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The chemical
CHAPTER
● Earth
● Biosphere
● Biome
● Ecosystem
● Community
● Population
● Organism
● Systems
● Organs
● Tissues
● Cells
● Organelles
● Molecules
● Atoms
composition
of cells
Chapter 2 The chemical
composition of cells
Key knowledge
General role of the enzymes in biochemical activities of cells
● Composition of cells: major groups of organic or inorganic substances, including carbohydrates,
proteins, lipids, nucleic acids, water, minerals, vitamins; their general role in cell structure and function
●
If you have ever ventured into the Australian bush, you may have come across an
intriguing sight: a wriggling knot of what seems like hundreds of caterpillars hanging
onto a eucalypt branch. These are the larvae of the sawfly wasp. They are better known
as spitfires and for a very good reason. If they are distracted in any way from their
writhings, they spit out a horrible, smelly mixture of green slime. This is made up of
the juice of the eucalypt leaf on which
it feeds. On a good day, this can travel
as far as 20 cm.
Spitfires, like all other living things,
are made up of cells. If you investigate
their cell structure you will find that
it is very like your own cell structure.
As you have seen in Chapter 1, animal
cells share a lot of common organelles.
The way that spitfire cells function is
also very similar to your own cells.
If we were able to zoom in on the
cell of the spitfire and its organelles,
and investigate the types of chemicals
that make up and are used in those
structures, we would see even more
similarities between different types
of cells.
Figure 2.1 A wriggling knot
of sawfly wasp larvae.
The chemicals in cells
Organic and inorganic molecules
Many of the molecules found in living matter are larger and more complex than those
in non-living material. With the exception of water, most of the molecules in living
organisms contain the element carbon. These complex carbon-containing molecules
are known as organic compounds. In the majority of cases, carbon (C) is joined with
hydrogen (H), oxygen (O), and in some cases, nitrogen (N) and phosphorus (P). The
organic compounds most commonly found in cells are carbohydrates, lipids, proteins,
and nucleic acids.
Other compounds are classified as inorganic. Of the inorganic compounds, water
and minerals are among the most important. Carbon dioxide is termed ‘inorganic’ as it
is not a complex molecule and does not contain hydrogen. Living things contain both
organic and inorganic compounds.
bioTERMS
organic
derived from living organisms;
complex carbon containing
compounds
inorganic
all other compounds that are
not organic
29
Investigating the chemicals in cells
Cells contain a large variety of organic and inorganic substances. It is possible to
identify which organic substances occur in which part of the cell by using biochemical
tests. The solutions that are used in these tests react with certain organic substances
and undergo a colour change described in Table 2.1.
Table 2.1 Biochemical tests for specific organic substances.
Table 2.2 Changes in the colours of
Benedict’s solution when reacted with
simple sugars.
Colour of
Benedict’s
reagent
Approximate
sugar
concentration
Blue
Nil
Light green
0.5–1.0%
Green to yellow
1.0–1.5%
Orange
1.5–2.0%
Red to red–brown
>2.0%
Substance tested for
Biochemical test used
Expected colour change if
chemical present
Monosaccharide (glucose)
Benedict’s solution
Green – orange – red
Starch
Iodine solution
Dark blue
Lipid
Sudan IV indicator
Pink – red
Protein
Biuret reagent
Pink – violet – purple
As shown in Table 2.1, Benedict’s solution is an indicator for monosaccharides.
Table 2.2 summarises the expected colour changes of Benedict’s solution when
reacted with simple sugars.
By using the tests outlined in Table 2.1 and other biochemical tests, it is
possible to create an image of a cell and the chemicals that it contains. In Chapter 1
you saw that bacterial cells do not contain membrane-bound organelles, but plant
and animal cells contain many similar organelles and some different organelles. You
would therefore expect the chemicals within bacterial, plant and animal cells to
show similarities and differences (see Figure 2.2(a), (b) and (c)).
aa
ribosomes – protein
and RNA
cell wall – peptidoglycan
(polysaccharide and protein)
ACTICA
PR
2.1
CTIVITY
LA
Figure 2.2 (a) (above) Bacterial cell
showing where inorganic and organic
substances occur (b) (opposite) Plant
cell showing where inorganic and
organic substances occur (c) (p.32)
Animal cell showing where inorganic
and organic substances occur.
capsule –
polysaccharide
and protein
30
Unit 1
chromosome
– DNA
plasma
membrane – −
phospholipid
bilayer
plasmid
– DNA
cytoplasm
– 80% water
Chapter 2 The chemical
composition of cells
chloroplast
DNA
carbohydrate – glucose
b
plasma membrane
lipid – phospholipids
– steroids
protein – receptor
– transport
cytoplasm
water
carbohydrates
lipid
protein – enzyme
RNA
nucleolus
protein – enzymes
nucleus
DNA
RNA
protein – enzymes
cell wall
cellulose
large permanent vacuole
water
mitochondrion
carbohydrate – glucose
DNA
protein – enzymes
31
c
cytoplasm
water
carbohydrates
lipid
protein – enzyme
RNA
plasma
membrane
lipid – phospholipids
– steroids
protein – receptor
– transport
endoplastic reticulum
proteins
mitochondrion
carbohydrate – glucose
DNA
protein – enzyme
ribosomes
RNA
protein
– enzyme
nucleolus
protein – enzymes
cell nucleus
DNA
RNA
protein
– enzyme
32
Unit 1
Chapter 2 The chemical
composition of cells
Table 2.3 Organic compounds in cells.
Organic compound
Elements
Examples
Where found in cell
Functions
Carbohydrates
Carbon
Hydrogen
Oxygen
Monosaccharides
– glucose
Disaccharides
– sucrose
Chloroplast
Mitochondrion
Cytoplasm
Polysaccharides
– starch
– glycogen
– cellulose
Plastid
Cytoplasm
Cell wall
Energy source and
storage
Strengthens plant cell
walls
Provides tough exterior
covering
Carbon
Hydrogen
Oxygen
Phosphorus (in
phospholipids)
Fats and oils
Cytoplasm
Phospholipids
Cell membrane
Steroids
Cell membrane
Waxes
Cell surface
Carbon
Hydrogen
Oxygen
Nitrogen
Sometimes sulfur and
phosphorus
Messenger proteins and
antibodies
Ribosome, endoplasmic
reticulum, Golgi
apparatus, vesicles
Receptor proteins
Transport proteins
Cell membrane
Structural proteins
Cell surface
Enzymes
Throughout the cell
DNA
Nucleus
Chloroplast
Mitochondrion
RNA
Nucleus
Cytoplasm
Ribosome
Lipid
Protein
Nucleic acid
Carbon
Hydrogen
Oxygen
Nitrogen
Phosphorus
Energy storage
Cell membrane
component
Chemical messengers
Protection
Immunity
Communication
Transport materials
Support cell and body
structures
Speed up chemical
reactions
Store hereditary
information (DNA)
Decode hereditary
information (RNA)
The plasma membrane
The cells in spitfires, like your own cells, are animal cells. The cells in the eucalypt
leaves that the spitfires eat are plant cells. The bacteria in the digestive system of the
spitfire that assist in digesting the eucalypt leaves are prokaryotic cells. As you saw in
Chapter 1, all these types of cells are enclosed by a plasma membrane. This membrane
forms the boundary between the inside of the cell and the environment on the outside
of the cell. In order for the cell to survive, it must be able to allow substances to pass
across it.
