THE CELL

The Cell is the basic structural and functional unit of all known living organisms.The branch of Biology ,which deals with the study of cell,is called Cytology.
Rpbert Hooke coined the term cell,when he saw honey comb-like structure in the section of cork.However ,he only discovered cell wall.The first living cell was discovered by Leeuwenhoek.A typical cell consist of protoplasm,i.e.,nucleus and cytoplasm.

Function of Cell

Cell wall provide shape and rigidity to the cell.Mitochondria is called the power house of a cell,because in it stepwise oxidation of fuel occurs,which results in release of chemical energy.This energy is stored in the form of ATP.
Plastids are present only in plant cells and are of three types - Chloroplasts,leucoplasts and chromoplasts.Chloroplast is the site of photosynthesis as it contains chlorophyll.Chloroplas is called the Kitchen of the cell.
Lysossomes are polymorphic organelles having hydrochloric enzymes .These enzymes function at acidic pH=5.These are sometimes called suicidal bags of the cell.
Nucleus was discovered by Robert Brown.It contains nucleoplasm,nucleolus and chromotin material.Chromatin is controlling centre of cell as it contains chromosomes.

Nucleic Acids

It contains the genetic instructions used in the development and functioning of all known living organisms.These are of two types DNA and RNA

Deoxiribo nucleic Acid(DNA)

It is a long polymer made from repeating units called nucleotides.Each nucleotide consist of a nucleoside and a phosphate group,joined together by ester bonds.It has four bases i.e.,adenine,guanine,cytosine and thymine.Adenine and guanine are the purine bases ; cytosine and thymine are pyramidine bases.

Ribo Nucleic Acid(RNA)

It is also made up of a long chain of nucleotides.Each nucleotides consists of a nitrogenous base, a ribose sugar and a phosphate group .Here thymine is replaced by Uracil(U)

Chromosome

Chromosome is thred-like structure,found in the nucleus.Bead like structure found on chromosome is called genes,which are made up of DNA.Number of pairs of chromosomes :
Human....23 (=46)
Dog....39 (=78)
Monkey....21 (=42)
Onion....8 (=16)

Types of Cell

This are two types

Prokaryotic Cells

This are primitive cell, lacking a nucleus and most of the other cell organelles e,g,,bacteria cell.

Eukaryotic cells

These have nucleus and membrane bound cell organelles.These are present in unicellular and multicellular plant and animal cells.

Differences between plant and animal cell

Plant cell.....Animal cell
It has cell wall (made from cellulose).....Cell wall is usually absent
Plastids are found.....Plastids are usually absent
A big vacuole is present.....Vacuole is absent or very small in size.

Cell division

It is the process,by which a cell increase in number.
It is of two types-Mitosis and Meiosis
1.Mitosis-It occurs in unicellular organism during a sexual reproduction .Each mitosis cell division results in the formation of two daughter cell having number of chromosomes equal to parent cell.
2.Meiosis-It occurs in reproductive division because of the presence of single set of chromosomes.It results in the formation of four daughter cells.Meiosis is a source of new genetic variation.