll is made up of spongy
cells, which are much more loosely arranged. Intercellular (air) spaces are common
among these cells. Both photosynthesis and cellular respiration occur in the spongy
cells. The many air spaces within the spongy mesophyll allow gases to move among
the cells and to diffuse relatively easily between the spaces and both palisade and
spongy cells. The network of intercellular spaces can make up anywhere from fifteen
to forty percent of the internal volume of particular leaves.
Cross Section of a Leaf
A major difference between plants and animals is that air
occupies most of the intercellular spaces in plants while
those of animals are fluid-filled.
The surfaces of many plant leaves and other plant parts are often covered with close
fitting epidermal cells and a waxy layer or cuticle. These are intended to reduce
water loss. However, gas movements are also restricted. To overcome this problem,
the branched air passages of many leaves are connected to special openings in the
epidermis called stoma (also called stomates; plural stomata). These openings are
regulated by the actions of bean-shaped guard cells which exist in pairs.
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Lesson 5
Guard Cells and Stomata
Guard cells and stomata could be located in both the upper and lower epidermis of a
horizontal leaf. The concentrations could vary according to plant species.



Land plants frequently have more
stomata on the undersides of leaves
or in the lower epidermis.
Vertical leaves of plants may have
equal numbers on both sides.
Some floating leaves of aquatic
plants have their stomata on the
upper surface where they are
exposed to air.
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Lesson 5
The size of an opening or stoma changes during a 24 hour day. Most stomata are
open during the day and closed at night. The actual opening and closing of a
stomata is related to the structure of guard cells and turgor changes in them.
Guard cells' epidermal walls next to the stoma are slightly thicker than the walls on
the other sides. When water enters the guard cells and increases their turgor
pressures, their outer walls are pushed outward more readily. This draws the inner
walls outward as well, causing a stoma to open. When water leaves the guard cells
and their turgor falls, the cell walls move inward reducing the stomatal opening or
causing it to close almost completely.
Open stomata during the day allow carbon-dioxide and oxygen to move in and out of
a leaf more readily. This is when photosynthesis is commonly at its peak and
movement of the gases (especially oxygen) is also greatest. Stomata could close in
very hot, dry conditions when guard cells lose more water then usual.
Stems
Stomates are also present in herbaceous stems where a considerable
amount of photosynthesis and cellular respiration takes place.
Gases can diffuse in and out of such stems through these openings.
Stems which increase in thickness or diameter year after year
usually have an epidermis which is transformed into bark cells.
Bark lacks stomates and is largely impermeable to movements of
water and gases. However, in such woody stems the total number of
living cells is no longer that large.
Much of the "wood" of trees consists of non-living vascular tissue
which does not require gas movements. A large number of the
still-living cells are concentrated in a thin layer beneath the bark.
Gas requirements for these cells are supplied by small, round areas
of loosely-packed cells in the bark. These often-raised, corky areas
create openings called lenticels through which gases can pass in
and out of stems.
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Lenticels
on a Twig
Lesson 5
Roots
The location of most roots beneath the soil surface or in shady, humid areas limits
the action of their cells largely to cellular respiration. Oxygen is needed for cellular
respiration to occur. Carbon dioxide is given off. Oxygen and carbon dioxide pass in
and out of roots through their epidermal cells and root hairs. The gases move from
cell to cell by simple diffusion.
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Lesson 5
Other Plant Parts
Exchange of gases occurs in various other plant parts, some of which have been
mentioned before. Flowers, fruits and seeds experience gas exchanges by diffusion
through the moist membranes of epidermal cells or "skins". In these structures the
main process is cellular respiration so that oxygen is taken in and carbon-dioxide
must be released. However, there are usually no special openings or special
adaptations for gas exchanges in these plant parts as there are in the other organs.
Respiration In Other Producers
There are significant numbers of non-vascular plants such as algae, mosses and
ferns. As well, there are non-plant producers which include many single-celled or
colonial organisms belonging to the Protist or Moneran kingdoms. All of these may
carry out both photosynthesis and cellular respiration, which are dependent on
different gas exchanges. Most of these organisms have no special structures
associated with respiration. Respiration or gas exchanges are accomplished mainly
by simple diffusion between atmosphere or soils and producer parts and then from
cell to cell.
