Cell is called a structural and functional unit of life of all living organisms capable of independent existence and can perform allfunctions of life.
To see cells clearly we need a microscope. Larger cells can be seen through simple microscope but to see smaller cells we require compound microscope. Simple microscope can magnify image 50 to 100 times but a compound microscope can do so 1000 times or more. In the microscope we use in the laboratory, a beam of light is used to make things visible hence it is light microscope. To see interior of cell we need electron microscope. It can magnify image 500000 times.
There is no typical shape of a cell. Cells may be spherical, rectangular, flattened, polygonal, oval, triangular, conical, columnar, etc.
Cell size varies greatly in various plants and animals. Some of them are not visible to naked eye. Some are barely visible while some are macroscopic. The smallest cell size can be seen in mycoplasma (0.3 µm in length), bacterial cell size is 3 to 5 µm, while the largest size of cell is seen in Ostrich egg (nearly 15cms). Longest cells are nerve cells. You already know that cell theory was proposed by Schwann and Schleiden. However, in this theory, there was no explanation about formation of new cells. It was Rudolf Virchow (1855) who explained for the first time that new cells are formed by cell division from pre existing cells (Omnis cellula- e- cellulla). In later years, advanced research in
cytology led to modification in cell theory, which is now known as Modern Cell Theory.
It is the capacity or the potential of living nucleated cell to differentiate and divide to form any other type of cell and thereby a complete new organism.
A cell is totipotent because it has the entire genetic information of the organism in its nucleus. Embryonic animal cells are totipotent and termed as stem cells. Stem cells have great medical applications including cure for diseases.
Kinds of cells :
Living organisms are grouped into two main categories the Prokaryotes and Eukaryotes. The prokaryotes have simple cellular organization while eukaryotes exhibit high degree of organization.
A. Prokaryotic cells :
The cell in prokaryotes show following main features. It has chemically complex protective cell envelop. However, it does not have well-defined nucleus and other membrane bound cell organelles.
Cell envelop is a three-layered structure with outer glycocalyx, middle cell wall and inner plasma membrane. Glycocalyx is present as either slime layer (loose sheath) or capsule (tough). Bacteria are better observed when stained.
The most followed staining method is ‘Gram staining’ developed by Danish bacteriologist Hans Christian Gram. The cell wall is made up of peptidoglycan (in Gram positive bacteria) and murein (in Gram negative bacteria). It gives mechanical strength to the cell. Cell membrane is a phospholipid bilayer. All these structures give protection to the cell and also help in inter-cellular transport. In motile bacteria either cilia or flagella are found. Both are driven by rotatory movement produced by basal body (which works as motor). Other parts are filament and hook. Some other surface projections are the tubular pili (which help in inter-cellular communication) and fimbriae (for clinging to support).
The cell membrane shows infoldings called mesosomes, which help in cell wall formation and DNA replication. Some bacteria especially photosynthetic cyanobacteria show more longer extensions called chromatophores.
They carry photosynthetic pigments. The cytoplasm contains dense particles called ribosomes helping in protein synthesis. Ribosomes are described by their sedimentation rate in Svedberg units. Bacterial ribosome are 70S (composed of a larger subunit 50S + smaller subunit 30S).
B. Eukaryotic Cells :
Cells in which the nucleus has a definite nuclear membrane are known as Eukaryotic cells. These cells exhibit presence of membrane
bound cell organelles. e.g. Cells of Protists, Plants, Animals and Fungi. The eukaryotic cells have different shape, size and physiology but all the cells are typically composed of plasma membrane, cytoplasm and its organelles viz. Mitochondria, Endoplasmic Reticulum, Ribosomes, Golgi complex, etc. and a true nucleus.