Cells can be divided into two types based on the structure of the organism.

1. Plant cell: All the cells that form the plant body are called plant cells. Plant cells contain cell wall and plastids. It has fewer mitochondria and usually no centrioles. Their stored food is starch and fat. It lacks microvilli and pinocytic cavities. It contains small and large cells. It maintains cell junctions through plasmodesmata.
2. Animal cell: All the cells that form the body of an animal are called animal cells. Animal cells do not have cell walls and plastids. It contains more mitochondria and centrioles are always present. Their stored food is glycogen and fat. It contains microvilli and pinocytic cavities. It does not contain glyoxysomes. It maintains cell junctions through desmosomes.

Dougherty (1975) divided cells into two groups based on the structure of the nucleus.

1. Prokaryotic cell: The word Prokaryotic is formed from Greek word pro meaning origin and karyon meaning nucleus. Prokaryotic means original nucleus or pseudonucleus. Cells that do not have a well-formed nucleus are called primitive cells. Adiocytes are small in size and less than 5 micrometers. Its cell wall consists of peptidoglycan. It contains 70S ribosomes. Primary cells do not contain chromosomes. It contains only DNA. Its nucleoid is called nucleoid or xenophore. It is low in histone proteins and non-enveloped. It does not undergo cytoplasmic movement or cyclolysis. However, transcription occurs in the cytoplasm. Protozoa like bacteria, blue green algae (BGA), mycoplasma (PPLO), spirochetes, rickettsia etc.

2. Eukaryotic cell: The word Eukaryotic is formed from the Greek word eu meaning actual and karyon meaning nucleus. Eukaryotic means true nucleus. Cells that have a well-formed nucleus are called true cells. Cysts are large in size and more than 5 micrometers. The actual cell size is 10 times larger than the original cell due to the presence of the nucleus alone. Its cell wall consists of cellulose. It contains 80S ribosomes. Cells contain chromosomes and DNA. Cyclosis occurs in its cytoplasm and transcription in the nucleus. Cell division in normal cells occurs in the process of mitosis and meiosis. Algae, fungi, bryophytes, pteridophytes, gymnosperms and angiosperms, somatic cells, germ cells, muscle cells, nerve cells etc.

A type of cell between protocell and protocell is called mesokaryotic cell. Mesokaryotic cells have a nuclear membrane. Its chromosomes are composed of DNA and acidic proteins. For example – Gymnodinium (Algae), Peridineum (Algae), Noctiluca (Protozoa) etc.

Cells are divided into two groups based on the number of chromosomes.
1. Haploid cells: Cells which have haploid (n) number of chromosomes are called haploid cells. Haploid cells divide by the process of meiosis. These cells have half the number of chromosomes. For example, sperm and egg.
2. Diploid cells: Cells which have diploid (2n) number of chromosomes are called diploid cells. Diploid cells divide in the process of mitosis. Diploid cells have the same number of chromosomes. For example, root, stem, muscle, xylem, phloem etc.

Cells are divided into two types based on physiological function.

1. Organism: The cells that form the body of multicellular organisms are called organelles. It is diploid in nature. Cells divide in the process of mitosis. That is, an equal number of chromosomes divides. Body cells such as root, stem, leaf, xylem, phloem, muscle cell, nerve cell (neuron), epithelial cell, blood cell etc.
2. Germ cells: Cells which take part in the reproduction of an organism are called germ cells. It is haploid in nature. Germ cells divide by the process of meiosis. Germ cells have half the number of chromosomes. That is, the reductive division of chromosomes takes place. Sperm and ovum are the units of sexual reproduction and spores or molecules are the units of asexual reproduction.

Cells are divided into two types based on physiological function.

