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A cell is the smallest unit of living organisms.It is composed of structures called organelles, which perform diverse functions.

For example, mitochondria is responsible for cellular respiration and chloroplast for photosynthesis.

This course covers following:

The cell was first discovered and named by Robert Hooke, in 1665. He remarked that it looked similar to cellula (small rooms), which monks inhabited, thus coining the name “cell” for his discovery.

Hooke observed the dead cell walls of plant cells (cork) as it appeared under the microscope.

Cork shaped cell observed by Robert Hooke under the microscope
Figure 1. Cork-shaped cell (Robert Hooke, Micrographia, 1665)

Each cell acts as a self-contained and self-maintaining unit. It takes nutrients in, converts these nutrients into energy, carries out specialized functions, and reproduces as required.

Every living thing has cells within its body. The composition, distribution, and number of cells present in an organism determine its type.

Unicellular organisms are those made of a single cell, whereas multicellular organisms contain many (sometimes millions) cells.

The table below shows basic differences in unicellular and multicellular organisms.

Characteristics Unicellular Multicellular
Number of cells One More than one
Organization Simple Complex
Division of labor At organelle level therefore low level of operational efficiency At cellular, tissue, organelle, and organ level therefore the operational efficiency is very high
Exposure Exposed to the environment Contains cell wall no cell organelles inside the cell are not exposed
Size Small Large
Extreme environment More likely to be found Rare
Function Single cell performs all life processes Different cells specialized for to carry out specific functions
Reproduction Asexual Mainly Sexual
Injury Can lead to death of the organism No death and lost parts can be replaced in some multicellular organisms.
Life span Short Long because of division of labor
Example Amoeba


neuronNerve Cell

Cell theory

Cell theory is a widely accepted explanation of the relationship between cells and living things. Hooke, Leeuwenhoek, Schleiden, Schwann, and Virchow developed the classical cell theory.

Today, cell theory is considered to be a foundation of biology. According to this theory:

  • All living organisms are composed of one or more cells.
  • The cell is the basic unit of structure and organization in organisms.
  • Cells arise from pre-existing cells.

Modern cell theory

Over time, new discoveries and concepts have refined and modified the scientific view of cells to form the modern cell theory.

Following are the important concepts of this theory:

  • The cell is the fundamental unit of structure and function in living things.
  • All cells come from pre-existing cells by division.
  • Energy flow occurs within cells.
  • Cells contain hereditary information (DNA), which is passed from cell to cell during cell division
  • All cells are basically the same in chemical composition.
  • All known living things are made up of cells.
  • Some organisms are unicellular, made up of only one cell.
  • Other organisms are multicellular (composed of countless number of cells).
  • The activity of an organism depends on the total activity of independent cells.

Some well-known exceptions to the theory are given below:

  • Although viruses are living organisms, they are not made up of cells.
  • The first cell did not originate from a pre-existing cell.
  • Mitochondria and chloroplasts have their own genetic material. They both reproduce.independently from the rest of the cell.