In order to maintain cellular respiration, substances such as oxygen and glucose
move into the living cell, and waste products such as carbon dioxide and excess
water move out into the surrounding environment where they are disposed. Carbon
dioxide and water must be able to move into the plant cell for use in photosynthesis,
33
BIO
Figure 2.3 A phospholipid molecule.
The hydrophilic heads are attracted to
water whereas the hydrophobic tails
repel water.
LIN
K
Plasma membrane
and glucose and oxygen must be able to move out. Other cellular processes require
different inputs and produce different wastes and products. As you learnt in
Chapter 1, other substances, such as proteins, enzymes and hormones, must also be
able to move across the plasma membrane.
The plasma membrane has other interesting features. It needs to be a flexible
structure so the cell can change shape easily. It also needs to be able to grow and
expand as the cell contents increase, especially during cell division. If the plasma
membrane is punctured, some of the cytoplasm will leak out, but the hole will be
sealed quickly.
What makes up the plasma membrane to allow it to act as a regulatory boundary
between the inside of the cell and the outside? How is material selected to move across
the membrane? How does it reseal a puncture? To answer these questions, we need to
look at the property of the chemicals that make up the plasma membrane.
One remarkable property of plasma membranes is their ability to change shape,
expand and contract. During cell division and vesicle formation, membranes can break
and reassemble themselves. This is because membranes are actually two-dimensional
fluids, constantly flowing and changing shape.
The plasma membrane, and in fact all
membranes surrounding organelles within
hydrophilic
the cell, are made up of a double layer of
head containing
phosphate
phospholipid molecules, the phospholipid
bilayer, as shown in Figure 2.3. Phospholipids
are part of a larger group of molecules known
hydrophobic tail
as
lipids (see below). Phospholipids alone
made of fatty acid
would not allow this required flexibility,
side chains
as strong inflexible bonds naturally form
between the lipid tails.
Another type of lipid called cholesterol
is interspersed among the phospholipid molecules. Cholesterol prevents the lipid tails
forming strong bonds and therefore prevents inflexibility. Therefore, it is cholesterol
that is responsible for the flexibility of membranes.
Cholesterol belongs to a special group of lipids called steroids. Cells can remodel
cholesterol to form other steroids such as the sex hormones progesterone, oestrogen
and testosterone and bile salts that assist with fat digestion in the small intestine.
Vitamin D (see page 36) is another steroid derived from cholesterol.
REVIEW
1 Organise the following list of substances under either the heading
of organic or inorganic:
water, lipid, protein, minerals, carbon dioxide, carbohydrate,
nucleic acid
2 How do scientists determine which chemicals occur within cells?
3 Explain why the plasma membrane needs to be able to allow some
substances to pass through it.
34
Unit 1
4 Explain what chemicals are responsible for each of the following
features of the plasma membrane:
a ability to change shape
b flexibility
c ability to reseal a puncture
Chapter 2 The chemical
composition of cells
Lipids
Lipids form a larger class of compounds containing fats and oils. Fats are solid at room
temperature and oils are liquid. They all contain the elements carbon, hydrogen and
oxygen and are insoluble in water. You have probably noticed that if you mix oil and
water together, the oil will float on top of the water. As lipids are complex molecules
and contain carbon, they are an example of an organic compound.
The most common type of lipid is the
triglyceride. As its name (tri-) suggests it is
composed of three fatty acids and one glycerol (see
Figure 2.5). If the fatty acid contains only single
bonds between the carbon atoms, it is described
as a saturated fat. Single bonds tend to be difficult
to break down by the cells in your body. As such
they can also withstand much higher temperatures.
This is why you will find fast-food outlets cooking
with saturated animal fat such as lard instead of the
more unstable unsaturated fats such as peanut oil
and sunflower oil.
If there are double bonds between some carbon
atoms then the fat is described as unsaturated. If
there are many double bonds between the carbon
atoms then it is described as polyunsaturated.
Double bonds are more easily broken down
bioBYTE
by the cells in your body than are
single bonds.
Fats are an essential part of our diet.
Fats that originate from plants and seafood
Phospholipids differ to triglycerides
(such as soy, nuts and vegetable oils) are
because one of the fatty acids is replaced
the healthiest type of fat you can eat. Fats
by a phosphate group. This causes one
from animals such as steak and chicken
end of the phospholipid molecule to be
are healthy in moderation, and the worst
fat that you can eat is fat from processed
‘water-loving’ (hydrophilic) and the other
foods (hydrogenated oil).
end to be ‘water-hating’ (hydrophobic)
(see Chapter 3).
Figure 2.4 Saturated fats are used
for cooking at high temperatures;
unsaturated fats will break down at
these temperatures.
bioTERMS
fatty acid
a type of organic acid that
combines with glycerol to
form fat
glycerol
a molecule that combines with
three fatty acids to form fat
a
H
H
H
H
H
H
H
H
H
H
H
H
H
C
C
C
C
C
C
C
C
C
C
C
C
H
H
H
H
H
H
H
H
H
H
H
H
O
C
OH
Examples:
beef
pork
cheese
coconut oil
butter
saturated fatty acid
b
H
H
H
H
H
H
H
H
H
H
H
H
H
C
C
C
C
C
C
C
C
C
C
C
C
H
H
H
H
H
H
unsaturated fatty acid
O
C
OH
Examples:
olive oil
peanut oil
almonds
sunflower oil
corn oil
fish
mayonnaise
most margarines
Figure 2.5 Structural diagram
of (a) saturated fat showing
single bonds between carbon
atoms (b) unsaturated fat
showing double bonds between
some carbon atoms.
35
Vitamins
Fats are often regarded to be an unhealthy inclusion in the diet. They are, however,
essential in the absorption of many important vitamins. Vitamins are organic
molecules that are needed by the body in minute amounts. Vitamins help your body
grow, assist in the normal functioning of many metabolic processes and change food
into energy. Vitamins are either water-soluble (vitamins B group and C) or fat-soluble
(vitamins A, D, E and K). Your body cannot store water-soluble vitamins and as such
they must be eaten every day. If these vitamins are not used immediately by your body
they are lost in the urine. Fat-soluble
bioBYTE
vitamins are stored within your fatty
tissue, but if you eat too much of
Controlled supplements of vitamin K
may help to retain calcium in the bones
certain vitamins they can accumulate
during menopause. Newborn babies
and eventually become harmful.
are given an injection of vitamin K to
Table 2.4 shows some of the essential
help prevent haemorrhagic disease.
vitamins that need to be included in
(Haemorrhagic disease is a condition
where excess bleeding occurs either
your diet to ensure normal healthy
internally, in any part of the body, or
cell functioning.
from an open wound.)
BIOBOX 2.1
MERTZ OF THE ANTARCTIC
BIO
Vitamin A toxicity results from an excessive intake of the vitamin. The amounts ingested need to be
well above the recommended daily intake and occur over several months. As vitamin A is fat-soluble, it
is not excreted in the urine but, instead, is stored in the liver, where it may reach toxic levels.
