Why is more than one type of enzyme required to complete the digestion process? 

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Enzymes are sometimes referred to as organic catalysts because they are substances in the body that speed up chemical reactions. 

An important characteristic of catalysts is that they are very specific, meaning that particular enzymes work on specific substrates. 

The food we eat contain different nutrients, e.g. proteins, carbohydrates,and fats.

Each of these represent a different substrate to an enzyme and therefore different enzymes are needed to breakdown each nutrient into smaller molecules that can be absorbed into the blood stream for use by the body.

For example: lipases break down fats into fatty acids and glycerol; proteases break down proteins to amino acids; and amylases break down carbohydrates to glucose.

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Enzymes are mostly proteins (some are RNA molecules) that catalyze chemical reactions. They speed up or activate chemical reactions by providing alternate pathways.

Our digestion depends on a large number of enzymes. It is estimated that there are 50-70 thousand different enzymes in our body that support our metabolism. The enzymes are needed to break down our food into nutrients, because our digestive system is incapable of absorbing food directly (it can only absorb nutrients). These enzymes are produced in pancreas, small intestine, salivary glands and stomach. 

The reason we need more than one enzyme for digestion of food is the specificity of enzymes. We can think of this specificity as lock and key arrangement. An enzyme has an active site that can only bind with a specific food (or substrate) and hence can only break down that specific substrate. This specificity exists for particular reaction, particular functional group, particular chemical bond, etc. And hence we need more than one type of enzyme for completion of digestion.

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