3.3.1-2- Digestion

Question 1

Some people suffer from lactose intolerant
They can be caused by the inability to break down lactose in the upper small intestine

a) suggest which disaccharidase enzyme is deficient or missing in people who are lactose intolerant (1)
b) How are the digestion products of lactose absorbed across the epithelial cells of the ileum? (2)

Answer 1

a) Lactase
b) The digestion products of lactose are glucose can galactose are absorbed across the epithelial cells by active transport with sodium ions

via cotransporter proteins

Question 2

​A student investigated the effect of chewing on the digestion of starch in cooked wheat.
He devised a laboratory model of starch digestion in the human gut. This is the method he
used.
1. Volunteers chewed cooked wheat for a set time. The wheat had been cooked in boiling
water.

2. This chewed wheat was mixed with water, hydrochloric acid and a protein-digesting
   enzyme and left at 37 °C for 30 minutes.
3. A buffer was then added to bring the pH to 6.0 and pancreatic amylase was added. This
   mixture was then left at 37 °C for 120 minutes.
4. Samples of the mixture were removed at 0, 10, 20, 40, 60 and 120 minutes, and the
   concentration of reducing sugar in each sample was measured.
5. Control experiments were carried out using cooked wheat that had been chopped up in a
   blender, not chewed.

(a) What reducing sugar, or sugars, would you expect to be produced during chewing? (2)

Give a reason for your answer.

b) In this model of digestion in the human gut, what other enzyme is required for the
complete digestion of starch? (1)

c) What was the purpose of step 2, in which samples were mixed with water, hydrochloric
acid and pepsin? (1)

(d) Explain what these results suggest about the effect of chewing on the digestion of starch in
wheat.

Answer 2

a) 1. Maltose
2. Salivary amylase breaks down starch

b) Maltase
c) (Mimics / reproduces) effect of stomach;

(d)
1. Some starch already digested when chewing / in mouth;
2. Faster digestion of chewed starch;
3. Same amount of digestion without chewing at end;

Question 3

​Some people have a medical condition called pancreatitis. This can lead to their pancreatic duct
becoming blocked. As a result, a high concentration of amylase is found in their blood.
At 12-hour intervals, a doctor measured the concentration of amylase in the blood of a person
suffering from a blocked pancreatic duct. He also measured the concentration of amylase in
the blood of a healthy person.
The figure below shows his results.

e.g. Person with disease - 1800 arbitary units of amylase in blood
Healthy- 800 in blood

(a) (i) The changes in concentration of amylase in the blood of a person with a blocked
pancreatic duct are different from those of a healthy person during the period
shown in the figure above.

Describe two​ of these differences. (2)

ii) In a person with a blocked pancreatic duct, starch digestion is affected.
Explain how.

Answer 3

ai-
1. At 0 h / start higher than healthy person / higher than healthy person
throughout;
2. Rises then falls whereas healthy person falls then rises;
3. At 48 h / end, below the starting value whereas healthy person is the
same (as at start);

ii) 1. Little / less / no amylase can enter small intestine;

Accept gut or intestine but reject wrong locations e.g. stomach
2. Little / less / no starch digested (in intestine);

Question 4

Healthy people have amylase in their blood.
This does not cause any harmful effects in
the body.
Explain why.
(2)

Answer 4

1. Amylase is specific (to starch);

2. No starch in human blood / cells / tissues / starch only in plants;

Question 5

Pancreatitis can lead to the release of protein-digesting enzymes into the blood. This is
harmful to the body.
Suggest one​ reason why.
(2)

Answer 5

1. Could digest own body / own proteins;

e.g. ‘could digest carrier proteins in body cells’ would score 2
marks
e.g. ‘could digest antibodies in blood’ would also score 2 marks
2. Example of protein digested e.g. membrane protein, antibody, named
protein in blood;

Question 6

Sucrose monosaccharides are

Lactose monosaccharides are

Answer 6

1. Glucose fructose

2. Glucose and galactose

Question 7

Where is amylase produced and what is the Product​ ​of​ ​the
reaction​ ​catalysed
by​ ​the​ ​it?

Where is Maltase produced and what is the Product​ ​of​ ​the
reaction​ ​catalysed
by​ ​the​ ​it?

Answer 7

1. (Amylase) pancreas, produces maltose;
   Place and product = 1 mark
   (mark horizontally)
2. (Maltase) in / on epithelium (of small intestine), produces glucose;

Ignore references to salivary glands or saliva
Accept wall / lining of small intestine
Ignore reference to cells alone
Ignore reference to ribosomes / rER

Question 8

In humans, triglycerides are the main form of dietary lipids. They are digested in the gut and
the products of digestion are absorbed by the small intestine.

(b) Describe a
biochemical test that could be performed on a sample of food to determine
whether it contained triglycerides.

Answer 8

dissolve in / add ethanol then mix with water;

emulsion / white colour indicates triglycerides present;

Question 9

Name R and S

Answer 9

(ii) R​ = tissue fluid / interstitial fluid / extracellular fluid / intercellular space;
S​ = lymph(atic) vessel / lymph capillary / lacteal;

Question 10

Describe the role played by organelle U​ ​(in the formation of chylomicrons.
(2)

Answer 10

proteins are synthesised by U​;
involvement of ribosomes;
protein isolation / transport (inside RER);
vesicle formation;

Question 11

Suggest how the chylomicrons (The triglyceride molecules
are formed into chylomicrons in organelle T.​ Chylomicrons are made from many
triglyceride molecules surrounded with protein molecules.) leave the epithelial cell. Give a reason for your

answer. (2)

Answer 11

exocytosis / description of;

because of size / too large to leave by other methods;

Question 12

(ii) Explain why sodium ions and amino acids are not​ ​absorbed from the lumen of the small
intestine in the presence of DNP.

