CELL — STRUCTURE AND FUNCTIONS
- All organisms are made of smaller parts called organs.
- Organs are made of still smaller parts. The smallest living part of an organism is a ‘cell’.
- Cell: The smallest structural and functional unit of an organism, which is typically microscopic and consists of cytoplasm and a nucleus enclosed in a membrane.
- Cells were first observed in cork by Robert Hooke in 1665.
Fig:Cork cells as observed by Robert Hooke
- Cells exhibit variety of shapes and sizes.
- Number of cells also varies from organism to organism.
- Some cells are big enough to be seen with the unaided eye.
Ex :Hen’s egg
- Some organisms are single-celled, while others contain large number of cells- multi- cellular.
- The single cell of unicellular organisms performs all the basic functions performed by a variety of cells in multicellular organisms.
- The cell has three main parts:
(i) the cell membrane,
(ii) cytoplasm which contains smaller components called organelles,
(iii) the nucleus.
- Cell membrane: The basic component of a cell. The cytoplasm and nucleus are enclosed within cell membrane. There is an outer thick layer in cells of plants called cell wall.
Cytoplasm: The jelly-like substance present between the cell membrane and the nucleus.
Various organelles present in the cytoplasm are:
(i) Mitochondria
(ii) Plastids (present only in plant cell)
(iii) Endoplasmic reticulum (ER)
(iv) Ribosome
(v) Lysosome
(vi) Vacuole
(vii) Golgi body
(viii) Centrosome (present only in animal cell).
Nucleus:
Nucleus is separated from cytoplasm by a nuclear membrane.
It is generally spherical in the centre of the cell.
Nuclear membrane:
Nucleus is separated from the cytoplasm by a membrane called the nuclear membrance.
Nucleolus:
- Nucleus contains a still smaller round body known as nucleoulus.
Vacuoles:
Are the empty or blank looking structures in the cytoplasm
Chromosomes:
- Nucleus contains thread-like structures called chromosomes.
These carry genes.
- Gene is a unit of inheritance in living organisms.
- It controls the transfer of a hereditary characteristic from parents to offspring.
prokaryotic cells.
Cells without well organised nucleus, i.e. lacking nuclear membrane, are called prokaryotic cells.
Size of cells:
- The size may be as small as a millionths of a metre or may be as large as a few centimetres.
- Size of the cells has no relation with the size of the body of the animal or plant.
- It is related to its function.
Comparison of Plant and Animal Cells
