Question

describe the structure of a generalized plasma membrane . Include the major components (lipids,proteins,glycolipids, polysaccharides).Describe transport mechanisms by which materials move into and out cells.

Answer 1

Answer:

Plasma membrane:

Plasma membrane is a dynamic, quasi fluid structure and forms the external boundary of the cells. It acts as a selectively permeable membrane and regulates the molecular traffic across the cell boundary.

Chemical constituents (Composition) of the Plasma membrane:

All plasma membranes, regardless of their source; are made of proteins and lipids. The ratio of protein to lipid varies enormously depending on the cell types. As an example, the plasma membrane of human RBC contains 43% lipid and 49% protein by weight; whereas plasma membrane of mouse liver cells contains 54% lipid and 46% protein by weight.

Carbohydrates are also present in the plasma membrane and comprise only 5-10% of the total membrane mass. Carbohydrates are bound either to proteins as constituents of glycoproteins or to lipids as constituents of glycolipids. Carbohydrates are especially abundant in the plasma membranes of eukaryotic cells.

General structure of the plasma membrane:

Different models were proposed to explain the structure of the plasma membrane however the most accepted one is the Fluid Mosaic Model as proposed by Jonathan Singer and Garth Nicolson in 1972.

In this model, membranes are viewed as quasi-fluid like structures in which proteins are inserted into the lipid bilayers. It describes both the mosaic arrangement of proteins embedded throughout the lipid bilayer as well as the fluid movement of lipids and proteins alike.

Phospholipid bilayer:

The basic structure of the plasma membrane is the phospholipid bilayer. This bilayer is composed of two leafletsof amphipathic phospholipid molecules whose polar head groups are in contact with the intra or extracellular aqueous phase; whereas their non-polar tails face each other; constituting the hydrophobic interior of the membrane.

Thus, the hydrophilic heads lies outside and the hydrophobic tails lies at the inside of the plasma membrane.

Two types of phospholipids occur in the bilayer; the glycerophospholipids, in which the hydrophobic regions are composed of two fatty acids joined to glycerol and the sphingolipids, in which a single fatty acid is joined to sphingosine.

The lipid bilayer is not composed exclusively of the phospholipids. In addition to the phospholipids, the plasma membrane also contains glycolipids and sterol.

Forces involved:

The primary physical forces for organizing the lipid bilayer are the hydrophobic interactions. These interactions result in the formation of the phospholipid bilayer, a sheet containing two layers of phospholipid molecules whose polar head groups face the surrounding watery surface while the fatty acyl chains form a continuous hydrophobic interior.

Further, sterols are structural lipids present in the membranes of most eukaryotic cells. The basic structure of sterol is a steroid nucleus, consisting of four fused rings, three with six carbons and one with five. Cholesterol is the major sterol present in the plasma membrane of animal cells. Also, lipids are not randomly mixed in each leaflet of a bilayer. Certain lipids in the plasma membrane, particularly cholesterol and sphingolipids, are organized into aggregates; called the lipid rafts.

Thus, the points mentioned above explain the structure of the plasma membrane.

Function of the plasma membrane:

Since plasma membrane is a selectively permeable membrane, the entry and exit of the substances to and from the cells; that is their transport is mediated with the help of the plasma membrane.

Plasma membrane forms a barrier that blocks the free exchange of the molecules between the cytoplasm and the external environment of the cell.

The transport through the plasma membrane is done with the help of special classes of the transport proteins; channel and the carrier proteins which are responsible for transferring such solutes across the cell membranes. Thus, the transport if the substances across the plasma membrane can occur either through the lipid bilayer through the transport proteins.

Transport across the plasma membrane is of two types;

1. Passive Transport: This transport occurs along the concentration gradient and without the use of metabolic energy. It may be two types;

1. Simple diffusion: It is a nonselective process by which any molecule able to dissolve in the phospholipid bilayer is able to cross the plasma membrane and equilibrate between inside and outside of the cell. That is the movement of the molecules occurs along the concentration gradient by the process of diffusion. Examples include; diffusion of gases such as carbon dioxide and oxygen are able to diffuse across the plasma membrane.
2. Facilitated diffusion: Facilitated diffusion, like the simple passive diffusion involves the movement of the molecules towards their concentration gradient that is from their higher concentration to their lower concentration. However, the passage of the molecules is mediated by a set of the transport proteins called Permeases and is highly selective in nature. Also, the rate of the transport of the molecules across the membrane is far greater in facilitated diffusion as compared to the simple diffusion. Examples include; diffusion of water within the cells occurs with the help of a set of permeases called; aquaporin. Facilitated diffusion also allows the polar and charged molecules such as carbohydrates, amino acids, nucleosides, and ions to cross the plasma membrane.

2. Active Transport: It occurs against the concentration gradient and is mediated by the carrier proteins. Metabolic energy is used to move the ions or molecules against the concentration gradient. It results in the accumulation of the solute on one side of the membrane. Active transport is further of two types;

1. Primary active transport: It is a direct active transport wherein the transport of the molecules is directly coupled to the metabolic source of energy such as ATP hydrolysis. Example includes the movement of the Sodium-potassium pump by ATP hydrolysis.
2. Secondary active transport: It is an indirect transport wherein the uphill transport of one solute is coupled to the downhill flow of a different solute that was originally pumped uphill by primary active transport. Example includes the movement of the Sodium ions and glucose.

Thus, these were the different types of the transport mechanisms mediated by the plasma membrane.