a) Fig. 1.1 is a diagram of the digestive system.
Each letter may be used once, more than once or not at all.
State the letter of the part shown in Fig. 1.1:
- that produces bile
- that produces gastric juice
- that produces urea
- where maltose is digested
- where trypsin acts
(b) A student investigated the effect of bile on the digestion of fat in milk.
They set up three different test-tubes:
• test-tube A contained milk and bile
• test-tube B contained milk and lipase
• test-tube C contained milk, lipase and bile.
They used an indicator that is pink in alkaline solutions and colourless in acidic solutions.
They added the same volume of indicator to each test-tube.
The student observed and recorded the colour of the contents of each test-tube at 0 minutes, 20 minutes and 40 minutes.
Table 1.1 shows the results of the investigation
(i) Explain the results for test-tubes B and C in Table 1.1.
(ii) Explain the purpose of test-tube A in Table 1.1.
(c) The action of lipase is affected by temperature.
Fig. 1.2 shows the axes for a graph of the effect of temperature on the activity of lipase.
Complete the graph by:
• drawing a line to show the expected effect of temperature on the activity of lipase
• adding a label line and a label to show the point at which all the lipase has been denatured.
(d) Explain why lipase cannot be used to catalyse the breakdown of proteins.
▶️ Answer/Explanation
(a)
- that produces bile: K (Liver produces bile, stored in the gallbladder)
- that produces gastric juice: C (Stomach secretes gastric juice containing HCl and pepsin)
- that produces urea: K (Liver converts ammonia to urea in the ornithine cycle)
- where maltose is digested: H (Small intestine contains maltase for maltose breakdown)
- where trypsin acts: H (Trypsin is active in the small intestine)
(b)(i)
Test-tube B turns acidic due to lipase breaking down fats into fatty acids. Test-tube C turns acidic faster because bile emulsifies fats, increasing surface area for lipase action. The indicator changes from pink (alkaline) to colorless (acidic) as pH drops.
(b)(ii)
Test-tube A acts as a control to confirm that bile alone does not digest fats or change pH. It ensures observed changes in B and C are due to lipase activity.
(c)
The graph should show an increase in lipase activity up to an optimum temperature (~37°C), then a sharp decline as enzymes denature. Label the point where the curve meets the x-axis as “All lipase denatured.”
(d)
Lipase is specific to fats due to its active site’s complementary shape. Proteins cannot bind to lipase’s active site, preventing enzyme-substrate complex formation. Thus, lipase cannot catalyze protein breakdown.
(a) Fig. 1.1 shows part of the human digestive system.
(i) Using label lines and the letters shown, identify on Fig. 1.1:
- the organ that produces bile and label it A
- the organ that stores bile and label it B.
(ii) Describe physical digestion.
(iii) State the names of two places where physical digestion occurs in the body.
(b) Bile is involved in fat digestion.
(i) Complete the sentences about fat digestion.
Bile is released into the ………… in the digestive system. Bile increases the surface area of fats and oils. This is called ………… . This increase in surface area increases the rate of ………… digestion using the enzyme …………. .
(ii) State the names of the products of fat digestion.
(c) Explain the role of bile in the breakdown of protein by trypsin.
▶️ Answer/Explanation
(a)(i) Ans: liver labelled A ; gall bladder labelled B
The liver produces bile, while the gall bladder stores and concentrates it before release into the small intestine.
(a)(ii) Ans: the breakdown of food into smaller pieces without chemical change to the food molecules.
Physical digestion involves mechanical processes like chewing and churning, reducing food size for easier chemical digestion.
(a)(iii) Ans: mouth ; stomach
Teeth chew food in the mouth, and stomach muscles churn food into chyme. The small intestine also mechanically mixes food with enzymes.
(b)(i) Ans: small intestine / duodenum ; emulsification ; chemical ; lipase
Bile enters the duodenum, emulsifying fats into smaller droplets. Lipase then chemically breaks these into fatty acids and glycerol.
(b)(ii) Ans: fatty acids and glycerol
Lipase hydrolyzes fats into these absorbable products, which are used for energy or stored.
(c) Ans: Bile neutralizes stomach acid, providing the alkaline pH (pH 8) required for trypsin to digest proteins effectively.
Trypsin works optimally in alkaline conditions. Bile ensures the acidic chyme from the stomach is neutralized, enabling protein digestion.