 the basic structural unit of an organ, which is the cell.

 cells are assembled to make the body of every organism. Human body has trillions of cells which vary in shapes and sizes.





 Different groups of cells perform a variety of functions.

 A billion is a thousand million.



 A trillion is a thousand billion. Organisms made of more than one cell are called multicellular(multi: many; cellular : cell) organisms.



 The number of cells being less in smaller organisms does not, in any way, affect the functioning of the

organisms.

 You will be surprised to know that an organism with billions of cells begins life as a single cell which is the fertilized egg.



 The fertilised egg cell multiplies and the number of cells increase as development proceeds.

 The single-celled organisms 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.

 Amoeba has no definite shape, unlike other organisms.





 It keeps on changing its shape.

 Observe the projections of varying lengths protruding out of its body are called pseudopodia.



 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.

 Generally, cells are round, spherical or elongated.



 Some cells are long and pointed at both ends.

 They 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.

 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. 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.





 Each organ is further made up of smaller parts called tissues.

 A tissue is a group of similar cells performing a specific function.



 The basic components of a cell are cell membrane, cytoplasm and nucleus.

 The 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 jelly-like substance between the nucleus and the cell membrane is called cytoplasm.



 Cytoplasm is the jelly-like substance present between the cell membrane and the nucleus.

 Various other components, or organelles, of cells are present in the cytoplasm.



 These are mitochondria, golgi bodies, ribosomes, etc.

 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.



 With a microscope of higher magnification, we can see a smaller spherical body in the nucleus. It is 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.

 Gene is a unit of inheritance in living organisms.





 It controls the transfer of a hereditary characteristic from parents to offspring.

 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.



 The cells having nuclear material without nuclear membrane are termed prokaryotic cells.

 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).



blank-looking structures in the cytoplasm? It is called 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.



 several small coloured bodies in the cytoplasm of the cells of Tradescantia leaf.

 They are scattered in the cytoplasm of the leaf cells. These are called plastids.



 They are of different colours.

 Some of them contain green pigment called chlorophyll.





 Green coloured plastids are called chloroplasts. They provide green colour to the leaves. You may recall that chlorophyll in the chloroplasts of leaves, is essential for photosynthesis.