Respiration in Consumers
Organisms classed as consumers cannot make their own food. They lack the ability
to carry out photosynthesis and must obtain their organic requirements from other
organisms. Such individuals must still carry out cellular respiration in order to
release the energy from the organic nutrients or to put those nutrients into forms
that can be utilized for other purposes.
The actual exchange of gases (or respiration) in consumers is still by diffusion.
Molecules of oxygen and carbon-dioxide pass through cell membranes from areas of
higher to lower concentration. This movement by diffusion is the same as that found
in producers. For consumers which are unicellular, colonial or multicellular but
small in size, diffusion is often the only process responsible for movements of gases.
As consumers become larger in size, there is an increasing difficulty in getting
sufficient movements of gases between the outer bodies and all inner cells and
tissues. Consumers that maintain more active lives and therefore require higher
energy levels further add to this problem. Increased rates of cellular respiration
bring on greater oxygen demands while releasing larger amounts of waste carbon
dioxide.
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Lesson 5
Increased body sizes and increased activities among many of
the multicellular consumers lead to one of the more
important differences in gas exchange when comparing
consumers and producers – particularly animals and plants.
Some multicellular animals, in addition to diffusion, are
able to breathe.
Multicellular animals are able to carry out mechanical or physical actions designed to
move more air in and out of bodies faster. These breathing actions (and sometimes
the term "respiration" is incorrectly applied to mean "breathing") involve:



special muscles
air passages
tissues or organs such as gills or lungs.
As well, an internal transport (circulatory) system in some of these animals is
associated with breathing. While plants also have tissues designed to move
substances, their transport systems are not as actively involved. Transport systems
will be looked at and compared more fully in another lesson. The remainder of this
lesson will look at the methods and developments related to respiration in various
consumers.
Fungi
Fungi have the characteristic of being heterotrophic
or nutritionally dependent on other organisms. The
Fungus kingdom includes such diverse types as
slime molds, bread and fruit molds, yeasts and
mildews. Many of the larger fungal masses consist
of thread-like hyphae that may or may not have
cross walls.
slime mold
Whether fungi exist as single cells or as masses of
mycelia, respiration takes place by diffusion.
Maintaining moist membranes for proper diffusion is
a necessary requirement and this is why most fungi
prefer humid and dark or shady conditions.
bread mold
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Lesson 5
Unicellular "Animals"
Most unicellular animals are found either in aquatic or moist environments, which
could include the body fluids of host organisms. Exchanges of dissolved gases occur
by diffusion.
Multicellular "Animals"
Sponges, Cnidarians (Hydra) and Echinoderms
(Starfish)
Even though these particular organisms may
consist of many cells and possibly a number of cell
layers, their method of respiration is the same as
that of unicellular animals. Individual cells are
either in direct contact with the external
environment or not many cells removed from it.
This allows respiration by simple diffusion.
Worms – Platyhelminthes, Nematoda and Annelida
The three worm phyla receive oxygen and remove carbon dioxide by diffusion
through an epidermis.


For those normally found in water, the gases are dissolved in the liquid.
Worms that occupy terrestrial environments must maintain a moist epidermis for
gases to pass in or out. A dry skin would lead to respiratory failure and death.
Biology 30
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Lesson 5
Earthworms caught in warm, dry conditions could
quickly have their skins dry out. They would not
be able to receive oxygen nor would they be able to
get rid of carbon dioxide and nitrogen wastes. To
avoid too much drying, the worms try to remain in
moist soil conditions. They also secrete some
mucus onto the epidermal surface to help
maintain moistness. On the other hand, if their
burrows are completely submerged by rainwater,
earthworms will come to the surface where
diffusion of gases could be faster.

G
Parasitic worms use their hosts' body fluids to satisfy diffusion needs.
