Download The Task of Respiration

S E C T I O N
8.1
The Task of Respiration
E X P E C TAT I O N S
Describe gas exchange in
simple organisms.
Explain why some organisms
do not need specialized
respiratory systems.
List the respiratory
adaptations that enable
organisms to live on land.
Describe the main types of
animal respiratory systems.
Figure 8.1 Single-celled
organisms, like the amoeba
shown here, do not require a
specialized respiratory system.
Like most organisms on earth, the amoeba is aerobic
— that is, it requires oxygen to survive. Whether an
aerobic organism is unicellular, like the amoeba, or
multicellular, each cell in the organism must have
a supply of oxygen. Oxygen is necessary to carry
out cellular respiration, the process that releases
the energy needed to drive all cell functions. Each
cell must also rid itself of the carbon dioxide that is
the waste product of cellular respiration. The basic
function of the respiratory system is to make sure
that oxygen can enter each cell in the organism,
and that carbon dioxide can leave each cell. This
process is known as gas exchange.
REWIND
To review the concept of cellular respiration, turn to
Chapter 3, Section 3.3.
Gas Exchange at Its Simplest
Most single-celled aerobic organisms do not have a
distinct respiratory system. Such organisms,
including protists, algae, fungi, and some bacteria,
rely instead on diffusion to meet their gas exchange
requirements. These organisms need moist
environments, and are found either in aquatic
environments or in other moist places such as
within the body of a host organism. Oxygen, which
is dissolved in the water around the cell, diffuses
through the outer membrane of each cell and
thereby becomes available for cellular respiration.
At the same time, carbon dioxide diffuses out. For
these organisms, the cell membrane itself provides
a moist surface area big enough to accommodate
the organism’s gas exchange requirements.
Math
Different organisms have different kinds of
respiratory systems to accomplish the task of gas
exchange. Every respiratory system, however,
shares two requirements. First, the surface area
available for gas exchange, or respiratory surface,
must be big enough for the exchange of oxygen and
carbon dioxide to occur at a rate that will meet the
organism’s metabolic needs. Second, respiration
must take place in a moist environment, so that the
oxygen and carbon dioxide are dissolved. The
respiratory systems you will examine in this chapter
all demonstrate variations of these two basic features.
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LINK
A number of factors affect the rate of diffusion of dissolved
gases. These include the surface area available for diffusion,
the concentration gradient, the size of the diffusing molecules,
and the length of the diffusion path. In one set of experiments,
a group of researchers found that, under a particular set of
conditions, a molecule of oxygen dissolved in water diffused
1 µm in 0.0001 s, but took 100 s to diffuse 1 mm. Write an
equation showing the relationship between the diffusion time
and the length of the diffusion path. Now estimate the distance
from your nose to your writing hand when your arm is fully
extended. How long would it take oxygen to diffuse that distance,
assuming the same conditions as in the experiment above?
Colonial algae, such as volvox, and simple
animal organisms such as hydra (see Figure 8.2A
and B) also lack specialized tissues for gas
exchange. Although they are multi-celled organisms,
almost all of the cells of these organisms are in
direct contact with the surrounding aquatic
medium. As a result, each individual cell can
obtain sufficient oxygen through diffusion. In
contrast to volvox or hydra, the worm-like
planarian (see Figure 8.2C) has all three germ
layers, or tissue types, found in higher animals.
This tiny creature can still manage without a
respiratory system, however, because of its thin,
flat shape. A planarian’s body is at most only a few
cells deep, so oxygen has a short distance to travel
in order to diffuse into every living cell. At the
same time, the width of the body provides a large
surface area for gas exchange.
B
C
Figure 8.2 Volvox (A), hydra (B) and planarian (C). These
simple organisms can also depend on diffusion alone to
meet their gas exchange requirements.
A
Unicellular Terrestrial Organisms
Some unicellular terrestrial organisms also lack a
distinct respiratory system. Bacteria and fungi
living within the soil, as well as terrestrial protists,
depend on the air in the tiny spaces in the soil for
their source of oxygen. The organism’s requirement
for a moist respiratory surface is satisfied by water
derived from the soil solution. As long as there is
water in the soil, the air spaces will remain humid
enough for the cell membrane to stay moist.
