Absorption of digestion productsAQA A-Level Biology: Subtopic test

1. 1
   Cells lining the ileum absorb glucose from the gut lumen after a meal. As digestion proceeds, the concentration of glucose in the lumen can fall below that inside the epithelial cells. Glucose enters the cells together with sodium ions through a co-transporter protein in the cell-surface membrane.

   (a)

   How do sodium ions enter the epithelial cell through the co-transporter protein?

   [1 mark]

   • ABy active transport, using ATP hydrolysed by the co-transporter
   • BBy simple diffusion through the phospholipid bilayer
   • CBy facilitated diffusion down their concentration gradient
   • DBy exocytosis from vesicles in the cell

   (b)

   Which process maintains the low concentration of sodium ions inside the epithelial cells?

   [1 mark]

   • AActive transport of sodium ions out of the cell into the blood by the sodium-potassium pump
   • BFacilitated diffusion of sodium ions from the cell into the lumen
   • COsmosis of water into the cell from the lumen
   • DFacilitated diffusion of glucose out of the cell into the blood

   (c)

   Explain how glucose can be absorbed into the epithelial cells when its concentration in the lumen is lower than in the cells.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   After a meal containing fat, triglycerides in the small intestine are hydrolysed by lipase to monoglycerides and fatty acids. Bile salts, secreted by the liver, mix with these products and form tiny spherical structures that move to the surface of the epithelial cells lining the ileum.

   (a)

   What name is given to these structures?

   [1 mark]

   • AChylomicrons
   • BLysosomes
   • CVilli
   • DMicelles

   (b)

   Monoglycerides and fatty acids cross the cell-surface membrane without a carrier protein. Which statement explains this?

   [1 mark]

   • AThey are polar, so they dissolve in the water inside the membrane
   • BThey are lipid-soluble, so they diffuse through the phospholipid bilayer
   • CThey are too large to diffuse, so they enter by endocytosis
   • DThey are actively transported, using ATP supplied by the cell

   (c)

   Explain how these structures increase the rate at which monoglycerides and fatty acids are absorbed.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Pieces of ileum from a rat were placed in a solution containing glucose and sodium ions, and the rate of glucose uptake was measured. In the normal solution, uptake was 100 arbitrary units. When sodium ions were removed from the solution, uptake fell to 8 units. When the normal solution was supplemented with ouabain, a substance that inhibits the sodium-potassium pump but does not bind to the glucose co-transporter, uptake fell gradually to 15 units over 30 minutes.

   (a)

   Use the information to explain the role of sodium ions in the absorption of glucose.

   [3 marks]

   (b)

   Ouabain does not bind to the glucose co-transporter. Explain why it still reduces the uptake of glucose, and why the fall was gradual.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A patient has gallstones that block the bile duct, so very little bile reaches her small intestine. Her faeces are pale and contain a large amount of undigested fat, and she is losing weight. Blood tests show that she absorbs glucose and amino acids from her meals normally.

   (a)

   Explain why the absorption of lipids is reduced in this patient.

   [6 marks]

   (b)

   Explain why the absorption of glucose is unaffected by the lack of bile salts, by comparing it with the absorption of monoglycerides and fatty acids.

   [6 marks]

   Total for question 4: 12 marks

End of questions