Mollusks – Snails, Clams, Octopuses and Squids
The soft-bodied mollusks could follow a basic three part body plan (which is shown
by the top illustration in the following diagrams):



A head region
A muscular foot, which may remain as a
single unit or divide into tentacles (squids and
octopuses)
A visceral hump which includes all the
internal organs.
Important to all mollusk classes is a special covering layer of cells over the visceral
hump. This is called a mantle and it not only produces an exterior shell in some
classes but by moving away from the visceral hump it creates a mantle cavity. Inside
this mantle cavity water or air can circulate and diffusion can take place between the
mantle and other special structures (gills).
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Lesson 5
Arthropods
A.
Crustaceans
Larger crustaceans, such as the crayfish we have in Saskatchewan waters and
the look-alike but larger ocean lobster, carry out respiration by means of gills.
Gills are located just under a portion of the hardened exoskeleton called the
carapace. Water can enter along the lower edges and pass upward and forward
over the gills. Attachment to the legs allows some movement of the gills which
increases gas exchanges between the water and their capillary-rich filaments.
Water trapped under the carapace enables some of these organisms to
continue functioning should they come out of the water for short periods of
time.
Smaller water crustaceans such as Daphnia, copepods and
amphipods create water flows under their shells by leg and
body movements. Their smaller body sizes enable them to
satisfy respiration needs by diffusion between water and
cells directly, without the help of gills.
Land crustaceans—such as the sow bugs we may find in basements, around
foundation walls or under piles of wood or lumber—are usually small and must
remain in humid conditions. Openings along their flattened, ventral surfaces
permit air to enter their bodies and pass over a series of special plate-like gills
where diffusion occurs.
Bruce Marlin
A sow bug
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Lesson 5
B.
Arachnids – Spiders and Scorpions
Organisms adapted to a land or an air existence, as many of the arachnids are,
show other developments which help in respiration. Circulatory systems,
already evident in some previous groups, are very important in moving gases to
and from all body parts and specific exchange points. It is at these exchange
areas that transfers of gases between the circulatory systems and atmosphere
occur most readily.
Arachnids may show two developments different from the previous
groups.
1.
Along the ventral side of spiders is an opening to a chamber
called a book lung. Many flat, folded membranes extend
into this cavity (like the pages of a book). These membranes
allow circulating blood to easily make gas exchanges.
2.
Some species also have a pair of branched air tubes or
tracheae connected to the external surface by openings
called spiracles. These air tubes enable air to move farther
into the bodies and closer to the circulatory systems and
cells.
Different arachnid species may have both a book lung and
tracheae, just a book lung or just the tracheae.
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Lesson 5
C.
Insects, Chilopods (Centipedes) and Diplopods (Millipedes)
All three of these classes assist respiration with breathing movements which
force air in and out of a series of interconnected tracheae or air tubes and air
sacs.
Rhythmic movements of thoracic and abdominal muscles in a grasshopper
cause some of its front spiracles to open and allow air into the tracheal system.
Further contractions move air through the system and, while closing the front
spiracles, cause the spiracles located farther back along the midline of the
abdomen to open and let air be expelled.
The three classes have interconnected tracheae scattered quite extensively
throughout the bodies. This arrangement appears to be a means of assisting
the circulatory systems as much as possible. Enlargements of the tracheae
into larger sacs at various points in the bodies permit more air to move through
the system and to be exchanged.
The system of tracheal tubes, air sacs and spiracles is present in aquatic
insects also.
Some move about on water surfaces, so they have no great difficulty in
receiving the air they need. Some of the diving insects (and even some water
spiders) can take air with them below the water surface as bubbles attached to
their bodies (diving beetles) or as a thin layer of air completely around their
bodies. Some may carry the air supply until it is used up while others
temporarily store it in bubbles beneath various underwater structures.
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Lesson 5
Vertebrates
A.
The Fishes
Almost all varieties of fish have blood vessels leading to a concentration in a gill
area where they can be close to moving water. Water movement increases the
rates of diffusion by bringing oxygenated water to the gill surfaces and
removing water which has taken on carbon and nitrogen wastes. Water moves
across the gills when a fish moves or when the fish uses a "gulping" action to
suck water into its mouth and then force it out across the gills.
