Cambridge International A Level Biology Revision Guide

Cambridge International A Level Biology
272
move sodium and potassium ions across the membrane
in opposite directions. For each ATP used, two potassium
ions move into the cell and three sodium ions move out of
the cell. As only two potassium ions are added to the cell
contents for every three sodium ions removed, a potential
difference is created across the membrane that is negative
inside with respect to the outside. Both sodium and
potassium ions leak back across the membrane, down their
diffusion gradients. However, cell surface membranes are
much less permeable to sodium ions than potassium ions,
so this diffusion actually increases the potential difference
across the membrane.
This potential difference is most clearly seen as
the resting potential of a nerve cell (page 333). One of the
specialisations of a nerve cell is an exaggeration of the
potential difference across the cell surface membrane as a
result of the activity of the sodium–potassium pump.
The importance of active transport in ion movement
into and out of cells should not be underestimated. About
50% of the ATP used by a resting mammal is devoted to
maintaining the ionic content of cells.
Respiration
Respiration is a process in which organic molecules act
as a fuel. The organic molecules are broken down in a
series of stages to release chemical potential energy, which
is used to synthesise ATP. The main fuel for most cells is
carbohydrate, usually glucose. Many cells can use only
glucose as their respiratory substrate, but others break
down fatty acids, glycerol and amino acids in respiration.
Glucose breakdown can be divided into four stages:
glycolysis, the link reaction, the Krebs cycle and oxidative
phosphorylation (Figure 12.7).
The glycolytic pathway
Glycolysis is the splitting, or lysis, of glucose. It is a
multi-step process in which a glucose molecule with six
carbon atoms is eventually split into two molecules of
pyruvate, each with three carbon atoms. Energy from
ATP is needed in the first steps, but energy is released in
later steps, when it can be used to make ATP. There is a
net gain of two ATP molecules per molecule of glucose
broken down. Glycolysis takes place in the cytoplasm of a
cell. A simplified flow diagram of the pathway is shown in
Figure 12.8.
In the first stage, phosphorylation, glucose is
phosphorylated using ATP. As we saw on page 269,
glucose is energy-rich but does not react easily. To tap
the bond energy of glucose, energy must first be used to
make the reaction easier (Figure 12.3, page 269). Two ATP
molecules are used for each molecule of glucose to make
first glucose phosphate, then fructose phosphate, then
fructose bisphosphate, which breaks down to produce two
molecules of triose phosphate.
Hydrogen is then removed from triose phosphate and
transferred to the carrier molecule NAD (nicotinamide
adenine dinucleotide). The structure of NAD is shown in
Figure 12.12, page 275. Two molecules of reduced NAD are
produced for each molecule of glucose entering glycolysis.
The hydrogens carried by reduced NAD can easily be
transferred to other molecules and are used in oxidative
phosphorylation to generate ATP (page 273).
The end-product of glycolysis, pyruvate, still contains a
ATP
ATP
glucose (hexose) (6C)
fructose phosphate (6C)
fructose bisphosphate (6C)
2 molecules of triose phosphate (3C)
phosphorylation
glycolysis
2ATP
anaerobic
pathways
to ethanol
or lactate
glycolysis
Link
reaction
Krebs cycle
aerobic pathways
in mitochondria
oxidative
phosphorylation
intermediates
2H
2NAD
2 reduced
NAD
2ATP
2 molecules of pyruvate (3C)
–2ATP
+4ATP +2 reduced NAD
Figure 12.7 The sequence of events in respiration.
Figure 12.8 The glycolytic pathway.