Cell membrane structureEdexcel International A Level Biology: Mind map

Exam questions on Cell membrane structure

1. Cell surface membranes are made mainly of phospholipids and proteins. Each phospholipid molecule has a phosphate-containing head that attracts water and two fatty acid tails that repel water.
   Explain why sodium ions cannot diffuse directly through the phospholipid bilayer.2 marks
2. In an experiment on cell fusion, a mouse cell and a human cell were joined to make one hybrid cell. The membrane proteins of the mouse cell were labelled with a red fluorescent dye and those of the human cell with a green dye. Immediately after fusion the red and green labels were in separate halves of the hybrid cell. After 40 minutes at 37 °C the two labels were completely mixed over the whole surface. When the experiment was repeated at 0 °C the labels stayed in separate halves.
   Explain how the results at 37 °C support the fluid mosaic model of membrane structure.2 marks
3. A student investigates the effect of temperature on the permeability of beetroot cell membranes. She cuts identical discs of beetroot, rinses them, and places one disc in 10 cm³ of distilled water in each of five test tubes held in water baths at 20, 30, 40, 50 and 60 °C for 10 minutes. She then removes the discs and measures the absorbance of the liquid in each tube with a colorimeter. Absorbance rose slightly between 20 °C and 40 °C and then rose sharply above 50 °C.
   Explain why the absorbance of the liquid increases when the membranes of the beetroot cells become more permeable.3 marks