Chapter 3
Answers
1
QUICK-CHECK questions
Which organic compounds provide most of the energy required by living
organisms?
The organic compounds that provide most of the energy required by living organisms
are carbohydrates and fats.
2
Write the equation for each of the following and indicate the direction of
movement of energy:
a
an exergonic reaction
Energy is released by an exergonic reaction.
b
an endergonic reaction.
Energy is required to drive an endergonic reaction.
3
How does the shape of an enzyme molecule affect its function?
According to the ‘lock-and-key’ theory of enzyme action, the shape of part of the
substrate on which an enzyme can act is complementary to the shape of the active site
of the enzyme. As a result, the shape of its active site determines the specificity of the
enzyme so that a particular enzyme can act only on one kind of substrate. (See
figure 3.10.)
4
In relation to enzymes, what are cofactors and coenzymes?
Cofactors and coenzymes are non-protein substances that are required for the action
of some enzymes. Cofactors are inorganic substances including metallic ions, such as
Mg2+. Coenzymes are organic molecules, such as certain vitamins.
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Chapter 3: QUICK-CHECK answers
5
Name two factors that affect the rate of enzyme actions.
Factors that affect the rate of enzyme action include:




6
temperature
pH
enzyme concentration
substrate concentration, but only to a point where all active sites on the enzyme
molecules are occupied.
Do all cells of a plant carry out photosynthesis? Explain.
No. Only those cells that contain chloroplasts can carry out photosynthesis since the
chlorophyll that is contained within chloroplasts is essential for photosynthesis.
7
Where in a cell is light energy transformed to chemical energy?
Light energy is transformed to chemical energy in the chloroplasts of cells.
8
What features of a leaf structure equip the leaf to carry out photosynthesis?
Features of a leaf structure that equip it to carry out photosynthesis include:
 presence of many chloroplasts in cells to capture and transform the radiant energy
of sunlight
 flat and thin shape to maximise exposure to sunlight
 presence of stomata to allow entry of the reactant carbon dioxide
 presence of air spaces within the leaf to facilitate the diffusion of carbon dioxide to
the photosynthetic cells
 existence of a network of xylem vessels to supply water, another reactant in
photosynthesis.
9
In which stage of photosynthesis is oxygen formed? Why is it termed a waste
product?
Oxygen is a product of photosynthesis that is formed during the light-dependent stage
of photosynthesis when water is split. Oxygen that is produced during photosynthesis
from the splitting of water is termed a waste product because it is not used in the
second stage of photosynthesis but is released into the atmosphere. This is in contrast
to the H+ ions, which are also produced from the splitting of water and which are
required in the second stage of photosynthesis.
10
List two differences between C3 and C4 plants
Differences between C3 and C4 plants include:
 location of chloroplasts: In C3 plants, chloroplasts are located only in the mesophyll
cells whereas, in C4 plants, they are also present in cells surrounding the vascular
bundles of the leaf.
 carbon dioxide concentration gradient: Because carbon dioxide occurs in C4 plants
in deeper tissues than in C3 plants, the carbon dioxide concentration gradient in
C4 plants is steeper than in C3 plants.
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Chapter 3: QUICK-CHECK answers
 rate of photosynthesis: Because the carbon dioxide concentration gradient is
greater in C4 plants than in C3 plants, C4 plants are more efficient in taking up
carbon dioxide and have a higher rate of photosynthesis than C3 plants.
 intermediate product of photosynthesis: In C3 plants, the second stage of
photosynthesis is the Calvin cycle, which produces a 3-carbon intermediate product
(PGA). C4 plants produce a different intermediate 4-carbon product that can be
broken down to a 3-carbon product with the release of carbon dioxide.
 duration of opening of stomata: Because C4 plants can produce carbon dioxide
from the breakdown of their 4-carbon compounds during the night, C4 plants can
keep their stomata closed during hot, dry conditions during the day but still produce
sugars. C3 plants lack this ability and can carry out photosynthesis only when their
stomata are open to allow entry of atmospheric carbon dioxide.
 survival in hot climates: Because they can keep their stomata closed for long
periods during the day, C4 plants are better able to survive in hot, dry climates than
C3 plants.
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True or false? Explain your choice.
a
Sugars are formed simply by joining molecules of carbon dioxide together.
False: The process of sugar formation is a cyclic reaction involving the formation
of intermediate compounds. Each carbon dioxide molecule enters the Calvin cycle
by reacting with a 5-carbon compound (ribulose diphosphate) resulting in the
formation of two 3-carbon compounds (phosphoglycerate (PGA)). Sugars are
formed from two PGA molecules.
b
The energy of sunlight is directly used to drive the Calvin cycle.
False: The energy required to drive the Calvin cycle comes from the ATP that is
generated during the light-dependent phase of photosynthesis.
c
Water is a waste product of the Calvin cycle.
True: Water molecules are produced during the formation of some intermediate
products in the Calvin cycle. This happens when PGA or phosphoglycerate
(C3H7O7P) is converted to PGAL or phosphoglyaldehyde (C3H5O6).