1. Organism: The cells that form the body of multicellular organisms are called organelles. It is diploid in nature. Cells divide in the process of mitosis. That is, an equal number of chromosomes divides. Body cells such as root, stem, leaf, xylem, phloem, muscle cell, nerve cell (neuron), epithelial cell, blood cell etc.
2. Germ cells: Cells which take part in the reproduction of an organism are called germ cells. It is haploid in nature. Germ cells divide by the process of meiosis. Germ cells have half the number of chromosomes. That is, the reductive division of chromosomes takes place. Sperm and ovum are the units of sexual reproduction and spores or molecules are the units of asexual reproduction.

1. Viruses, virions and prions have no cellular location.
2. Bacteria, archaebacteria and blue-green algae do not have a well-structured nucleus.
3. Many algal and fungal cells are multinucleated.
4. The protoplasm of many cells is dead.
5. Red blood cells and cytoplasm are without nucleus.
6. Connective tissue contains a lot of extracellular matrix.

In 1838-39 German botanist Jacob Schleiden, zoologist Theodor Schwann and later in 1855 Rudolf Virchow provided the theory of cells called cell theory. Cell theory is called Cell doctrine.

Fundamentals of Cell theory

1. All organisms are made up of cells.

2. Cell is the basic unit of life.

3. Cell is the structural, organizational and physiological unit of an organism.

4. It is the unit of inheritance.

5. New cells are formed from old cells.

6. All body functions take place inside cells.

7. The biological functions of the body depend on the functions of all cells.

8. All members of the same species have the same chemical nature of cells.

9. Cells contain the genetic information of organisms and are transmitted from cell to cell during cell division.

10. All biological functions of multicellular organisms are the sum of interactions of body cells.

The units of cell measurement are meter (M), decimeter (dm), centimeter (cm), millimeter (mm), micrometer or micron (µ), nanometer (nm) and angstrom (Å).
1 meter = 100 cm = 1000 mm = 1,000,000 micrometers = 1,000,000,000 nanometers
1 cm = 10 mm = 1/100 meter
1 mm = 1/10 cm = 1/1000 meter
1 micrometer = 1/10,000 cm = 1/1,000,000 meter
1 nanometer = 1/10,000,000 cm = 1/1,000,000,000 meter
1 nm = 0.001 µm = 10 Å
Cells larger than 100 μm are visible to the naked eye.
A light microscope can see from 0.2 μm to a maximum of 40 μm and a magnification of up to 2,000 times.
Electron microscopes can see from 0.5 μm to 20 μm at a maximum magnification of 500,000 times.
Electron microscopes use electron beams at shorter wavelengths. So many tiny objects are visible.

Each cell has a specific lifespan. Cell lifespan depends on environmental conditions and genetic regulation or genetic clock. The life span of bacteria is a few hours. The life span of intestinal lining cells is 2-4 days. The average lifespan of a human cell is 10-12 days. White blood cells live for 1-15 days, platelets for 8-12 days and red blood cells for 120 days. Muscle and blood cells live throughout life.

Amoeba, protozoa, bacteria, algae and fungi are made up of single cells. Organisms are composed of a few to several thousand cells. The number of cells in a newborn human baby is about 2×10¹². Human body has 100 trillion cells. A human weighing 80 kg has approximately 60×10¹⁵ (6-10 lakh crore) cells.

Most cells are microscopic and 20-30 micrometers. A typical body cell is about 10 µm in size and weighs 1 ng. The smallest cell is Mycoplasma (Mycoplasma gallisepticum, PPLO-Pleuro Pneumonia Like Organism) with a diameter of 0.1 micrometer. The largest cell is the ostrich egg (17×12.5 cm). The smallest cell in the human body is the sperm and the largest cell is the egg (diameter 0.1 mm). The diameter of red blood cells is 7-8 micrometers. A human pea neuron cell is 1.37 meters long (from the base of the spinal cord to the big toe). Acetabularia algae are the largest cells in the plant kingdom. Its length is 5-10 cm. The longest cell is the bark fiber of Bochmeria nivea plant. Its length is 55 meters. The cells of plant fibers such as cotton, jute, palm etc. are quite long.