Active transport and co-transportAQA A-Level Biology: Mind map

Exam questions on Active transport and co-transport

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.
   Explain why the root hair cells could not take up nitrate ions by simple diffusion.2 marks
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.
   Use your knowledge of co-transport to explain why glucose absorption fell when the sodium-potassium pump was inhibited.2 marks
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.
   Explain why the rate of uptake levelled off at a maximum value.3 marks