A2 Biology - Module 6: DIGESTION ANSWERS

A2 Biology - Module 6: DIGESTION ANSWERS

1. Different concentrations of maltose were placed in the small intestine of a mammal. The amount of glucose appearing in the blood and in the small intestine were measured. The results are shown in the graph.

(a) (i) Give the name of the blood vessel most likely to have been sampled for glucose.

Hepatic portal vein

(1)

(ii) By what chemical process is a molecule of maltose converted into two molecules of glucose?

Hydrolysis

(1)

(b) The enzyme maltase converts maltose into glucose. This enzyme is found in the cell surface membrane of the epithelial cells of the small intestine.

(i) Explain the evidence from the graph which supports the view that the breakdown of maltose does not occur in the lumen of the small intestine.

Very little/no increase in amount of glucose in the lumen;
if breakdown occurred in lumen, would increase here/
would take time to diffuse to wall;

(2)

(ii) Suggest an explanation for the shape of the curve showing the change in the amount of glucose in the blood.

More glucose more active sites occupied;
curve flattens as enzyme becomes limiting;
at any one time;
all active sites are occupied; (3)

A cow obtains most of its nutritional requirements from mutualistic microorganisms in its rumen. The diagram summarises the biochemical processes carried out by these microorganisms.

(i) the relation between the cow and the microorganisms which live in its rumen may be described as mutualistic;

Both organisms have a nutritional advantage;
cow gains fatty acids/protein;
microorganisms gain cellulose/protein/urea;

(2)

(ii) it is possible for a cow to survive on a diet which is poor in protein;

Obtain non-protein nitrogen;
microorganisms convert this into microbial protein;
which cow can digest;

(2)

(iii) ruminant animals such as cows are less efficient than non-ruminant animals in converting energy in food into energy in their tissues.

Some food energy used by microorganisms;
some lost in methane;

(2)

2. Answer should be written in continuous prose. Credit will be given for biological accuracy, the organisation and presentation of the information and the way in which the answer is expressed.

The diagram shows the nitrogen cycle.

Describe how nitrogen-containing compounds in producers are digested in the gut of a mammal.

Digestion involves hydrolysing / breaking down peptide bonds / bonds
between amino acids / protein;
endopeptidases followed by exopeptidases;
endopeptidases break protein from inside / into smaller polypeptides /
peptides;
pepsin in stomach / gastric juice;
trypsin from pancreas / acting in small intestine;
exopeptidases break off single amino acids / dipeptides from end;
dipeptidases in epithelial cells / on cell surface membrane / brush border /
microvilli;
May include up to 2 marks for further detail
conditions of pH under which pepsin / trypsin works;
activation of trypsinogen / pepsinogen;
nucleic acid digestion;
Correct reference to microbial digestion of protein;

(Total 7 marks)

3. The flow chart represents the breakdown of starch in the human gut.

(a) Name two organs which produce amylase in humans.

Organ 1 Pancreas and

Organ 2 Salivary glands

(1)

(b) Describe how the release of amylase from each of these organs is controlled.

Organ 1 Salivary glands - nervous reflex / nervous [stimulation];
in response to food in mouth / smell of food /sight of food
/ thought of food;

Organ 2 Pancreas - in response to food in duodenum;
hormonal [stimulation] / (hormone is) CCKPZ /
pancreozymin;

(3)

In (cell surface) membrane of / Brush border/microvilli;

epithelial cell / in ileum / small intestine;

(2)

(Total 6 marks)

4. (a) The diagrams show fatty acid molecules in saturated and unsaturated triglycerides.

Complete the diagrams to show the difference between a saturated fatty acid and an unsaturated fatty acid.

Carbon to carbon bonds correct in both;

Carbon to hydrogen bonds correct in both;

(2)

(b) In the human gut, a triglyceride may be converted to glycerol and fatty acids by hydrolysis.

(i) Explain what is meant by hydrolysis.

Breakdown involving the addition of (a molecule of) water;

(1)

(ii) Describe the part played by bile in the hydrolysis of triglycerides.

Emulsifies fats / breaks fats down into smaller droplets/large surface area;
for enzyme / lipase action;
produces optimal / alkaline pH (for lipase);

(2)

(Total 5 marks)

5. Inorganic ions form a minor but essential part of the human diet.

(a) Give two functions of each of the following ions in the human body.

