Enzymes
Enzymes and the lock and key model
are biological catalysts — they speed up chemical reactions in the body without being used up themselves. Enzymes are large proteins with a specially shaped .
The describes how a substrate molecule (the "key") fits into an enzyme's active site (the "lock") because their shapes are complementary. Once bound, the enzyme speeds up the reaction that turns the substrate into a product, which is then released.
Key terms
Temperature, pH and enzyme activity
Enzyme activity increases as temperature rises, up to an optimum, because particles move faster and collide more often. Above the optimum, the enzyme : the active site changes shape, the substrate no longer fits, and the reaction stops.
Each enzyme also has an optimum pH. Too acidic or too alkaline an environment denatures the enzyme in the same way as excess heat, changing the active site's shape.
Exam tip
Write that the enzyme is because its active site has changed shape. Never say it has been "killed": enzymes aren't alive.
Key terms
Calculating rate of reaction
If you are given the time taken for a fixed amount of product to form or substrate to be used up, rate of reaction can be calculated as:
rate = 1 ÷ time
If you know the amount of product formed or substrate used, use instead:
rate = amount ÷ time
A faster reaction has a shorter time and therefore a higher rate.
Digestive enzymes and bile
Three digestive enzymes break down large food molecules into small, soluble molecules that can be absorbed:
- breaks down starch into sugars. Made in the salivary glands, pancreas and small intestine.
- breaks down proteins into amino acids. Made in the stomach, pancreas and small intestine.
- breaks down lipids into fatty acids and glycerol. Made in the pancreas and small intestine.
, made in the liver and stored in the gall bladder, neutralises stomach acid and emulsifies fats — breaking them into smaller droplets to increase the surface area for lipase to act on.