The cell can be defined as the structural and functional unit of life. It is the smallest unit that can carry out all activities of life. Cells are building blocks of complex multicellular organisms.
Emergence and Implication of Cell Theory
Study of cell (cell biology) began with the discovery of cell by Robert Hooke (1665), who reported his work in his famous publication Micrographic. He prepared and studied thin section of cork (of dead plant material) under his self –made compound microscope.
He observed that the cork is composed of minute honey comb like compartments which he termed as Cells (Fig.4.1). According to Hooke, cell is an empty space bounded by thick walls. Very little information was added to this idea in the following century. The work again started in the beginning of 19th century.
Lorenz Oken (1805) a German scientist, believed that “all living beings originate from or consist of vesicles or cells”. Jean Baptist De-Lamarck (1809) expressed similar idea and said “no body can have life if its constituent parts are not cellular tissue or are not formed by cellular tissue”.
In 1831 Robert Brown reported the presence of nucleus in the cell. Due to this discovery Hook’s idea about the cell as an empty space was changed. It was later established that cell is not an empty space. A German zoologist Theodor Schwann (1839) and a German botanist Schleiden (1838), working independently, came out with a theory called the Cell Theory.
Fig 4.1: The microscopic structure of a piece of cork
They found that the cell consisted of 3 basic parts, viz nucleus, the fluid (cytoplasm) surrounding the nucleus, and an outer thin covering or membrane (plasma membrane). The cell wall, they said, was an additional structure, present only in plant cell. Keeping in view this definition of cell, the cell could be observed in plants as well as in animal according to cell theory, all living organisms are composed o f cells and cell products.
The cell theory is one of the most fundamental generalizations in Biology. It has wide ranging effect in all fields of biological sciences. After the cell theory was presented, many details of cell were studied ,as a result of which the cell theory was extended.
Rudolph Virchow (1885), a German physician, hypothesized that new cell were formed only by the division of previously existing living cells (to put it in Virchow’s words: “ommis cellular e cellular”)
It was contrary to the idea of “abiogenesis” (living things arise spontaneously from non-living things), one of the prevailing but controversial ideas about origin of life, at that time. Louis Pasteur (1862), one of the greatest scientists of all times, supplied experimental proof for Virchow’s hypothesis by demonstrating that microorganisms (bacteria) could be formed only from existing bacteria.
Original cell theory and Virchow’s hypothesis gave us the basis for working definition of living things: living things are chemical organizations composed of cell and capable of reproducing themselves’. August Weismann (1880) said all presently living cells have a common origin because they have basic similarities in chemical composition, metabolic activities and structure, although they differ in many respect. Cells are basically similar but extraordinarily versatile. Cell is not only the structural but also the functional unit of living organisms. So cell theory is a very important unifying concept.
The human naked eye can differentiate between two points, which are at least 1.0mm apart. This is known as resolution of eye. The resolution can be increased with the aid of lenses. In a typical compound microscope the resolution is 2.0um, which is about 500X that of naked eye. A compound microscope is a typical laboratory microscope with at least different magnification power power. The typical laboratory microscope with at least different magnification powers.
The typical ocular lenses could be 5X and 10X but others also exist. Likewise different type of objective lenses viz. 20X, 40X, 100X etc exist. The magnification power of microscope is determined by multiplying X values of the objective lenses. Therefore, a microscope with 10X ocular lens ans 40X objective lens will have (10X40+400X) 400X magnification power. The resolution will, however, remain the same, which is 500X that of the naked eye. The source of illumination is such microscope is visible light. In electron microscope rages between 2-4 Angstrom, which make it 500X greater than that of the compound microscope and 250,000X greater than that of the naked eye. This means that two points which are 2-4 Angstrom apart can be differentiated with the help of electron microscope. The revelation of complexity of structure of various cellular organelles is closely linked with the development of microscopy and improvement in the resolution power of the microscope.
The salient features of cell theory in this present from are:
- All organisms are composed of one or more cells.
- All cells arise from pre-existing cells.
- Cell is the basic structural as well as functional unit for all organisms.
- Cell as a unit of structure and function
A cell is a unit of structure ans function in living organisms. In multicellular organisms there is a division of labor among cells. Different cell are specialized fore different functions. The function of the organism as a whole is the result of the sum of axtiviries ans interactions of different cells ans of different components of the cell. In animals e.g., muscle cell contract and relax, nerve cells transmit impulses, gland cells secrete, red blood cell carry oxygen and some stomach cells secrete gastric juice similarly in plants xylem cell conduct water and mineral salts from soil to the aerial parts of the plants.
Phloem cells translocate food, sclerenchymatous, cell give support to surplus food and meristematic cells produce new cell for growth and development of the plant. As they perform different functions they show great variation in shape and size. Despite the structural and functional diversity, the plant cells as well as animal cells have a common plan of organization.