CELL MEMBRANE
Introduction
A cell is the basic biological, functional and structural unit of living things. Cells mediate the transport of molecules and ions within tissues. They are also responsible for metabolic processes, movement and reproduction. In this sense, it is the activities occurring within cells that support life (Stein, 2012). Each animal or plant cell has a semipermeable membrane that encloses its components, which include the endoplasmic reticulum, Golgi apparatus, ribosomes, lysosomes, centrioles, pinocytic vesicles, mitochondria and the nucleus. The cell membrane was discovered by Carl Naegeil and C. Cramer in 1855 (Edidin, 2003). Studies on the functional and structural elements of the cell membrane led to the discovery of the fluid mosaic model by G L Nicolson and S J Singer in 1972. The fluid mosaic model provides that a cell membrane takes the form of a two-dimensional liquid that controls the movement of molecules into and out of the cell (Cooper and Hausman, 2000). In this paper, I describe the various components of the cell membrane in line with their structure and function. I also compare plant and animal cells in this paper.
Components of a Cell Membrane
Lipids, proteins and carbohydrates are the main components of a cell membrane. The lipids of the cell membrane are of two types: phospholipids and cholesterol. Each phospholipid molecule has two ends: head and tail. The head is hydrophilic. This is because it contains a phosphate group, which makes it to attract water molecules. On the other hand, the tail is hydrophobic. It repels water molecules as it contains fatty acid chains made of carbon atoms and hydrogen (Stein, 2012). Phospholipids in animal and plant cell membranes are arranged in a manner that they form a double layer referred to as a lipid bilayer. Hydrophobic tails are arranged in a way that they keep away from intracellular and intracellular fluids (Figure 1). The arrangement of phospholipids is important as enables the cell membrane to be selective in allowing specific solutes, such as water, ions and proteins, to enter the cell (Cooper and Hausman, 2000). Therefore, phospholipids within the cell membrane play the leading role in controlling movement of molecules to and out of the cell. Cholesterol contained in animal cell membranes is attracted to their fatty acid chains proximal to phospholipids. This arrangement provides for the immobilization of the outer part of animal cell membranes and also makes them less permeable to small hydrophilic molecules (Brown and London, 2000). Cholesterol specifically plays the role of ensuring that the integrity of animal cell membranes is maintained (Stein, 2012).
Proteins in the cell membrane are of two types: integral and peripheral proteins. Integral membrane proteins are permanently attached to the cell membrane while peripheral proteins are temporarily anchored to the cell membrane (Figure 2). Proteins serve several functions. For example, receptor proteins relay signals between the extracellular and intracellular environments of a cell. There are also transport proteins which play the role of moving ions and molecules across the lipid bilayer (Cooper and Hausman, 2000). Proteins also act as membrane enzymes. They are responsible for catalyzing biological activities within the cell membrane. Furthermore, there are cell adhesion proteins which enable cells, such as those involved in immune responses, to recognize each other and interact (Brown and London, 2000). The carbohydrates of the cell membrane are attached to integral proteins from the external surface. This arrangement allows them to play their role of holding adjacent cells together. Carbohydrates also act as sites where chemical messengers to the cell, including hormones attach (Stein, 2012).
Animal and Plant Cell Membrane
The cell membrane of plant cells is covered by a cell wall. This is unlike animal cells which lack cell walls. The function of the cell wall in plants is to support cells and to protect the cell membrane. Cellulose is the main structural component of the cell wall. Notably, the structure of the plant cell membrane is similar to that of the animal cell membrane. This is because both plant and animal cell membranes have phospholipids, cholesterol, carbohydrates and cholesterol serving specific structural and functional roles as described above. However, plant cell membranes differ from animal cell membranes as they lack cholesterol (Brown and London, 2000).
Animal cells need cholesterol for its important role of providing extra support to the cell membrane. The support cholesterol provides to animal cell membranes is important as they lack cell walls. It is the rigidity of cholesterol that makes it suitable in providing extra support to animal cell membranes (Stein, 2012). Cholesterol provides support by immobilizing some lipid molecules contained in animal cell membranes. Additionally, the rigidity made possible by cholesterol is important to animal cell membranes as it makes them less permeable to small molecules. Animal cell membranes act as effective barriers to specific ions due to the presence of cholesterol (Brown and London, 2000). Furthermore, animal cells need cholesterol to help them in keeping the fluidity of their cell membranes. Cooper and Hausman (2000) explain that cholesterol within animal cell membranes generates space between other lipid components, which allows them move freely.
Conclusion
Phospholipids, proteins and carbohydrates are the main structural and functional components of both plant and animal cell membranes. Animal cell membranes also contain cholesterol. This is unlike plant cell membranes which lack cholesterol. The role of phospholipids in both plant and animal cell membrane is to form a lipid bilayer, which acts to control the movement of compounds, molecules and ions into and out of the cell. Cholesterol in animal cell membranes serves an important function of maintaining their fluidity and providing support. Notably, plant cells membranes lack cholesterol but are enclosed by a rigid cell wall made of cellulose, which provides support and therefore acts as a protective layer. Proteins in both plant and animal cell membranes serve a wide range of functions depending on their type. Functional proteins within the cell membranes of plant and animal cell include carrier proteins, receptor proteins, channel proteins, enzymatic proteins and cell recognition proteins. Carbohydrates in plant and animal cell membranes play roles related to cell adhesion and also act as sites into which important chemical massagers to the cell, such as hormones, attach in the process of influencing cellular activities.
References
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.
Cooper, G.M. and Hausman, R.E., 2000. The cell. Sunderland: Sinauer Associates.
Edidin, M., 2003. Lipids on the frontier: a century of cell-membrane bilayers. Nature Reviews Molecular Cell Biology, 4(5), pp.414-418.
Stein, W., 2012. Transport and diffusion across cell membranes. Elsevier.
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