Components of the cell:
The major components of the cell are; cell membrane, cytoplasm and nucleus.
Cell membrane (Plasma membrane):
Every cell is bound by a thin delicate membrane called cell membrane or plasma membrane. Plasma membrane is the outermost covering of the cell which separates the contents of a cell from its external environment. Structurally, plasma membrane is very flexible. The flexibility of plasma membrane helps the cell to engulf in (take in) food and other substances from its external environment. This process is known as endocytosis. Protozoans like Amoeba get its food through endocytosis. The plasma membrane is made of proteins and lipids and several models were proposed regarding the arrangement of proteins and lipids. The fluid mosaic model proposed by Singer and Nicholson (1972) is widely accepted and according to model; the plasma membrane consists primarily of a bilayer of phospholipid molecules into which a variety of globular proteins are embedded, each phospholipid molecule has two ends, an outer head hydrophilic (water attracting) and the inner tail pointing centrally hydrophobic (water repelling) and protein molecules arranged in two ways; peripheral proteins or extrinsic proteins (present on the outer and inner surfaces of lipid bilayer) and integral proteins or intrinsic proteins (penetrate lipid bilayer partially or wholly).
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.
Transport of small molecules (such as glucose, amino acids, water, mineral ions etc.):
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.
Transport of large molecules (bulk transport):
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.