Cell membrane and it's structure and functions

Membrane of cell

1. by Dr.absar ullah khan Cell membrane

2. The cell membrane is a protective sheath, enveloping the cell body. It is also known as plasma membrane. Cell membrane

3. Cell membrane This membrane separate the fluid out side the cell called extracellular fluid(ECF) and the fluid inside the cell called intracellular fluid(ICF). The cell membrane is a semi permeable membrane. So, there is free exchange of certain substances between the ECF and ICF .

4. COMPOSITION OF CELL MEMBRANE The cell membrane is composed of three types of substances; 1.proteins(55%) 2.lipids(40%) 3.carbohydrates(5%)

5. Structural model of the cell membrane Three models explained the structure of cell membrane 1.Danielli-davson model 2.unit membrane model 3.The fluid mosaic model

6. The fluid mosaic model Later in 1972, SJsinger and GL Nicholoson proposed THE FLUID MOSAIC MODEL. According to them the membrane is a fluid with mosaic of proteins(mosaic means pattern formed by arrangement of different colored pieces of stone,tile,glass or other such materials). This model is accepted by the scientist till now. In this model, the proteins are found to float in the lipid layer instead of forming the layers of sandwich type model.

7. Cell membrane

8. Lipid layer of the cell membrane The central lipid layer is a bilayered structure. This is formed by a thin films of lipids. The characteristic feature of lipid layer is that, it is fluid in nature and not a solid structure. So, the portions of the membrane moves from one point to another point along the surface of the cell. the material dissolved in lipid layer also move to all the areas of the cell membrane. The major lipids are; 1.phospholids 2.cholesterol

9. 1.phospholipids The phospholipids are the lipid substances containing phosphorus and fatty acids. The phospholipids of the lipid layer are amino phospholipids, Sphingomyelins, phosphotidyl choline, phosphotidyl etholamine,phosphotidyl glycerol and phosphotidyl inositol.

10. phospholipids The phospholipids molecules are arranged in two layers . Each phospholipid molecule resemble the headed pin in shape. The outer part of the phospholipids molecule is called head portion and the inner portion is called tail portion. The head portion is the polar end and it is soluble in water and has strong affinity for water(hydrophilic). The tail portion is nonpolar end. It is insoluble in water and repelled by water(hydrophobic)

11. 2.cholesterol The cholesterol molecule are arranged in between the phospholipid molecule. The phospholipids are soft and oily structure, and cholesterol helps to “pack” the phospholipids in the membrane. So, cholesterol is responsible for structural integrity of lipid layer of the cell membrane

12. Function of the lipid layer The lipid layer of the cell membrane forms a semi permeable membrane. It allows only the fat soluble substances to pass through it. thus only the substances like oxygen , carbon dioxide and alcohol can pass through the lipid layer. And, this layer forms a barrier to the water soluble materials such as glucose, urea and electrolytes.

13. Proteins layer of the cell membrane The proteins layers gives the protection to the central lipid layer. The proteins present in these layers are mostly the glycoprotein. These proteins molecules are classified into two categories: 1.integral proteins Peripheral proteins

14. 1.Integral proteins The integral proteins are the proteins that pass through the entire thickness of the cell membrane from one side to the other side. These proteins are tightly bound with the cell membrane. Examples of integral proteins; Cell adhesions proteins channel proteins some hormone receptors, antigens and some enzymes.

15. 2.Peripheral proteins The peripheral proteins are the proteins which are partially embedded in the outer and inner surfaces of the cell membrane and do not penetrate the cell membrane. The peripheral proteins are loosely bound with the integral proteins of the cell membrane. So, these proteins molecule dissociate readily from the cell membrane. Examples Proteins of cytoskeleton, some carrier proteins and some enzymes.

16. Function of proteins in cell membrane 1.Integral proteins Integral proteins molecules provide the structural integrity of the cell membrane. 2.Channels proteins Some integral proteins molecules function as channels for the diffusion of water soluble substances like glucose and electrolytes. So, these proteins are called channel proteins 3.pumps. Some carrier proteins acts as pumps by which ions are transported actively across the cell membrane.

17. Functions of proteins in cell membrane 4.Carrier proteins Some proteins molecules help in in the transport of substances across the cell membrane by means of active or passive transport. These proteins are called carrier proteins. 5.Receptor proteins Some proteins molecule serve as the receptor sites for hormones and neurotransmitter such proteins are known as receptor proteins. 6.Enzymes Some of the proteins molecules forms the enzymes which control chemical reactions within the cell membrane.

18. Functions of proteins in cell membrane 7.Antigens Some proteins acts as antigen and induce and induce the process of antibody formation. 8.cell adhesion Some integral proteins are responsible for attachment of cells for their neighbors .

19. Carbohydrates of the Cell Membrane Some of the carbohydrate molecules present in cell membrane are attached to proteins and form glycoproteins (proteoglycans). Some carbohydrate molecules are attached to lipids and form glycolipids. Carbohydrate molecules form a thin and loose covering over the entire surface of the cell membrane called glycocalyx .

20. FUNCTIONS OF CELL MEMBRANE 1. Protective function : Cell membrane protects the cytoplasm and the organelles present in the cytoplasm. 2.Selective permeability: Cell membrane acts as a semipermeable membrane, which allows only some substances to pass through it and acts as a barrier for other substance

21. 3.Absorptive function : Nutrients are absorbed into the cell through the cell membrane. 4.Excretory function : Metabolites and other waste products from the cell are excreted out through the cell membrane. 5.Exchange of gases : Oxygen enters the cell from the blood and carbon dioxide leaves the cell and enters the blood through the cell membrane. 6.Maintenance of shape and size of the cell : Cell mem-brane is responsible for the maintenance of shape and size of the cell.