The cell can be defined as structural and functional unit of life. It is the smallest unit of the body. It can perform all the activities of life. The cells are building block of complete multicellular organisms. Following scientists played their role in the formulation of the cell theory:
He discovered cell in 1665. His work on cell was published in “Micrographia”. He prepared a thin section of cork. Cork is made up of dead plant material. Hooke studied this section under his self-made compound microscope.
He observed that the cork was composed of minute honey comb compartments. He called them cells. He described cell as an empty space. They are covered by thick wall.
He was a German Scientist. He believed that all the organisms originate from vesicles. So these organisms consist of vesicles or cells.
He also believed that a body cannot have life without cellular tissues. All its parts are formed by cellular tissues.
He discovered nucleus in cell in 1831. It changed the Hooke’s idea that the cells have empty spaces. He proved that the cell is not an empty space.
Theodor Schwann and Schleiden
Schwann was a German Zoologist (1839) and Schleiden was a German Botanist (1838). Both these scientists were working independently. They formulated Cell Theory. They find that cell consists of 3 basic parts:
- Nucleus, present in middle.
- Cytoplasm, it surrounds the nucleus.
- Plasma Membrane, it forms outer thin covering.
The cell theory states, “All the living organisms are composed of cells and cell products”. It is a most generalized theory in biology. This theory has great effects on the biological research. Many details of the cell were studied. The cell theory was extended on the basis of these studies.
Rudolph Virchow (1855) (Extension in Cell Theory)
He was German Physician. He made a hypothesis that the new cells are formed only by the division of previously living cells. In Virchow’s words, “Omnis Cellula e Cellula”. At that time there were two ideas about the origin of life.
- Abiogenesis: It states that the living things arise from the non-living things. It was most popular idea of that time.
- Biogenesis: It states that the living things arise only from the living things.
Virchow’s hypothesis supported the idea of biogenesis and opposed the idea of Abiogenesis. On the basis of Virchow’s hypothesis, a new working definition of the Cell Theory was formed:
It states that “Living things are chemical organizations. These are composed of cells and capable of reproducing themselves”.
He was one of the greatest scientists of his time. He supported the Virchow’s hypothesis with experiments. He proved that microorganisms (bacteria) could be formed from existing microorganisms.
He said, “All presently living cells have a common origin because they have basic similarities in structures and functions”. There are fundamental similarities in the chemical composition, metabolic activities, and structures of the organisms. Some small differences may be present.
Therefore, the cells are basically similar but extraordinarily versatile. Thus, cell is the structural and functional unit of the living organisms. So, cell theory is a very important unifying concept.
Salient Features of Cell Theory
Cell theory has following salient (important) features:
- All organisms are composed of one or more cells.
- All cells arise from pre-existing cells.
- Cell is the basic structural and functional unit of all the organisms.
Cell as unit of Structure and Function
A cell is unit of structure and function in living organisms. There is a division of labour among cells in the multicellular organisms. Different cells are specialized for different functions. Different cells and different components of the cells perform different activities.
It results in the functioning of the whole organism. These cells perform different functions. So, they show great variations in shapes and structures. Despite these variations, the plant cells and animal cells have a common plan of organization.
- Muscle Cells contract and relax.
- Nerve Cells transport impulses.
- Gland cells secrete hormones.
- Red Blood cells carry oxygen.
- Some Stomach Cells secrete gastric juice.
- Xylem cells conduct water and mineral salts from soil to plant body.
- Phloem cells translocate food.
- Sclerenchymatous cells give support to the plant.
- Chlorenchymatous cells carry out photosynthesis.
- Parenchymatous cells store surplus food.
- Meristematic cells produce new cells for growth and development.
Resolution of Eye and Microscopes
The human eye can differentiate between two points, which are at least 1.0mm apart. This is known as resolution of eye. This resolution can be increased with the help of lenses. These lenses are used in microscopes.
Microscopes have played an important role in the discovery of different cellular organelles. The improvement in the resolution power has increased the discovery of different new organelles. There are two types of microscopes.
The source of illumination for a compound microscope is visible light. A compound microscope is a typical laboratory microscope. Its resolution is 2μm. Its resolution is 500X (times) of human eye.
Microscope has two lenses: ocular (eye piece) and objective. The typical power of ocular can be 5X and 10X and so on. The power of objective can be 20X, 40X and 100X and so on. The magnification of a microscope can be determined by multiplying X values of ocular with the X values of the objective. For example, the microscope with 10X ocular lens and 40X objective lens have (10×40 = 400) 400 magnifying power. The resolution will remain the same i.e. 500X of the naked eye.
The source of illumination for an electron microscope is a beam of electrons. The resolution of the electron microscope is between 2-4 Angstrom. It makes the object 500X greater than the compound microscope and 250,000 greater than the naked eye. It means that two points which are 2-4 Angstrom apart can be differentiate with the help of an electron microscope.