Enzymes as biological catalysts, the lock and key model and the active site, the effect of temperature, pH and substrate concentration on enzyme activity, denaturing, and the breakdown of carbohydrates, proteins and lipids by digestive enzymes.

What this topic is asking

OCR wants you to explain that enzymes are biological catalysts, describe the lock and key model, explain how temperature, pH and substrate concentration change the rate, define denaturing, and describe how digestive enzymes break down the large food molecules.

Enzymes as biological catalysts

Living cells run thousands of chemical reactions, together called metabolism, and most would be far too slow at body temperature without enzymes. Because an enzyme is not used up, one molecule catalyses the reaction over and over again. The lock and key model explains the specificity: the substrate fits into the active site like a key fits a lock, the enzyme holds the substrate while the reaction happens, and the products are then released, leaving the active site free again. A given enzyme only works on a substrate whose shape matches its active site, which is why amylase breaks down starch but not protein.

Temperature, pH and concentration

The rate of an enzyme reaction depends on the conditions:

• Temperature. Raising the temperature gives molecules more energy, so the enzyme and substrate collide more often and with more energy, and the rate rises up to the optimum (about $37$ degrees Celsius for many human enzymes). Above the optimum the enzyme denatures: the bonds holding its shape break, the active site changes shape, and the substrate no longer fits, so the rate falls sharply.
• pH. Each enzyme has an optimum pH. Moving away from it changes the active site shape and slows the reaction; a large change denatures the enzyme. Stomach protease (pepsin) works best in acidic conditions, while amylase in the mouth and small intestine prefers a neutral or slightly alkaline pH.
• Substrate concentration. Increasing the substrate concentration speeds the reaction up because collisions are more frequent, until all the active sites are working as fast as they can, after which the rate levels off.

Digestive enzymes

Large food molecules are too big to be absorbed, so digestive enzymes break them into small soluble molecules. Amylase (made in the salivary glands and pancreas) breaks starch into sugars such as maltose. Proteases (such as pepsin in the stomach) break proteins into amino acids. Lipase (made in the pancreas) breaks lipids (fats and oils) into fatty acids and glycerol. Bile, made in the liver and stored in the gall bladder, is not an enzyme but it neutralises stomach acid and emulsifies fats into small droplets, giving a larger surface area so lipase works faster. The small soluble products are then absorbed into the blood across the wall of the small intestine.