CHAPTER - 8 CELL – STRUCTURE AND FUNCTIONS

1) Discovery of the cell In 1965 Robert Hooke observed slices of cork from the bark of a tree under a simple magnifying glass. He noticed small boxes or compartments in the cork slice. The boxes were separated from each other by a wall or partition. He named each box as a Cell. The cells which Hooke observed were actually dead cells of plants.

Cells are the basic structural units of living organisms. 2) The Cell Cells are the basic structural units of living organisms. Some organisms are made up of a single cell and some are made up of many cells. The number of cells in a tall tree or an animal like an elephant or human being are billions and trillions of cells. Organisms made up of many cells are called multicellular organisms. Eg: a tree, an elephant. Organisms made up of only one cell are called unicellular organisms. Eg: amoeba, paramaecium.

3) Shape of cells Cells are of different shapes. Some cells keep changing their shape. Eg : amoeba, white blood cells in human beings. Some cells are spherical, round, elongated, spindle shaped, or branched.

4) Size of cells Cells in living organisms are of different sizes. They may be as small as millionth of a meter (micrometre or micron) or large as a few centimetres. The smallest cell is the cell of bacteria (0.1 to 0.5 micrometre). The largest cell is the egg of an ostrich (17cm x 13cm).

Onion peel cells as seen under a microscope

Human cheek cells as seen under a microscope

Structure of a plant cell

Structure of an animal cell vacuole

Structure of an animal cell

5) Parts of a cell :- The main parts of a cell are cell membrane (plasma membrane), cytoplasm and nucleus. The cell membrane gives shape to the cell, protects the inner parts of the cell and allows movement of materials in and out of the cell. In plant cells the cell membrane is covered by another thick covering called cell wall. The cytoplasm is a jelly like substance between the cell membrane ad nucleus. It contains other components (organells) like mitochondria, golgi bodies, ribosomes, vacuoles etc. The nucleus is a spherical body generally found in the centre of the cell. It has a membrane called nuclear membrane. It has a smaller spherical body called nucleolus and thread like structures called chromosomes. The chromosomes carry genes which transfer characters from the parents to the off springs. Vacuoles are blank looking structures in the cytoplasm. Vacuoles are large in size in plant cells and small in size in animal cells. Plant cells have several small coloured bodies in the cytoplasm called plastids. The green coloured plastids are called chloroplasts. They contain chlorophyll which helps in photosynthesis.

Structure of an animal cell and a plant cell Vacoule

Comparison of Plant cell and Animal cell

Comparison of Plant cell and Animal cell Sl.No. Part Plant cell Animal cell 1 Cell membrane Present 2 Cell wall Absent 3 Nucleus 4 Nuclear membrane 5 Cytoplasm 6 Plastids 7 Vacuole (smaller in size (larger in size)