Plasma Membrane or Cell Membrane: Its structure and function

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Hi, it’s all about an important topic of cell; Plasma Membrane or Cell Membrane ,its structure and function.

In this article ,You will find about the information about

  • Definition
  • Discoverer
  • Components
  • Features
  • Structure
  • Function

of cell membrane.

Definition:

The Plasma Membrane ( also known as cell membrane or cytoplasmic membrane or plasmalemma) is a living, semi-permeable, elastic biological membrane ,located on the outside of the protoplasm of each living cell that separates the interior of all living cell from external environment.

Discoverer:

The term plasmalemma was coined by scientist J.Q.Plowe.

The cell membrane was discovered by Swiss botanist Carl Naegeli and C Cramer in 1855.

Components of Plasma Membrane:

Plasma membrane is composed of phospholipids,proteins and carbohydrates.

A double layer of phospholipids forms the main membrane skeleton. Proteins can be found sprinkled inside this layer or clinging to the outer faces. Carbohydrates molecule cling to integral proteins or phospholipids.

Features:

The main features of cell membrane are as below_

  • Present in all cell.
  • It is living and semi-permeable.
  • Made up of protein and lipid.
  • Takes part in osmosis and diffusion.
  • Cell organelles originates from cell membrane.

Structure of Plasma Membrane:

The mostly accepted model for the structure of plasma membrane is the fluid mosaic model. This model has been given the most importance in discussing the characteristics of plasma membrane.

Fluid Mosaic model of Plasma Membrane

It was proposed by Singer and Nicolson in 1972.

According to this model cell membrane is composed of phospholipid bilayer and the globular proteins are embedded in it.The plasma membrane actually forms a quasi fluid structure in which the proteins and lipids are arranged in a mosaic pattern.

They believe that location of lipid and protein result due to hydrophobic interaction. The protein molecules serves as enzyme, ion receptor and act as permeability barrier to the liquid layer. The main feature of this model is that all biomolecules are Quasi fluid in nature and proteins and lipids are arranged in mosaic pattern.

According to this model the phospholipid are arranged serially and globular proteins are embedded in it like icebergs in the sea. The globular proteins float in the the sea of phospholipid in the plasma membrane. According to Singer and Nicolson bonding between lipid and protein is hydrophobic and this hydrophobic bond is responsible for Quasi fluid nature of plasma membrane. The phospholipids and globular proteins are amphipathic molecule that is both hydrophobic and hydrophilic ends are present in the same molecule.

Membrane Proteins:

Membrane proteins are of two types: integral proteins remain embedded in the lipid bilayer up to various depth and peripheral proteins remain bound to the outer or inner surface of the lipid bilayer. Some of the integral proteins are transmembrane proteins passing across the lipid bilayer. The peripheral proteins remain associated with membrane by ionic interaction with the hydrophilic heads of the lipid bilayer. The integral proteins have two parts: the hydrophilic part remains close to the polar head groups of lipid, while the hydrophobic part remain buried in the lipid bilayer due to hydrophobic interaction with the hydrophobic tales of lipid.

Membrane proteins are mostly globular proteins having α helical organisation instead of being β pleated sheets.

Integral Proteins:

These proteins penetrate the lipid bilayer and have domain that protude from both extracellular and cytoplasmic side of the membrane. The segment of protein that pass through the lipid bilayer is called transmembrane segments. Proteins that possess one transmembrane segment and cross the membrane a single time are called bitopic protein. Polytropic proteins possess more than one transmembrane segment. Monotopic proteins are rare that remain embedded in the lipid bilayer.

Integral proteins are amphipathic molecule having both hydrophobic and hydrophilic end.

Peripheral Proteins:

These proteins are located entirely outside the lipid bilayer on either extracellular or cytoplasmic surface and are associated with membrane by non covalent bonds. They can be solubilized by extraction with aqueous solution of salt or alkaline pH.

Lipid-anchored Proteins:

These are located outside the lipid bilayer but are covalently linked to a lipid. They are of two types,one type present on the external face and another type on cytoplasmic face.

Membrane Carbohydrates:

Carbohydrates in the plasma membrane are present in the form of covalently linked molecules with proteins and lipids. They are of two types:

  • Glycoproteins
  • Glycolipids.

The common sugar associated with the proteins are D-glucose, D-galactose, D-mannose etc which are oligosaccharide complexes. Besides simple sugars ,sugar derivatives such as N-Acetyl-D- glucosamine and N-acetyl-D-Galactosamine are also present

Function of Cell Membrane:

1) Providing a selectively permeable barrier:

Plasma membrane prevents the free interchange of material from one side to other, and it ensures that the appropriate substances are allowed into the cytoplasm from the external space and inappropriate substances are kept out.

2) Transporting solutes

The plasma membrane contains the machinery for physically transporting substances from one side of the membrane to another. Transport across the membrane may be active or passive and occur via the phospholipid bilayer or by the help of specific integral membrane proteins, called permeases or transport proteins. Passive transport may be of three types of osmosis, simple diffusion and facilitated diffusion.

3) Transporting macromolecules:

Cells routinely import and export large molecules across the plasma membrane through different process like;

Exocytosis: Macromolecules such as proteins ,lipids or carbohydrates are secreted out from the cell by exocytosis.

Phagocytosis: The process of ingestion of large sized solid substances by the cells is known as phagocytosis.

Endocytosis: In endocytosis small regions of plasma membrane fold inwards until it has formed new intracellular membrane limited vesicles. In eukaryotes endocytosis is of two types: pinocytosis receptor mediated endocytosis.

4) Energy transduction:

Membranes are intimely involved in energy transduction, a process by which one type of energy is converted to other. For example, during photosynthesis energy in sunlight is absorbed by membrane bound pigments and converted into chemical energy content by carbohydrates.

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