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How do enzymes digest food, and how does the digestive system absorb nutrients?

The action of enzymes as biological catalysts, the lock and key model, the effect of temperature and pH on enzyme activity, the products of digestion, and the role of bile, with the food tests for the required practical.

A focused answer to AQA GCSE Biology 4.2.2, covering enzymes as biological catalysts, the lock and key model, the effect of temperature and pH, the products of digestion, the role of bile, and the food tests required practical.

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  1. What this dot point is asking
  2. Enzymes as biological catalysts
  3. Effect of temperature and pH
  4. Products of digestion and the role of bile
  5. Required practical: food tests
  6. Try this

What this dot point is asking

AQA wants you to explain how enzymes work using the lock and key model, describe the effect of temperature and pH on enzyme activity, name the three groups of digestive enzymes and their products, explain the role of bile, and carry out the food tests required practical.

Enzymes as biological catalysts

Because the active site fits only one substrate, enzymes are specific: amylase breaks down starch but not protein. If the substrate fits the active site, the enzyme can catalyse the reaction; if the active site changes shape, the substrate no longer fits and the enzyme stops working. Enzymes work in digestion (breaking food down) and inside cells (for example building large molecules and controlling respiration).

Effect of temperature and pH

  • Temperature: activity increases as temperature rises, because the enzyme and substrate molecules have more kinetic energy, move faster and collide more often. This continues up to an optimum (around 37 ∘C37\,^{\circ}C in humans). Above the optimum, the enzyme denatures: the active site changes shape and the substrate no longer fits, so the rate falls sharply.
  • pH: each enzyme has an optimum pH. Too acidic or too alkaline changes the active site shape and denatures the enzyme. Stomach protease works best in acidic conditions (around pH 2), whereas enzymes in the small intestine prefer slightly alkaline conditions, which is why bile is needed.

Products of digestion and the role of bile

  • Carbohydrases (for example amylase, made in the salivary glands, pancreas and small intestine) break carbohydrates into simple sugars such as glucose.
  • Proteases (for example pepsin in the stomach) break proteins into amino acids.
  • Lipases (made in the pancreas and small intestine) break lipids into fatty acids and glycerol.

Required practical: food tests

  • Starch: add iodine solution; a blue-black colour shows starch is present.
  • Sugars (reducing): add Benedict's solution and heat in a water bath; a colour change from blue to green, yellow, orange or brick-red shows sugar.
  • Protein: add Biuret reagent; a purple or lilac colour shows protein.
  • Lipids: add ethanol then water (the emulsion test); a cloudy white emulsion shows lipid.

Try this

Q1. Explain what happens to an enzyme above its optimum temperature. [2 marks]

  • Cue. It denatures; the active site changes shape so the substrate no longer fits and the reaction slows or stops.

Q2. Name the products formed when lipase digests a lipid. [2 marks]

  • Cue. Fatty acids and glycerol.

Exam-style practice questions

Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

AQA 20194 marksExplain the effect of temperature on the rate of an enzyme-controlled reaction, including what happens above the optimum temperature.
Show worked answer →

A 4-mark explain question rewards the rise, the optimum and denaturing.

As temperature increases from low values, the rate of the enzyme-controlled reaction increases, because the enzyme and substrate molecules have more kinetic energy, move faster and collide more often, so more successful collisions occur. The rate is highest at the optimum temperature (about 37 degrees Celsius in humans). Above the optimum, the rate falls sharply, because the high temperature changes the shape of the enzyme's active site, so the substrate no longer fits; the enzyme is denatured and this is permanent.

Markers reward the rate rising due to more frequent collisions, the optimum, and denaturing changing the active site so the substrate cannot bind.

AQA 20214 marksA student tested a food sample for starch, sugar, protein and lipid. Describe the test and the positive result for each of the four food groups.
Show worked answer →

A 4-mark required-practical question rewards the correct reagent and positive colour for each.

For starch, add iodine solution; a positive result changes the orange-brown iodine to blue-black. For reducing sugars, add Benedict's solution and heat in a water bath; a positive result changes the blue solution to green, yellow, orange or brick-red. For protein, add Biuret reagent; a positive result turns the blue reagent purple or lilac. For lipids, add ethanol and then water (the emulsion test); a positive result is a cloudy white emulsion.

Markers reward the correct reagent and positive colour change for each of the four tests, and heating for the Benedict's test.

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