All living cells be it prokaryotic cell or a eukaryotic cell consists of a plasma membrane. This membrane actually separates the cytoplasm of one cell from the other cells in other words it is a boundary of the cell. The plasma membrane is permeable only to specific molecules. It allows nutrients and other essential elements to enter the cell and waste materials to leave the cell. Small molecules like oxygen, carbon dioxide, and water can pass freely across the membrane and on the other hand the entry of larger molecules like amino acids and sugars are regulated.
Plasma membrane has the ability to sense the external signals and helps the cell to react according to the environment. The plasma membrane is also called as cytoplasmic membrane, cell membrane or plasmalemma. The term cell membrane was coined by Carl Nageli and Carl Eduar Cramer in 1865. The term plasmalemma was coined by Janet Plowe in 1931.
Chemical composition of Plasma membrane
Plasma membrane is very thin in nature and this is the reason it cannot be observed under light microscope. Based on the chemical analysis of the cell membrane of human red blood cells, it was believed that cell membrane is composed of lipids arranged in two layers. These lipids are arranged within the membrane with the polar head towards the outer sides and the non-polar tails towards the inner side. This arrangement makes sure that the nonpolar tail of saturated hydrocarbons is protected from the aqueous environment.
After the invention of electron microscope the detailed structure of the cell membrane was studied. Biochemical investigation clearly revealed that the cell membranes also consist of protein and carbohydrate. The ratio of carbohydrate, protein and lipid varies significantly in different cell types. Consider the following table:
|Plasma membrane of
A) Proteins: Proteins are the main components of all the biological membranes they may be about 50% of the constituents of the plasma membrane. The amount of proteins present in the membrane varies considerably depending on the location and function of the cell. Membrane of the nerve cell has less than 25 percent of protein in it whereas the internal membranes of cells involved in energy transduction like mitochondria and chloroplast have about 75% of protein. Proteins may act as enzymes, antigens, receptor molecules, regulatory molecules etc.
Position of the proteins and ease of extraction
Depending on the position of the protein and the ease of extraction, membrane proteins can be classified into two types namely, integral proteins or peripheral proteins.
- These proteins are also called as extrinsic proteins.
- They lie on the surface of membrane.
- Peripheral proteins have weak association with the membrane. They are associated to the membrane lipids by electrostatic interaction.
- These proteins are also called as intrinsic proteins.
- They are partially or totally submerged in the membrane.
- Intrinsic proteins have strong association with the membrane.
Peripheral and integral proteins may be ectoproteins (proteins lying on external side of cytoplasmic surface of the plasma membrane) or endoproteins (proteins lying on internal side of cytoplasmic surface of the plasma membrane).
Function of the proteins
Depending on the function of the proteins in the membrane they may be of three main types namely, structural proteins, transport proteins and enzymes.
They are extremely lipophilic in nature. They form the backbone of the plasma membrane.
These are the proteins which help in transport of specific substances across the plasma membrane and also other cellular membranes.
Enzymes: Enzymes are protein molecules present in the cell and act as catalysts. Enzymes of the plasma membrane can be either ectoenzymes (enzymes lying on external side of cytoplasmic surface of the plasma membrane) or endoenzymes (enzymes lying on internal side of cytoplasmic surface of the plasma membrane). Examples of enzymes present on the plasma membrane are phospholipase A, Maltase, Lactase, Alkaline phosphatase, Acetyl phosphatase etc.
B) Lipids: There are mainly three types of lipids present on the plasma membrane namely,
All the above lipid types are amphipathic molecules with hydrophilic and hydrophobic domains. Just like proteins, the amount of lipids present on the plasma membrane also varies considerably depending on the location and function of the cell.
- Phospholipids may be acidic (for example, sphingomyelin) or neutral (for example, phosphatidyl choline, phosphatidyl serine). Generally phospholipids are loosely arranged and they form about 20-79% of the cell membrane. The main types of phospholipids included in the plasma membrane are phosphatidyl choline, phosphatidyl ethanolamine, sphingomyelin, phosphatidyl serine.
- Phosphatidyl choline and sphingomyelin have no net charge and are distributed more on the outer layer whereas phosphatidyl serine, phosphatidyl ethanolamine, phosphatidyl inositol, cardiolipin, sulphalipin and phosphatidyl glycerol with net negative charge are distributed more in the inner layer of the plasma membrane.
- Cholesterol is abundant in plasma membrane of mammalian cells. Cholesterol is ABSENT in prokaryotic cell. It gives stability and preserves the structure of the plasma membrane. The function of cholesterol is temperature dependent. At higher temperatures, cholesterol stops the movement of fatty acids. At lower temperatures cholesterol avoids freezing of the membrane by interfering with hydrophobic interactions.
C) Carbohydrates: Carbohydrates in the plasma membrane are short, unbranched or branched chains of sugars. They are found either attached to ectoproteins or to the polar ends of phospholipids at external surface of the plasma membrane. They may be either associated with the lipids (glycolipids) or with proteins (proteins). It is important to note that, carbohydrates are not located at the cytoplasmic or inner surface of the plasma membrane. The six basic glucose derivative sugars namely D-galactose, D-mannose, L-fucose, N-acetylneuraminic acid, N-acetyl-D-glucosamine and N-acetyl-D-galactosamine.
- Explain the chemical composition of plasma membrane.
- What are the types of glucose derivatives present in the plasma membrane.
- Write about the types of proteins occuring in the plasma membrane.
- Discuss about phospholipids.
- What are the enzymes present on the plasma membrane.
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