The cells are the fundamental structural and functional units of all living organisms, and anything less than a complete cell has no independent existence.
In 1665, Robert Hooke observed a thin section of cork under a compound microscope, noticed a honey-comb like compartments. He coined the term “Cell”.
In 1674, Anton Von Leeuwenhoek first saw and describe the living cell.
Later, in 1831, Robert Brown discovered the nucleus in the root cells of orchids.
The invention of the microscope & its improvement leading to the electron microscope revealed all the structural details of the cell.
In 1838, Matthias Schleiden found that all the plant cells have essentially similar structure and have a cell wall.
In 1839, Theodore Schwann studied different types of animal cells and reported that it had a thin outer layer which is today known as the ‘plasma membrane’.
He also concluded that the presence of the cell wall is a unique character of the plant cells. On the basis of this Schwann proposed the hypothesis that the bodies of animals & plants are composed of cells & its product.
Schleiden and Schwann together formulated this theory. This theory, however, didn’t explain how they were formed.
In 1855, Rudolf Virchow first explained that the cells divided and new cells are formed from pre-existing cells. He modified the hypothesis of Schleiden and Schwann to give the cell theory a final shape.
This theory as understood today is:-
All living organisms are composed of cells and their products.
All cells arise from pre-existing of itself.
It is the outermost, it is a thin layer, transparent, elastic & semi-permeable membrane. It helps in the exchange of materials between the cytoplasm and extracellular fluid.
The nucleus also called the director of the cell, is the most important part of it which directs and controls all the cellular functions
The eukaryotic cell consists of following cell organelles:
- Endoplasmic Reticulum
- Golgi apparatus
ENDOPLASMIC RETICULUM – Endoplasmic Reticulum (ER) is a network of tiny tubular structures scattered in the cytoplasm. The ER bearing Ribosomes on their surface is called Rough Endoplasmic Reticulum (RER). In the absence of Ribosomes, they appear smooth and are called Smooth Endoplasmic Reticulum (SER).RER is actively involved in protein synthesis and secretion. The SER is the major site for lipid synthesis & lipid-like steroidal hormones.
RIBOSOMES – These are granular structures, composed of RNA and proteins & they are surrounded by any membrane. The eukaryotic Ribosomes are the 80S while prokaryotic Ribosomes are 70S. These are the sites of protein synthesis.
GOLGI BODIES – They consist of many flat, disc-shaped sacs or cisternae of 0.5 1 micron diameters, stacked parallel to each other. It involved cell secretion, the formation of hormones, biosynthesis of glycolipids & glycoproteins.
MITOCHONDRIA – It is a double membrane-bound structure with the outer membrane and inner membrane dividing its lumen into two compartments. They are called powerhouses of the cell as these are sites of ATP formation.
LYSOSOMES – These are vesicular structures of the cytoplasm that are involved in intra-cellular digestive activities; contain all types of hydrolytic enzymes, also known as the suicidal bag of the cell.
FIBRILS – They are present in the various cells. It helps to maintain the cell shape and is also responsible for the contractility as they are present in the muscle fibers.
MICROTUBLES – They become part of mitotic spindles in dividing cells.
CENTRIOLES – These are the pair of short rod-shaped bodies found adjacent to the nucleus lying at right angles to each other. centrioles also give rise to cilia or flagella.
CILIA & FLAGELLA – Cilia & Flagella are microscopic, hair or thread-like motile structures present extra-cellularly but originate intra-cellularly and help in locomotion, feeding, circulation, etc.
INCLUSIONS – These are pigments like melanin or lipofuscin, storage granules such as glycogen and fat, and secretion granules.