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Class 8 Science NCERT-Biology Lesson 4
Chapter 8: Cell Structure and Functions
What are cells? * We know that all living organisms carry out certain basic functions. Different sets of organs perform these functions. The basic structural unit of an organ is the cell * Cells may be compared to bricks. Bricks are assembled to make a building. Similarly, cells are assembled to make the body of every organism Discovery of the Cell: Robert Hooke in 1665 observed slices of cork under a simple magnifying device. Cork is a part of the bark of a tree. He took thin slices of cork and observed them under a microscope. He noticed partitioned boxes or compartments appearing like a honeycomb in the cork slice. He also noticed that one box was separated from the other by a wall or partition Hooke coined the term 'cell' for each box.
** Very little was known about the cell for the next 150 years after Robert Hooke's observations. Today, we know a lot about cell structure and its functions because of improved microscopes having high magnification. Basics of cell: * Both, bricks in a building and cells in the living organisms, are basic structural units. The buildings, though built of similar bricks, have different designs, shapes and sizes. Similarly, in the living world, organisms differ from one another but all are made up of cells. * Cells in the living organisms are complex living structures unlike non-living bricks. The egg of a hen represents a single cell and is big enough to be seen by the unaided eye. Otherwise, scientists observe and study the living cells using microscopes which magnify objects.
** There are millions of living organisms. They are of different shapes and sizes. Their organs also vary in shape, size and number of cells. The number of cells in an organism runs into billions and trillions. Human body has trillions of cells which vary in shapes and sizes. Different groups of cells perform a variety of functions Unicellular and multicellular organisms: Organisms made of more than one cell are called multicellular organisms. The number of cells being less in smaller organisms does not, in any way, affect the functioning of the 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.
* Organisms that are made up of a single cell are called unicellular organisms. A single-celled organism performs all the necessary functions that multicellular organisms perform A single-celled organism, like amoeba, captures and digests food, respires, excretes, grows and reproduces. ** Similar functions in multicellular organisms are carried out by groups of specialised cells forming different tissues. Tissues, in turn, form organs. Shape of Cells: ** Amoeba has no definite shape, unlike other organisms. It keeps on changing its shape. The projections of varying lengths protruding out of its body. These are called pseudopodia. * The change in shape of Amoeba is due to formation of pseudopodia which facilitates movement and help in capturing food. 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 Humans have different cells such as blood, muscle and nerve cells. The different shapes of these pecific functions. Generally, cells are round, spherical or elongated. Some cells are long and pointed at both ends. They cells are related to their s exhibit spindle shape Cells sometimes are quite long. Some are branched like the nerve cell or a neuron The nerve cell receives and transfers messages, thereby helping to control and coordinate the working of different parts of the body
Nucleus White blood cell (leucocyte) Cell Body Schwann cell Dendrite (Myelin Sheath) Axon Red blood cell (erythrocyte) Nodes of Ranvier Axon Terminals
* Components of the cell are enclosed in a membrane. This membrane 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. Bacterial cell also has a cell wall. ize of Cells $ Most of the cells are microscopic in size and are not visible to the unaided eve. 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 cells has no relation with the size of the body of the animal or plant. he size of the cell is related to its function. For example, nerve cells, both in the elephant and rat, are long and branched. They perform the same function, that of transferring messages