The signs and symptoms of vitamin A toxicity include nausea, loss of hair, drying and scaling of
skin, bone pain, fatigue and drowsiness. Blurred vision and headaches along with growth failure and
enlargement of the liver can also be observed.
Xavier Mertz accompanied Douglas Mawson on the 1911 Australasian Antarctic Expedition.
When food supplies ran out, Xavier killed and ate the husky dogs who had been pulling the sleds.
Consumption of too many husky livers led to vitamin A toxicity and eventually to Xavier’s death.
Today, over-consumption of vitamin A can result from the ingestion of unsuitable amounts
of synthetic vitamin preparations. This can lead to a yellowing of the skin, a condition known as
carotenaemia.
 Define a vitamin excess.
 Explain why a large intake of vitamin A can lead to it building up to toxic levels in the body.
 List the symptoms of vitamin A toxicity.
LIN
K
Icy explorers
36
Unit 1
Chapter 2 The chemical
composition of cells
Table 2.4 Vitamins – their source and functions.
Vitamin
Source
Function
Fat-soluble vitamins
A (retinol)
Fish-liver oils, butter and margarine,
green and yellow vegetables,
carrots, yellow-fleshed fruits,
tomatoes, egg yolk, liver and
kidneys, whole milk
Epithelial tissues – skin, linings of
nose, mouth, digestive and urinary
tracts; vision in dim light – forms
visual purple in retina of the eye
D (calciferol)
Liver, eggs, and can be made by the
body
Stimulates absorption of calcium
and phosphorus in bone and teeth
formation
E (tocopherols)
Wheat germ, butter and margarine,
bread, green leafy vegetables,
whole-grain products
Prevents damage to cell
membranes; protects fats and
vitamin A from destruction by
oxidation
K (phytomenadione)
Green vegetables, tomatoes,
cabbage, cauliflower, potatoes,
cereal, eggs
Formation of prothrombin, essential
for blood clotting
Water-soluble vitamins
B1 (thiamine)
Seafood, meat, whole-grain
products
The release of chemical energy from
carbohydrates
B2 (riboflavin)
Meat, eggs, green vegetables,
mushrooms, whole-grain products,
pasta
Protein metabolism; helps maintain
healthy skin and eyes; essential for
growth of new body tissue
B3 (niacin)
Leafy vegetables, whole-grain
products, peanut butter, potatoes,
tuna, eggs
Enzyme systems that convert
carbohydrates, proteins and fats
into energy; aids synthesis of
hormones
B6 (pyridoxine)
Yeast, wheat germ, cereal, wholegrain products, liver, meat,
soybeans, peanuts, egg yolk
Many enzyme reactions, including
protein and fat metabolism, the
conversion of tryptophan to niacin,
and for development of red blood
cells
B12 (cyanobalamin)
Liver, meat, eggs, oysters, sardines
Development of red and white
blood cells; also involved in protein,
fat and carbohydrate metabolism
Folic acid
Liver, kidneys, yeast, mushrooms,
leafy green vegetables
Development of red blood cells and
metabolism of protein; linked with
vitamin B12
Biotin
Liver, meat, fish, egg yolk, milk,
smaller amounts in grains and
vegetables
Metabolism of fat and protein;
growth and function of nerve cells
Pantothenic acid
Liver, meat, fish, egg yolk, yeast,
cereals, peanuts and vegetables
Metabolism of carbohydrate, fat and
protein
C (ascorbic acid)
Citrus and other fruits, tomatoes,
leafy vegetables, potatoes, rosehips
Connective tissues, bones, teeth;
promotes wound healing and
absorption of iron
37
BIOBOX 2.2
PELLAGRA
Pellagra is a nutritional disorder caused by a deficiency of vitamin B3 (niacin). Symptoms of the
disease include dermatitis, diarrhoea and dementia. Dermatitis is the first symptom seen. Skin lesions
occur as a result of an increased sensitivity to sunlight. Gastrointestinal symptoms usually consist of
alternate constipation and diarrhoea. These symptoms can be accompanied by inflammation of the
mouth and tongue. The dementia or neurological symptoms are the last to appear and may include
nervousness, confusion, depression, apathy and delirium. Niacin deficiency is treated effectively with
vitamin supplements and recovery is quick. Pellagra is usually a disease seen in poorer nations, and,
in more affluent societies, the disease may result from chronic alcoholism. In developed nations all
commercial breads are fortified with niacin to assist in preventing this disease.
 List the symptoms of pellagra.
 How can pellagra be prevented?
Minerals
Minerals are inorganic compounds present in
the food we eat and incorporated into many
structures of the body, such as bones, teeth and
blood. Over 20 minerals are required by the
human body (see Table 2.5).
BIOBOX 2.3
bioBYTE
Chronic iron toxicity can occur in
people who readily consume alcoholic
beverages brewed in unlined iron vessels,
such as beer. The iron in the vessel
dissolves in the beverage and, when
ingested, is absorbed and deposited
in the liver. The condition is known as
haemosiderosis. It may also occur as a
result of cooking in iron pots.
TYPES OF MINERALS
The term ‘mineral’ was used in the 19th century to describe the inorganic nutrients left in ash after
plant or animal matter was burned. Now we know that minerals are really inorganic ions needed for
life. Organisms need some minerals in relatively large amounts. This is the case for calcium (Ca2+),
iron (Fe2+ or Fe3+), magnesium (Mg2+), potassium (K+), sodium (Na+), chloride (Cl–), nitrate (NO3–),
phosphate (H2PO4–) and sulfate (SO42–). These ions are referred to as major mineral elements.
Some minerals are needed in smaller amounts and are called trace elements or micronutrients.
They include cobalt (Co2+), copper (Cu2+), manganese (Mn2+), molybdenum (generally found as
MoO42–) and zinc (Zn2+). However, there is no hard and fast dividing line between major mineral
elements and trace elements. Further, some trace elements needed by animals are not required
by plants, and vice versa. For instance, iodine (I–) is an essential trace element for many animals,
including ourselves, but does not seem to be required by plants.
38
Unit 1
Chapter 2 The chemical
composition of cells
Table 2.5 Mineral elements required by humans.