Jawless fishes such as lampreys and cartilaginous fishes such as sharks,
mantas and rays have anywhere from five to fourteen pairs of individual gill
slits along the sides of the anterior ends.
The greater number and variety of fresh water and marine bony fishes
(Osteichthyes) have a number of gill arches, gill rakers and gill filaments on
each side of the head. Unlike the two other classes of fish, bony fish have their
gill structures grouped close together with only one gill slit and protective
covering (operculum) on each side.
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Lesson 5
B.
Amphibians
Frogs, toads, salamanders and other amphibians may have up to three
different structures or organs associated with respiration or gas exchange.
1. A thin, moist skin allows direct diffusion between the skin cells and
capillaries and the surrounding air or water. Reduced respiration
requirements for hibernating or estivating amphibians enable many of them
to survive by means of diffusion through the skin alone, even while they
remain completely submerged.
2. External or internal gills are common in the life cycles of many amphibians.
In some varieties, gills remain throughout their life cycles.
mud puppy
3. The third area where respiration takes place in some amphibians is in a pair
of internal lungs connected by gullet and nostrils to the exterior.
Thin-walled and lined with many blood vessels, the lungs also have many
internal infoldings which increase the area through which diffusion occurs.
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Lesson 5
C.
Reptiles
Many of the reptilian groups have adapted to a land existence. To do so
required a development to reduce or prevent the loss of water through the skin.
This came about with the formation of a thick and largely impermeable skin
covered with scales.
This characteristic eliminated one of the means of respiration shown
by groups before them – that of diffusion through the skin.
After embryonic development and hatching, gills were also lacking.
The only major difference between amphibian and reptilian lungs is that the lungs
of the latter group have many more internal infoldings.
These greatly increase the surface area across which diffusion can occur and
makes up for the loss of gill or skin respiration. Minor adaptations may
develop, as in some snakes, where one lung almost disappears while the other
greatly enlarges or lengthens.
D.
Birds
Birds expend a great deal of energy in sustaining certain body actions,
particularly those involving flight. The large energy requirements lead to high
rates of cellular respiration and demands for large volumes of air. The
respiratory system of a bird consists of a cartilage-strengthened trachea
leading to an enlarged structure called the syrinx ("voicebox"). From here the
air tube splits into two smaller ones called bronchi.
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Lesson 5
Each bronchus makes its way into a lung.
A unique feature about the birds' respiratory systems is the presence
of numerous air sacs which extend out from the lungs and into
various body cavities. Some of these even extend into some of the
larger bones. Each bronchus extends through the lung and to the
posterior air sac. Along the way, tubes branch off into the air sacs
and the lung itself.
Expansion of the thoracic cavity, especially on the upbeat of a bird's wings,
draws air into and through the lungs to the air sacs. Some diffusion occurs as
the air passes through the tubes in the lungs. Very little, if any, diffusion
occurs in the air sacs. Muscle contractions then force much of the fresh air
back from the air sacs and through many capillary-lined tubules in the lungs
before final expulsion. The presence of the air sacs thus enables fresh air to
pass through the lungs on both inhalation and exhalation. Air sacs are also of
value since their presence reduces the weight of birds and forms storage areas
for air when birds fly at oxygen-scarce, high altitudes.
E.
Mammals
A number of major differences exist between the respiratory systems of birds
and those of mammals.
1. The first involves the presence in
mammals of a flat sheet of
muscle which divides the body
cavity into two. The upper or
anterior thoracic cavity
contains the heart and lungs.
The cavity below or posterior to
the muscle sheet is the
abdominal cavity containing the
liver, stomach, pancreas,
intestines and organs of the
urogenital system. The
separating muscle sheet or
diaphragm is important as part
of the actual breathing process.
Biology 30
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Lesson 5
2. Missing from mammalian systems are the air sacs extending out from the lungs
which are common to birds.
