Cells are the basic unit bounded by the membrane that consists of the fundamental molecules of life of which all living organisms are made up of. Cell Biology encompasses everything about cells from its basic structure to the functions of every cell organelle. A single cell is an organism in itself such as yeast or bacterium; other cells gain special functions soon after they mature. These acquire unique functions as they mature. They cooperate with other cells and become the building blocks of multicellular organisms such as in humans and animals. A group of small bacteria called mycoplasmas and they are the smallest known cells. Some of these unicellular organisms are spheres of about 0.3 micrometers in diameter with a mass of 10-14 gram.

Definition of a Cell:

A cell can be defined as the smallest unit of life. It is the structural, functional and biological unit of all living beings. A cell can replicate itself independently and are thus referred to as the building blocks of life. Each cell contains a cytoplasm which is enclosed by a membrane and contains several biomolecules like proteins, nucleic acids, etc.

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The cell was first discovered in 1665 by Robert Hooke. Then in 1839, the Cell Theory was developed. 

Cell Theory :

Cell Theory is one among the basic principles of biology and was proposed by the German scientists named Theodor Schwann and Matthias Schleiden. The cell theory states that:

  1. All living species on Earth are either composed of a single cell or many cells.
  2. All cells have certain common structures like their plasma membrane and genetic material.
  3. A cell is the smallest living unit of life.
  4. Every cell arises from its pre-existing cells.

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Types of Cells

Cells are similar to small factories with different laborers and departments that work all the time to make life possible. Various kinds of cells perform different functions like protein synthesis and energy production.There are two major kinds of living organisms based on their cellular structure namely: prokaryotes and eukaryotes.

  • Prokaryotes are made up of cells with no nucleus. They all are single-celled microorganisms including archaea, bacteria and photosynthetic blue-green algae which are also known as cyanobacteria.


  • Eukaryotes consists of cells with a nucleus. This large category involves all plants, fungi (such as molds, yeast, and mushrooms), protozoa (Plasmodium falciparum and parasite that cause malaria) and animals. The plasma membrane is responsible for monitoring the transport of nutrients and electrolytes in and out of the cell and also responsible for cell to cell communication.


Cellular life is entirely dependent on the various chemical process for survival. These chemical reactions mainly occur in a watery solution within the cell known as cytoplasm.

Difference Between Prokaryotic and Eukaryotic Cells

There are several difference features between Eukaryotic and Prokaryotic cells. The below table states the comparison between cell organelles and structures found in a typical animal eukaryotic cell and prokaryotic cell.   

Cell Structure Eukaryotic Cell  (Typical Animal Cell) Prokaryotic Cell
Ribosomes Yes Yes
Nucleus Yes No
Golgi Complex Yes No
Mitochondria Yes No
Cilia or Flagella Yes (Complex) Yes (Simple)
Cell Membrane Yes Yes
Centrioles Yes No
Endoplasmic Reticulum Yes No
Peroxisomes Common No
Cell Wall No Yes

A cell is bounded by a plasma membrane that forms a selective barrier allows nutrients to enter and leave waste products.

The Function and Characteristics of Cells

  • The cell interior is organized into different special compartments or organelles surrounded by a separate membrane.
  • The nucleus(major organelle) holds genetic information necessary for reproduction and cell growth.
  • Every cell has one nucleus and other types of organelles exist in multiple copies in the cytoplasm.
  • Mitochondria, a double membrane-bound organelle is mainly responsible for the energy transactions vital for survival od the cell.
  • Lysosomes digest unwanted materials in the cell.
  • Endoplasmic reticulum plays a significant role in the internal organization of the cell by synthesizing selective molecules and processing, directing and sorting them to their appropriate locations.

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Practise This Question

A wire of resistance 0.5Ωm1 is bent into a circle of radius 1 m. An identical wire is connected across a diameter AB as shown in fig. The equivalent resistance is