Describe transport mechanisms across cell membranes. Give an example of each.
Candidates were able to list types of transport across cell membranes but frequently
described them incorrectly or gave an incorrect example. In a number of answers, there was
confusion between facilitated diffusion and secondary active transport. Though diagrams
were not required,several Candidates used a diagram of the cell very effectively to illustrate
the mechanisms of transport across the membrane. For a good answer, some mention and
description of exocytosis, endocytosis, ion channels, facilitated diffusion, passive diffusion,
primary and secondary active transport was expected.
Diagrams were not required, they said. But clearly they didn't hurt those "several Candidates". Obviously one has only a minute or two to hastily sketch a few transport systems in this scenario. This minimum of artistic effort is represented here with the aid of a cheap nasty Illustrator diagram.
- Transport of molecules into (and out of) the cell can take three main forms:
- 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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Yang, Nicole J., and Marlon J. Hinner. "Getting across the cell membrane: an overview for small molecules, peptides, and proteins." Site-Specific Protein Labeling. Humana Press, New York, NY, 2015. 29-53.
Stein, Wilfred. Transport and diffusion across cell membranes. Elsevier, 2012.
Cussler, E. L., Rutherford Aris, and Abhoyjit Bhown. "On the limits of facilitated diffusion." Journal of membrane science43.2-3 (1989): 149-164.
Wu, Ling-Gang, et al. "Exocytosis and endocytosis: modes, functions, and coupling mechanisms." Annual review of physiology 76 (2014): 301-331.