1.7–1.13BThe human digestive system
The digestive system is an example of an organ system in which several organs work together to digest and absorb food. Digestive enzymes convert food into small soluble molecules that can be absorbed into the bloodstream.
Enzymes catalyse specific reactions in living organisms due to the shape of their active site. Each enzyme has an active site of a particular shape, and only a substrate that fits it will react — which is why one enzyme does one job.
Enzymes are not only digestive. Metabolism is the sum of all the reactions in a cell or in the body, and enzymes catalyse those reactions too — building new carbohydrates, lipids and proteins from the products of digestion, and breaking glucose down in respiration. The digestive enzymes here are one visible corner of it.
You are expected to use the lock and key theory as a simplified model to explain enzyme action. Simplified is the operative word — it explains specificity and it explains denaturing, and it is not a picture of what a real enzyme looks like. If a question offers you another model, use that one on its own terms.
Temperature and pH matter because both change the shape of the active site. Raising the temperature speeds the reaction up at first, because molecules collide more often; past the optimum the enzyme denatures, the active site loses its shape, and the substrate no longer fits. The wrong pH does the same thing. A denatured enzyme has not been "killed" — it was never alive — its shape has changed.
The products of digestion are used to build new carbohydrates, lipids and proteins, and some glucose is used in respiration. Word equations are enough here — no chemical symbol equations are required.
Bile is made in the liver and stored in the gall bladder. It does two things, and questions usually want both. It is alkaline, so it neutralises hydrochloric acid from the stomach, giving the enzymes of the small intestine the alkaline conditions they work best in. It also emulsifies fat into small droplets, which increases the surface area. Together, the alkaline conditions and the large surface area increase the rate of fat breakdown by lipase.
Common mistake
Bile is not an enzyme and it does not digest fat. It emulsifies fat — breaking it into droplets — so that lipase can work faster. Saying "bile breaks down fat" loses the mark.
Required practical 4
Use qualitative reagents to test for a range of carbohydrates, lipids and proteins
- Benedict's test for sugars: add Benedict's solution and heat in a water bath at about 75 °C. Blue means none; green, yellow, orange or brick red means increasing amounts of sugar.
- Iodine test for starch: add iodine solution. Orange-brown stays orange-brown if there is no starch; blue-black means starch is present.
- Biuret test for protein: add Biuret reagent. Blue means no protein; purple or lilac means protein is present.
- Emulsion test for lipids: add ethanol, shake, then add water. A cloudy white emulsion means lipid is present.
Watch out: These are qualitative tests — they tell you whether something is there, and Benedict's roughly how much, but they are not measurements. Say "turns blue-black, showing starch is present", naming the colour change and the conclusion.
Required practical 5
Investigate the effect of pH on the rate of reaction of amylase enzyme
- Set up starch solution with amylase and a buffer at a known pH, in a water bath so temperature is controlled.
- Every 30 seconds, take a drop of the mixture and add it to iodine solution on a spotting tile — this is the continuous sampling technique.
- The iodine stays orange-brown once all the starch has been digested. Record the time at which that happens.
- Repeat at a range of pH values and compare the times.
Watch out: Temperature must be controlled with a water bath or electric heater, or you cannot claim the change in rate was caused by pH. The shortest time means the fastest rate, so a graph of time against pH is upside down compared with one of rate against pH.
Worked example
Starch is completely digested in 90 seconds at pH 7, and in 150 seconds at pH 5. Calculate the rate of reaction at each pH in s⁻¹, and say which is faster.
- Rate of reaction = 1 ÷ time.
- At pH 7: 1 ÷ 90 = 0.011 s⁻¹.
- At pH 5: 1 ÷ 150 = 0.0067 s⁻¹.
Answer: pH 7 is faster — 0.011 s⁻¹ against 0.0067 s⁻¹ — because a shorter time means a higher rate.
- Digestive system
- an organ system in which several organs work together to digest and absorb food
- Enzyme
- a biological catalyst that speeds up a specific reaction in a living organism because of the shape of its active site
- Active site
- the part of an enzyme where the substrate binds; only a substrate with a matching shape will react, which is why one enzyme does one job
- Denatured
- describes an enzyme whose active site has lost its shape, usually through high temperature or the wrong pH, so the substrate no longer fits
- Metabolism
- the sum of all the reactions in a cell or in the body
- Bile
- an alkaline liquid made in the liver and stored in the gall bladder, which neutralises stomach acid and emulsifies fat into small droplets to increase the surface area for lipase
1.14BMeasuring the energy in food
Biology only — not on the Combined Science specification
Food stores energy, and calorimetry measures how much. The principle is simple: burn a known mass of food, use the heat released to warm a known volume of water, and measure how much the temperature of the water rises.
The method is worth learning as a sequence, because the marks are in the measurements you take rather than in the burning. Measure out a volume of water into a boiling tube and record its starting temperature. Weigh the food. Set light to the food and hold it under the tube until it stops burning. Record the highest temperature the water reaches.
Two calculations follow, and they must be done in this order.
First the energy transferred to the water: energy in joules = mass of water in grams × 4.2 × temperature change in °C. The 4.2 is the specific heat capacity of water — the energy needed to raise one gram by one degree. One cm³ of water has a mass of one gram, so a volume in cm³ can be used directly as a mass in grams.
Then the energy per gram of food: divide the energy transferred by the mass of food burnt. A question that gives you a mass of food is asking for this second step, and an answer that stops at the first one is incomplete.
Worked example
0.5 g of a crisp is burnt under a boiling tube holding 20 cm³ of water. The temperature of the water rises from 21 °C to 36 °C. Calculate the energy released per gram of crisp.
- Temperature change = 36 − 21 = 15 °C.
- 20 cm³ of water has a mass of 20 g.
- Energy = 20 × 4.2 × 15 = 1260 J.
- Energy per gram = 1260 ÷ 0.5.
Answer: 2520 J/g, or 2.52 kJ/g.
Common mistake
Every calorimetry result is an underestimate, and saying so is usually worth a mark. Heat escapes to the air and warms the boiling tube and the clamp instead of the water, and the food may not burn completely. If you are asked to improve the experiment, insulate the tube, shield it from draughts, and stir the water — do not say "be more accurate".
Before you practise
- An enzyme is specific because only a substrate with a complementary shape fits its active site.
- Denaturing is a change in the shape of the active site — the enzyme is not "killed".
- Rate against temperature rises to an optimum and then falls sharply; rate against pH peaks at the optimum for that enzyme.
- Bile emulsifies fat and neutralises stomach acid; it is not an enzyme.
- Learn the food tests as reagent, positive colour, and what it shows.
- Energy in food = mass of water × 4.2 × temperature rise, then divide by the mass of food burnt.
- Every calorimetry result underestimates, because heat is lost to the surroundings.