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Human Cell: Chapter 6-8 Summary
Cell metabolism encompasses all the enzyme-catalysed reactions that occur in a cell.
Anabolism consists of biosynthetic metabolic pathways that build up complex molecules
from simpler constituents and need a supply of energy; catabolism consists of metabolic
pathways that break down larger molecules into smaller ones and usually release energy.
For a metabolic reaction to occur, activation energy is needed to form a transition state
from which end products are produced. Enzymes catalyse biochemical reactions by
lowering the activation energy needed by the reactants to form their transition state.
Substrate molecules have an affinity for the active site on an enzyme. The active site’s
shape determines the orientation of the reactants on it and it binds to them closely with an
induced fit.
The enzymes controlling a metabolic pathway usually work as a group. Although some
steps are irreversible, most metabolic reactions are reversible. The direction in which the
reaction occurs depends on factors such as concentration of the substrate and removal of a
product as it becomes converted to another metabolite.
Each step in a metabolic pathway is regulated by an enzyme that catalyses a specific
reaction. Each enzyme is under genetic control.
Some metabolic pathways are required continuously and the genes that code for their
enzymes are always switched on. Other pathways are only needed on certain occasions.
To prevent resources being wasted, the genes that code for their enzymes are switched on
or off as required in response to signals from within the cell and from its environment.
Molecules of a competitive inhibitor resemble the substrate in shape. They become
attached to the active site and slow down the reaction. Their effect is reversed by
increasing the concentration of substrate.
Some regulatory molecules stimulate enzyme activity or inhibit it non-competitively by
changing the structure of the enzyme molecule and its active site(s).
Some metabolic pathways are controlled by end product inhibition, a form of negative
feedback control.
ATP is a high energy compound able to release and transfer energy when it is required for
cellular processes. ATP is regenerated from ADP and Pi by phosphorylation using energy
released during cellular respiration. Phosphorylation also occurs when Pi and energy are
transferred from ATP to a reactant in a pathway.
Cellular respiration begins with glycolysis, the breakdown of glucose to pyruvate. This
consists of an energy investment phase and an energy payoff phase with a net gain of two
molecules of ATP.
In the presence of oxygen, pyruvate is broken down into carbon dioxide and an acetyl
group. With the help of coenzyme A, the acetyl group enters the citric acid cycle by
combining with oxaloacetate to become citrate.
As one respiratory substrate is converted to another in the citric acid cycle, carbon dioxide
is released, ATP is formed and pairs of hydrogen ions along with high-energy electrons are
removed and passed to coenzymes NAD and FAD forming NADH and FADH2.
NADH and FADH2 pass their high-energy electrons to electron transport chains where the
energy released is used to pump hydrogen ions across inner mitochondrial membranes.
The return flow of these ions makes part of each ATPsynthase molecule rotate and catalyse
the synthesis of ATP. Oxygen, the final electron acceptor, combines with hydrogen ions and
electrons to form water.
Complex carbohydrates, proteins and fats can all be used as respiratory substrates if they
are first converted to suitable intermediates able to enter the pathway.
Phosphofructokinase is an enzyme that catalyses an irreversible step in glycolysis. Its
activity is inhibited by high levels of ATP and citrate. These inhibitory feedback mechanisms
help to synchronise and regulate the respiratory pathway.
During strenuous activity, creatine phosphate in muscle cells breaks down, releasing energy
and phosphate used to convert ADP to ATP. The ATP formed fuels muscle contraction for a
few seconds.
During a period of vigorous exercise, muscle cells do not receive the adequate supply of
oxygen needed for the electron transport chain to operate. Instead of entering the citric
acid cycle, pyruvate is converted to lactic acid, which causes fatigue of muscles. Any oxygen
debt that develops is repaid when vigorous exercise comes to a halt.
Skeletal muscle contains fast-twitch fibres, good for short-lived power events, and slowtwitch fibres, good for endurance activities.