(i) Calcium

Enzyme reactions;
blood clotting/explanation e.g. prothrombin ® thrombin;
bones/teeth/explanation;
muscle contraction/explanation;
synaptic transmission/explanation;

(2)

(ii) Iron

haemoglobin/myoglobin;
R prevent anaemia
enzyme reactions;
cytochromes/electron transport chain;

(2)

The table shows how the estimated average daily requirements for calcium and iron in the diet vary with age and sex.

Age/years / Estimated average requirement/mg day–1
calcium / iron
Both sexes 1 - 3
4 - 6
7 - 10 / 275
350
425 / 5.3
4.7
6.7
Males 11 - 14
15 - 18
19 - 50
50+ / 750
750
525
525 / 8.7
8.7
6.7
6.7
Females 11 - 14
15 - 18
19 - 50
50+ / 625
625
525
525 / 11.4
11.4
11.4
6.7

(b) Explain the change in the estimated average daily requirements for calcium and for iron between the ages of 4 and 10.

Calcium associated with increased skeletal growth/bone growth/bone
development/increased calcium for bone growth;
R just growth
iron with increase in blood volume/amount of red cells/haemoglobin;
R blood supply

(2)

(i) calcium;

More calcium required by males who have larger skeletons/example/more
bone mass;
only during puberty;

(ii) iron.

Due to menstrual blood loss;
decline at/after menopause;

(3)

The graph shows the results of an investigation of the effect of calcium ion concentration on their rate of uptake by epithelial cells from the small intestine.

(d) Explain the evidence from the graph that active transport is involved in the absorption of calcium ions from the lumen of the intestine.

Higher when no respiratory poison/lower or stops with respiratory poison;
ATP required for active transport/no ATP production;

(2)

(e) For Curve Y, obtained when a respiratory poison was added:

(i) explain the shape of the curve between concentrations B and C;

Diffusion occurring; (allow in either)
rate of diffusion/uptake linked to increasing (diffusion)
gradient/concentration of calcium ions/directly proportional;

(ii) suggest why no calcium ions were taken up between concentrations A and B.

More calcium in cells than outside/negative concentration gradient;

(3)

The diagram shows some of the ways in which other substances are absorbed in the small intestine.

Research has shown that microorganisms may cause diarrhoea by shutting down the pathway that transports sodium ions into the epithelial cells. These microorganisms do not, however, have any effect on the mechanism responsible for the “co-transport” of glucose and sodium ions into the cells.

(f) (i) Explain, in terms of water potential, why a failure to remove sodium ions from the lumen of the intestine causes diarrhoea.

Lower/more negative water/solute potential in lumen/gut;
water moves from tissues/into lumen by osmosis/diffusion

(2)

(ii) Oral rehydration therapy (ORT) is used to treat diarrhoea. It involves giving the patient a solution of sodium salts and glucose. Using the information in the diagram, explain how this is effective in treating diarrhoea.

Stimulates uptake through co-transport channel;
so water moves out of lumen/follows;

(2)

(g) Attempts have been made to produce more effective oral rehydration therapy. Suggest why using starch instead of glucose reduces the faecal output for a much longer period.

Starch broken down to produce glucose;
digestion is a slow process/takes time;
so stimulating uptake of solute for longer period/glucose present for longer;
starch osmotically inactive;

(2)

(Total 20 marks)

6. (a) Trypsin is a protein-digesting enzyme. It is produced as inactive trypsinogen in the pancreas. Trypsinogen is secreted when food enters the duodenum.

(i) Describe the way in which the secretion of trypsinogen is stimulated by food entering the duodenum.

Receptors in duodenum/small intestine detect presence of food;
secrete CCK-PZ/pancreozymin;
hormone which travels in blood to pancreas;

(2)

(ii) Describe how trypsinogen is converted to trypsin in the small intestine.

Enterokinase/enteropeptidase;
splitting off peptide/amino acids from trypsinogen/hydrolysis;

(2)

(b) Trypsin is used in the tanning industry for the production of leather. It was first obtained by extracting it from the faeces of domestic animals. Explain why faeces contain digestive enzymes such as trypsin.

Trypsin not digested/broken down;
not absorbed (by intestine);
too large;

(2)

(Total 6 marks)