All cells are enclosed in a membrane that serves as their outer boundary, separating the cytoplasm from the external environment; this membrane is known as plasma membrane. It allows the cell to take up and retain certain substances while excluding others.
All biological membranes have the same basic molecular organisation. They consist of a double layer of (bilayer) of phospholipids interspersed with proteins. The phospholipids molecules in the plasma membrane are arranged in two parallel layers. Their non polar hydrophobic ends face each other, whereas their polar hydrophilic ends are associated with carbohydrate protein etc. Plasma membrane also contains several types of lipids like cholesterol. In certain animal cells cholesterol may constitute upto 50% of the lipid molecules in plasma membrane. It is absent from plasma membrane of most plat and bacteria cells.
In 1972 Singer and Nicolson proposed working model of plasma membrane known as fluid mosaic model. In this model the lipid bilayer is retained as the core of the membrane. These fluid molecules are present in a fluid state capable of rotating and moving laterally within the membrane. The structure and arrangement of membrane.
Proteins in the fluid mosaic model are like ice bergs in the sea. Proteins occur as a mosaic of discontinuous particles that penetrate deeply into and even completely through the lipid sheet. The components of the plasma membrane are mobile and capable of coming together to engage in various types of semi permanent interactions.
Proteins associated with lipid bi-layer can be divided into two large groups,
(a) Integral Proteins (Intrinsic Proteins):
A class of proteins that are directly incorporated within the lipid bi-layer. Some of these proteins are believed to provide a channel through which water soluble substances such as lions, can pass back and forth between the extra cellular and intracellular compartment.
(b) Peripheral Proteins (Extrinsic Proteins):
A class of protein located entirely outside of the lipid bi-layer or either the extra cellular or cytoplasmic surface, exhibit a loose association eith membrane surface. These proteins which may possess lipid (lipprotein) or carbohydrates (Glycoproteins and Protoglycon) side chains are arranged as mosics within cell membrane.
The plasma membrane performs several function but the main functions are Protection of cell cytoplasm and to regulate the flow of solution and material in and out of the cell with certain limitation. The limitations or check flow across the membrane is called differential or selective permeability. Transport across membranes is necessary to maintain suitable pH ionic concentration for enzyme activity and excrete toxic substances. Foe entry or exit there are two main processes, Passive transport i.e. diffusion and osmosis and active transport. The passive process do not require energy while active require energy with these, there are two other phenomenon i.e. endocytosis and exocytosis.
(i) DIFFUSION OR PASSIVE TRANSPORT
It occurs spontaneously and no extra energy is required to bring it about. A few substances freely diffuse across plasma membrane e.g. respiratory gases (O and CO2) diffuse in and out of the cells.
(ii) OSMOSIS
It maintains a balance between the osmotic pressure of the intracellular fluid and that of interstitial fluid known as osmoregulation.
(iii) ACTIVE TRANSPORT
Movement of molecules from lower concentration ot the higher concentration by consuming energy called active transport.
(iv) ENDOCYTOSIS
It is the process in which the cell membrane helps to take in material by infolding in the form of vacuole. Endocytosis may be (a) Phagocytosis in which solid particles are picked up and ingested by the cell e.g. W. B. C. picked up foreign bodies blood stream. In this way they destroy the harmful bacteria which enter into our body. It is also called cell eating process (b) Pinocytosis when liquid material in bulk in the form of vesicles is taken in by endocytosis, the process is called Pinocytosis which is also called drinking process.
(v) EXOCYTOSIS
The process of membrane fusion and the movement of material out of a cell is called endocytosis.
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