Element
Functions
Effects of deficiency
Sources
Sodium
Major extracellular ion, maintains
body fluid concentrations, carries nerve
impulses
Cramp in muscles, loss of appetite,
impeded brain function
All foods, table salt
Potassium
Major intracellular ion, maintains body
fluid concentrations, carries nerve
impulses
Lethargy, muscular weakness
Fruits, vegetables, legumes,
whole grains, meat
Chlorine
Maintains body fluid concentration,
aids digestive juice formation
Cramp in muscles, loss of appetite
Table salt, all foods
Calcium
Bone and tooth formation, role in
muscle contraction and cell function
Stunted growth, rickets, convulsions
Milk and other dairy products,
whole grains, legumes, nuts,
whole fish (sardines)
Phosphorus
Bone and tooth formation, part of many
cellular compounds
Loss of calcium from bone, weakness,
poor growth
Milk and other dairy products,
meat, whole grains, legumes, nuts
Magnesium
Involved in many cellular functions
Reduced growth, mental depression,
muscular weakness, sometimes
convulsions
Milk and other dairy products,
meat, seafood, whole grains,
legumes, nuts
Sulfur
Involved in cellular functions, part of the
essential amino acid methionine
Reduced growth
Animal and plant proteins
Iron
Component of haemoglobin and some
respiratory enzymes
Anaemia (lack of haemoglobin in
blood), reduced resistance to infection
Liver, meat, eggs, whole grains*,
dark green vegetables*, legumes
Iodine
Component of thyroid hormones
Goitre, cretinism
Seafood, iodised salt
Copper
Component of melanin and
haemoglobin
Anaemia, bone disorders
Seafood, liver, meat, legumes,
whole grains
Zinc
Component of digestive enzymes
Retarded growth, reduced resistance
to infection, skin disorders
Seafood, oysters, liver, meat, eggs,
whole grains, legumes, nuts
Selenium
Component of selenium enzyme
Reduced growth in young animals,
liver and muscle damage
Seafood, whole grains*,
legumes*, meat
Chromium
Helps maintain glucose concentration in
the body
Excess glucose in blood
Whole grains, brewer’s yeast,
meats
Nickel
Involved in membrane structure, helps
in absorption of iron
Reduced growth, anaemia
Legumes, whole grains
Silicon
Cross-linking agent in matrix of
early bone
Reduced growth and skeletal
development
Whole grains, nuts, eggs
Manganese
Activation of enzymes
Reduced growth and fertility, bone and
neurological disorders
Whole grains, legumes, leafy
vegetables
Molybdenum
Component of molybdenum enzymes
Reduced growth and uric acid
synthesis
Legumes, whole grains, milk,
organ meats, leafy vegetables
Fluorine
Accumulates in tooth enamel and bones
Increased tooth decay
Fish, tea, fluoridated water
Macro elements
Micro elements
* Vary with soil.
Source: Australian Academy of Science, The Common Threads, Part 1, Canberra: AAS, 1990: 198.
39
REVIEW
5 How do you tell a fat from an oil?
6 Why are lipids also called triglycerides?
7 If you wanted to cook chips at a high temperature, would it be
best to use lard (a saturated fat) or sunflower oil (an unsaturated
fat)? Explain your answer.
8 Explain why it is important to include some fat in a healthy diet.
9 Why do vitamins need to be eaten in small amounts?
10 If a person was suffering from anaemia, which mineral(s) would
they be lacking in and what would be the effects?
11 List all the vitamins and minerals you would obtain by eating
whole grains.
Proteins in cells
bioTERMS
proteins
large organic molecules,
containing nitrogen, essential to
the structure and function
of living things
amino acids
nitrogen-containing compounds
that are the building blocks of all
proteins
Your body produces over 1 million new red blood cells each second. A range of
different proteins is found embedded within the membranes of these cells. Channel
proteins, for example, penetrate through the membrane and select substances to pass
through the membrane. This means that your existing cells need to produce proteins
for 1 million plasma membranes and organelle membranes each second.
Like lipids, proteins are made up of the elements carbon, hydrogen and oxygen,
but they are different to these compounds in that they always contain nitrogen. In
addition, sulfur is often present, and sometimes phosphorus and other elements.
These elements combine to form building blocks called amino acids. There are
over 20 different types of amino acids and they join together to form a protein or
polypeptide. It is the order and number of amino acids that make different types
of protein. The ordering of amino acids in proteins is determined by the genes in
our chromosomes.
O
H
N
H
R
C
H
Table 2.6 Essential and
non-essential amino acids.
40
Unit 1
Essential
Non-essential
Isoleucine
Leucine
Lysine
Methionine
Phenylalanine
Threonine
Tryptophan
Valine
Histidine (for
children)
Cysteine
Tyrosine
Glycine
Arginine
Proline
Glutamic acid
Aspartic acid
Serine
Alanine
Asparagine
Glutamine
C
O
H
Figure 2.6 Chemical diagram of
an amino acid where the R group
can represent a number of different
chemicals. Many amino acids join
together to form a polypeptide.
Your body produces most of the amino
acids required for protein production,
but you must include eight essential
amino acids in your diet. Failure to
do this will result in several proteindeficiency diseases.
Chapter 2 The chemical
composition of cells
Biobox 2.4
PKU – a protein disease
Phenylketonuria (PKU) is a genetic disease of metabolism. It is detectable during the first days of life with appropriate
blood testing (every newborn in Australia is screened for PKU). The disease is characterised by the absence or deficiency
of an enzyme that is responsible for processing the essential amino acid, phenylalanine. In people not suffering from
PKU, phenylalanine is converted to another amino acid (tyrosine), which is then used by the body. However, when the
enzyme that breaks down phenylalanine (hydroxylase) is absent or deficient, phenylalanine accumulates in the blood
and can rise to such a level that it is toxic to brain tissue.
Without treatment, most infants with PKU develop mental retardation. To prevent this happening, treatment
consists of a carefully controlled diet restricting foods that contain phenylalanine. This diet is begun during the
first days or weeks of life. Most experts suggest that this diet should continue lifelong. Foods that are high in
phenylalanine and therefore need to be excluded from the diet include cheese, eggs, fish, meat, almonds, peanuts,
chocolate and any food containing wheat flour.
Regulating movement – transport proteins
Membrane proteins control movement of substances into and out of organelles and
between the cell and its external environment.
Many chemicals are unable to move directly through the plasma membrane.
Glucose, for instance, is a soluble compound and, as such, fails to pass through the
phospholipid layer of a membrane directly. How do substances such as water get
through the membrane? The answer lies in special channels made of transport proteins
that act like gates, facilitating movement across the membrane.
high concentration
concentration
gradient
low concentration
channel protein open
channel protein closed
Figure 2.7 Channel protein.
41
Structural proteins
Other proteins, called structural proteins, link membranes, cytoplasm and the nucleus,
allowing for communication. Structural proteins such as the insoluble fibrous protein
keratin are found in hairs, feathers, nails, hooves and horns. Another fibrous protein,
collagen, is the most abundant protein in vertebrates, making up a third of their total
protein mass. Humans are largely held together by collagen as it is found in bones,
cartilage, tendons, ligaments, connective tissue and skin. Collagen is even found in the
cornea of the eye. Collagen fibres have a tensile strength greater than that of steel.
Figure 2.8 Nails, horns and hooves are all
made of the protein keratin.
BIO
Controlling metabolism – enzymes
LIN
K
Enzyme activity
42
Unit 1
A cell is essentially a busy chemical factory. Because of the membrane-bound
organelles, many different types of reactions can happen in different parts of the cell at
the same time. Different types of reactions happen in different types of cells. What is it
that controls the type and duration of these reactions?
Enzymes are one of the most important groups of proteins. Without enzymes the
reactions that occur in living organisms would be so slow as to hardly proceed at all;
this would be incompatible with the maintenance of life.
Enzymes do more than merely speed up the reactions; they also control them.
Over 1000 different reactions can take place in an individual cell. The functional
organisation that this demands is achieved by a specific enzyme in a particular place
within the cell acting as a catalyst for each individual reaction.
