Cell theory

Cell Theory

  • Matthias Schleiden (1838):

A German botanist observed that all plants are composed of various types of cells, which together form the tissues of the plant. 

  • Theodore Schwann (1839):

- A British zoologist, he studied animal cells and noted the presence of a thin outer layer, now known as the plasma membrane.

- He also recognized the unique characteristics of cell walls in plant cells. 

- Schwann proposed that both plants and animals are composed of cells and their products, contributing to the formulation of Cell Theory. 

  • Rudolf Virchow (1855):

- He added a crucial aspect to Cell Theory by explaining that new cells are formed through the division of pre-existing cells. 

- His famous phrase "Omnis cellula-e cellula" means that every cell originates from another cell. 

- Virchow's contribution finalized the Modern Cell Theory, consisting of two main principles:

1. All living organisms are composed of cells and their products. 

2. All cells arise from pre-existing cells through cell division. 

Overview of Cell

  • Cells are the fundamental structural and functional units of all living organisms. 
  • Unicellular organisms can independently exist and perform essential life functions. Anything less than a complete cell structure cannot ensure independent living.
  • Anton Von Leeuwenhoek was the first to observe and describe live cells, and Robert Brown later discovered the nucleus. 
  • The invention and improvement of microscopes, including electron microscopes, have revealed the intricate structural details of cells, advancing our understanding of life at the cellular level.

 Cell Structure:

- Cells come in various forms, but two main types are plant cells and animal cells.

-Plant cells have a distinctive cell wall as the outer boundary, followed by the cell membrane.

-In contrast, animal cells have a cell membrane as their outer boundary.


- Both plant and animal cells contain a nucleus, a dense membrane-bound structure.

- The nucleus houses chromosomes, which carry genetic material in the form of DNA.

- Cells with membrane-bound nuclei are known as eukaryotic, while those lacking such a nucleus are prokaryotic.


- Within cells, a semi-fluid matrix known as cytoplasm fills the interior.

- It serves as the primary site for various cellular activities, including essential chemical reactions. 


- Eukaryotic cells have membrane-bound organelles such as the endoplasmic reticulum (ER), Golgi complex, lysosomes, mitochondria, microbodies, and vacuoles.

- Prokaryotic cells lack these structures.


- Non-membrane-bound organelles called ribosomes are present in all cells, both eukaryotic and prokaryotic.

- They can be found in the cytoplasm, as well as within organelles like chloroplasts (in plants) and mitochondria, or attached to rough ER. 


- Animal cells have another non-membrane-bound organelle known as the centrosome, which plays a role in cell division. 

Cell Variation:

- Cells differ in size, shape, and function.

- For example, the smallest cells like Mycoplasmas are only 0.3 µm in length, while bacteria can range from 3 to 5 µm.

- The largest isolated single cell is the ostrich egg.

- Among multicellular organisms, human red blood cells have a diameter of about 7.0 µm.

- Nerve cells are among the longest.

- The shape of cells may vary according to their specific functions.