Question 16

College Answer

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.

Discussion

A decent diagrammatic representation of the cell membrane which is hastily scrawled during the primary exam would probably look a bit like this:

Though it is difficult to reconstruct the expectations of the examiners from their laconic comments ("generally well covered by most candidates" is all we get), one can - from looking at textbooks - assume that the basic elements you need to include would be:

• A lipid bilayer with clearly labelled hydrophobic heads and hydrophilic tails
• Transmembrane, integral and surface proteins
• Polysaccharides
• A membrane thickness 
• Surface negative charge and adsorbed ions

As for the membrane transport...

In pictureless written format, the transport of molecules into (and out of) the cell can be said to take three main forms:

• Diffusion:
  • Passive ("simple") diffusion: occurs along a concentration gradient directly through the lipid bilayer. Example: Oxygen and carbon dioxide molecules.
  • Facilitated diffusion: occurs along a concentration gradient, but requires a protein channel as a conduit. Example: aquaporins
  • Ion channels:  selective conduit proteins, usually gated, which only allow the passage of specific ions, usually in response to a triggering stimulus. Example: voltage-gated sodium channels.
• Active transport:
  • Primary active transport: mediated by a "pump" protein which uses chemical energy stored in ATP to facilitate the transport of molecules (usually against their concentration gradient). Example: sodium and potassium transport by Na⁺/K⁺ ATPase.
  • Secondary active transport: mediated by an exchaner or co-transporter which facilitates the movement of molecules using the energy of a concentration gradient set up by another (primary) ATP-powered transport process. Example: sodium and glucose co-transport.
• Vesicle transport
  • Endocytosis: where the transport of substances into the cell occurs by formation membrane-bounded vesicles containing the substance. Example: catecholamine neurotransmitter reuptake.
  • Exocytosis: the opposite of endocytosis, where vesicles transport molecules to the cell surface and empty their contents into the extracellular fluid.  Example: catecholamine neurotransmitter release.

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