There are as many enzymes in living organisms as there are types of chemical
reactions. They are divided into two broad groups: intracellular and extracellular.
Intracellular enzymes occur inside cells, where they speed up and control metabolic
reactions. Extracellular enzymes are produced by cells but achieve their effects outside
the cell; they include digestive enzymes, which break down food in the gut (see
Chapter 5).
Chapter 2 The chemical
composition of cells
The properties of enzymes
Enzymes are nearly always proteins. The main properties of enzymes are given below:
1 Enzymes generally work very rapidly. One of the fastest enzymes is catalase (the
name of an enzyme generally ends in –ase). This enzyme is found in several organs
and tissues, including the liver, where its job is to speed up the breakdown of
hydrogen peroxide (H2O2) into oxygen and water:
ACTICA
PR
bioTERMS
2H2O2  2H2O + O2
substrates
substances that enter a
reaction; also called
reactants or precursors
Hydrogen peroxide is a waste of cellular metabolism. If it is allowed to accumulate
it can be toxic. Its rapid conversion to water and oxygen is, therefore, important.
2 Enzymes are not destroyed or altered by the reactions that they catalyse, so they can
be used again. This is not to say that a given molecule of an enzyme can be used
indefinitely, for the action of an enzyme depends critically on its shape, which is
readily affected by changes in temperature, acidity and so on.
3 Enzymes can work in either direction as metabolic reactions are generally
reversible. The direction in which the reaction proceeds at any given time depends
on the relative amounts of substrates and products present. For example:
products
substances at the end of
a metabolic reaction
denatured
describes a protein, the
structure of which has been
altered so that it no longer
functions in the way it was
meant to
A+B  C+D
Substrate Products
Figure 2.10 The sooty grunter fish in
Central Australia has enzymes that can
function at high temperatures, thus
enabling it to live in hot springs.
4.0
Rate of reaction (mg of products per unit time)
If there is a lot of A and B compared with C and D, the reaction
will go from left to right until an equilibrium between the
substrates and products is reached.
4 Enzymes are affected by temperature and have an optimal range
in which they operate. In mammals this is 35–40°C. Below this
range, they do not function efficiently. Above this range, they
are denatured; the enzyme changes shape so that it can no longer
function efficiently.
The effect of temperature on the rate of an enzymecontrolled reaction is shown in Figure 2.9. Up to about
40°C the rate increases smoothly, a 10°C rise in temperature
being accompanied by an approximate doubling of the rate
of reaction. Above this temperature the rate begins to fall off
and then declines rapidly, ceasing altogether at 60°C. The
reason for this is that enzymes, being proteins, are denatured
at high temperatures. Because of the susceptibility of enzymes
to heating, few cells can tolerate temperatures higher than
approximately 45°C. Organisms that live in environments where
the temperature exceeds 45°C either have
heat-resistant enzymes (Figure 2.10)
or are able to regulate their body
temperature.
2.2
CTIVITY
LA
3.0
2.0
1.0
10
20
30
40
50
Temperature (°C)
Figure 2.9 The effect of temperature
on the rate of an enzyme-controlled
reaction. All other variables were
kept constant.
43
bioTERMS
5 Enzymes are sensitive to pH. Every enzyme has its own range of pH in which
it functions best. Most intracellular enzymes function optimally around neutral
(pH 7). Excessive acidity (less than pH 7) or alkalinity (more than pH 7) denatures
them and makes them inactive. Digestive enzymes behave differently. The
protease enzyme pepsin functions more effectively in an acidic environment, such
as that found in the stomach, while trypsin functions effectively in an alkaline
environment, such as that found in the duodenum (see Chapter 5).
6 Enzymes are usually specific to particular reactions. Normally, a given enzyme will
catalyse only one reaction. This is generally the case with intracellular enzymes,
which work on one particular substrate. Extracellular enzymes, such as pancreatic
lipase, will digest a variety of fats.
Enzyme specificity also explains how organisms manage to digest proteins
within a container made of protein. For example, the various proteases in the
stomach digest the proteins in the food but not the proteins in the stomach wall.
pH
a measure of how acidic or
alkaline a solution is
lock-and-key mechanism
a model suggesting that the
shape of a substrate molecule
is an exact fit to the shape of an
enzyme’s active site
STU
organic catalysts
enzymes
DE
NT C D
Enzymes
We can explain the properties of enzymes by suggesting that, when an enzymecontrolled reaction takes place, the enzyme and substrate molecules become joined
together for a short time to form an enzyme–substrate complex. The substrate
molecules then react together and the end product leaves the enzyme. The enzyme,
unchanged by the reaction, can be used again.
enzyme + substrate(s)  enzyme–substrate complex  enzyme + end product(s)
Figure 2.11 With the lock-and-key
mechanism, the substrate fits into a
specific active site on the surface of the
enzyme, where the reaction takes place.
enzyme molecule
active site
substrate
molecule
enzymesubstrate
complex
product
molecules
It is thought that each enzyme molecule has a precise place on its surface to which
the substrate molecules become attached. This is called the active site. This model of
enzyme action is known as the lock-and-key mechanism (see Figure 2.11).
The lock-and-key mechanism suggests that there is an exact fit between the
substrate and the active site of the enzyme. However, recent research has
suggested that the active site may
not necessarily be exactly the right
enzyme molecule
shape to begin with. It is believed
that when the substrate combines
with the enzyme, it causes a small
change to occur in the shape of
the enzyme molecule, thereby
substrate
enabling the substrate to fit closely
molecule enters
the active site
into the active site. This is called
the induced-fit hypothesis (see
Figure 2.12).
substrate molecule
Because of its specific shape,
an enzyme will act only on one
type of chemical reaction, making
it proceed much faster than it
normally would. It is the type of
enzymes in a cell that ultimately
control what functions the cell
enzyme molecule
closes round
can carry out. Enzymes are said to
the substrate
be organic catalysts.
molecule
Figure 2.12 The induced fit hypothesis. The
substrate molecule enters the enzyme’s active site,
causing the enzyme molecule to change its shape so
that the two molecules fit together more closely.
44
Unit 1
Chapter 2 The chemical
composition of cells
Denaturation
When cells are exposed to high temperatures or an environment that is more or less acidic
(outside its pH range) than normal, they often cease to function. This is because proteins
within the cell change their three-dimensional shape. This change in shape, caused by the
environmental conditions, is called denaturation. If the shape has changed enough to break
bonds between the connecting units of amino acids, proteins cannot return to their original
shape when conditions revert to normal. In this case, the protein is destroyed.
This has repercussions, both dangerous and useful, for us. If our body temperature
rises too much during an infection, critical enzymes in our brain could denature,
leading to seizures and possible death. On the other hand, we are able to chew and
digest meat more easily after cooking. Raw meat is difficult to chew because of fibrous
proteins contained in the muscle cells. By heating the meat, proteins are denatured,
making it easier to chew and digest.
REVIEW
12 Draw a concept map to illustrate what a polypeptide is made of,
and why it is an important part of a healthy diet.
13 Why are enzymes important for the maintenance of life?
14 Explain why there are thousands of different types of enzymes in
the human body.
