The Cell Membrane - Essay Example

?Explain how the molecular structure of cell membranes accounts for their selective permeability. The cell membrane is a very important structure which forms the outer covering of the cells. Its integrity and normal working is very essential for the proper functioning of the human body as well as the carrying out of normal metabolic functions. The main components of the cell membrane are the lipids and the proteins with the proteins forming a major component of the membrane. Fifty five percent of the cell membrane is constituted by proteins. That is for approximately fifty molecules of lipids, there is one protein molecule present. Other than proteins and lipids the cell membrane also consists of carbohydrates which form a layer outside the cell membrane to serve specific functions of the body. The proteins have a very important role to play in maintaining the shape of the cell membrane which is crucial for maintaining the form and working of the cell as well. They have other important roles not only restricted to the maintenance of functioning and integrity of cell membrane but also in the intracellular metabolic activities. This is because proteins act as receptors for several hormones as well as transmitters which then alter the metabolic activities taking place within the cell. Another important aspect of proteins is that most of the enzymes in the human body are proteins. Most of the intracellular reactions that take place are catalyzed by enzymes and hence proteins are central to the normal functioning of the cells. In the cell membrane certain proteins act as enzymes and they assist in carrying out reactions on the cell membrane surface. Proteins also act as cell adhesion molecules. These molecules possess the capability of attaching the cells to each other and also attaching the cell to the basal lamina. Proteins also have the ability to work as pumps for the passage of ions by the process of active transport which works against the normal diffusion gradient. They also act as carriers and work in moving substances by the process of facilitated diffusion which involves the movement of secondary substances in association with proteins for the purpose of transport. Another important role is as ion channels which only work upon stimulation and activation and allow the movement of ions in the intracellular or extracellular space. Glycoprotein’s present on the cell membrane also have functions related to the immune system. They function in the activities of the antibodies within the body and assist in recognizing the cells of the body from the foreign cells. Thus they protect the cells of the body from undergoing an autoimmune attack. The lipid bilayer of the cell membrane also plays an integral role in the selective permeability of the cell membrane. The bilayer is composed of phospholipids which are hydrophobic at one end and hydrophilic at one end (Guyton & Hall 2006; Ganong 2005). The question now arises as to how these two important molecules help in the selective permeability of the cell membrane. The lipid layer acts as a barrier for water soluble products such as glucose and urea whereas fat soluble substances can be moved through this lipid bilayer because of its properties. The steroid cholesterol plays an important role in maintaining the permeability of the cell membrane. Cholesterol has properties of lipids with a steroid nucleus and hence it only allows fat soluble products to pass through. It is through this lipid bilayer that the cell membrane gets the property of impermeability. The lipid bilayer of the cell membrane also does not allow the complex processes of active transport to occur through it. It only allows passive diffusion of certain substances inside the cell. One may ask as to how the cell survives without essential water soluble products of glucose when the lipid bilayer does not allow them to move inside the cell. The answer to this lies in the complexity of the protein molecules embedded in the cell membrane. It is for this purpose that specific carrier proteins have been designated inside the cell membrane. These carrier proteins help in the transport of specific ions through the cell membrane so that the integrity of the cell is maintained (Guyton & Hall 2006). The proteins that are found in the cell membrane may be of two types. Proteins that span the entire cell membrane are referred to as integral proteins or transmembrane. On the other hand another class of membrane proteins is the peripheral proteins. These proteins as the name indicates are only found on either the inner side of the membrane or the outer side of the membrane but do not span the entire length of the membrane as do the integral proteins. The integral proteins serve to function as a path for the exchange of substances between the fluids that lie within the cell and the fluids outside the cell membrane. The integral proteins then play their role of transporters and act as paths for these substances. They also have the capability of allowing transport not only from a region of higher concentration to a lower concentration but also in the other direction by a process referred to as active transport. Hormones which are not lipid soluble use the integral proteins as receptors. The integral proteins undergo changes in their structure after the binding of the hormones. This results in changes in the integral proteins on the intracellular side and there is transmission of signals from outside the cell to the internal side and a cascade of reactions is initiated within the cell. Hence integral proteins also perform the role of carrying of information from the extracellular compartment to the intracellular one. These proteins also serve certain enzymatic functions. The integral proteins have pores through which specific water soluble substances can pass inside the cell. Thus proteins are the molecules which help in transporting specific water soluble substances inside the cell which are otherwise repelled by the lipid bilayer (Ganong 2005; Sherwood 2006; Guyton & Hall 2006). The protein channels for essential products are also selectively permeable in the cell membrane. They are of different types and form