CELL-STRUCTURE AND FUNCTION

CELL-STRUCTURE AND FUNCTION

*       Cell is the basic structural and functional unit of an organism.

DISCOVERY OF THE CELL

*       Robert Hooke in 1665 observed honey comb like boxes or partitioned compartments under simple magnifying device from the thin slices of cork. He coined the term ‘CELL’ for each compartment. Cells in the cork are seen as dead cells by him.

*       Improved microscope with high magnification power lead to discovery of cell- its structure and functions.

*       Many cells are microscopic and some cells like egg of hen, duck or ostrich are single celled which can be seen by naked eye (unaided eye).

ORGANISMS SHOW VARIETY IN CELL NUMBER, SHAPE, AND SIZE

*       Specialised cells make tissue, tissues form organ, Organs make an organ system and organ systems make an organism.

*       Each organ performs specific and specialised function.

NUMBER OF CELLS

*       Human body is made up of trillions of cells which very in shapes and sizes.

*       Different group of cells perform different functions.

*       Organisms may be unicellular made up of only one cell or multicellular having more than one cell.

*       Every organisms begins life from single cell i.e. fertilised egg (which multiplies and developed to form an organism).

*       Unicellular organisms can perform almost all life processes like digestion, growth, reproduction, excretion, respiration e.t.c.

SHAPE OF CELLS

*       Microbes are irregular  in shape. Example, Amoeba has varying projections/protrusions coming out of its body. These are called PSEUDOPODIA (Pseudo: False, Podia: feet). Pseudopodia appear and disappear as amoeba moves or feeds.

*       White blood cell (WBC) can change its shape.

*       Shape of cell determines the specific function.

*       Cells may be round, spherical, elongated, long, pointed at both ends, spindle shaped, branched.

*       Nerve cell/Neuron is a branched cell which receives and transfers messages, thereby helping to control and coordinate the working of different parts of the body.

*       Cell membrane and cell wall gives rigidity and shape. Cell wall is found in plants and bacteria not in animal cell.

SIZE OF CELLS

*       Most cells are microscopic and invisible to naked eye.

*       Smallest cell measures 0.1-0.5 micrometer. Example: Bacterium

*       Largest cell is egg of Ostrich egg measuring 170X130 mm.

*       Size of cell has no relation with size of the body of animal and function.

PARTS OF CELL

*       Basic components of cell are: Cell membrane, Cytoplasm and Nucleus.

*       Cell Membrane/Plasma membrane/Plasmalemma: Enclose cytoplasm and nucleus. It separates from one another and from surrounding medium. It is porous and allows the movement of substances or materials both inward and outward. Cell membrane gives shape to the cell and is surrounded by cell wall in bacteria and plant cells. Cell wall gives protection to cell against variations in temperature, high wind speed, atmospheric moisture, etc. Vacuole is found large in plant cell and little in animal cell. Plastids are found in plant cells of different colour. Green coloured plastids containing chlorophyll are chloroplasts. They provide green colour to plant leaves and helps in photosynthesis.

*       Cytoplasm: It is Jelly like substance present between the cell membrane and the nucleus. Various other organelles of cells like mitochondria, golgi bodies, ribosome, Vacuole, chloroplast (found only in plant cells and some prokaryotes), etc are present in it.

*       Nucleus: It is most important component found as spherically shaped at the centre of cell. Nuclear membrane surrounds nucleus and separates from cytoplasm. The membrane is porous allowing movement of materials from in and out of nucleus. Nucleolus is the spherical body inside nucleus composed of thread-like structures called chromosomes. These carry genes and helps in inheritance or transfer of characters from the parents to the offspring.  Role of nucleus is inheritance and controls all the activities of the cell.

*       Protoplasm is living content of cell composed of nucleus and cytoplasm.

*       GENE: It is unit of inheritance in living organisms and controls transfer of a hereditary characteristic from parents to offspring. Different genetic combination result in different characteristics.

*       Prokaryotes (Pro: primitive; Karyon: Nucleus) are organisms without well organized nucleus i.e don’t have nuclear membrane. Example; Bacteria and Blue green algae.

*       Eukaryotes (Eu: True; Karyon: Nucleus) are organisms with well organized nucleus with nuclear membrane. All organisms other than bacteria and blue green algae.

COMPARISON OF PLANT CELL AND ANIMAL CELL

PART/COMPONENT	PLANT CELL	ANIMAL CELL
Cell Membrane	present	Present
Cell Wall	Present	Absent
Nucleus	Present	Present
Nuclear Membrane	Present	Present
Cytoplasm	Present	Present
Plastids	present	Absent
Vacuole	Bigger vacuole present	Present in smaller size