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Knowledge required from other Units: from Unit 1 molecules, especially carbohydrates roles of
enzymes structure of a mitochondrion.
The term metabolism refers to all the chemical reactions taking place inside cells. It includes catabolism,
where complex molecules are broken down into simple molecules with the release of energy, and
anabolism, in which complex molecules are built up from their simpler components.
Each reaction takes place in a series of small steps, known as a metabolic pathway. Each reaction in the
pathway is catalysed by a specific enzyme and the reactions take place in a precise sequence. In this way,
metabolic reactions within cells are controlled.
Living organisms require energy for movement, active transport and the synthesis of macromolecules. This
energy is obtained by cellular respiration and provides cells with adenosine triphosphate (ATP). An ATP
molecule consists of the nucleotide adenine, the fivecarbon sugar ribose and three inorganic phosphate
groups. There are phosphoanhydride bonds between the phosphate groups and these yield free energy
when ATP is hydrolysed. ATP is resynthesised from ADP (adenosine diphosphate) and inorganic
phosphate when energy is released from the oxidation of glucose. ATP is the immediate source of energy
within cells. It links energyrequiring and energyyielding processes.
Reactions in a metabolic pathway, such as cellular respiration, may involve oxidation of a substrate by the
removal of electrons or hydrogen atoms. Electrons are transferred to electron carriers (or hydrogen
carriers), which become reduced. Nicotinamide adenine dinucleotide (NAD+) is an important electron
carrier. When a substrate is oxidised, that substrate loses two hydrogen atoms. NAD+ accepts one of the
hydrogen atoms and two electrons (both the electrons from the two hydrogen atoms lost from the
substrate), becoming NADH. The other hydrogen atom, minus its electron (so it is H+), appears in solution.
The reaction is written out in the equation below.
NAD+ + 2H+ + 2e- NADH + H+
NAS Biology Online Teachers' Guide Nelson Thornes © 2002 1
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Reduced electron carriers transfer their electrons to other electron carriers in an electron transport chain. In
aerobic respiration, the electrons are eventually transferred to oxygen, which is reduced to form water. As
the electrons are passed from carrier to carrier, the change from the reduced form of the carrier to the
oxidised form is linked to the synthesis of ATP.
Glycolysis occurs in the cytoplasm of cells. It consists of a series of enzymecontrolled reactions during
which monosaccharides are converted to pyruvate.…read more
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1. Define metabolism and explain what is meant by a metabolic pathway.
2. Give an example of a catabolic reaction that takes place inside cells. How does this type of reaction
differ from an anabolic one?
3. List as many anabolic reactions that take place in cells as you can.
4. Describe the components of an ATP molecule. Where are the phosphoanhydride bonds located?
5. Explain why ATP is important in cells.
6. Make a diagram to show the structure of a mitochondrion.…read more