15 What is the difference between an intracellular enzyme and an
extracellular enzyme?
16 List the main properties of enzymes. To what extent can each
property be explained by the lock-and-key mechanism?
17 Explain why doctors get worried if their patient develops a
temperature in excess of 42°C.
The cell wall
Plant cells have an extra coating on their cells to provide the cell with protection and
strength. This is the cell wall and it is made from cellulose. Cellulose belongs to a
larger group of molecules known as carbohydrates.
Carbohydrates
Figure 2.13 Sugar, flour, rice and
celery all contain large amounts of
carbohydrates.
Carbohydrates are members of a large group of molecules
that contain the elements carbon, hydrogen and oxygen.
The carbohydrate glucose is the most common single sugar
molecule and is known as a monosaccharide. It contains carbon,
hydrogen and oxygen in the ratio 1:2:1. It has six atoms of
carbon, 12 atoms of hydrogen and six atoms of oxygen; hence its
chemical formula is C6H12O6.
The original source of all glucose molecules is
photosynthesis. As you saw in Chapter 1 photosynthesis
occurs in the chloroplasts of plant cells. During this process,
the inorganic molecules, carbon dioxide and water, are broken apart
using light energy from the Sun and enzymes found in the thylakoid
membrane system and stoma (see Figure 1.25) of the chloroplast.
The atoms are rejoined to form glucose, water and oxygen.
45
bioBYTE
Sugar that people buy for use in cooking
is almost pure sucrose. Fructose, a
monosaccharide, is sweeter than sucrose and is often
used in the manufacture of sweets and diet foods,
as the same sweetness can be obtained for fewer
calories. However, sugars are not the only compounds
that are sweet. Saccharin is 500 times sweeter than
sucrose, although chemically it is quite distinct
from sugars. Certain proteins taste even sweeter
than saccharin. One such protein is obtained from
serendipity berries, the fruit of a West African plant.
This protein is 2500 times as sweet as sucrose. New
low-calorie sweeteners are being developed by the
food industry. One is aspartame. This compound is
200 times sweeter than sucrose and lacks the slightly
bitter aftertaste often found with saccharin.
Figure 2.14 Cellulose is a
polysaccharide. It is composed of
many glucose molecules joined
together.
glucose
If two monosaccharide molecules join together, they form
a disaccharide (‘two sugars’). Sucrose or common table sugar
is an example of a disaccharide.
If many monosaccharides join together, or two or more
disaccharides join together, or two or more polysaccharides
join together, they form a polysaccharide (‘many sugars’).
Cellulose, found in plant cell walls, is composed of many
glucose molecules joined together. The different linking
patterns between the glucose molecules mean that cellulose
fibres are tough, insoluble and resistant to being crushed and
bent. Hence, they are ideal as the structural components of
plant cell walls. The efficient functioning of plants is very
much bound up with the properties of their tough, cellulose
cell walls.
+
glucose
+
glucose
Cellulose
This toughness also means that when plant material is eaten by humans, the cellulose
cannot be broken down easily. Humans do not produce the enzymes necessary to break
down cellulose. It passes through our digestive system virtually unchanged. It is important
to us in digestion though, as a source of fibre or ‘roughage’. Fibre assists by holding water
in the large intestine, thereby aiding in defecation.
Starch is another example of a plant polysaccharide made up of amylose and
amylopectin, two other polysaccharide molecules. They are linked together to form
long chains. Some starches contain over 6000 glucose molecules and hence are perfect
molecules to act as glucose storage. Most plants store excess starch in their roots and
break it down into glucose when they require it for respiration. Potatoes, for example,
are rich in starch.
Sugars move around in the phloem of the plant in the form of sucrose (a disaccharide)
rather than glucose. Phloem, being made up of living cells, would use any available
glucose for respiration. Being in the less reactive form of sucrose ensures that the sugar
will reach other cells that need it.
Animals tend to store excess carbohydrate in the form of glycogen, a polysaccharide.
Glycogen is very similar to starch except for some slight differences in the way that
the polysaccharide molecule is bonded together (see Figure 2.15). Glycogen is stored
in the liver and muscles and is converted back into glucose as the concentration of
glucose in the blood begins to drop.
46
Unit 1
Chapter 2 The chemical
composition of cells
a
Figure 2.15 (a) Amylose
is a polysaccharide starch
(b) Glycogen molecule. Note
the difference in branching to
amylose.
b
The prokaryotic boundary
Figure 2.16 Bacterial cell dividing by
binary fission, causing more cell wall to
be produced.
All prokaryotic cells are surrounded by a plasma membrane. On the
outside of this is the cell wall. As in plant cells, the main function
of the cell wall is to protect and provide strength. The prokaryotic
cell wall is composed of peptidoglycan, which is composed of
polysaccharides and protein. The peptide links between the proteins
provide tremendous strength. When the bacterial cell divides by
binary fission, the size of the cell wall must also grow. This is done
when the links between the peptidoglycan are broken and new
building blocks that are synthesised in the cytosol are inserted.
Around the cell wall is a polysaccharide, and sometimes protein,
jelly-like capsule. This is often seen in disease-causing bacteria and
offers protection to the cell. It is thought to stop phagocytosis by
white blood cells.
REVIEW
18 Draw a concept map to illustrate the chemical composition, production, building up and
breaking down of carbohydrates.
19 Where, and in what form, is excess carbohydrate stored in animals? Plants?
20 What chemical substances are found in the prokaryotic boundary?
47
Cytoplasm
bioTERMS
solvent
a substance in which other
substances can be dissolved, the
most common being water
extracellular fluid
fluid surrounding and bathing
a cell
cohesive
describes behaviour that causes
resistance to rupturing when
placed under tension
surface tension
bonds between surface
molecules that stop or slow
down molecules leaving or
objects penetrating the surface
The mitochondria in actively respiring cells are in constant need of oxygen. The
carbon dioxide that is produced needs to be removed from the area so that it does not
build up and inhibit its diffusion.
The cytoplasm provides the fluid medium for substances to move about within the
cell. Cytoplasmic streaming ensures that organelles are constantly provided with their
requirements and any wastes or products that they are producing are removed. The
cytoplasm is made up mainly of the inorganic molecule water (H2O) in which other
molecules are either dissolved or suspended. Your cells are approximately 80% water.
Cells will die if they drop below a certain percentage of water.
Water as a solvent
Water is unusual among small molecules by being a liquid at room temperature. It is a
good solvent for most salts in that they dissolve in it readily. However, water is a poor
solvent for proteins, lipids and other large organic molecules in living cells. This means
that liquid water can transport nutrient elements to and within living cells without
dissolving and destroying the organic molecules of which cells are made. Water also
transports waste elements away from cells through the extracellular fluid that
bathes all cells.
Increased water content leads to increased chemical activity within
the cell because all the chemical reactions that take place in cells do so
in aqueous solution. Many chemical reactions require water as part of
their reactants.
Temperature-stabilising effects of water
The range of temperatures in which biochemical processes can proceed
is quite narrow, and most cells cannot tolerate wide variations in
temperature. The presence of water helps keep the temperature of cells
fairly stable.
Figure 2.17 Light micrograph of
a section through cartilage.