Answer 12

Concentration of Na
+
inside cell no longer less than concentration in

gut lumen / no longer a concentration gradient;
No (facilitated) diffusion of NA
+
ions possible / amino acid absorption

requires diffusion of Na
+
ions into cell;

Question 13

Maltose is hydrolysed by the enzyme maltase.

Explain why maltase catalyses only this reaction. (3)

Answer 13

1. Active site (of enzyme) has (specific) shape / tertiary structure / active
   site complementary to substrate / maltose;
   Reject active site on substrate.
   Must have idea of shape
   Assume “it” = maltase
   Accept (specific) 3D active site
   Reject has same shape
2. (Only) maltose can bind / fit;
   Accept “substrate” for “maltose”
3. To form enzyme substrate complex.
   Accept E−S complex

Question 14

Describe how proteins are digested in the human gut. (5)

Answer 14

1. Hydrolysis of peptide bonds;
2. Endopeptidases break polypeptides into smaller peptide chains;
3. Exopeptidases remove terminal amino acids;
4. Dipeptidases hydrolyse / break down dipeptides into amino acids.

Question 15

n the control experiments, cooked wheat was chopped up to copy the effect of
chewing.
Suggest a more appropriate control experiment. Explain your suggestion.

Answer 15

1. Add boiled saliva;

2. Everything same as experiment but salivary amylase denatured

Question 16

Describe how you would test a sample of food for the presence of starch (2)

Answer 16

1. Add iodine / potassium iodide solution to the food sample;
2. Allow ‘iodine’
3. Must be in the context of the correct reagent
4. Blue / black / purple indicates starch is present;

Question 17

The concentration of glucose in the blood rises after eating a meal containing
carbohydrates.
The rise is slower if the carbohydrate is starch rather than sucrose. Explain wh
y (3)

Answer 17

1. Starch digested to maltose / by amylase;
   Ignore ‘hard to digest / easily digested’
2. Maltose digested to glucose / by maltase;
3. Digestion of sucrose is a single step / only one enzyme / sucrase;
4. Accept converse for starch
5. Do not accept digestion of sucrose is faster

Question 18

The glycaemic load (GL) of a diet is a measure of how much digestible carbohydrate
it contains. The higher the GL of a diet the more quickly it raises the blood glucose
concentration after a meal. A diet with a high GL also increases the concentration of
harmful lipids in the blood.
Scientists investigated the relationship between diets with different glycaemic loads
and the risk of developing coronary heart disease (CHD) in women.
The scientists determined the glycaemic loads of the diets of a large number of
women. They then divided the women into 5 groups. Group 1 had diets with the
lowest glycaemic load and group 5 had diets with the highest glycaemic load. The
scientists determined the risk of developing CHD in each group.
The graph shows their results

c) The scientists excluded women who smoked from the study. Explain why. (1)

(d) (i) What do these data show about the effect that glycaemic load of the diet has
on the risk of developing CHD? (2)

ii) Use the information provided to explain the effect that glycaemic load of the
diet has on the risk of developing CHD. (2)

Answer 18

(c) 1. Smoking increases risk of CHD / introduces another variable;

(d) (i) 1. No effect on risk with diet group 1 and 2 / lowest glycaemic load;
Simple statement of correlation is not enough for this mark
2. Above diet group 2 / in higher groups, risk increases as glycaemic
load increases

1. (Higher GL diets lead to) more (harmful) lipids (in blood), so greater
   risk of atheroma;
   Ignore reference to lipids in diet
2. Atheroma leads to blockage of coronary artery / increased risk of
   blood clot in coronary artery;
   Ignore references to myocardial infarction / heart attack

Question 19

Mammals have some cells that produce extracellular proteases. They also have
cells with membrane-bound dipeptidases.
Describe the action of these membrane-bound dipeptidases and explain their
importance.
[2 marks]

Answer 19

1. Hydrolyse (peptide bonds) to release amino
   acids;
2. Amino acids can cross (cell) membrane;
   OR
   Dipeptides cannot cross (cell) membrane;
   OR
   Maintain concentration gradient of amino acids
   for absorption;
   OR
   Ensure (nearly) maximum yield from protein
   breakdown;

Question 20

1.1- Use your knowledge of lipid digestion to explain the differences in the result for samples A and B shown in table 1

You should assume no absorption has occurred (3)

Answer 20

• Triglyceires decrease because of action of lipase
• hydrolysis
• because of digestion

Question 21

1.3.- Describe the role of micelles in absorption of fats into the cells lining the ileum (3)

Answer 21

Include bile salts and fatty acids
soluble
diffusion

Question 22

1.2- After collecting samples, the scientist immediatly heated them to 70 degrees for 10 mins explain why (2)

Answer 22

denatured- no further digestion can occur

Question 23

Explain the advantages of lipid droplet and micelle formation.
[3 marks]

Answer 23

1. Droplets increase surface areas
2. Faster hydrolysis / digestion
3. Micelles carry fatty acids and monoglycerides to (epithelial) cell;

Question 24

Name structure Q in Figure 2 and suggest how it is involved in the absorption of
lipids. (4)

Answer 24

1. Golgi apparatus
2. Modifies triglycerides
3. Combines triglycerides with proteins
4. Packages for exocytosis