2.2.1: The human digestive system
Not started yet — this one needs some love.
Enzymes: biological catalysts; made of protein; each has a specific active site shape.
Lock and key model: only the substrate with a complementary shape to the active site can bind; the enzyme itself is unchanged — only the substrate is converted to products.
Effect of temperature: rate ↑ up to an optimum; above the optimum → the enzyme denatures (active site permanently changes shape).
Effect of pH: each enzyme has an optimum pH; too acidic or alkaline → denatures (see RP5: Amylase and pH).
Carbohydrases (e.g. amylase): starch → simple sugars (maltose/glucose); made in salivary glands, pancreas, small intestine.
Proteases: proteins → amino acids; made in the stomach (pepsin), pancreas, small intestine.
Lipases: lipids → glycerol + fatty acids; made in the pancreas and small intestine.
Bile: made in the liver; stored in the gall bladder; released into the small intestine.
emulsifies fat → small droplets → ↑ surface area for lipase → faster fat breakdown
alkaline → neutralises hydrochloric acid from the stomach → optimum pH for intestinal enzymes
Products of digestion are used to build new carbohydrates, lipids and proteins; some glucose is used in respiration.
Food tests (see RP4: Food tests):
Benedict's reagent (heat in a hot water bath) → reducing sugars turn orange/brick-red
iodine solution → starch turns blue-black
Biuret reagent → protein turns purple
Common exam mistakes
Bile is NOT an enzyme — it is an emulsifier/alkaline substance. It cannot digest fat.
Bile neutralises acid in the small intestine, not in the stomach.
Benedict's test: must be heated (in a hot water bath) — adding Benedict's at room temperature will not produce a colour change even if reducing sugar is present.
Iodine solution (not just 'iodine') for starch test — must specify 'solution'.
Lipases produce glycerol AND fatty acids — not fatty acids alone.
Amylase breaks down starch (not glycogen, not protein); it is a carbohydrase, not a protease.
Amylase is produced in salivary glands, pancreas and small intestine — NOT the stomach.
Lock and key: substrate fits into active site; just saying they 'fit' or 'match' without naming the active site is insufficient.
Do not confuse enzyme and substrate: amylase is the enzyme; starch is the substrate — never say starch is an enzyme or that amylase fits into starch.
Starch is NOT broken down to form glucose for photosynthesis — glucose is MADE in photosynthesis and then converted to starch for storage.