The Structure and Function of Cell Membranes
The cell (or plasma) membrane is a phospholipid bilayer that forms the boundary between a cell and its environment (Buehler, 2015). The cell membrane is essential in enabling cell-environment interaction, in addition to obtaining oxygen and such other nutrients as amino acids, carbohydrates, and lipid molecules, among others. Mauseth (2011) opines that “all lipid/protein cell membranes are differentially permeable” (p. 269), meaning that they only permit certain substances to pass through. This enables cell membranes to perform their varied functions. The premise of this essay is to examine the structure of the cell membrane, and the different functions that it performs. Some of the functions of the cell membrane that will be examined include its protective role of the cell from the external environment, regulation of the passage of material in and out of the cell, and its intricate role in facilitating cell-to-cell-communication.
The cell membrane refers to a phospholipid bilayer characterised by a fluid consistency in which wholly or embedded protein molecules are dispersed across the membrane (Karp, 2009). The protein molecules form a mosaic-like structure or what is popularly known as the fluid-mosaic model. According to Buehler (2015), the fluidity of the cell membrane is desirable as it aids in the functioning of proteins like enzymes which are often inactivated once the membrane solidifies. Since the two halves of the plasma membrane are not identical (Cooper, 2000), it is thus described as being asymmetrical. The glycolipids consist of proteins and carbohydrates chain that is only found on the outside surface. Conversely, cytoskeletal filaments are to be found on the inside surface where they attach to proteins. The phospholipids instinctively position themselves into hydrophilic and hydrophobic layers. The hydrophilic (or water loving) non-polar layer normally face the inside of the cell towards the water, while the water fearing (or hydrophobic) non-polar tails face each other (Karp 2009).
Other than the phospholipids, the plasma membrane also consists of two other forms of lipids: glycolipids and cholesterol (Alberts et al., 2002). According to Lodish and Zipursky (2011), glycolipids share a similar structure to that of the phospholipids, save for the fact that the hydrophilic head consists of different forms of sugars joined together to form a branching or straight carbohydrate chain. On the other hand, animal plasma membrane also contains a lipid known as cholesterol while plant plasma membrane contains steroids (Karp, 2009). The role of cholesterol in the plasma membrane is to reduce its permeability to most biological molecules.
The plasma membrane also contains proteins that exist either as integral proteins or peripheral proteins. Peripheral proteins are to be found on the inside or outside surface of the plasma membrane (Buehler, 2015). Some of the peripheral proteins are attached to the plasma membrane via covalent bonding (Jamieson & Robinson, 2014), while other are attached by non-covalent bonding that may be distorted through a change of PH or by gentle shaking. In contrast, integral proteins are located within the plasma membrane. They consist of hydrophilic and hydrophobic regions. Whereas the hydrophilic regions are lodged within the plasma members (Karp, 2009), the hydrophilic regions on the other hand, extend from either surfaces of the bilayer.
A key aspect of membrane activity entails transporting materials through it. In this way, the plasma membrane constantly permits the passage of water, mineral ions, monosaccharides, amino acids, and other nutrients (Alberts et al., 2002). The plasma membrane is differentially or selectively permeable (Jamieson & Robinson, 2014). In other words, the cell membrane permits the free passage of some materials through it, while excluding the passage of others. Some materials can only leave or enter the cell by utilising energy. For instance, such small hydrophobic molecules as O2, small lipids, and CO2, dissolve in the plasma membrane, and this facilitates their passage through the membrane (Alberts et al., 2002). Conversely, such tiny polar molecules as alcohol and H2O can also pass through the phospholipid molecules, albeit minimally (Jamieson & Robinson, 2014). Although ions, along with most nutrient molecules have been shown to experience a limitation in terms of freely moving through the plasma membrane, they are usually carried via the transport protein channels (Buehler, 2015), a process that may or may not require energy expenditure.
The cell membrane further plays a crucial role in securing the cytoskeleton, effectively giving shape to the cell (Jamieson & Robinson, 2014). It also anchors to the extracellular matrix, as well as other cells, thereby aiding in grouping cell together (Lodish & Zipursky, 2011), thus forming tissues.
The cell membrane also plays a protective function in that it protects the cell form entry of harmful chemicals in the external environment. According to Lodish and Zipursky (2011), the cell membrane further protects the cells from a possible loss of its valuable biological macromolecules. The cell membrane further plays an active transport function that involves the movement of substances through it, a process that expends energy. Active transport entails the conversion of ATP energy into ADP to move materials out of and into the cell (Alberts et al., 2002). Passive transport may also occur, in which a substance moves from a region of high concentration through a cell membrane into one of lower concentration without having to expend energy.
The ability of the plasma membrane to facilitate cellular signalling is among its most intricate functions. Complex proteins are usually involved in cellular signalling (Jamieson & Robinson, 2014), either as receptors of extracellular inputs, or as markers of intracellular processes to facilitate cell-cell recognition (Buehler, 2015). This is important for enabling cellular signalling processes that facilitate organ and tissue development. Membrane receptors also function as extracellular sites for the attachment of growth factors and hormones. These effectors are important in activating intracellular responses.
In sum, the plasma membrane consists of a phospholipid bilayer that makes it differentially permeable. Consequently, it permits the passage of certain nutrients and materials into the cells and not others. This happens through active transport, osmosis, or diffusion. The ell membrane also performs a complex function of facilitating cell-to-cell communication, thereby aiding in organ and tissue development.
Alberts, B., Johnson, A., Lewis, J., et al. (2002). Molecular Biology of the Cell (4th ed.). New
York: Garland Science.
Buehler, K. (2015). Cell Membranes. New York: Garland Science.
Cooper, G.M.(2000). The Cell: A Molecular Approach. 2nd edition. Sunderland (MA): Sinauer.
Jamieson, G.A., & Robinson, D.M. (2014). Mammalian Cell Membranes: Volume 2: The
Diversity of Membranes. Amsterdam: Elsevier.
Karp, G. (2009). Cell and Molecular Biology: Concepts and Experiments. London: Wiley.
Lodish, H., Berk, A., Zipursky, L.S., et al. (2004).Molecular Cell Biology (4th ed.). New
York: Scientific American Books.
Mauseth, J.D. (2011). Botany: An Introduction to Plant Biology. Burlington, Massachusetts:
Jones & Bartlett.