Cell membranes and passive transportEdexcel A-Level Biology B: Mind map

Exam questions on Cell membranes and passive transport

1. A liver cell takes up and releases many substances across its cell surface membrane, including testosterone (a lipid-soluble steroid hormone), glucose, sodium ions and amino acids. The membrane is described by the fluid mosaic model.
   Explain why sodium ions cannot cross the membrane by simple diffusion, and state how they can cross it by a passive process.2 marks
2. A student investigated the effect of temperature on the permeability of beetroot cell membranes. She cut discs of beetroot of identical size, rinsed them, and placed one in each tube of distilled water at 20, 30, 40, 50 and 60 °C for 20 minutes. She then measured the absorbance of the water in each tube with a colorimeter, as a measure of the red pigment, betalain, that had leaked out. The absorbance readings were 0.05, 0.08, 0.15, 0.62 and 1.10.
   Explain the large increase in absorbance between 40 °C and 50 °C.2 marks
3. Glucose enters red blood cells through carrier proteins in the cell surface membrane. A scientist measured the rate of glucose uptake at different external glucose concentrations. The rate rose in proportion to the concentration up to 5 mmol dm⁻³, but at higher concentrations it levelled off at a maximum. The uptake was not changed when a respiratory inhibitor was added to the cells.
   Explain why the rate of glucose uptake levels off at higher glucose concentrations.3 marks