3.3 digestion and absorption

Question 1

Describe the processes involved in the absorption and transport of
digested lipid molecules from the ileum into lymph vessels. (5)

Answer 1

1. Micelles contain bile salts and fatty acids/monoglycerides;
   Ignore other correct components of micelles
2. Make fatty acids/monoglycerides (more) soluble (in water)
   OR
   Bring/release/carry fatty acids/monoglycerides to cell/lining (of the
   iluem)
   OR
   Maintain high(er) concentration of fatty acids/monoglycerides to
   cell/lining (of the ileum);
   Accept lipid/fat for fatty acid/ monoglyceride
3. Fatty acids/monoglycerides absorbed by diffusion;
   Reject if absorbed by facilitated diffusion
   Ignore if micelles themselves are being absorbed
4. Triglycerides (re)formed (in cells);
   Accept chylomicrons form
5. Vesicles move to cell membrane;

Question 2

Answer 2

1. (ATP to ADP + Pi ) Releases energy;
   Reject ‘produces/makes/creates energy’.
2. (energy) allows ions to be moved against a concentration gradient
   OR
   (energy) allows active transport of ions;

Question 3

The movement of Na+ out of the cell allows the absorption of glucose into
the cell lining the ileum.
Explain how.

Answer 3

1. (Maintains/generates) a concentration/diffusion gradient

for Na+ (from ileum into cell);
Accept ‘(Maintains/generates) a lower concentration
of Na+ inside the cell compared with outside the
cell’.

2. Na+ moving (in) by facilitated diffusion, brings glucose with it
   OR
   Na+ moving (in) by co-transport, brings glucose with it;

Question 4

(3)

Answer 4

1. Triglycerides decrease because of the action of lipase

OR
Fatty acids increase because of the action of lipase;
2. Triglycerides decrease because of hydrolysis (of triglycerides)
OR
Fatty acids increase because of hydrolysis (of triglycerides);
3. Triglycerides decrease because of digestion of ester bonds (between
fatty acid and glycerol)
OR
Fatty acids increase because of digestion of ester bonds (between
fatty acid and glycerol);

Question 5

After collecting the samples, the scientist immediately heated them to 70
°C for 10 minutes.
Explain why. (2)

Answer 5

1. To denature the enzymes/lipase;

Accept description of denaturation in terms of
change in tertiary structure.

2. So no further digestion/hydrolysis/catalysis occurred;

Question 6

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

Answer 6

1. Micelles include bile salts and fatty acids;
   Ignore other correct components of micelles.
2. Make the fatty acids (more) soluble in water;
   For ‘fatty acids’ accept fats / lipids.
3. Bring/release/carry fatty acids to cell/lining (of the ileum);

For ‘fatty acids’ accept fats/lipids.

4. Maintain high(er) concentration of fatty acids to cell/lining (of the
   ileum);
5. Fatty acids (absorbed) by diffusion;

Question 7

Describe the role of enzymes in the digestion of proteins in a mammal.(4)

Answer 7

1. (Reference to) hydrolysis of peptide bonds;
2. Endopeptidase act in the middle of protein/polypeptide
   OR
   Endopeptidase produces short(er) polypeptides/ increase number of ends;
3. Exopeptidases act at end of protein/polypeptide
   OR
   Exopeptidase produces dipeptides/amino acids;
4. Dipeptidase acts on dipeptide/between two amino acids
   OR
   Dipeptidase produces (single) amino acids;

Question 8

Scientists investigated how the diet of rabbits affected their digestion and
absorption of protein. The scientists fed rabbits an identical mass of food but
varied the percentage of protein in the food.
The scientists measured the mean mass of protein fed to the rabbits that was
absorbed, which they then expressed as a percentage value.
The scientists’ results are shown in Figure 1.
The error bars show ± 2 standard deviations.
± 2 standard deviations cover 95% of the data.
(3)

Answer 8

1. No significant difference (in protein absorption);
2. (because ± 2) SDs overlap;
   Accept error bar for SD
3. (So mean) percentage absorbed not affected by percentage in diet;
4. Amount of protein (in diet) is not a limiting fact
   OR
   Something else is limiting factor eg amount of protease;
5. (But) small range of protein in diet
   OR
   (Should) Investigate wider range;

Question 9

(3)

Answer 9

1. More/remaining/undigested (protein) broken down;

Accept all (protein) broken down
2. (So more) amino acids absorbed;
3. (Because) protein/food passes again through stomach/ileum;

Question 10

name of the process by which fatty acids and
glycerol enter the intestinal epithelial cell.
Name structure Q in the diagram above and suggest how it is involved in
the absorption of lipids.

Answer 10

Diffusion
1. Golgi (apparatus);
2. Modifies / processes triglycerides;
3. Combines triglycerides with proteins;
4. Packaged for release / exocytosis
OR
Forms vesicles;

Question 11

Explain the advantages of lipid droplet and micelle formation. (3)

Answer 11

1. Droplets increase surface areas (for lipase /

enzyme action);
2. (So) faster hydrolysis / digestion (of
triglycerides / lipids);
3. Micelles carry fatty acids and glycerol /
monoglycerides to / through membrane / to
(intestinal epithelial) cell;

Question 12

Cells lining the ileum of mammals absorb the monosaccharide glucose by
co-transport with sodium ions. Explain how. (3)

Answer 12

1. Sodium ions actively transported from ileum cell to blood;
2. Maintains / forms diffusion gradient for sodium to enter cells from gut
   (and with it, glucose);
3. Glucose enters by facilitated diffusion with sodium ions;