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Enzymes are biological catalysts: this means that they speed up
the chemical reactions in living things. Without enzymes, our guts would take weeks and
weeks to digest our food, our muscles, nerves and bones would not work properly and so on
- we would not be living!
A catalyst is any substance which makes a chemical reaction go
faster, without itself being changed. A catalyst can be used over and over again in a
chemical reaction: it does not get used up. Enzymes are very much the same except that
they are easily denatured (destroyed: but do NOT use this word since the protein molecule
is not broken down into amino-acids, it just loses it shape and will not work any more) by
heat. Our enzymes work best at body temperature. Our enzymes also have to have the correct
pH.
All enzymes are made of protein; that is why they are sensitive
to heat, pH and heavy metal ions. Unlike ordinary catalysts, they are specific to one
chemical reaction. An ordinary catalyst may be used for several different chemical
reactions, but an enzyme only works for one specific reaction.
Human saliva contains an enzyme called amylase. This enzyme helps
to turn starch into a sugar called maltose. When you swallow a mouthful of food, the
amylase stops working because it is much too acid in the stomach pH 2. Amyalse works best
in neutral or slightly alkaline conditions, i.e. at about pH 7. When your food gets into
the small intestine, more amylase is made by the pancreas and this turns the remaining
starch into maltose. Another enzyme (maltase) turns all this maltose into glucose. Glucose
is then absorbed into the blood.
Enzymes in the human alimentary canal and what they
digest:
| Enzyme |
Substrate |
| Amylase |
Starch |
| Maltase |
Maltose |
| Sucrase |
Sucrose |
| Lipase |
Fats |
| Pepsin |
Proteins |
All animals, green plants, fungi and bacteria produce enzymes: so
enzymes are not just about digesting food. The enzymes which we use to digest our food are
extra-cellular, that means they are found outside cells. We also have enzymes inside our
cells; these are intra-cellular enzymes. Enzymes are used in ALL chemical reactions in
living things; this includes respiration, photosynthesis, movement growth, getting rid of
toxic chemicals in the liver and so on.
Viruses are rather different, but you do not need to know much
about them for GCSE, so just make sure that you don't catch any!
Enzymes must have the correct shape to do their job. They are
made of proteins, and proteins are very easily affected by heat, pH and heavy metal ions.
Some people say that enzymes work like a key in a lock. If the key has been twisted by
heat, or dissolved in acid or stuck up with chewing gum it will not work. Enzymes change
their shape if the temperature or pH changes, so they have to have the right conditions.
Copper ions are poisonous: if you get copper ions in your blood they will block up some of
the important enzymes in red and white blood cells.
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