3. Another difference is that mammals have the bronchi dividing into smaller tubes
which eventually end up in small, "deadend" air pockets called alveoli. In birds,
the tubules are continuous in the lungs so that air could keep moving straight
through while diffusion is occurring.
The mammalian system is less efficient in that exhalation
forces air that is "spent" or no longer fresh back the way it
came in.
How Breathing Happens
The inhalation process of breathing in mammals mainly involves expansion of the
thoracic cavity. Outward movement of the rib cage is caused by the contraction of
muscles attached to the sides of the ribs and extending towards the back. As this is
happening, the diaphragm muscle also contracts, drawing it downward or away from
the lungs. Abdominal muscles
may relax somewhat as the
diaphragm pushes against the
visceral organs. These actions
result in an increase in the size
of the thoracic cavity and
reduced pressure immediately
around the lungs, causing them
to expand and to have air move
in.
Biology 30
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Lesson 5
Lung expansion is also partly due to the presence of a double, pleural membrane.
This membrane lines the inner surface of the thorax and the outer surface of the
lungs. Moisture and mucus between the thoracic and lung membrane linings cause
them to stick or to adhere to one another, although they can slide over each other.
Therefore, when the thorax becomes larger or moves outward, it pulls the lung
surface with it.
Diseases, hard blows or other accidents to the thorax could
cause a separation between the membranes resulting in a
collapsed lung which can no longer function effectively. A
collapsed lung is sometimes brought about deliberately in
humans by medical means to give it a better opportunity to
heal from some injury or disease. Gradual absorption of air
from between the two membranes by the body will usually
bring the outer surface of the lung and the inner thoracic
surface into contact once more.
In normal exhalation, relaxation of the outer rib muscles allows the thorax to move
back (inward). Relaxation of the diaphragm allows it to resume its concave shape
towards the thoracic cavity, also reducing its volume. The natural elasticity of the
lungs then causes those organs
to shrink, forcing out the air. An
organism may consciously force
more air out by contracting
another set of rib muscles
drawing the chest or rib area in
even farther. At the same time,
contraction of abdominal muscles
pushes the viscera against the
diaphragm moving it farther into
the thoracic cavity.
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Lesson 5
Air enters and leaves a body through the nostrils or mouth. When it enters, it moves
to the back of the throat area or pharynx. At this point there is an opening called the
glottis which leads into a cartilage-strengthened trachea. In most mammals, a
relatively prominent larynx or voicebox (containing vocal cords) is at the beginning of
the trachea.
A cartilage structure, the epiglottis, serves to cover the
tracheal opening or glottis when an organism swallows. This
prevents saliva or food from entering the respiratory system
to cause choking, although there are occasions when the
epiglottis may not close in time.
Air moves along the trachea, which splits into bronchi leading to the right and left
lungs. Inside the lungs, the bronchi split into smaller branches called bronchioles.
The bronchi and larger bronchioles continue to be strengthened by horseshoe-shaped
rings of cartilage.
Bronchioles finally end in clusters of tiny, thin-walled sacs. It is through the walls of
these alveoli and surrounding capillaries that diffusion of gases occurs (see diagram
above). Exhalation simply reverses the pathway of air movement. In normal
breathing, an average human adult inhales and exhales approximately 500 ml of air.
Forced, deep breathing can increase this nine to ten times. (This is called vital
capacity.)
The total surface area of all the alveoli is considered to be
about 70-90 square meters (750-975 square feet), which
means that the area over which diffusion can occur is quite
large. (This is the size of a small house).
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Lesson 5
The rate of breathing appears to be controlled by an area of the brain called the
medulla oblongata. This area is sensitive to the amount of carbon dioxide carried in
the blood. Blood leaving the capillaries surrounding the alveoli carries the same
carbon dioxide concentration as the air in those alveoli. If the medulla senses a
carbon dioxide concentration above a certain level in the blood, it sends impulses to
the diaphragm and rib muscles to speed up breathing. A higher carbon dioxide
concentration also causes the walls of the bronchioles to expand. This dilation of the
air pathways allows a greater volume of air to move through them. As the carbon
dioxide levels fall, the medulla's impulses and the rate of breathing slow down as
well. In this way, the carbon dioxide levels in the blood and the medulla oblongata
that senses them act as homeostatic devices to control breathing rates.
