Cell surface is bounded by very thin osmotically active membrane called cell membrane or plasma lemma or plasma membrane. It is thin, elastic, porous and semi permeable in nature.
About the structure of plasma membrane different theories were proposed by some workers. To describe the structure of cell membrane, three structural models have been described which are as follows:
(1) Bi-molecular lipid layer:
Gorter and Grendel in 1925 suggested that a membrane is made up of two layers of lipid molecules. Hydrophobic ends of each molecule point towards the interior while hydrophilic ends point towards the outer boundary of the membrane.
(2) Trilaminar structure:
In 1935 Danieth and Davson proposed trilaminar nature of membrane which was further refined in 1938 by Harvey
(3) Fluid Mosaic Model:
In 1972 S. Jonathan Singer and Garth Nicloson developed fluid mosaic model of membrane structure. According to this model, a membrane is double layer of bi-layer of proteins and phospholipids and is fluid rather than solid. Phospholipids layer forms an ocean or sea in which specific proteins float like ice bergs. Being fluid the membrane is in constant state of flux shifting and changing, while retaining its uniform structure.
This word mosaic refers to many different kinds of proteins dispersed in phospholipids bi-layer. Important points are:
(1) Phospholipids have one polar end and one non-polar end. Polar ends are oriented on one side towards outside of cell and into the fluid cytoplasm on other side, and the non-polar ends face each other in middle of the layer. Taits of both layers of phospholipids molecules attrat each other and are repelled by water and as such are hydrophobic or water dreading. As a result the polar spherical leads the phosphate protein is located over the cell surfaces both the outer and inner and are hydrophobic or water attracting.
(2) Cholesterol is present in plasma membrane and organelle membranes of eukaryotic cells. Cholesterol molecules are embedded in interior of membrane and help to make the membrane less permeable to water soluble substances. Rigid structures of cholesterol molecules helps to stabilize the membrane.
(3) Membrane proteins are individual molecules attached to inner or outer membrane surfaces and termed as peripheral proteins or extrinsic proteins. Some are embedded within lipid bi-layer of membrane and is termed as intrinsic proteins. Some of them are linked to sugar protein makers on cell surface. Other intrinsic protein help to move ions or molecules across the membrane to cells inner part the cytoskeleton or to various molecule outside the cell.
(4) When carbohydrates unit with proteins they form glycoproteins and when they unite with lipids they form glycolipids on the surface of plasma membrane. Surface carbohydrates and portions of proteins and lipids make up glycocalyx or cell coat.
Function of plasma membrane:
(1) Regulation of material through cell membrane: Plasma membrane is differentially permeable membrane. It allows some selective molecules to pass through it. Different molecules may move from outside the cell to inside the cell or from inside to outside the cell.
(2) Transport material: Oil or fat soluble molecules enter through the lipid portion of membrane where as the water soluble molecules pass through the proteins.
(3) Cellular homeostasis: Plasma membrane help in homeostasis by regulating movement of water molecules and ions.
(4) Mechanical support: Plasma membrane provides mechanical support due to its flexible nature and helps in maintaining specific shape of the cell.
(5) Acts as barrier: It forms the boundary of the cell and separates the inside of the cell from outside. It also separates various organelles within the cell.
(6) Provide receptor site for specific chemicals: It provides large surface area on which specific chemical reactions can occur. Some proteins act as receptor sites, which bind specific chemicals. This binding helps the cell to begin specific function such as synthesis of a hormone.
(7) Cell identity makers: Cell identity makers are the sites proteins in nature that differentiate one type of cell form the other.
Movement across membrane: Molecules can cross membranes in number of ways both by using their energy and by relying on outside energy source. Various ways are diffusion & osmosis. Transport across membranes is necessary to maintain suitable pH, ionic concentration for enzymes activity and excrete toxic substances. For entry or exit, there are two main processes passive transport i.e. diffusion and osmosis and active transport. The passive processes do not require energy which 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. Few substances freely diffuse across plasma membrane e.g. the respiratory gases (O2 and CO2) diffuse in and out of cell.
(ii) Osmosis: It maintains a balance between osmotic pressure of the intracellular fluid and that of interstitial fluid known as osmoregulation.
(iii) Active transport: Movement of molecules from lower concentration to higher concentration by consuming energy called active transport.
(iv) Endocytosis: It is the process in which 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 and ingested by the cell e.g. W. B. C. picked up foreign bodies from the blood stream. In this way they destroy harmful bacteria. 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 cell drinking process.
(v) Exocytosis: The process of membrane fusion and the movement of material out of a cell is called exocytosis.
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