Cell biology is the study of cell with respect to its physiology, biochemistry and genetics. This became possible after the invention of ultramicroscope. Although they differ in many respects, plant and animal cells almost share a common morphology. A typical eucaryotic cell is very complex, consisting of living protoplasm enclosed by a plasmamembrane. A plant cell further contains a rigid, porous cellulosic cell wall outside the membrane. The protoplasm contains a central nucleus and the outer cytoplasm inside which floats various cell organelles responsible for specific functions.
Presence of cell wall makes plant cells unique. Formed during cell division it provides mechanical support to the cell. Matured cell walls show depositions of cutin, lignin, etc. which provides additional strength.
Plasmamembrane is a continuous lipid bilayer intercalated with extrinsic and intrinsic proteins. Its primary function is to exercise selective control over the passage of molecules in and out of the cell.
Plastids are large cytoplasmic organelles found in plants. Chloroplasts are green, chromoplasts are coloured and leucoplasts are colourless. Chloroplast Contains chlorophyll and is bounded by a double membrane. The internal matrix called stroma bears additional stacked membranous thylakoids which is the aggregation of a number of grana. The green chlorophyll pigments responsible for photosynthesis are found in thylakoid membrane.
Mitochondria is the site of energy production. Enclosed by a double membrane, these are called ‘power house’ of cells. The inner membrane forms many inward foldings called cristae which penetrate into the matrix. A number of protein particles present on inner membrane and cristae contain the enzymes of electron transport system. Mitochondrion serves as the site of aerobic respiration.
Endoplasmic reticulum (ER) remains inside cytoplasm as an interconnecting membrane system consisting of cisternae, tubules and vesicles. Rough ER contains ribosomes attached to its membrane and helps in synthesis of secretory and membrane proteins. Smooth ER lacks ribosomes and synthesise lipids and sterols.
Ribosomes are found freely or as polysomes in the cytoplasm or remain on membrane of rough ER. They contain r-RNA and proteins and are sites of protein synthesis in cytoplasm, mitochondria and chloroplast. Golgi complex constitutes stacks of membranous cisternae associated with tubules and vesicles. Secretion, package and transport of materials are its main functions.
Lysosomes are membranous vesicles which store acid hydrolase and are responsible for digestion of substances within the cell. They are called ‘suicide bag’ of the cell.
Micro-bodies like peroxisome and glyoxysome are membranous vesicles contain peroxide-producing and peroxide-destroying enzymes. They are protective in function.
Centrosome contains a pair of cylindrical bodies present at one pole of interphase nucleus. Being the characteristic of animal cells, they contain 9 sets of microtubule triplets. They make spindle fibres during cell division and basal bodies associated with cilia and flagella.
Cytoskeletal structures like microtubules and microfilaments form the structural framework of the cell. These proteinous bodies function in the movement of cilia, flagella and microvilli. Cilia and flagella are the organ of locomotion. They bring about movement of free cell in the surrounding medium.
Vacuoles are fluid-filled vesicles found in cytoplasm of plant cells. It maintains cell turgidity.
Nucleus is one of the principal organelles enclosed by a double walled nuclear envelope with many octagonal pores. The interior contains nucleoplasm with one or more nucleoli and the chromatin. Chromatin is the genetic material which condenses to definite chromosomes during cell division. The nucleus controls all cellular activities. And the chromosomes are the bearers of genes.
Ergastic substances include carbohydrates like starch grains, glycogen and inulin granules; nitrogenous sub stances like aleurone grains, and fat and oils. Alkaloids, gums, glucosides, latex, etc. are also secreted by the cytoplasm. Mineral crystals like raphides and cystoliths are very often found in certain cells.