Note: This explains why the complete balanced equation for photosynthesis (see
page 69) has water on both sides of the equation as both a reactant and a product
of photosynthesis. However, the essential nature of photosynthesis is summarised
in the following more simple equation:
6CO2 + 6H2O  C6H12O6 + 6O2
d
The Calvin cycle can occur only in the dark.
False: The Calvin cycle cannot occur in the dark. The Calvin cycle and the lightdependent stage of photosynthesis are interdependent because the Calvin cycle
requires products generated by the light-dependent stage, namely NADPH and
ATP. In the dark, the light-dependent stage stops and so the production of
NADPH stops.
e
In the Calvin cycle, carbon undergoes an oxidation reaction.
False: In the Calvin cycle, carbon undergoes a reduction reaction. This reaction
changes the state of carbon from highly oxidised carbon dioxide to the more
reduced sugar (C6H12O6).
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Chapter 3: QUICK-CHECK answers
12
True or false? Explain your choice.
a
The chemical energy in glucose is transferred with 100 per cent efficiency to
ATP.
False: The efficiency of transfer of energy from glucose to ATP is about 40 per
cent. The remainder of the energy is not lost but appears as heat energy.
b
Fats are the usual energy source for cells.
False: The most usual source of energy for cells is the sugar glucose.
c
Cellular respiration overall is an energy-requiring process.
False: Overall, cellular respiration is an energy-producing process.
13
List three differences between anaerobic and aerobic respiration.
Differences between anaerobic and aerobic respiration include:
 Anaerobic respiration occurs in the absence of oxygen, while aerobic respiration
can occur only in the presence of oxygen.
 Anaerobic respiration is less efficient in the transfer of energy from glucose to ATP
than aerobic respiration, with anaerobic respiration producing 2 mole ATP per mole
of glucose compared with 36 mole ATP per mole glucose for aerobic respiration.
 ATP is produced more rapidly in anaerobic respiration than in aerobic respiration.
 The end products of anaerobic respiration in humans are lactate and water while
the end products of aerobic respiration are carbon dioxide and water.
14
Prepare a table with the following headings:
Location
Starting compound
Finishing compound
Amount of energy
produced as ATP
Compare glycolysis, the Krebs cycle and electron transport under each of these
headings.
Location
Starting
compound
Finishing
compound
Amount of energy
produced as ATP
glycolysis
cytosol
glucose
pyruvate
2 mole ATP per mole of
glucose
Krebs
cycle
mitochondria
pyruvate
carbon dioxide
(and loaded
acceptors)
2 mole ATP per mole of
glucose
electron
transport
chain
mitochondria
NADH and
FADH2
water
32 mole ATP per mole of
glucose
(Note: The amount of energy produced as ATP shown in the last column is based on
starting with one mole of glucose. Remember that this one mole of glucose produces
2 mole pyruvate (see figure 3.28). The pyruvate then enters the Krebs cycle and each
mole of pyruvate produces 3 mole NADH and 1 mole FADH2 (see figure 3.30). These
carrier molecules then enter the reactions of the electron transport chain (see
figure 3.31).)
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Chapter 3: QUICK-CHECK answers
15
What effect would you predict on cells in which the cytochromes were inhibited
by a poison?
It is reasonable to predict that, if the cytochromes in cells were inhibited by a poison,
the reactions of the electron transport chain could not occur and that this would cause
cellular respiration to stop, resulting in death of the cells.
16
When and where does anaerobic respiration occur in human beings?
In human beings, anaerobic respiration can occur for a limited period in muscle tissue
during strenuous exercise. Normally muscle carries out the more efficient aerobic
respiration but, when the circulation cannot supply sufficient oxygen to the muscles
during strenuous exercise, aerobic respiration begins.
17
Why do human muscle cells produce lactate during anaerobic respiration while
yeast cells produce an alcohol and carbon dioxide?
Human muscle cells have different enzymes from those present in yeast cells with the
result that, during anaerobic respiration, humans and yeast produce different end
products.
Note: In anaerobic respiration in humans, pyruvate is converted to lactate by the
enzyme lactate dehydrogenase. In anaerobic respiration in yeast, pyruvate is
converted in a two-step reaction to ethanol and carbon dioxide, and the second step is
catalysed by a different enzyme, alcohol dehydrogenase.
18
How are the ‘holes’ in a slice of bread formed?
The ‘holes’ in bread result from the production of carbon dioxide gas by yeast cells in
the bread dough when they carry out anaerobic respiration. The carbon dioxide is
produced when yeast cells convert pyruvate to ethanol and carbon dioxide.
19
Where do fatty acids enter the aerobic respiration pathway?
Fatty acids arising from the digestion of fats can enter the aerobic respiration pathway
as acetyl coenzyme A, which is an intermediate in the Krebs cycle.
20
What is the last group of organic compounds to be used as an energy source
during starvation?
During starvation, the last organic compounds to be used as an energy source are the
proteins.
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