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Lesson 1 Definition of Cell and Types of Cell


The cell is defines as living unit of life or basic unit of life or structural and functional unit of life or fundamental unit of life or autonomous and self contained unit of the life.

In 1665, Robert Hook discovered cell. In his experiment, he cut the cork pieces. Then he observed under his microscope and found many small compartments in it. He named each compartment Cell.

In 1838, German Botanist Schleiden occluded that the plant body is also composed of cells. In addition, in 1839 German Zoologist Theodor concluded that the animal body is also composed of cells.

Why cell is considered as basic unit of life or why cell is called self contained or autonomous unit?

The cell is smallest unit of life. A single cell can perform different life processes as it can digest food, respire, excrete, reproduce, and grow. The cell carries different metabolic activities. The body function is out come of the different activities of the cell. So the cell is considered as basic unit or self contained or autonomous unit of life.

Cell Theory

  • Schleiden in 1838 and Schwann proposed cell theory in 1839.
  • It states that Cell is a mass of protoplasm containing nucleolus and is bounded by a membrane.
  • All the cells are similar in chemical composition and metabolic activities.
  • Living things are composed of cells,
  • The function of organisms is the outcomes of cell activities and interaction of cell components.
Objection of cell theory

  • Bacteria and cyan bacteria do no contain nucleus.
  • Some fungi contain many nuclei in branched body.
  • There is no cellular organization in virus

To include all the living organisms the cell theory proposed by Scleidan and Schwann can be modified which is called cell principle

  • Cell is a mass of protoplasm containing nucleus or in lack of nucleus nuclear materials is taken.
  • Living things are made up of cells.
  • Cell can store genetic information and express from generation to generation.
  • Cell sometimes assume forms, which no longer have all the characteristics of the cell.
What is cellular totipotency

Even a single cell of a plant body has a power to regenerate entire plant body. This is called cellular totipotency. In 1950, Steward conducted an experiment on carrot.

In this experiment, He removed small piece of carrot phloem, kept the piece in liquid medium. Phloem cells divided producing cell clusters. The clusters are kept on solid medium, after few days a plant grew in it.

Difference between unicellular and multicellular organisms:

In unicellular organisms the body is single cell, single cell perform all the life activities, Cells are larger in size, easily infected and susceptible for damage, i.e. Amoeba, Paramecium

In multicellular organisms, number of different types of cells forms body, the body function is outcome of the cellular activities of different cells, cells are usually smaller is size, not easily susceptible to damage, higher plants and animals.

Types of cells:

There are two types of cells found in organisms. The differences are as follows.

Prokaryotic Cell Eukaryotic Cell
  1. Small in size (range from 100-10000nm) Usually gelatinous sheath is present.
  2. Cell wall is made up of amino sugar and muramic acid.
  3. Mesosome is present, the power house of the cell Nuclear membrane and nucleolus is absent.
  4. DNA is found freely in cytoplasm.

  5. Histone protein is absent in DNA, So called naked DNA.
  6. Cell organelles are absent. Flagella is simple.
  7. Pili are present.

  8. Ribosomes are small and 70 S type. Plasmid may be present.

  9. Cyclosis is absent.

  10. Cell division is amitosis. Cell cycle is short Spindle fibers are absent.

  11. Eg. Bacteria and Cyanobacteria.
  1. Larger in size (range from 10000-100000nm) Usually gelatinous sheath absent.

  2. In animal cell, cell wall is absent but in plant cell it is made up of cellulose.

  3. Mesosome is absent. Mitochondria are power house of the cell.

  4. Nuclear membrane and nucleolus are present DNA is bounded by nuclear membrane.

  5. Histone protein is present in DNA so called closed DNA.

  6. Cell organelles are present.

  7. Flagella are compound (9+2 pattern) Pili absent.

  8. Ribosomes are large and 80S type Plasmid is absent.

  9. Cyclosis is present.

  10. Cell division is mitosis and meiosis Cell cycle is long.

  11. Eg. Spindle fibers present in Higher plants and animals.

Structure of Eukaryotic Cell:

Shape: The shape is variable like round oval irregular spherical elongated or tubular etc.

Size: The size is also different in unicellular organisms it range from 1µm to 1 mm. in multicellular organisms it range from 1- 100µm.


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