channels which are for specific products. Sodium channels are an example of such a channel formed by protein which carries a negative charge within itself. These negative charges help in attracting the positive sodium molecules inside the cell when they are required by the cell itself. In a similar manner other channels are also formed by these proteins which can be potassium channels, sodium glucose channels and calcium channels. These protein channels work in a complex manner known as gating. The channels are gated by specific charges which help in the influx or efflux of ions as needed by the cell. These protein channels can carry out different transport mechanisms through which the substances can enter the cell. They either use osmosis, diffusion or active transport to transfer molecules inside the cell. Some of the channels are formed to carry diffusion while some carry out osmosis and active transport. Sodium Potassium pump is an example of a protein channel which uses active transport to transfer sodium and potassium in and out of the cell. The protein channels have a special capability of co-transporting molecules inside the cell. An example of a co-transport carrier is Sodium glucose channel which transports sodium along with a glucose molecule. Similarly counter transport mechanism is also used by the protein channels in which a certain molecule is pumped outside the cell and in response another molecule is taken up inside the cell. An example of counter transport channel is Sodium Calcium channel (Guyton & Hall 2006). Peripheral proteins on the other hand exist mainly attached to the integral proteins. They have two significant roles which include working as enzymes. The cell membrane consists of pores as well. The peripheral proteins act as regulators of the substances that make way through these pores. The arrangement of the proteins in the cell membrane is such that the proteins which do not carry any charge and are hydrophobic lie on the inner side of the cell membrane and the proteins which are charged and hydrophilic lie on the external surface of the cell membrane. This is important in maintaining the shape of the cell as the configuration of the lipids of the membrane is in a similar manner. The attachment of the peripheral proteins to the cell membrane is by means of either glycosylphospatidylinositol (GPI) anchors or they are attached with lipids. The proteins which are attached via the GPI anchors are enzymes, antigens and cell adhesion molecules. Enzymes include alkaline phosphatise. The proteins that are a part of the cell membrane are not consistent in all cells. This inconsistency is not only for different cells but the proteins in the membranes of organelles within the cells also differ. The basic structure of the cell is also made up by the proteins. A layer of fibrils joins together and is known as the basal lamina of the cell. The extracellular matrix and the basal lamina possess the function of keeping the cells in place and maintain their growth as well. They are made up of proteins which include collagens, laminins and proteoglycans (Guyton & Hall 2006; Harper 2009). Carbohydrates also form an important part of the cell membrane and they also function together to provide the function of selective permeability to the membrane. They are in combination with proteins or lipids and form an outer layer of the cell membrane. Proteoglycans are carbohydrates combined with proteins in the cell membrane and they are known as glycocalyx when covering the cell membrane. They have important functions in repelling and attracting specific molecules towards the lipid bilayer and protein channels. The glycocalyx coat has a negative charge all over it because of which it repels negative objects. This coat also helps in attracting positive objects towards it which need to be transported inside the cell. They also act as receptors so that specific substances can be bound to them. Mostly these substances are hormones which help in initiating a series of reactions to fulfil the function of the cell. Other than this the carbohydrates of the cell membrane also play a role in immune functions of the cell. Different cells attach to each other with the help of this layer of glycocalyx (Sherwood 2006; Guyton & Hall 2006). Cell membrane is serving an important function in the body of selective permeability in the cell. It allows keeping out unwanted ions and particles outside the cell whereas retaining the important ones. Proteins, carbohydrates and lipids all play an important role in maintaining the important function of the cell membrane. Proteins are an essential component of the human body. They have many important roles to play in the functioning of the cell membrane as well as many important activities within the cell. They form a major part of the cell membrane and perform vital activities which include the exchange of ions and other substances. The protein channels have specific charges on them which help in attracting ions towards it and maintaining the cell. They also function as receptors for transmitting signals from hormones as well as neurotransmitters from the extracellular part to the intracellular portion. They also function as enzymes and enzymatic activity is very essential for the metabolic reactions that take place within the cell. Similarly lipids play an important role in maintaining the specific permeability of the cell so that some molecules are kept outside the cell. Lastly carbohydrates also play an important role of receptors inside the cell with specific charges on them so that certain molecules can be attracted to the cell membrane and on the same hand certain molecules are repelled by the cell (Ganong 2005; Guyton & Hall 2006; Sherwood 2006). REFERENCES Top of Form Ganong, W. F. (2005). Review of medical physiology. New York: McGraw-Hill Medical. Bottom of Form Top of Form Guyton, A. C., & Hall, J. E. (2006). Textbook of medical physiology. Philadelphia: W.B. Saunders. Bottom of Form Murray, Robert K. (2009) Harper's Illustrated Biochemistry. New York: McGraw-Hill Medical. Top of Form Sherwood, Lauralee. (2006). Sherwood's Human Physiology: From Cells to Systems. Brooks/Cole Pub Co. Bottom of Form Read More