Cell was first discovered by Rober Hook in 1665. Cell is the structural and functional unit of a living thing. All living things are made of microscopic units i.e. cells.

This theory was put forwarded by Schwann (1838 to 1839) and later on by Virchow (1858). According to this theory:

1. All living thing organisms are made up of cells, these cells are the structural and functional unit of life.
2. Cells are produced spontaneously by their own (De nove). Later they are produced by Cell Division.
3. Each cell contains certain "Hereditary material" which is responsible for the passage of characters from one generation to the next.

Basically cells are of two types namely:

1. Prokaryotes
2. Eukaryotes

These cells also have got other membrane-bounded organelles like mitochondria, colgibodies etc. Examples of Eukaryotes are Algae, Fungi and other plants and animals.

The basic structure of plant and animal cell is almost the same except certain differences. The basic structure of a cell is composed of the following components.

1. Cell Membrane
2. Cytoplasm
3. Nucleus

However in plants, a rigid "Cell wall" is present outside the cell membrane or plasma membrane.

It is present in Bacteria and plant cells. The cell wall lies outside the cell membrane.

Chemically, it is made up of dead substances like cellulose and Pectine secreted by the protoplasm. Sometimes another substance Lignin is also present.

Functionally, cell wall serves to protect the protoplasm and to give a definite and rigid shape to the cell.

Structurally, cell wall is many-layered thick. Finally, Middle lamella is laid down between the two cells; Secondly, Primary cell wall is deposited between Middle lamella and the cell membranes of both the cells. Finally, Secondary layer of cell wall is deposited on the inner surface of Primary Cell Wall. The fibres of each layer are laid at different angles to provide maximum tensile strength to the cell wall.

Cell wall is also has got pores and is permeable in nature.

It is also called Plasma membrane. In animal cell, it is the outermost boundary whereas in plant cell, it lines the cell wall internally. It surrounds the cytoplasm in both types of cells.

Chemically, it is made up of Proteins and Lipids. Structurally, it is made up of three distinct layers, an outer and inner layer of proteins and a sandwiched layer of lipid. This three-layered structure was proposed by Dawson and Donelli.

The sandwiched layer of lipid is bimolecular whereas the protein layers are monomolecular. Lipid layer forms the gross structure of membrane. Lipids of membrane are of four types namely Phospholipid, Glycolipid, Cholesterol and Sterol. Out of these, phospholipid is the most important structural component. Each molecule of phospholipid has got two ends i.e.

1. Water soluble Hydrophilic end or Polar end.
2. Water insoluble Hydorphobic end or Non Polar end.

Hydrophilic end is made up of Phosphate while hydrophobic end is composed of fatty acid joined with the Glycolipid. The two layers of hydrophobic ends face each other while phosphate ends faces proteins layers. Cholesterol serves to join the phosphate molecules.

It was observed that Protein molecules are located in the outer and inner surface and are called "Extrinsic Molecules" while other penetrates the membrane called "Intrinsic Molecules". So it can be said that bimolecular layer of lipid serves as a sea in which protein molecules float as icebergs. This hypothesis was suggested by Singer and Nicholson in 1970 who named it "Fluid Mosaic Bilayer Model."

• Cell membrane serves to protect living cytoplasm and its organelles.

• Cell membrane also regulates passage of different substances to and from the cell, this property is called permeability.

The living ground material of the cell filling the space between the cell membrane and nucleus is called cytoplasm. It is fluid to semi solid in consistency. Living cytoplasm shows streaming movement along cell membrane called Cyclosis. To perform different functions, cytoplasm contains different kinds of organelles. These organelles are as follows;

1. Endoplasmic Reticulum
2. Ribosomes
3. Microtubules
4. Mitochondria
5. Colgibodies
6. Centrosomes
7. Plastids
8. Lysosomes
9. Vacuoles

It is a complex network of channels extending from cell membrane to nuclear membrane. These channels are made up of lipoproteins.

Endoplasmic reticulum is of two types:

1. Smooth Endoplasmic Reticulum
2. Rough Endoplasmic Reticulum

Smooth endoplasmic reticulum is not coated with ribosomes and is generally found in Adipose tissue, muscle cells and glycogen storage cells. Generally both types of Endoplasmic Reticulum are present in most of the cells.

Rough or Granular Endoplasmic Reticulum is coated with ribosomes and is generally found in protein synthesising cells such as Pancreatic, hepatic and other glandular cells.

Endoplasmic reticulum forms the structural frame work of the cell, it provides space for the attachement of Ribosomes and increases the surface area to enhance different chemical reactions by means of enzymes attached with the membranes of their channels. The channels serve as pathway for the transport of various substances from one place or organelle to another.

It is a characteristic cytoplasmic organelle of plants, totally absent in animals. It is also a membrane-bounded organelle serving as a chemical synthesizer, photo-battery and storage body.

Each plastid is a plate like structure or a spindle shape body containing different pigments, Plastids are classified according to their pigment into three types, i.e.

1. Chloroplast
2. Leucoplast
3. Chromoplast

These are coloured plastids found in coloured parts of plant such as flower, fruits and certain leaves. Different pigments present in chromoplasts are b Carotene (orange), Xanthophyll (Yello), Anthocyanin (blue) etc.

Vacuoles are bubble like cavities filled with different substances surrounded by "Tonoplasts". Vacuoles are large and conspicuous in plant cells filled with water, reserved food, enzymes and minerals.

In animal cells, especially lower animals or Protozoans vacuoles are of two types i.e.

1. Food vacuoles which store food substances.
2. Contractile Vacuole serve to store and excrete waste products.

Robert Brown in 1831 discovered a central tiny ball or spherical structure in the centre of cell, he named it nucleus.

Later on it was discovered that nucleus is the most important part of the cell and controls all its vital activities and division i.e. Reproduction.

Nucleus is generally single, circular and bounded with a membrane called "Nuclear Membrane". It has got pores called "Nucleopore" through which the cytplasm communicates with the ground substance of nucleus called "Nucleoplasm" or "Karyolymph". It is made up of proteins, in this dense liquid substances are scattered a number of delicate thread like structures called "Chromatin Network". This mesh of chromatin is made up of DNA and contains the hereditary information.

The DNA strands or Chromatin filaments condense to form assorted thicker threads called chromosomes during cell division.

Each specie contains a fixed number or definite number of chromosomes generally denoted as "2n" and called diploid number, eg 46 in humans, 80 in sugar cane.

Within the nucleus one or two spherical bodies are also found known as "Nucleolus". Each nucleolus contains RNA and serves to produce Ribosomes.