The cell membrane

All living organisms are made of cells and cells arose from pre-existing cells

For those that are not into life sciences, the above line belongs to what we life scientists know to be the cell theory. We are all made of cells, although human cells are organized into tissues, organs, and system within their bodies. The same goes for other organisms, including plants. While a lot of organisms exist as single cells, whole lots are made up of several cells with or without differentiation into tissues, organs or system.

^{By CNX OpenStax - CC BY 4.0, Link}

Cells can also be prokaryotic or eukaryotic depending on the presence or absence of certain organelles. Prokaryotic cells have been opined to be the first to have evolved and as such, are more primitive compared to their eukaryotic counterparts. They lack nucleus and membrane-bound organelles among some other smaller features.

The plant and animals cells are both categorized as eukaryotic cells. They are characterized by nucleus and membrane-bound organelles. However, plant cells tend to differ from animal cells in that the former is characterized with the presence of protective rigid cell walls composed largely of polysaccharides. In addition, the plant cell contains a radiation-absorbing molecule in form of chlorophyll which is usually housed within the chloroplast organelle along with the presence of larger vacuole as compared to that of animal cells.

Enough of the comparison and straight to today's business. Be it prokaryotic, plant or animal cell; all cells have a component that regulates the movement of materials in and out of them by being selective in its permissibility. The work of this part can be likened to that of a bouncer in a nightclub tasked with preventing people without proven modes of identification from gaining access into the club. Such is the function of the cell membrane within the cell.

More specifically, the cell membrane separates the contents of cells from their external environments by controlling exchange of materials such as nutrients and waste products between the two interfaces. In addition, cell membranes are important in that they enable separate compartments to be formed within the cells in which specialized metabolic processes such as photosynthesis and aerobic respiration can take place as it is obtainable in the eukaryotic cells. Hence, the chloroplasts and the mitochondria of the eukaryotic cells have their own membranes.

Cell membrane test from Alexandra Gordon on Vimeo.

Various chemical reactions within the cell actually take place on the surface of the membrane. For example, the light reaction of photosynthesis during which Adenosine Triphosphate and Nicotinamide Adenine Dinucleotide Phosphate are synthesized takes place on the surface of the chloroplast's membrane. In some cases, membranes also act as receptors to hormonal identification, neurotransmitters and several other chemicals which could be from the exterior of cells or within other parts of the organism itself.

Membranes are actually partial in their permissibilty unlike semi-permeability that we were thought in our high school's study of osmosis/diffusion. While semi-permeable membranes allow only the passage of water and other small molecules, cell membranes in addition permits the diffusion of other substances such as glucose, amino acids, fatty acids, glycerols, ions among several other substances. However, the membrane is generally configured to know when, how and the amount of each of those molecules to allow in or out of the cells.

An inquisitive mind would already be wondering what confers such control power on cell membranes. The structure of the cell membrane is best explained using the fluid-mosaic model which was propounded by Singer and Nicolson in 1972. Early work showed that organic solvents such as ether, chloroform, etc. pass through the membrane more easily than ordinary water, an observation that served as an indication that the cell membranes have non-polar portions and lipids.

The non-polar and lipid hypothesis was later confirmed by chemical analysis, the outcome of which revealed that membranes are made almost entirely of proteins and lipids, then little portions of cholesterol and carbohydrates. Specifically, the lipid portion of the membranes are referred to as phospholipids. Hence, the cell membrane is described as a mosaic of phospolipids, proteins, carbohydrate and cholesterol.

A detailed enquiry into the structure of the phospolipid molecule revealed a phosphate polar head which has an uneven distribution of charges within it, making it water solubles. In addition to the polar head are two non-polar hydrocarbon (cholesterol to be precise) tails from the fatty acids used to make the molecule. Hence, I like to view the phospholipid layer of the mosaic model of the membrane as two contrasting ends in one body; a water-loving or hydrophilic head and two water hating or hydrophobic tails.

^{By Boumphreyfr - CC BY-SA 3.0, Link}

Consequently, if a thin layer of phospholipid molecule is spread over the surface of water, they will arrange themselves into a single layer with the water-hating tails projecting out of the water while their water hating heads lie in the surface of the water. Thus, the basic structure of the cell membrane are the phospholipids molecules which are always arranged and organized in a double layers otherwise known as lipid bilayers.

There are two groups of protein molecules as far as the cell membrane structure is concerned. The first group is located within the lipid laters while the second group is located outside of it. They are otherwise referred to as integral and pheripheral proteins respectively. Understanding how the protein fits into the structure of the cell membrane (the phopholipid layers) was made possible by the freeze-fracturing technique.

The free-fracture technique involves rapid freezing and fracturing of cells with a sharp metal blade. The fracture usually allows membranes to be split and the surfaces to be examined. With the technique, researchers have been able to determine the presence of particles, mainly proteins which penetrate into and often right through the phopholipid bilayer. Also, it has been found that the more metabolically active a membrane is, the more protein particles contents it will have. Hence, the chloroplast's membranes have been found to contain about 75% protein whereas the metabolically unreactive myelin sheath was found to contain just 18%.

Associated with the extracellular side of the membrane are carbohydrate/sugar molecules which together form a structure known as glycolax. Generally, the glycolax serves cushion and protective functions within the plasma membrane as well as playing an important role in cell recognition.

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Conclusion

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All cells, be it prokaryotic or eukaryotic, plant or animal cell, have cell membrane in common although eukaryotic cells have other forms of membrane such as mitochondia and chloroplast's membrane. These membranes are best described as partilly permeable in controlling the movement of materials in and out of the cell. The function is made possible by the structure of the membrane which has been described using the fluid mosaic' model proposed by Singer and Nicolson.

Thank you all for reading.

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References

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study.com

ncbi

khanacademy

thoughco.com

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