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Enzymes
Enzymes are protein molecules which change the rate of a chemical reaction
without themselves being permanently changed by the reaction. Enzymes
help change a substrate (the molecule onwhich they are acting) into a
product.
How is the rate of enzyme activity changed?
Four major factors influence the rate at which enzymes work:
1. Enzyme concentration
2. pH
3. Temperature
4.Substrate concentration

Enzyme concentration
Influencing the rate of a biochemical reaction by changing the enzyme
complex is expensive in terms of energy. The cell has to make complex
enzymemolecules from simpler molecules; this process involves the
consumption of energy and also takes time and may therefore be
unsatisfactory for urgent responses.

pH and temperature variation
It isimportant to think about which external (outside the enzyme) conditions
are likely to be most influential within the body or cell of an organism. The
wide range of pH and temperature variation in alaboratory test tube may
have little relevance to an organism in which body temperature and pH are
closely controlled e.g. human blood is usually maintained within a pH
range of 7.35 - 7.45 and atemperatue range of 37- 380C. Even reptiles are
capable of maintaining their body temperature within quite narrow limits.
However, in practice, the body rarely attempts to control the rate of
metabolicpathways by deliberately altering pH or temperature.

Metabolic Pathways
There are two types of metabolic reactions:

Anabolic - in which larger molecules are made from smaller ones e.g.proteins from amino acids.

Catabolic - in which larger molecules are broken down into smaller ones
e.g. proteins into amino acids during digestion.
Often several reactions are linked together in alinear chain, with several
different types of molecules, known as metabolites, being formed between
substrate and final product. Each stage is often controlled by a different
enzyme:

substrate...