Enzymes are biological catalysts: they speed up chemical reactions without changing or being used up.
They can break large molecules into smaller ones - this happens in digestion before absorption.
They can build up large molecules from smaller ones - glucose molecules can be joined together by enzymes which work to form storage molecules such as starch, or structural molecules such as cellulose.
They can convert one small molecule into another as many reactions require changing molecules by adding or removing atoms.
Enzymes are made of protein. Each enzyme catalyses one reaction and they are affected by temperature and pH.
The action of enzymes can be described with the 'lock and key' model - the active site (where the reaction takes place) will only accomodate a substrate (the substance the enzyme works on) that fits its shape.
Proteins are influenced by surroundings so if the conditions change too much from the optimum temperature or pH (where the enzymes work best) they will change shape.
At a higher temp or different pH the bonds holding the enzyme molecule together will break down and change the shape of the active site - the substrate won't fit, so the enzyme is said to be denatured.
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Enzymes in industry
Only small quantities of enzymes are needed for reactions, and they often work at lower temperatures than other catalysts so reduce fuel costs.
In the production of beer: enzymes in barley seeds convert starch into maltose, which is used by yeast in fermentation to produce alcohol.
Pectinases break down pectins (molecules that act as a glue in plant cell walls). This is used for extracting fruit juices.
Fermentation: in industry this is a process using microorganisms to produce a useful product.
Penicillin is produced by fermentation: a fermentation vessel is filled with a medium containing sugars and ammonium salts and the fungus Penicillium is added. An air supply provoides oxygen for aerobic respiration. A stirrer ensures that the microorganisms are suspended so always have access to nutrients and oxygen. A water-cooled jacket keeps temperature constant. Probes monitor the pH. The fungus grows well in these conditions and produces penicillin.
In biological washing powders, modified enzymes (to withstand higher temperatures and alkaline conditions) are used. Proteases break down protein stains e.g. blood, grass and egg. Lipases break down fats e.g. butter, lipstick and mayonnaise. Amylases break down starch, and cellulases break down cellulose fibres to remove dirt attached to them.
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