At high altitudes where oxygen is scarce, the brain control could be fooled because the
levels of carbon dioxide produced would be low as well. Breathing would not speed up
even though the body is not receiving enough oxygen. A back-up control exists in
receptors in the aorta and carotid arteries. These can detect lower oxygen levels in
the blood causing them to send impulses to the medulla, which in turn speeds up
breathing.
Some Respiratory Disorders
Before concluding mammalian respiration, brief mention will be made of some
respiratory disorders that can affect humans. As mentioned earlier, the total
diffusion area of human lungs could be up to 90 square meters. Any kind of
situations or disorders that reduce this area could create a condition of oxygen
starvation where not enough oxygen is diffusing into the blood system.
Pneumonia infections lead to accumulations of lymph and mucus in alveoli and
bronchioles that prevent air from reaching the walls of the alveoli.
Emphysema is a condition that can develop slowly over a number of years.
Emphysema is caused almost always by cigarette smoking. Air pollution may also
play a role. An inherited form is rare.
Emphysema is a Greek word
meaning ‘inflation’.
Emphysema is the over-inflammation and eventual destruction of the tiny air sacs,
the alveoli in the lungs. The elasticity of the alveoli and walls of the airways is
destroyed and air becomes trapped within the lungs. The transfer of oxygen from the
air to the blood stream is affected. A person affected by Emphysema becomes very
short of breath. Extra strain is placed on the heart.
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Lesson 5
Chronic bronchitis is also a condition that may develop from smoking. Healthy air
passages are lined with (epithelial) cells possessing hair-like cilia. These cells secrete
mucus that is swept along by the waving action of cilia. Mucus and cilia are
designed to trap and then remove particles from the breathing passages. Tars from
smoke can settle around the cilia and slow down their actions, leading to irritation of
the epithelial cells. These cells swell and are even further irritated by accumulations
of mucus. This can lead to extra coughing and clearing of the throat as one attempts
to clear the mucus and any other irritants from the air passages.
Smoking increases the incidence of cancer, where abnormal lung cells develop and
interfere with diffusion and blood circulation. Such cancerous cells are initially hard
to detect and are easily spread by the circulatory system from the lungs to other body
areas.
Summary
All autotrophs and heterotrophs are constantly involved in the process of obtaining
nutrients for their cells and bodies. These nutrients are the sources of matter for
building and repairing cells. They are also the sources of energy for carrying out all
the necessary life-sustaining activities. For most green plants, the nutrients are of
an inorganic nature and are obtained in soluble form through cell membranes.
Heterotrophic organisms also take in inorganic nutrients (water, dissolved
minerals...) but a large amount is organic – from plant and animal bodies.
Manners of actually obtaining nutrients vary greatly. Once nutrients, especially of an
organic nature, are taken in or ingested, the processing and general outcomes are
fairly similar among many species. The end result is to change them into forms
which are soluble and readily usable.
Synthesizing or breaking down organic compounds requires particular gases while
releasing others. Respiration, or the exchange of gases at the cell membrane level, in
all organisms occurs by the process of diffusion. Differences occur in the manners of
trying to regulate concentrations of gases near membranes. As body sizes and
complexities increase, especially among land animals, greater specializations appear
in order to increase volumes and speeds of gas movements. Physical movement or
"breathing" becomes apparent in larger animals and this is one of the major
distinctions between animal and plant respiration. Along with breathing, animal
orders also develop special structures or organs where gases can be rapidly
exchanged with circulatory systems which transport those gases throughout the
bodies. Gills, tracheae, book lungs and lungs make their appearances in various
groups as adaptations to certain environments.
The importance of respiration to organisms is often emphasized by conditions that
slow or stop this action. Fish, crayfish or water plants soon die if left on land. Land
plants and animals "drown" in water. Diseases or injuries which affect breathing
passages or organs could impair the general health and normal actions of organisms
involved and could also cause death.
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Lesson 5