Cell membranes form the outer, but living boundary of a cell. As such, the membrane plays roles such as regulating molecule passage in and out of a cell hence maintaining cellular homeostasis, and regulating a cell’s mechanical strength and shape. In this context, a review of the structure of a cell membrane will provide insight on how it is related to its function.
Structure of the Cell Membrane
The cell membrane comprises of four different types of molecules-namely phospholipids, cholesterol, proteins and carbohydrates (Singer, and Nicolson 2012, p.8). Notably, Lodish, et al. (2000) points out that phospholipids are arranged as a lipid bilayer that comprises of hydrophobic tails, which are repelled by the extracellular and cytosol fluids, and hydrophilic heads, which are attracted towards the cytosol and extracellular fluids. As such, the lipid bilayer is partially permeable; therefore, only specific molecules diffuse across the membrane. Further, Cholesterol is another key component found in animal cell membranes. Brown and London (2000, p. 17222) point-out that cholesterol molecules are dispersed amongst the membrane’s phospholipids. Their main role is to prevent the stiffening of the cell membrane.
Additionally, different types of proteins are found in the plasma membrane, and they perform various functions. More precisely, Lodish, et al. (2000) specifies that: receptor proteins enable cells to communicate with the extracellular environment through neurotransmitters, hormones and signaling molecules; structural proteins provide cells with support and enable them to attain suitable shapes; and transport proteins facilitate the movement of molecules across the cell membrane. In addition, Engelman (2005, p.579) highlights that glycoproteins also form part of the cell membrane. Their main role is to enable communication between cells, and to facilitate the transfer of molecules across the membrane. Lastly, glycolipids, which are attached to a carbohydrate sugar and are positioned on the plasma membrane’s surface, also form part of the cell membrane. Singer and Nicolson (2012, p8) point out that glycolipids facilitate the recognition of other cells.
Transport across the Cell Membrane
The processes of transporting molecules across the semipermeable cell-membrane can be categorized into passive and active forms of transport.
Passive transport
According to Lodish et al. (2000) passive transport includes two processes; osmosis, and diffusion. Ore elaborately, Osmosis refers to the diffusion of water molecules, through a semipermeable membrane, to an area of a higher solute concentration (BIOL1 2016, np).This process is aided hydrophilic channels in the lipid bilayer. Additionally, diffusion refers to the movement of substance molecules down a concentration gradient, and across a membrane until equilibrium is reached (Singer and Nicolson 2012, p.11). Though, in facilitated diffusion, the passage of ions down the concentration gradient is enabled by an ion channel that is developed by transmembrane proteins (BIOL1 2016, np).
Active transport
Lodish et al. (2000) indicates that active transport involves the movement of molecules against a concentration gradient, using ATP. This transport process mainly facilitates the movement of materials that cannot penetrate the lipid bilayer, and it entails the movement of materials by binding them to particular carrier proteins (BIOL1 2016, np).This process is important in the uptake of ions in plant roots.
Conclusion
Notably, various components of the cell membrane play significant role in passive and active transportation of molecules into and out of a cell. Illustratively, hydrophilic channels in the lipid bilayer aid osmosis, while transmembrane proteins facilitate the diffusion of ions down the concentration gradient. Additionally, carrier proteins facilitate the active transportation of materials that cannot penetrate the lipid bilayer. Therefore, the structure of a cell membrane is closely related to its function.
- BIOL1, (2016). The structure of plasma membranes enables control of the passage of substances across exchange surfaces.Biologyguide.net. Available at: http://www.biologyguide.net/biol1/3b_exchange.htm [Accessed 2 Nov. 2016].
- Brown, D.A. and London, E., 2000. Structure and function of sphingolipid-and cholesterol-rich membrane rafts. Journal of Biological Chemistry, 275(23), pp.17221-17224.
- Engelman, D.M., 2005. Membranes are more mosaic than fluid. Nature, 438(7068), pp.578-580.
- Lodish, H., Berk, A., Zipursky, S.L., Matsudaira, P., Baltimore, D. and Darnell, J., 2000. Transport across cell membranes.
- Singer, S.J. and Nicolson, G.L., 2012. The fluid mosaic model of the structure of cell membranes. Membranes and Viruses in Immunopathology; Day, SB, Good, RA, Eds, pp.7-47.