Many bacteria and fungi live on the surface of
the soil, often in association with organic debris.
These organisms depend on simple diffusion of
gases to and from the surrounding air. As in their
aquatic or soil-borne counterparts, this exchange
takes place directly across the cell membrane. In
general, it is the availability of moisture (either as
soil moisture or as water vapour in the surrounding
air) that restricts the habitat of such organisms.
Figure 8.3 This environment supports many unicellular
terrestrial organisms.
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The Specialized
Respiratory System
The larger an organism is, the more oxygen it
requires to supply its cells, and the greater the
distance over which that oxygen must travel in
order to reach all of the cells. This raises two
problems. First, diffusion of oxygen is only effective
over a distance of a few cells, so an organism with
a body more than a few cells thick must use another
mechanism to bring oxygen to all its cells. Second,
as body parts become differentiated for uses such
as locomotion or reproduction, the body surface
that can be dedicated to gas exchange is reduced.
Different kinds of adaptations have made it
possible for larger, more complex organisms to
meet their need for oxygen. All of the higher
animals have evolved a specialized respiratory
system. An animal’s respiratory system includes a
respiratory surface, any passageways that connect
this surface to the external environment, and any
muscular structures that are used to help bring the
respiratory medium into contact with the
respiratory surface. The following pages examine
several types of specialized respiratory structures
found in animals.
PAUSE
mouth
carbon dioxide diffuses out
oxygen diffuses in
network of blood
vessels and capillaries
lying beneath the skin
gut
Figure 8.4 The earthworm makes use of its moist skin
surface to exchange gases between its circulatory system
and its environment.
The exchange of gases across the skin of the
animal is called skin respiration. As in volvox and
planaria, the gas exchange occurs across the
organism’s entire body surface. The crucial
difference is that the earthworm uses a circulatory
system to carry oxygen to the cells that could not
rely on diffusion to meet their needs.
RECORD
Multicellular animals have evolved ways of storing nutrients
and, to a lesser extent, water in their bodies. Your own body
can survive for several days without water, and for weeks
without food, but only for a few minutes without oxygen.
What factors might prevent your body from storing oxygen?
List your ideas and explain briefly. Hint: what does the term
“oxidization” mean?
Skin Respiration
One adaptation is demonstrated by animals in the
phylum Annelida, which includes the segmented
worms such as earthworms and leeches. Segmented
worms can survive only in water or in damp earth,
because their skin must remain moist. To meet
their oxygen requirements, earthworms make use of
the moist surface and their simple circulatory
mechanism. The skin is lined with many tiny
capillary vessels, and these capillaries make contact
with the skin surface all over the body of the worm.
At each of these points of contact, respiratory gases
diffuse into and out of the circulatory vessels
(Figure 8.4). The oxygen is then carried through the
circulatory vessels to other parts of the organism.
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Figure 8.5 The aquatic tube worm cannot exchange gases
through its protective casing.
Gills
In contrast to the earthworm, whose whole body
surface is available for respiration, most of the
body of a tube worm is covered with a protective
casing. Many other aquatic animals also have
protective coats. These coats provide defence
against predators or help maintain the animals’
internal environments. In such organisms the
relative area over which diffusion of respiratory
Figure 8.6 The fish draws water through its
mouth and over its gills. As water passes
over the gills, oxygen diffuses into the
adjacent blood vessels. At the same time,
carbon dioxide diffuses out.
gills
water and
oxygen
gill
water and
carbon dioxide
artery
to gills
artery
from gills
oxygenated
blood
deoxygenated
blood
water
flow
gill
filament
gases can occur is considerably reduced. Two
adaptations help these organisms accomplish the
task of respiration. First, structural changes have
increased the surface area of the body part involved
in gas exchange. Second, a mechanism has evolved
that enables the animal to ventilate this surface —
that is, to move the oxygen-containing aquatic
medium over the respiratory surface. This allows
the respiratory surface to be constantly exposed to
a fresh supply of oxygen.
