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Cell membrane or plasma membrane consists of a double layer formed from proteins and lipids. It surrounds the cell and separates the cytoplasm (the cell content) from its neighboring environment. The cell membrane depicts a selective permeability, meaning it only allows certain molecules to enter and exit. It equally possess the ability to control the amount of particular substances that enter and leave the cell. Cell membrane is present in all cells.
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Function of the Cell Membrane
Cell membrane controls the materials entering and leaving the cell and gives the cell its structure. It serves as a drawbridge, which serves to protect the castle and keep out wrong people. The cell membrane selectively allows particular molecules to go in or leave. Oxygen, which play an important role in the cell metabolic functions like cellular respiration, and carbon dioxide that came as a byproduct of these functions easily find their way through the membrane. Water is the other material that freely crosses the membrane although at a slower rate. For the highly charged molecules such as ions, things are much different, and so is the case with large macromolecules like amino acids and carbohydrates. These materials cannot pass directly through the cell membrane but instead pass through protein layer, which is embedded in the membrane. It is through this system that the cell controls the rate of diffusion of such substances (Kamp, and op den, p.15-17).
Cells membrane can also bring molecules in through endocytosis processes. These processes includes pinocytosis (“cell drinking”) and phagocytosis (“cell eating”). These processes works by the cell membrane forming a small depression and surrounding the particle that it intends to engulf. This if followed by “pinching off” to create a small sphere of membrane referred to as vesicle that contains the molecules. These molecules are then transported to wherever it is to be used within the cell. Vesicles can also be formed from the cell membrane in the absence of docytosis, where they become useful in the transportation of molecules to various points within the cell. Exocytosis, which is the reverse of endocytosis, serves as a process by which the cell get rid of molecules. During this process, vesicles move to the surface of the cell membrane, join it, and discharge their contents to the outside of the cell. Ejection of cells waste products or parts of molecules that have no use to the cell is done through a process called Exocytosis (Kotyk, and Janáček, p-4-7).
In addition, the Cell membrane plays an important role in communication and signaling. Here, receptor proteins found on the cell membrane attach themselves to molecules of substances released from other body areas like hormones. Once molecules are attached to its target receptor located on the membrane, it triggers a signal transduction pathway within the cell that convey the signal to the respective molecules. The cell then responds by performing actions that are informed by the signal molecule. This can be boosting or stopping the production of particular protein.
Phospholipids is a type of lipid molecule that forms the major part of the cell membrane. Lipids molecules can either be made of fats, some vitamins, waxes and other materials. Each phospholipid comprises of a phosphate group, two fatty acids and a glycerol molecule. Phosphate molecules have a tendency of forming spontaneous two-layered membrane, thanks to its properties.
Each phospholipid is comprised of a phosphate group head and two fatty acid tails. The Fatty acid are formed from long chains of carbon and hydrogen, while phosphate groups comprises of a phosphorous molecule joined to four oxygen molecules. A third glycerol molecule connects the two-phospholipid components.
Phospholipid Bi layer
Phospholipids ability to form cell membranes comes from the nature of its component i.e. phosphate group head, which is hydrophilic (attracted to water or water loving), and the fatty acid which is hydrophobic (water hating). These properties makes it possible for the phospholipids to automatically arrange themselves creating particular pattern in water and thus form the cell membrane. While forming membranes, phospholipids form a line close to each other with each head positioning itself on the outside of the cell while their tails remain in the inside. Phospholipids forms a second layer with the heads pointing to the inside of the cell while the tail face away. Based on this formation, a double layer is created with phosphate group heads located on the outside, and the fatty acid tail facing positioned in the inside. The double layer, also termed a lipid bilayer, comprises the main part of the cell membrane. The membrane surrounding the cell’s nucleus or nuclear envelope is also formed from phospholipids created from a lipid bilayer, like in the membrane of mitochondria (Stein, and Smith p.12-18).
Functions of Phospholipids
As part of the membrane, phospholipids are semi-permeable (also called selectively permeable), implying that only specific, molecules can go through it to either enter or exit the cell. Here molecules that dissolve in fat can pass through with less difficulty, while those that dissolve in water cannot. Carbon dioxide, oxygen and urea are part of the molecules that readily find their way through the membrane. Large molecules such as glucose or ions like potassium and sodium cannot go through easily. This selective permeability helps in keeping the content of the cell working appropriately and divides the inside of the cell from the outside environment (Stein, and Smith p.12-18).
Cells can break down Phospholipids and generate energy for the body. The same phospholipids can be broken into smaller molecules referred to as chemokine, that help in then regulation of different cell activities like production of specific proteins and movement of cells to various areas within the body. In addition, they are located in areas such as the joints and lungs, where they serve to lubricate the cells.
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Phospholipids have a very important role in the pharmaceuticals. Here, they form part of the drug delivery systems. These systems help in the transportation of drugs throughout the body to areas that it is intended to affect. Such is the case that phospholipids presents a higher bioavailability (or they are easily absorbed in the body). Valium is the best example of a treatment that employs a delivery system, which is founded on phospholipid.
In conclusion, Cell membrane or plasma membrane consists of a double layer formed from proteins and lipids. Cell membrane controls the materials entering and leaving the cell and gives the cell its structure. Phospholipids is a type of lipid molecule that forms the major part of the cell membrane. Lipids molecules can either be made of fats, some vitamins, waxes and other materials. Each phospholipid is comprised of a phosphate group head and two fatty acid tails. Phospholipids form part of the delivery systems that help in the transportation of drugs throughout the body to areas that it is intended to affect (Stein, and Smith p.12-18).
- Kotyk, Arnošt, and K Janáček. Cell Membrane Transport. Boston, MA: Springer US, 1988. Print.
- Stein, J, and G Smith. Phospholipids. County Clare: Elsevier/North-Holland, 1982. Print.
- Kamp, Jos A. F. op den. Membrane Biogenesis. Berlin: Springer-Verlag, 1988. Print.