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Metabolism and energy
Metabolism: chemical reactions in cells
Countless chemical reactions take place in cells and are responsible for all the actions of organisms. Together, these reactions make up an organism's metabolism. The chemicals taking part in these reactions are called metabolites.
In all reactions:
- chemical bonds in the reacting molecules are broken; this takes in energy
- new chemical bonds form to make the products; this gives out energy
When a chemical reaction takes place energy is either taken in or released. This depends on the relative strengths of bonds being broken and bonds being formed.
In an exergonic reaction, energy is released to the surroundings. The bonds being formed are stronger than the bonds being broken.
In an endergonic reaction, energy is absorbed from the surroundings. The bonds being formed are weaker than the bonds being broken.
You may also come across the terms exothermic and endothermic reactions. These describe exergonic and endergonic reactions when the energy released or absorbed is heat energy. In an exothermic reaction the temperature of the surroundings increases. In an endothermic reaction the temperature of the surroundings decreases.
Anabolism and catabolism
Two types of metabolic reactions take place in the cell: 'building up' ( anabolism ) and 'breaking down' ( catabolism ).
Anabolic reactions use up energy. They are endergonic. In an anabolic reaction small molecules join to make larger ones. For example, the following condensation reactions that occur in cells are anabolic:
- amino acids join together to make dipeptides:
e.g. NH 2 CHRCOOH + NH 2 CHRCOOH NH 2 CHRCONHCHRCOOH + H 2 O
and the process continues as large protein molecules are built up
- small sugar molecules join together to make dissacharides:
e.g. C 6 H 12 O 6 + C 6 H 12 O 6 C 12 H 22 O 11 + H 2 O
and the process continues as large polysaccharide molecules are built up
- glycerol reacts with fatty acids to make lipids:
e.g. CH 2 OHCH(OH)CH 2 OH + C 17 H 35 COOH CH 2 OHCH(OH)CH 2 OOCC 17 H 35
and the process continues as the trigyleride is produced via similar reactions with the other two hydroxyl groups of the glycerol molecule
It is more common for reactions between molecules to take place in a series of consecutive steps. After each step a reaction intermediate forms. Unlike an activated complex this has a real existence. For each step an activated complex is formed and there is an associated activation energy. The step with the highest activation energy is the rate-determining step in the reaction and controls how fast the overall reaction is.
