CELL- Unit of Life


Unicellular organisms are capable of -
(i) independent existence and
(ii) performing the essential functions of life.
Anything less than a complete structure of a cell does not ensure independent living. 
Hence, the cell is the-
fundamental structural and functional unit of all living organisms.

  • Anton Von Leeuwenhoek first saw and described a live cell. 
  • Robert Brown later discovered the nucleus. 
  • The invention of the microscope and its improvement leading to the electron microscope revealed all the structural details of the cell.

Cell theory-
In 1838- Matthias Schleiden, [a German botanist]-

  • Examined a large number of plants and observed that all plants are composed of different kinds of cells which form the tissues of the plant.

Schwann (1839), [a British Zoologist]-

  •  studied different types of animal cells and reported that cells had a thin outer layer which is today known as the plasma membrane
  •  He also concluded, based on his studies on plant tissues, that the presence of cell wall is a unique character of the plant cells. On the basis of this, Schwann proposed the hypothesis that the bodies of animals and plants are composed of cells and products of cells.

Schleiden and Schwann together formulated the cell theory.
This theory however, did not explain as to how new cells were formed.

Rudolf Virchow (1855)-

  • first explained that cells divided and new cells are formed from pre-existing cells (Omnis cellula-e cellula). He modified the hypothesis of Schleiden and Schwann to give the cell theory a final shape. Cell theory as understood today is:

(i) all living organisms are composed of cells and products of cells.
(ii) all cells arise from pre-existing cells.

Overview of Cell-

Inside each cell is a dense membrane-bound structure called nucleus. This nucleus contains the chromosomes which in turn contain the genetic material, DNA. Cells that have membrane-bound nuclei are called eukaryotic whereas cells that lack a membrane-bound nucleus are prokaryotic. In both prokaryotic and eukaryotic cells, a semi-fluid matrix
called cytoplasm occupies the volume of the cell. 

  • The cytoplasm is the main arena of cellular activities in both the plant and animal cells. Various chemical reactions occur in it to keep the cell in the ‘living state’.

Ribosomes are non-membrane bound organelles found in all cells both eukaryotic as well as prokaryotic. Within the cell, ribosomes are found not only in the cytoplasm but also within the two organelles chloroplasts (in plants) and mitochondria and on rough ER.

  • Animal cells contain another non-membrane bound organelle called centrosome which helps in cell division.

Mycoplasmas, the smallest cells, are only 0.3 μm in length while bacteria could be 3 to 5 μm. The largest isolated single cell is the egg of an ostrich. Among multicellular organisms, human red blood cells are about 7.0 μm in diameter. Nerve cells are some of the longest cells. Cells also vary greatly in their shape. They may be disc-like, polygonal, columnar, cuboid, thread-like, or even irregular. 
  • The shape of the cell may vary with the function they perform.

Prokaryotic cells-
The organization of the prokaryotic cell is fundamentally similar even though prokaryotes exhibit a wide variety of shapes and functions. prokaryotes have a cell wall surrounding the
cell membrane except in mycoplasma. The fluid matrix filling the cell is the cytoplasm. There is no well-defined nucleus. The genetic material is basically naked, not enveloped by a nuclear membrane. In addition to the genomic DNA (the single chromosome/circular DNA), many bacteria have small circular DNA outside the genomic DNA. This smaller DNA are called plasmids. 

  • The plasmid DNA confers certain unique phenotypic characters to such bacteria. One such character is resistance to antibiotics.

No organelles, like the ones in eukaryotes, are found in prokaryotic cells except for ribosomes. Prokaryotes have something unique in the form of inclusions. A specialized
differentiated form of cell membrane called mesosome is the characteristic
of prokaryotes. They are essentially infoldings of the cell membrane.

Cell Envelope and its Modifications-
The cell envelope consists of a tightly bound three-layered structure i.e., the outermost glycocalyx followed by the cell wall and then the plasma membrane. Although each layer of the envelope performs a distinct function, they act together as a single protective unit.
Bacteria can be classified into two groups on the basis of the differences in the cell envelopes
and the manner in which they respond to the staining procedure developed by Gram viz., those that take up the gram stain are Gram-positive and the others that do not are called Gram-negative bacteria.

Glycocalyx differs in composition and thickness among different bacteria. It could be a loose sheath called the slime layer in some, while in others it may be thick and tough, called the capsule. The cell wall determines the shape of the cell and provides strong structural support
to prevent the bacterium from bursting or collapsing. The plasma membrane is selectively permeable in nature and interacts with the outside world. This membrane is similar structurally to that of the eukaryotes.

A special membranous structure is a mesosome which is formed by the extensions of the plasma membrane into the cell. These extensions are in the form of vesicles, tubules, and lamellae. 

  • They help in cell wall formation, DNA replication and distribution to daughter cells. They also help in respiration, secretion processes, to increase the surface area of the plasma membrane and enzymatic content.
  •  In some prokaryotes like cyanobacteria, there are other membranous extensions into the cytoplasm called chromatophores which contain pigments. The bacterial flagellum is composed of three parts – filament, hook and the basal body. The filament is the longest portion and extends from the cell surface to the outside.

Pili and Fimbriae are also surface structures of the bacteria but do not play a role in motility. The pili are elongated tubular structures made of a special protein. The fimbriae are small bristle-like fibers sprouting out of the cell. In some bacteria, they are known to help attach the bacteria to rocks in streams and also to the host tissues.

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