Figure 2.18 A water strider supported
by surface tension.
48
Unit 1
Water’s cohesion
Swimming on a hot summer night can be refreshing, if you do not mind all the nightflying insects that hit the water’s surface and float about on it. Because of the attractive
forces holding water molecules together, liquid water shows cohesive behaviour. It
resists rupturing when it is placed under tension.
Surface tension is the resulting force that causes the
surface of water to act as if it has an elastic skin. At
ordinary temperatures, water has the highest surface
tension of any known liquid except mercury, and
this is of considerable biological significance.
Due to the strong cohesive forces, water is
incompressible. Cells filled with water become
swollen and distended and, because of the resulting
rigidity, plants can support themselves.
Chapter 2 The chemical
composition of cells
The strong cohesive forces between water molecules play an important part in
the movement of water up the xylem in the stems of plants. Were these forces much
weaker, trees could not be so tall. Surface tension also allows the surface film of still
water to support – and provide a habitat for – certain aquatic organisms such as the
water strider pictured in Figure 2.18.
Freezing properties of water
Most liquids increase in density, and so decrease in volume, on solidifying. Water is
most unusual in that the reverse is the case. Cells that freeze increase in volume so
much they normally burst. So does the environment have to be higher than freezing
point for a cell to survive?
It seems not, as nearly all the Antarctic continent is permanently covered with
ice but quite a lot of ‘lower’ plant groups – like moss, fungi, liverworts – and lichens
live there. Many marine invertebrates live in the below-freezing temperatures of the
Antarctic marine environment. How is it that these organisms can survive the extreme
cold of Antarctica?
To help answer this, we can look at
how cars are designed to prevent damage
to engines in freezing conditions. An
automobile’s engine is cooled by jackets of
water. In winter, water in these channels
can freeze and expand. The expansion is
so forceful it can crack an engine block.
Antifreeze is a liquid solute (ethylene glycol)
that dissolves easily in water. In solution and
added to a car’s engine, it lowers the freezing
point of water, preventing it from freezing
and ruining the engine.
Glucose, like the antifreeze ethylene
glycol, dissolves in water and depresses the
freezing point of water. In cells, lowering
the freezing point of the liquid protects it
from freezing and ripping open with fatal
results. Antarctic marine invertebrates deal
with their situation by accumulating salts
and organic compounds, such as glucose and
amino acids, which lower the freezing point
of the body fluids.
Figure 2.19 Moss growing at
Bailey Peninsula, Antarctica.
REVIEW
21 If the water content of a cell drops below a certain point, the cell will die. Explain why this happens.
22 List the dissolved and suspended substances in the cytoplasm of a cell.
23 Give two reasons why water is necessary for efficient enzyme functioning.
49
Cytoskeleton
Red blood cells carry oxygen from our lungs to our cells. As they move through our
smallest blood vessels, the capillaries, they are squashed and bent as they squeeze
through the narrow tubes. When they reach a larger blood vessel, they return to their
regular shape.
It is the cytoskeleton that provides the cell with structure and strength. But it
cannot be so rigid that the cell cannot change its shape. The cytoskeleton must also
allow for flexibility in cell shape.
The cytoskeleton is made up of microfilaments and microtubules. They are made
of the protein actin. Actin is a globular protein that is joined together to form long
filaments or tubes. There are capping proteins at either end of the rod and, depending
on which end they occur, they either stabilise the filament or assist in its disassembly.
Powering the power supply
ACTICA
PR
2.3
CTIVITY
LA
bioTERMS
ATP (adenosine
triphosphate)
the main energy-carrier molecule
in cells, produced during cellular
respiration
The mitochondrion is a membrane-bound organelle and commonly known as
the power supply of the cell. Inside the mitochondrion occurs a series of chemical
reactions where glucose molecules combine with oxygen and are broken down into
carbon dioxide and water. Along the way, energy is released from the chemical bonds
in the glucose molecule. This energy is stored temporarily in a chemical called ATP
(adenosine triphosphate). When a cell requires instant energy, it is the ATP molecule
that is broken apart to supply it.
The rate of cellular respiration is dependent upon the specific enzymes present
and the temperature in the cell. These enzymes are found on the infolded projections
of the mitochondria called cristae (see Figure 1.14). These infoldings significantly
increase the mitochondria’s surface area and hence the space for the enzyme to work.
More enzymes working means a greater rate of cellular respiration and more energy is
released for use by the cell.
In the case of animals and fungi the sugar molecules that are broken down by the
mitochondria come from the food that the organism ingests. In the case of plants, the
sugar is produced during the chemical process of photosynthesis. Glycogen or starch,
both complex carbohydrates, store the sugar source (usually glucose) in animals and
plants respectively.
Glucose is not the only source of energy in cellular respiration. Fats and proteins as
well as other types of carbohydrates can be used. For some cells in our body, glucose
is the only starting material. A good example is the brain. A significant drop in blood
sugar levels can be fatal to the brain because the brain does not store carbohydrate and
cannot fall back on either lipids or amino acids as energy sources.
Endoplasmic reticulum
The endoplasmic reticulum is a series of interconnected canals that transport material
throughout the cytoplasm. The canals are composed of parallel membranes that are made
up of phospholipid bilayers (see page 14). Rough endoplasmic reticulum is studded
50
Unit 1
Chapter 2 The chemical
composition of cells
with ribosomes, which produce proteins. The endoplasmic reticulum in the cells of the
pancreas, for example, transport digestive enzymes. Smooth endoplasmic reticulum
has no ribosomes, and produces lipids and fats.
The Golgi apparatus looks like a series of flattened balloons, with the outer membrane
being made up of a phospholipid bilayer (see page 15). As materials move from
the endoplasmic reticulum into this structure, the ends pinch off to form smaller
vesicles filled with proteins, mainly enzymes. These vesicles move towards the plasma
membrane and, through the process of exocytosis, join with the plasma membrane to
release their contents to the outside.
BIO
Golgi apparatus and lysosomes
LIN
K
Golgi apparatus
REVIEW
24 The cytoskeleton is made up of the protein, actin. Explain why actin is a suitable molecule for
the cytoskeleton to be made of. (Hint: Think of what the function of the cytoskeleton is.)
25 What happens to the energy in glucose before it is utilised by the cell?
26 How does the structure of the mitochondrion increase enzyme efficiency?
27 List the molecules that you would expect to find in the endoplasmic reticulum.
Nucleus and nucleic acids
Many television programs now feature crime scenes where the victim is identified, or
the killer is found, through the process of DNA analysis. DNA is deoxyribonucleic
acid, a compound found mainly in the nucleus of eukaryotic cells. In prokaryotic cells,
the DNA is found floating freely in the cytoplasm or in small rings called plasmids.
DNA belongs to a larger class of organic molecules called nucleic acids. Also found
in this class is RNA (ribonucleic acid).
DNA is made up of sugars, phosphate groups and four bases containing nitrogen
– adenine, guanine, cytosine and thymine. A sugar, a phosphate group and a base are
bonded together to form a subunit called a nucleotide. But how can this help forensic
scientists identify one person from another?