Many aquatic organisms, including molluscs,
crayfish, mud puppies, tadpoles, and fish, have
developed gills, which are feathery tissue structures
that invariably consist of numerous delicate
branches. When the gills are exposed to water, gases
are exchanged across the thin gill membranes. The
oxygen diffuses into the organism’s vascular system
for circulation around the body. Gills ensure that a
considerable surface area is available for gas
exchange in a very limited space.
Gilled animals have evolved various means of
ventilation. In some animals, such as the tube worm,
the gill is moved through the water. In others, such
as fish and clams, water is drawn or pumped over
the gill. In most gilled animals, the water flows
only one way over the gills. This reduces the
amount of energy the animal must expend to move
the water over the respiratory surface.
Figure 8.6 shows the respiratory organ of a
typical fish. Water moves in through the mouth and
out behind the animal’s head, passing over the gills
on the way. Adjacent to the gills are a network of
tiny capillaries that absorb oxygen and release
carbon dioxide from the animal’s circulatory system.
The ventilation mechanisms of
some aquatic animals are linked with
other functions. In a number of
species, including many molluscs,
feeding and ventilation are
accomplished together. Food
particles are captured as the gill
moves through the water, and are
filtered out by the organism at the
same time that gas exchange takes
place. Other animals use the currents
produced by respiratory action for
locomotion. A squid, for example, can move rapidly
forward or backward by forcing water out of its
gill siphon.
Moving out of the Water
Given how much equipment we need in order to
breathe under water, it may be hard to imagine
that, from an evolutionary perspective, respiration
in air is actually the bigger challenge. In the water,
the need for a respiratory surface that stays moist is
rarely a problem. On land, however, moisture from
a respiratory surface exposed to the air will
evaporate, so the animal would have to release
more water constantly to keep the surface moist. A
terrestrial animal that made a large moist surface
area available for gas exchange would therefore risk
excessive water loss. Before living organisms could
Figure 8.7 People must take air with them in order to
breathe under water, along with special equipment to
dispense it.
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begin to colonize terrestrial environments,
adaptations were needed that could solve this
critical problem. In much the same way that scuba
divers take some of their usual respiratory medium
(air) along on the dive, all terrestrial animals have
evolved some way to carry a moist respiratory
environment wherever they go.
In the previous paragraphs, you learned that
most aquatic animals have developed some way to
ventilate their respiratory surface. The same is true
of most terrestrial animals. The act of ventilating a
respiratory surface with air is called breathing. The
breathing of terrestrial organisms relies on a basic
law of physics: air will move from a region of
higher pressure to a region of lower pressure until
equilibrium is reached (Figure 8.8). Different
terrestrial animals have evolved different ways to
make use of this principle in their respiratory
systems.
both containers same size, same pressure
The Tracheal Respiratory System
In insects, the problem of how to maintain a moist
respiratory surface without losing too much water
is resolved by an internal respiratory system that
consists of a series of external pores called
spiracles. The spiracles lead to an internal network
of tubes called tracheae (singular trachea). All of
the spiracles are controlled by valves. Although the
specific configuration of spiracles and tracheae
varies considerably from one species to the next,
the basic elements of the tracheal system are
common to most insects and many other nonvertebrate terrestrial animals.
If you can, observe a living grasshopper. Note
that the abdomen contracts and relaxes
rhythmically. These are the breathing movements
of the animal. As described in Figure 8.9, the
animal’s breathing causes an orderly movement of
air through the tracheae, from the front of the
animal to the back.
trachaeole
O2
A At equilibrium, air pressure is equal in both vessels.
left container larger, lower pressure; air moves in
cell
O2
air
+ O2
spiracle
(pore through skin)
A As the abdomen expands, air pressure drops within the
tracheae. At the same time, the anterior four pairs of
spiracles open, while the posterior six pairs of spiracles
remain closed. Air therefore flows in through the anterior
spiracles.
trachaeole
B If the volume of one vessel increases, the air pressure
inside it decreases. Air flows from the vessel with the
higher pressure into the vessel with the lower pressure.
