Cell structure and functions- BASICS

CELL STRUCTURE AND FUNCTIONSThis includesDiscovery of cellCellNumber of cellShape of cellSize of cellCell structure and functionsCell membraneCytoplasmNucleusDISCOVERY OF THE CELLRobert Hooke in 1665 observed slices of cork under a simple magnifying device.Cork is a part of the bark of a tree.He observed partitioned boxes or compartments in the cork slice.These boxes appeared like a honeycomb.He also noticed that one box was separated from the other by a wall orpartition. Hooke coined the term ‘cell’ for each box.CELLCells are the basic structural units of living organisms.Organisms differ from one another but all are made up of cells. Cells in the living organisms are complex living structures.Cells can be observed only under microscope.Stains (dyes) are used to colour parts of the cell to study the detailed structure.NUMBER OF CELLSHuman body have trillions of cells which vary in shapes and sizes.Different groups of cells perform a variety of functions.Organisms made of more than one cell are called multicellular (multi :many; cellular : cell) organisms.An organism with billions of cells begins life as a single cell which is the fertilised egg. The fertilised egg cell multiplies and the number of cells increase as development proceeds.The single-celled organisms are called unicellular (uni : one; cellular : cell) organisms. A single-celled organism performs all the necessary functions that multicellular organisms perform.Amoeba and Paramecium are single celled organisms.SHAPE OF CELLSAmoeba has no definite shape, unlike other organisms.It keeps on changing its shape.It has projections of varying lengths protruding out of its body.These are called pseudopodia (pseudo: false;podia : feet ).These projections appear and disappear as Amoeba moves or feeds.A white blood cell (WBC) in human blood is another example of a single cell which can change its shape. But while WBC is a cell, amoeba is a full fledged organism capable of independent existence.The different shapes of cells are related to their specific functions.Generally cells are round, spherical or elongated. Some cells arelong and pointed at both ends. They exhibit spindle shape. Some cellsare quite long. Some are branched like the nerve cell or a neuron.Components of the cell are enclosed in a membrane. Thismembrane provides shape to the cells of plants and animals.Cell wall is an additional covering over the cell membrane in plant cells. It gives shape and rigidity to these cells.SIZE OF CELLMost of the cells are microscopic in size and are not visible to the unaided eye. They need to be enlarged or magnified by a microscope.The smallest cell is 0.1 to 0.5 micrometre in bacteria. The largest cell measuring 170 mm ×130 mm, is the egg of an ostrich.The size of the cell is related to its function.CELL STRUCTURE AND FUNCTIONPART OF THE CELLThe basic components of a cell are cell membrane, cytoplasm and nucleus.CELL MEMBRANEThe cytoplasm and nucleus are enclosed within the cell membrane,also called the plasma membrane.The membrane separates cells from one another and also the cell from the surrounding medium.The plasma membrane is porous and allows the movement of substances or materials both inward and outward.The cell membrane gives shape to the cell. In addition to the cell membrane, there is an outer thick layer in cells of plants called cell wall. This additional layer surrounding the cell membrane is required by the plants for protection.Plant cells need protection against variations in temperature, high wind speed, atmospheric moisture, etc. They are exposed to these variations because they cannot move.CYTOPLASMIt is a jelly-like substance present between the cell membrane and thenucleus.It also has other components,or Organelles present inside them.These are mitochondria, golgi bodies, ribosomes, etc.NUCLEUSIt is generally spherical and located in the centre of the cell.Nucleus is separated from the cytoplasm by a membrane called the nuclear membrane.This membrane is also porous and allows the movement of materials between the cytoplasm and the inside of the nucleus.Nucleus has a Smaller spherical body inside it called the nucleolus. In addition, nucleus contains thread-like structures called chromosomes. These carry genes and help in inheritance or transfer of characters from the parents to the offspring. The chromosomes can be seen only when the cell divides.Nucleus, in addition to its role in inheritance, acts as control centre of the activities of the cell. The entire content of a living cell is known as protoplasm. It includes the cytoplasm and the nucleus. Protoplasm is called the living substance of the cell.VACOULECytoplasm has a blank-looking structure inside it, called as vacuole. It could be single and big as in an onion cell.Cheek cells have smaller vacuoles. Large vacuoles are common in plant cells. Vacuoles in animal cells are much smaller.PLASTIDSPlastids are several small coloured bodies in the cytoplasm. They arescattered in the cytoplasm of the leaf cells. They are of different colours. Some of them contain green pigment called chlorophyll.Green coloured plastids are called chloroplasts. They provide greencolour to the leaves.PROKARYOTES AND EUKARYOTESThe nucleus of the bacterial cell is not well organised like the cells ofmulticellular organisms. There is n nuclear membrane. The cells havingnuclear material without nuclear membrane are termed prokaryoticcells. The organisms with these kinds of cells are called prokaryotes (pro :primitive; karyon : nucleus). Examples are bacteria and blue green algae. The cells, like onion cells and cheek cells having well organised nucleus with a nuclear membrane are designated as eukaryotic cells. All organisms other than bacteria and blue green algae are called eukaryotes. (eu :true; karyon: nucleus).