plasma-membrane

What is Plasma Membrane? Structure, Chemical Composition & Functions of Cell Membrane

Plasma Membrane

In this article, we will discuss the chemical composition, structure, and functions of the plasma membrane but firstly we have to know about the plasma membrane.

What is the plasma membrane?

Plasma membrane or cell membrane is the external most layer of the cell. However, in a lot of plant cells, it is covered by a cell wall. The cell membrane is chemically composed of lipids and proteins; 60 – 80% are proteins, while 20-40% are lipids. In addition, there is a small quantity of carbohydrates.

plasma-membrane

Chemical Structure of Plasma Membrane

The membrane is primarily composed of lipids, proteins, and carbohydrates. Water makes about 29% of the total weight. Robertson (1959) proposed that the plasma membrane is a three-layered structure where proteins form the external and inner layers of membrane that confines lipids to form a unit membrane.

1.Lipid

The lipids identified in the plasma membrane include cholesterol, phospholipids, and galactolipids.

Phospholipids

The phospholipids include phosphatidylcholine, phosphatidylethanolamine, and sphingomyelin. The phospholipids are discovered to be related to the external protein shell in the plasma membrane. Glycerol and fatty acid make up lipid molecules.

In the membrane, the lipid molecules consist of two parts– a head and two tails. The head is composed of glycerol and is hydrophilic whereas the tails are composed of fatty acids that are hydrophobic. The head and tail are normally designated as polar and nonpolar end respectively. The proteins, in the membrane, exist in 2 layers and the lipids are present in between them.

The lipid particles are oriented in 2 (bimolecular) layers with their hydrophilic polar ends directed towards protein and the hydrophobic nonpolar ends face each other. Hydrogen bonds, ionic linkages or electrostatic forces bind the protein and lipid parts.

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2.Carbohydrates

They are present in the form of glycolipids and glycoproteins. Both of these types are confined solely to the external membrane surface area. Bell (1962) is of the viewpoint that polysaccharides provide some stability to the lipoprotein complex in the membrane.

3.Protein

In the membrane, it exists as enzyme protein, carrier protein, and structural protein. The enzyme proteins have catalytic activity. The carrier proteins assist to transport materials in and out of the cell throughout the membrane. The structural proteins play a crucial function to form the structure of the membrane.

Models of the structure of Plasma Membrane

Various designs were put forward about the physiochemical nature of the plasma membrane.

Old Concepts
1.Charles Overton

He found that the substances which dissolved in lipids get in more rapidly than substance insoluble in lipid. Therefore, he concluded that the plasma membrane was made up of lipid.

2.I. Langmuir

He made artificial membranes by including phospholipids in Benzene to water. Benzene vaporized and phospholipids membrane was formed.

E.Gorter and F. Grendel

They explained that cell membrane is in fact phospholipid bilayers. The hydrophobic ends of the phospholipid molecules are directed inward and hydrophilic ends of the membrane are directed outward. Gorter and Grendel also attempt to measure the precise contents of the plasma membrane. But they failed.

Modern Concept

Fluid Mosaic Model

  1. Jonathan Singer and Garth Nicholson established the fluid mosaic model of membrane structure in 1972. Cell membranes are represented according to a fluid-mosaic model, due to the fact that they are:

Fluid— the phospholipid bilayer is viscous and individual phospholipids can move position.

Mosaic— the phospholipid bilayer is embedded with proteins, leading to a mosaic of components.

The plasma membrane that surrounds these cells has 2 layers (a bilayer) of phospholipids (fats with phosphorous connected), which at body temperature resemble vegetable oil (fluid). And the structure of the plasma membrane supports the old stating, “Oil and water don’t mix.”

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Each phospholipid particle has a head that is attracted to water (hydrophilic: hydro = water; philic = loving) and a tail that repels water (hydrophobic: hydro = water; phobic = fearing). Both layers of the plasma membrane have the hydrophilic heads pointing towards the exterior; the hydrophobic tails form within the bilayer.

plasma-membrane-structure 

Because cells reside in a watery environment (extracellular fluid), and they include a watery solution inside of them (cytoplasm), the plasma membrane forms a circle around each cell so that the water-loving heads touch with the fluid and the water-fearing tails are secured on the inside.

Proteins and compounds such as cholesterol become embedded in the bilayer, offering the membrane the appearance of a mosaic. Because the plasma membrane has the consistency of vegetable oil at body temperature, the proteins and other compounds have the ability to move across it. That’s why the plasma membrane is described using the fluid-mosaic model.

The molecules that are embedded in the plasma membrane likewise serve a function. For example, the cholesterol that is stuck in there makes the membrane more stable and prevents it from solidifying when your body temperature is low. (It keeps you from actually freezing when you’re “freezing.”) Carbohydrate chains attach to the outer surface area of the plasma membrane on each cell. These carbohydrates specify to every person, and they supply qualities such as your blood type.

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Functions of Cell Membrane

Transport of products is one of the crucial roles it provides to the cell. It offers a barrier between the cell contents and their environment, allowing just selective substances to pass through it, thus it is called a differentially permeable or selectively permeable membrane.

The substances which are lipid-soluble cross it more quickly than others, therefore, it manages the flow of materials and ions to preserve a definite gradient. Many little gas molecules, water, glucose, etc. being neutral can quickly cross while ions, being charged particles, have some difficulty in crossing.

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Many substances which are not required, continuously get in the cell by passive transportation, others are taken up against the concentration gradient (they move from the area of low concentration to the area of high concentration). This uphill movement of materials needs energy and is described as active transport. The energy used for this motion is provided by ATP.

plasma membrane transportation

In numerous animal cells, the cell membrane assists to take in products by infolding in the form of vacuoles. This type of consumption is described as endocytosis which can be either phagocytosis (to engulf solid particles) or pinocytosis (to take in the liquid product).

In nerve cells (afferent neuron) the cell membrane transfers nerve impulses from one part of the body to the other to keep coordination.

 

Summary

The plasma membrane or cell membrane is the outermost layer of animal cell composed of 60 – 80 % proteins and 20 – 40 % lipids. Carbohydrates and phospholipids are also the part of plasma membrane. Carbohydrates are in the form of glycolipids and glycoproteins and are confines to outside of plasma membrane. Proteins are present as enzymes, carriers and structural proteins. Some models were also given for structure of plasma membrane. The most modern and accurate one is Fluid Mosaic Model. This was devised by SJ Singer and GL Nicolson. According to this, protein layers are not continuous and are not confined to the surface of membrane but are embedded in lipid bilayer in a mosaic manner. These protein molecules may function as a gateway for the transport of materials. Plasma membrane is differentially permeable or selectively permeable membrane, allowing only the selective substances to pass through it. Lipid soluble substances pass the membrane more easily than others.