The nucleotides are joined together by the sugar and phosphate groups to form
a long string (see Figure 2.20). Two strings are twisted around each other and held
together by the hydrogen bonding of the bases – adenine always bonds with thymine
and cytosine always bonds with guanine (see Figure 2.20). This is called a double
helix. It is the order in which these bases occur that is important. This order is unique
to individual people, unless they are one of identical siblings.
A segment of DNA is called a gene, and a gene is a chemical code for a protein,
usually an enzyme. It is the enzymes that determine which, and at what rate, specific
chemical reactions occur in cells. An example of this is the segment of DNA which
forms the gene that codes for the enzyme which produces brown pigmentation in skin
cells. This results in an organism having brown skin.
DNA is a large molecule and unable to move through the nuclear membrane.
How then do the instructions for protein production coded into DNA get to the
bioTERMS
plasmids
small, circular molecules of
extra bacterial DNA
RNA (ribonucleic acid)
a single-stranded nucleic acid
that functions in transcribing
and translating information
from DNA into proteins
51
Figure 2.20 A length of the
double helix of a DNA molecule.
The bases are bonded together
by hydrogen bonds.
ribosomes where the protein is produced? The
answer lies in the RNA. RNA also comprises
chains of the nucleotides adenine, cytosine
and guanine, but contains uracil instead of
thymine. Unlike DNA, RNA is usually a
single chain of unpaired nucleotides, and
therefore a much smaller molecule. RNA is
able to move through the nuclear membrane
and is found in both the nucleus and
cytoplasm. It has the job of taking information
encoded in DNA to the protein synthesising
organelles, the ribosomes. You will learn more
about DNA and RNA in Chapter 6.
Colouring our world –
pigments
It is the pigments in animal cells and plant
cells that provide us with the colourful
world we see around us. The beautiful green
of the leaves is due to the structure of the
plant pigment, chlorophyll. Chlorophyll
contains the elements carbon, hydrogen,
oxygen, nitrogen and magnesium. It is
very similar in structure to the haem group
found in haemoglobin in red blood cells (see
Chapter 5).
Chlorophyll absorbs the red and violet
wavelengths of the electromagnetic spectrum
most strongly. Green light is absorbed
poorly and thus reflected back to our eyes.
bioTERMS
nucleotides
organic compounds composed
of a sugar, a phosphate group
and a nitrogenous base.
Subunits of DNA and RNA
electromagnetic spectrum
consists of electromagnetic
waves ranging from a long
wavelength such as radio waves
to a short wavelength such as
X-rays and gamma rays; visible
light is a small part of this
spectrum
52
Unit 1
Figure 2.21 Two colourful
chameleons.
Chapter 2 The chemical
composition of cells
Chlorophyll also contains the carotenoid pigment. Carotenoids absorb the blue
wavelengths, therefore allowing chlorophyll to utilise a larger range of the light
spectrum. In leaves, the carotenoids are usually masked by chlorophyll, but in autumn
the chlorophyll begins to break down and the red and yellow of the carotenoids shows
through as autumn foliage.
Biochromes are microscopic pigments that are found in animal cells. Their
chemical make-up determines which wavelengths of light they absorb and which they
reflect. The colour of the pigment is determined by the combination of reflected and
absorbed wavelengths.
Non-cellular material
Living things are made up not only of cells,
but also of materials produced by cells that
are themselves non-cellular.
Hair is not made up of cells, although it
is produced from the cells in the hair follicle.
Hair is made of the protein keratin, as are
toenails, fingernails, horns and hooves.
One of the main problems faced by
terrestrial plants is water loss. To combat
this, many plants have leaves that are
coated in wax. Wax is made up of lipid
(see page 35), and as such is insoluble in
water. This makes it a suitable material to
stop water loss through the leaf surface.
Figure 2.22 Plant leaves are coated
in wax, which is insoluble in water.
REVIEW
28
29
30
31
Where in a cell would you expect to find DNA and RNA?
Explain why DNA is a suitable molecule to contain a chemical code.
How do chlorophyll and carotenoid pigments combine to enhance photosynthesis in leaves?
‘Living things are made up of cells and the products of cells.’ Explain what this means.
53
Visual summary
fingernails
and toenails
hair
hooves
horns
products
of cells
plasma
membrane
nucleus
Non-cellular
Structures
wax
cytoplasm
Cellular
Structures
water
cytoskeleton
Inorganic
Compounds
minerals
mitochondrion
Chemical
Composition
of Cells
triglycerides
unsaturated
fats and oils
lipids
endoplasmic
reticulum
Golgi apparatus
Organic
Compounds
saturated
fat soluble
vitamins
structural
proteins
water soluble
amino acids
carbohydrates
nucleic acids
RNA
enzymes
proteins
transport
proteins
disaccharides
monosaccharides
sucrose
DNA
polysaccharides
glycogen
starch
cellulose
storage in
animal cells
storage in
plant cells
found in
cell walls
glucose
54
Unit 1
Chapter 2 The chemical
composition of cells
Key terms
actin
glycogen
polysaccharide
active site
hydrophilic
polyunsaturated
ATP
hydrophobic
products
amino acid
inorganic
protein
biochromes
intracellular enzymes
RNA
carbohydrates
keratin
saturated
cholesterol
lipid
solvent
cohesive
lock-and-key mechanism
starch
collagen
minerals
steroids
denatured
monosaccharide
substrates
disaccharide
nucleotides
surface tension
electromagnetic spectrum
organic
triglyceride
enzyme–substrate complex
organic catalysts
unsaturated
extracellular enzymes
pH
vitamins
extracellular fluid
phagocytosis
fatty acids
phospholipid bilayer
glycerol
plasmids
Apply understandings
 Match up list A (simple molecule) with list B
(complex molecule) and list C (examples).
List A
List B
List C
Fatty acids and glycerol
Polypeptide
Keratin
Amino acids
Polysaccharide
Steroids
Glucose
Lipids
Cellulose
 Explain the difference between the concepts
in the following groups:
a monosaccharide, disaccharide,
polysaccharide
b substrate, product, enzyme
c organic, inorganic
d saturated fat, unsaturated fat.
 Give three reasons to explain how a potato
and a liver are similar both in structure and
function.
 A friend of yours decides to cut all fats out of
her diet. What advice would you give her?
 The pH of human blood and body fluids
(excluding the gastric juices) is around
6.8–7.0. Explain why maintaining this
level of pH is important.
 Figure 2.23 shows the effect of substrate
concentration on the rate of an enzymecontrolled reaction. The temperature was
kept constant.
a Use the lock and key theory to explain
why the rate of reaction levels out
although the substrate concentration
increases.
55
Rate of reaction
b Draw in another graph to show what
would happen if you doubled the amount
of enzyme present.
Concentration of substrate
Figure 2.23 The effect of substrate concentration
on the rate of an enzyme-controlled reaction.
Investigate and inquire
 The effect of anaesthetics on cells is very
quick. Find out whether anaesthetics are
water-soluble or lipid-soluble chemicals.
Suggest how they would best move through
the plasma membrane.
 Organisms such as the bacterium
Thermophilus can survive in hot springs at
about 80°C. Use reference material to find
out why some enzymes are more heat stable
than others.
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Unit 1