Active transport and co-transportAQA A-Level Biology: Subtopic test

1. 1
   Root hair cells of a barley plant absorb nitrate ions from the soil water. The concentration of nitrate ions in the soil water is 0.2 mmol dm⁻³, whereas the concentration inside the root hair cells is 12 mmol dm⁻³. When a researcher added a respiratory inhibitor to the roots, nitrate uptake fell to almost zero.

   (a)

   Which row correctly describes how nitrate ions enter the root hair cells?

   [1 mark]

   • AFacilitated diffusion down the concentration gradient using carrier proteins only
   • BOsmosis down a water potential gradient
   • CActive transport, against the concentration gradient, using ATP
   • DActive transport, down the concentration gradient, using ATP

   (b)

   Which statement best explains why nitrate uptake fell when the respiratory inhibitor was added?

   [1 mark]

   • AThe concentration gradient across the membrane was reversed
   • BLess ATP was produced, so less energy was available to change the shape of the carrier proteins
   • CThe carrier proteins were no longer specific for nitrate ions
   • DWater left the cells by osmosis, so the cells became flaccid

   (c)

   Explain why the root hair cells could not take up nitrate ions by simple diffusion.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   After a meal, epithelial cells lining the mammalian ileum absorb glucose from the lumen. Later in digestion the glucose concentration in the lumen can fall below that inside the epithelial cells, yet absorption continues. A researcher treated ileum tissue with ouabain, a drug that inhibits the sodium-potassium pump in the membrane facing the blood, and found that glucose absorption fell sharply.

   (a)

   What is the role of the sodium-potassium pump in the epithelial cells of the ileum?

   [1 mark]

   • AIt moves glucose from the epithelial cell into the blood by facilitated diffusion
   • BIt moves sodium ions from the lumen into the cell by diffusion
   • CIt transports glucose directly into the cell against its concentration gradient using ATP
   • DIt actively transports sodium ions out of the cell into the blood, keeping the sodium concentration inside the cell low

   (b)

   Glucose leaves the epithelial cells and enters the blood. Which process moves it?

   [1 mark]

   • AFacilitated diffusion through a carrier protein
   • BActive transport through the sodium-potassium pump
   • CCo-transport with sodium ions
   • DOsmosis through the phospholipid bilayer

   (c)

   Use your knowledge of co-transport to explain why glucose absorption fell when the sodium-potassium pump was inhibited.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Cells from a plant tissue were placed in solutions containing different concentrations of a radioactively labelled amino acid. The rate of uptake increased as the external concentration rose, then levelled off at a maximum value. Uptake was much lower when the cells were kept in oxygen-free conditions.

   (a)

   Explain why the rate of uptake levelled off at a maximum value.

   [3 marks]

   (b)

   Describe how a carrier protein uses ATP to move an amino acid into the cell against its concentration gradient.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Oral rehydration solution, given to people with severe diarrhoea, contains both glucose and sodium chloride dissolved in water. Doctors have found that it replaces lost water far more effectively than drinking water alone, or a solution containing only glucose without sodium ions.

   (a)

   Explain how glucose is absorbed from the lumen of the ileum into the blood, including the role of co-transport in the epithelial cells.

   [6 marks]

   (b)

   Evaluate why a solution containing both glucose and sodium ions rehydrates a patient more effectively than water alone or a solution containing glucose without sodium ions.

   [6 marks]

   Total for question 4: 12 marks

End of questions