S. M. Zaigham Zia 1
,8.1 Metabolism
Introduction
1. Define metabolic pathway
Metabolic pathways consist of chains or cycles of reactions that are catalysed by enzymes. Metabolism
includes all the chemical activities that keep organisms alive. Metabolic pathways may be very
complex, but most consist of a series of steps, each controlled by an enzyme. Simple pathways involve
the conversion of substrates to a final product:
Respiration and photosynthesis are two key metabolic pathways in ecosystems. Light energy from
the Sun is trapped as chemical energy in photosynthesis and then the energy is transferred through
food chains and released back to the environment as heat energy from respiration.
2. Explain that enzymes lower the activation energy of the chemical reactions that they catalyze
Enzymes work by lowering the activation energy of the substrate or substrates. In order for a
metabolic reaction to occur, the substrate has to reach an unstable, high-energy ‘transition state’
where the chemical bonds are destabilised, and this requires an input of energy, which is called the
activation energy.
Activation energy is used to break or weaken bonds in the substrates. When an enzyme catalyses a
reaction, the substrate binds to the active site and is altered to reach the transition state. It is then
converted into the products, which separate from the active site. This binding lowers the overall
energy level of the transition state. The activation energy of the reaction is therefore reduced. The
net amount of energy released by the reaction is unchanged by the involvement of the enzyme.
However, as the activation energy is reduced, the rate of the reaction is greatly increased, typically
by a factor of a million or more.
Most biological reactions release more energy than they take as activation energy and are said to
be exothermic. This is shown as energy change on the graph. Chemical reactions are not single-
step processes. Substrates have to pass through a transition state before they are converted into
products.
S. M. Zaigham Zia 2
, 3. Explain the difference between competitive and non-competitive inhibition, with reference to
one example of each
Enzyme inhibitors are substances that reduce or prevent an enzyme’s activity. Some inhibitors are
competitive and others non-competitive.
Competitive inhibitors have molecules whose
structure is similar to that of the substrate
molecule that normally binds to the active site.
They compete with the substrate to occupy the
active site of the enzyme and prevent the
substrate molecules from binding.
The inhibitors are not affected by the enzyme and
do not form products, so they tend to remain in
the active site. This means that the rate of
reaction is lower because substrate molecules
cannot enter the active sites of enzyme molecules
that are blocked by an inhibitor. At low
concentrations of substrate, competitive
inhibitors have a more significant effect than at
higher concentrations, when the substrate can
out-compete the inhibitor.
S. M. Zaigham Zia 3
,8.1 Metabolism
Introduction
1. Define metabolic pathway
Metabolic pathways consist of chains or cycles of reactions that are catalysed by enzymes. Metabolism
includes all the chemical activities that keep organisms alive. Metabolic pathways may be very
complex, but most consist of a series of steps, each controlled by an enzyme. Simple pathways involve
the conversion of substrates to a final product:
Respiration and photosynthesis are two key metabolic pathways in ecosystems. Light energy from
the Sun is trapped as chemical energy in photosynthesis and then the energy is transferred through
food chains and released back to the environment as heat energy from respiration.
2. Explain that enzymes lower the activation energy of the chemical reactions that they catalyze
Enzymes work by lowering the activation energy of the substrate or substrates. In order for a
metabolic reaction to occur, the substrate has to reach an unstable, high-energy ‘transition state’
where the chemical bonds are destabilised, and this requires an input of energy, which is called the
activation energy.
Activation energy is used to break or weaken bonds in the substrates. When an enzyme catalyses a
reaction, the substrate binds to the active site and is altered to reach the transition state. It is then
converted into the products, which separate from the active site. This binding lowers the overall
energy level of the transition state. The activation energy of the reaction is therefore reduced. The
net amount of energy released by the reaction is unchanged by the involvement of the enzyme.
However, as the activation energy is reduced, the rate of the reaction is greatly increased, typically
by a factor of a million or more.
Most biological reactions release more energy than they take as activation energy and are said to
be exothermic. This is shown as energy change on the graph. Chemical reactions are not single-
step processes. Substrates have to pass through a transition state before they are converted into
products.
S. M. Zaigham Zia 2
, 3. Explain the difference between competitive and non-competitive inhibition, with reference to
one example of each
Enzyme inhibitors are substances that reduce or prevent an enzyme’s activity. Some inhibitors are
competitive and others non-competitive.
Competitive inhibitors have molecules whose
structure is similar to that of the substrate
molecule that normally binds to the active site.
They compete with the substrate to occupy the
active site of the enzyme and prevent the
substrate molecules from binding.
The inhibitors are not affected by the enzyme and
do not form products, so they tend to remain in
the active site. This means that the rate of
reaction is lower because substrate molecules
cannot enter the active sites of enzyme molecules
that are blocked by an inhibitor. At low
concentrations of substrate, competitive
inhibitors have a more significant effect than at
higher concentrations, when the substrate can
out-compete the inhibitor.
S. M. Zaigham Zia 3