CO2
CO2
cell
same pressure in both (but lower than above)
air + CO2
spiracle
(pore through skin)
B When the abdomen contracts, the reverse situation
occurs. The anterior four pairs close and the posterior six
pairs open. Air pressure is now higher inside the animal
than in the surrounding environment, so air flows out
through the posterior spiracles.
C A new equilibrium is reached with air pressure once
again equal in each vessel.
Figure 8.8 Air will move from a region of higher pressure
to a region of lower pressure.
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Figure 8.9 The tracheal respiratory system of a grasshopper
In insects, the respiratory and circulatory
systems are separate from one another. The tracheal
system of an insect does not require a circulatory
system to transport the respiratory gases. Instead,
the many branches of the tracheal tubes ensure that
the respiratory surface is in close enough contact
with all of the living cells to allow gas exchange to
occur by diffusion across the moist tracheal walls.
Because most of the tracheal system remains
inside the animal, the risk of excessive water loss
is reduced.
lining of the mouth. This explains why a frog
cannot live for any extended period away from
either an aquatic or a very humid environment.
Because it uses its skin as a respiratory surface, it
has no satisfactory means of controlling water loss
through the skin.
The Lung
The third general type of respiratory system, and
the one that is characteristic of air-breathing
vertebrates, is the internal lung arrangement. The
lung is an internal respiratory surface connected to
the air by means of internal passageways. Lung
systems vary from species to species in terms of
both structure and efficiency. However, all lung
systems consist of three basic elements: one or two
lungs that have a moist respiratory surface; some
means of forcibly bringing air in contact with the
lung surface; and a circulatory system to carry the
gases between the lungs and the other cells of the
body. Figure 8.10 outlines the various general lung
types found in a number of animal species.
The frog uses a simple lung system ventilated by
the movement of muscles in the nostrils, mouth,
and abdomen. To breathe in, the frog first closes its
mouth and opens its nostrils or nares, then lowers
the floor of the mouth, drawing air into the mouth
cavity. The nostrils are then closed, and the floor of
the mouth raised. As a result the air is forced into
the lungs where gas exchange takes place. In the
frog, as in other amphibians, the lung system is
used in conjunction with both skin respiration and
the exchange of gases across membranes in the
SECTION
Figure 8.10 Examples of the types of lungs found in
animals such as amphibians, spiders, and reptiles. Spiders’
lungs are often referred to as “book lungs” because their
many folds are arranged like the pages of a book.
Snakes and other reptiles that flourish on land
(and even in desert environments) do not use the
skin as a site for the exchange of gases. They
confine this exchange to the internal lung or lungs.
All these organisms possess muscle and skeletal
arrangements that enable them to force air out of
the lungs and draw air back over the lung surface
to ventilate the respiratory tract.
REVIEW
1.
Describe how gas exchange takes place in an
amoeba.
2.
K/U Describe two ways in which the problem of
getting respiratory gases to individual cells has been
overcome in larger, more complex organisms.
3.
Imagine an argument between a tube worm and
a grasshopper about the best way to respire. Working
with a partner, try to create an imaginary dialogue
between the two animals (assume that the two
animals can speak the same language). Where are
the greatest misunderstandings likely to arise? How
would you suggest the two settle their argument?
K/U
C
lungs
4.
5.
Populations of grasshoppers and locusts
occasionally skyrocket, creating plagues that can
cause considerable damage to farm crops. How
could knowledge of the way grasshoppers breathe
be used to control their numbers?
MC
K/U
Can a frog respire under water? Why or why not?
6.
Account for the fact that you see many
earthworms on the ground after a rainstorm.
7.
I A new species of insect is discovered. This insect
has 12 pairs of spiracles along its abdomen. Design
an experiment to test whether the spiracles work in
concert, in sequence, or some type of intermediate
mechanism.
K/U
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