Describe, with the aid of a diagram, the structure of the cell membrane, (40% marks) and transmembrane transport processes. (60% marks)
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.
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
- 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:
- 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.
Rand, Richard Peter, and A. C. Burton. "Mechanical properties of the red cell membrane: I. Membrane stiffness and intracellular pressure." Biophysical Journal 4.2 (1964): 115-135.
Singer, S. Jonathan, and Garth L. Nicolson. "The fluid mosaic model of the structure of cell membranes." Science 175.4023 (1972): 720-731.
Gan, Lu, Songye Chen, and Grant J. Jensen. "Molecular organization of Gram-negative peptidoglycan." Proceedings of the National Academy of Sciences 105.48 (2008): 18953-18957.
Phillips, Rob. "Membranes by the Numbers." Physics of Biological Membranes. Springer, Cham, 2018. 73-105.
Langmuir, Irving. "The constitution and fundamental properties of solids and liquids. II. Liquids." Journal of the American chemical society 39.9 (1917): 1848-1906.
Takamori, Shigeo, et al. "Molecular anatomy of a trafficking organelle." Cell 127.4 (2006): 831-846.
Tarbell, John M., and L. M. Cancel. "The glycocalyx and its significance in human medicine." Journal of internal medicine 280.1 (2016): 97-113.
Raicu, Valerica, and Aurel Popescu. "Cell Membrane: Structure and Physical Properties." Integrated Molecular and Cellular Biophysics (2008): 73-99.
Movileanu, Liviu, et al. "Transbilayer pores induced by thickness fluctuations." Bulletin of mathematical biology 68.6 (2006): 1231-1255.
Verkman, A. S., et al. "Water transport across mammalian cell membranes." American Journal of Physiology-Cell Physiology270.1 (1996): C12-C30.
Mihailescu, Ella, et al. "Determining the Water Content of Lipid Membranes by Neutron Diffraction." Biophysical Journal 98.3 (2010): 286a.
Disalvo, Edgardo Anibal, et al. "Functional role of water in membranes updated: A tribute to Träuble." Biochimica et Biophysica Acta (BBA)-Biomembranes 1848.7 (2015): 1552-1562.
Van, EJ Zoelen, et al. "Non-electrolyte permeability as a tool for studying membrane fluidity." Biochimica et biophysica acta511.3 (1978): 335-347.
Dobrzyńska, Izabela. "Association equilibria of divalent ions on the surface of liposomes formed from phosphatidylcholine." The European Physical Journal E 42.1 (2019): 3.
Tamagawa, Hirohisa, and Sachi Morita. "Membrane potential generated by ion adsorption." Membranes 4.2 (2014): 257-274.
Heimburg, Thomas. "Physical properties of biological membranes." arXiv preprint arXiv:0902.2454 (2009).
Wilson, David B. "Cellular transport mechanisms." Annual review of biochemistry 47.1 (1978): 933-965.
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.