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Introduction to Cells

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Introduction to Cells - Lesson Summary

Cell is the structural and functional unit of all living organisms. 

A microscope is an instrument used to see objects too small for the naked eye. An English scientist, Robert Hooke was the first person to discover the existence of cells with the help of a microscope in 1665.


Types of microscopes
       •  Hand lens is a biconvex glass fixed in a frame with a handle. It is used to magnify small objects up to certain size.
       •  Compound microscope is the microscope designed to magnify objects that can be brought close to the device. It uses a pair of lenses to magnify the objects not visible to naked eye. Magnification is at about 50X, 100X, 200X etc. These microscopes magnify the specimens to about 1000 times more than their size.
       •  Scanning electron microscope (SEM) is used to examine the external parts of various organisms. It has a magnification range from 15X to 200,0000X. This type of microscope uses electrons instead of light. A beam of electrons interact with the sample and produce an image of the sample on a photographic plate.
       •  The transmission electron microscope (TEM) is used to view the internal structure of a cell and its organelles. TEM uses a beam of electrons instead of light. This microscope has special lenses called as electromagnetic lenses. Objects of the order of few angstrom are made visible by this microscope.


Types of organisms
Living organisms are broadly classified into unicellular and multicellular organisms based on the number of cells they possess. 

Unicellular organisms: Organisms that are made up of a single cell and perform all their vital activities are called unicellular organisms. A single cell performs all the metabolic functions like nutrition, respiration, excretion, reproduction etc.
       •  Amoeba is an example of unicellular organism performing different activities. It acquires its food by special structures called as pseudopodia. They use pseudopodia for locomotion also. It gives rise to new individual by binary fission.
       •  Paramecium is a single cell, built in such a way that it performs all its vital activities, like reproduction, locomotion, digestion, and so on.

Multicellular organisms: Organisms that are made up of more than one cell are called multicellular organisms.
       •  Multicellular organisms are made up of different cells of different shapes and different functions.
       •  Most life that can be seen with the naked eye is multi-cellular, as are all animals and plants.
       •  There is a division of labour among the cells of a multicellular organism.
       •  Complex level of organisation is observed in multicellular organisms.
       •  Human being is a multicellular organism made up of many cells. 

UNICELLULAR ORGANISMS MULTICELLULAR ORGANISMS Entire organism is made up of single cell. Many cells constitute the entire body. Life processes are carried out by single cell. Division of labour is observed in the organism. Entire cell is in direct contact with the environment. Only the cells lying in the outer layer are in contact with the environment. Organism has a short span of life. Life span is long due to complexity in the structure. Cell is capable of division. Some cells lose their capacity to divide. Injury to the cell may cause death of entire organism. Injury to some cells do not lead to death of the organism.

Shape of cells
Cells differ in their shape. Shape of the cells is related to the function they perform. Cells can be round, spherical, elongated, pointed, long and even branched.


       •  Amoeba is irregular in shape.
       •  Neuron, the nerve cell is a branched structure.
       •  White Blood Corpuscle is the only animal cell that changes its shape.
       •  Red Blood Corpuscles are  round  and flattened.
       •  Muscle cells are spindle-shaped.
       •  Plant cell has definite shape.

Size of cells
The size of the cell has no relation with the size of the organism.
       •  The single cell which can be seen with a naked eye is hen’s egg.
       •  The largest cell is the egg of an ostrich measuring 170 millimetres by 130 millimetres.
       •  The smallest cell is a bacterial cell measuring 0.1 to 0.5 micro-metres.

The size of the cell is related to the function it performs.
       •  Xylem cells of a plant are responsible to carry water molecules from the roots to the apical tip of the plant.
       •  Nerve cells in an animal are very long so as to carry impulses to long distances in the body.


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