Function: Plasma membrane bounds the semi-fluid content of the cells, protects the cell from injury and provides an outer boundary to the cell. Plasma membrane has carrier proteins for active transport, plasma membrane is selectively permeable; therefore it allows or permits the entry and exit of only selected substances. It also prevents the movements of some other substances across it.
Small molecules can be transported across the plasma membrane by any one of the following three methods: (i) Diffusion: movement of particles from an area of higher concentration to an area of lower concentration. This does not require energy. Example: absorption of glucose in a cell. (ii) Osmosis: Osmosis is a form of passive transport that’s similar to diffusion and involves a solvent moving through a selectively permeable or semipermeable membrane from an area of higher concentration to an area of lower concentration. (iii) Active transport: Active transport occurs across a semipermeable membrane against the normal concentration gradient, moving from the area of lower concentration to the area of higher concentration and requiring an expenditure of energy released from an ATP molecule.
During bulk transport the membrane changes its form and shape. It occurs in two ways; (i) endocytosis (taking the substance in), (ii) exocytosis (passing the substance out). Endocytosis is of two types; Phagocytosis and Pinocytosis.
Intake of solid particles.
Intake of fluid droplets.
Membrane folds outgoing round the particle, forming a cavity and thus engulfing the particle.
Membrane folds in and forms a cup like structure sucks in the droplets.
In addition to plasma membrane, plant cells contain a cell wall also. It is an outer, rigid, protective, supportive and semi-transparent covering of plant cells only. The cell wall lies outside the plasma membrane. Its thickness varies in different types of cells. The cell wall is mainly composed of cellulose. Cellulose is a complex substance and provides mechanical and structural strength to plants. The substance constituting the cell is not simply homogenous but it consists of fine threads or fibres called microfibrils. It may be thin and transparent as in the cells of onion peel.
Function: Provide a definite shape to the cell, protects plasma membrane and internal structures from the attack of pathogens and mechanical injury, counteracts the osmotic pressure, provides rigidly to the cell, there are breaks in the primary wall of the adjacent cells through which cytoplasm of one cell remains connected with the other. These cytoplasmic strands which connect one cell to the other one are known as plasmodesmata and Walls of two adjacent cells are firmly joined by a cementing material called middle lamella made of calcium pectate.