Cambridge International A Level Biology Revision Guide

Chapter 12: Energy and respiration
CO 2 + H 2 O
respiration
chemical potential
energy of ATP
Key
transfer of materials
transfer of energy
Figure 12.2 Transfer of materials and energy in an ecosystem.
Work
Work in a living organism includes:
■■
■■
■■
■■
light energy
autotrophs
e.g. green plants
heterotrophs
e.g. animals, fungi
and most bacteria
chemical potential
energy of carbohydrates
and other organic
molecules
thermal energy
work
the synthesis of complex substances from simpler
ones (anabolic reactions), such as the synthesis of
polysaccharides from monosaccharides, lipids from
glycerol and fatty acids, polypeptides from amino
acids, and nucleic acids from nucleotides
the active transport of substances against a diffusion
gradient, such as the activity of the sodium–potassium
pump (Figure 4.18, page 87)
mechanical work such as muscle contraction
(page 344) and other cellular movements; for example,
the movement of cilia and flagella (page 189), amoeboid
movement and the movement of vesicles through
cytoplasm
in a few organisms, bioluminescence and
electrical discharge.
Mammals and birds use thermal energy (heat) that is
released from metabolic reactions to maintain a constant
body temperature. Most animals are ectotherms. The
thermal energy that warms them comes from outside their
bodies. Mammals and birds are endotherms, releasing
enough thermal energy within their bodies to maintain
them above the temperature of their surroundings
when necessary. They also maintain a constant body
temperature through negative feedback loops (page 301).
For a living organism to do work, energy-requiring
reactions must be linked to those that yield energy. In
the complete oxidation of glucose (C 6
H 12
O 6
) in aerobic
conditions, a large quantity of energy is made available:
C 6
H 12
O 6
+ 6O 2
→ 6CO 2
+ 6H 2
O + 2870 k J
Reactions such as this take place in a series of small
steps, each releasing a small quantity of the total available
energy. Multi-step reactions allow precise control via
feedback mechanisms (Chapter 3). Moreover, the cell could
not usefully harness the total available energy if all of it
were made available at one instant.
Although the complete oxidation of glucose to carbon
dioxide and water has a very high energy yield, the
reaction does not happen easily. Glucose is actually quite
stable, because of the activation energy that has to be
added before any reaction takes place (Figure 12.3). In
living organisms, the activation energy is overcome by
lowering it using enzymes (page 56), and also by raising
the energy level of the glucose by phosphorylation
(page 272).
Theoretically, the energy released from each step of
respiration could be harnessed directly to some form of
work in the cell. However, a much more flexible system
actually occurs in which energy-yielding reactions in
all organisms are used to make an intermediary
molecule, ATP.
Increase in energy
C 6 H 12 O 6 + O 2
substrates
Progress of reaction
Figure 12.3 Oxidation of glucose.
CO 2 + H 2 O
products
activation
energy
available
energy
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