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Cell Membrane and Cell Wall

Cell Membrane

Cell membrane forms the main boundary of all cells. It controls the exchange between the cell and the external environment. Also called plasma membrane it is selectively or differentially a semi-permeable membrane regulating only certain ions or substance to pass through but not others.


It is elastic and pliable in some cells, quite rigid and unyielding in others. It is capable of limited repair if punctured by a needle and is also capable of growth as the cell enlarges.

Electron microscopic observations indicate that the cell membrane is a double-layered structure. Different models were proposed by different scientists but the most acceptable model is the fluid mosaic model given by S. Jonathan Singer and Garth Nicolson. Chemically the plasma membrane is composed of protein, bimolecular lipids and carbohydrates, which are arranged in a mosaic model in a fluid consistency.  There are two types of organelles, which serve as a covering for the cell, the cell membrane and the cell wall. All cells have a cell membrane, and certain cells also have a cell wall.


The main purpose of the cell membrane is to regulate the movement of materials into and out of the cell. For this purpose they have certain carrier molecules which bind with the molecules and transport them in a specific direction across the membrane. By doing so, the internal environment of the cell can be different from the external environment, since only certain materials can pass through. 

Cell membranes consist mainly of phospholipids. The phospholipids have two parts: a polar phosphate "head" and two nonpolar fatty acid "tails." These molecules are arranged in what is called a bilayer (a double layer) so that the polar heads face the external and the internal environments of the cell and the fatty acids form the inside of the membrane. The phospholipids are free to move around, often switching places with their neighbors. This allows the membrane to stretch and change shape.

Also contained within the membrane are large protein molecules. Some of them partially penetrate the lipid layer and stick out on either side of the membrane. These are called integral or intrinsic protein, while others are attached to only one side of the lipid layer.These are called extrinsic or peripheral protein.


Cell Wall

Cell wall is another structure that surrounds the cell membrane and is not found in all cells. Prokaryotes usually have a cell wall of some sort, and a type of eukaryotes called algae. They may also have a cell wall. The cell walls in each of these two types of organisms are different. All plant cells have cell wall.  

For prokaryotes, the cell wall usually contains large polymers called peptidoglycans. These molecules provide the strength for the bacterial cell wall. Some prokaryotic cell walls have two layers: an inner layer made of peptidoglycans and an outer layer composed of lipoproteins and lipopolysaccharides.

In eukaryotes, the cell wall has three main parts:
(i)The primary cell wall
(ii) The middle lamella and
(iii) The secondary cell wall. 

(i) The primary cell wall is located closest to the inside of the cell and it is the first formed cell wall. It is made mostly of cellulose, which allows the wall to stretch as the cell grows. Some primary cell walls also contain pectic polysaccharides and structural proteins. It is the outermost layer of the cell which is comparatively thin and permeable.

(ii) The middle lamella is composed of pectins, lignin, and some proteins. The cells of the plant tissues are generally cemented together with this middle lamella.

(iii) The secondary cell wall contains both cellulose and a strong material called lignin. Lignin strengthens the wall and gives the cell a somewhat rectangular shape. The secondary cell wall is thick and permeable.


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