14A16: Exam Report

Describe, with the aid of a diagram, the structure of the cell membrane, (40% marks) and transmembrane transport processes. (60% marks)

60% of candidates passed this question.

The structure of the cell membrane was generally well covered by most candidates.  Many had difficulties structuring an answer for the transmembrane transport processes.  Dividing this section into proteins (some receptors, channels etc.) and carbohydrates (some receptors, immune reactions etc) followed by a very brief discussion of each type of process would have

aided candidates towards providing a good answer.  

Ei / 14A16: Describe, with the aid of a diagram, the structure of a cell membrane (40 marks) and transmembrane transport processes (60 marks)


The plasma membrane is the semipermeable membrane that surrounds a cell and encloses the contents of cytoplasm to maintain structural integrity from ECF




  • Phospholipid bilayer
    • Hydrophilic heads
    • Hydrophobic tails
    • Cholesterol = stabilises lipid bilayer
    • Van der Waal forces between FA tails
    • Electrostatic and hydrogen bonds between hydrophilic heads and water
  • Proteins embedded along bi-layer
    • Membrane proteins = 50% plasma membrane mass
    • Some carbohydrates on lipids (glycolipid) and proteins (glycoproteins)
    • Proteins can be arranged as: transmembrane, monolayer, lipid linked, protein attached
  • Plasma membrane serves as a barrier to prevent loss of cell contents
  • Semi permeable to allow entry of nutrients and exit of waste

Transmembrane Transport Processes

Passive Transport

  • Simple Diffusion – movement of molecules down their concentration gradient
  • Osmosis – Movement of water from a high concentration of water to a low concentration of water, across a semi-permeable membrane
  • Facilitated diffusion – movement of molecules down their concentration gradient via a specific carrier protein ie Glut transporter transportation of glucose

Active Transport

  • Primary Active Transport – movement of molecular against their concentration gradient, with the expenditure of ATP
  • Secondary Active Transport – Using energy of an existing concentration gradient to transport molecules against their concentration gradient ie cotransport of Na/Glucose on PCT

Bulk Transport

  • Transport of a large amount of molecules with the use of ATP:
    • Exocytosis – Vesicle transport of molecules by fusion with plasma membrane ie ACh release
    • Endocytosis – cell membrane cleaves inwards to engulf substances ie Phagocytosis