ADDITIONAL RELATED READINGS ABOUT CELL
- A cell is the structural and functional unit of life.
- Discovery of cell is linked with the invention of microscope because most of the cells are microscopic.
- Robert Hooke discovered the cell in 1665 under his self-made compound microscope. According to Hooke, cell is an empty space surrounded by thick walls. He published his work in Micrographia in 1665.
- Lorenz Oken in 1805 (a German scientist) said “all living beings originate from or consist of vesicles or cells“.
- JEAN BAPTIST de-LAMARCK (1809) said “nobody can have life if it is not formed by cellular tissue.”
- Robert Brown said that nucleus is present in the cell and the cell is not an empty space. He discovered the nucleus in the cells of orchids.
- A German Zoologist Theodor Schwann (1839) and a German botanist Schleiden (1838) worked independently on the cell. They found that a cell has 3 parts:
(b) Cytoplasm (fluid surrounding the nucleus) and
(c) Plasmamembrane (outer thin membrane).
- Cell theory is: “All living organisms are composed of cells and cell products“
- The main features of CellTheory in its present form are:
(a) All organisms are composed of one or more cells.
(b) All cells arise from pre-existing cells.
(c) Cell is the basic structural and functional unit for all organisms.
- A German Physician Rudolph Virchow (1855) said “omniscellula e cellula” (new cells formed by the division of existing living cells). It was opposite to the idea of Abiogenesis
- Louis Pasteur (1862) said that bacteria are formed from existing bacteria.
- August Weismann (1880) said that all living cells have a common origin because they have basic similarity in structure and composition.
- The human naked eye can differentiate between two points, which are 1.0 mm apart. This is known as resolution of the eye. This resolution can be increased by lenses.
- In a compound microscope the resolution is 2.0 µm. It is 500X that of naked eye.
- A compound microscope has different magnification powers. The ocular lenses may be 5X and 10X while objective lenses may be 20X, 40X, 100X etc.
- The magnification power of microscope is determined by multiplying X values of ocular lens and X value of objective lens. For example: A microscope with 10X ocular lens and 40X objective lens will have 10 X 40 = 400 X magnifying power.
- The source of illumination in compound microscopes is visible light. In electron microscope the source of illumination is a beam of electrons.
- The resolution of electron microscope is 2 – 4 Angstrom. It is 500 X greater than that of the compound microscope and 250,000 X greater than that of the naked eye.
- In multicellular organisms there is a division of labour. Examples from animals are:
(a) Muscle cells contract and relax,
(b) Nerve cells transmit impulses,
(c) Gland cells secrete,
(d) Red blood cells carry oxygen,
(e) Some stomach cells secrete gastric juice,
(f) White blood cells (WBC) produce antibodies.
(g) Eye cells detect and respond to light.
Examples from animals are:
(a) Xylem cells conduct water and mineral salts from soil to the aerial parts of the plant
(b) Phloem cells translocate food,
(c) Sclerenchymatous cells give support to the plants,
(d) Chlorenchymatous cells carry out photosynthesis,
(e) Parenchymatous cells store surplus food and
(f) Meristematic cells produce new cells for growth and development of the plant.
- Due to different functions the newcells have different shapes and sizes.
- The function of an organism is due to activities and interactions of different cells and cell components.
- To study cell parts, modern techniques are used. Most modern technique is cell fractionation.
- The process of grinding to get a uniform composition/structure is called homogenization.
- The tissues are taken and are homogenized by special instruments (like homogenizers).
- The process of separation of different parts of the cell into different layers (on the basis of their size and weight and density of the medium) in the centrifuge tube using a centrifuge machine at medium speed is called density gradient centrifugation.
- The process of separation of different parts of the cell into different layers (on the basis of their size and weight and density of the medium) in the centrifuge tube using a centrifuge machine at high speed is called density gradient ultracentrifugation.
- Cell size is measured in micrometer (µm). One µm is 0.000,001 meter or 1 x 10-6 of a meter.
- Ostrich’s egg is the biggest egg.
- In most plant cells, cell membrane is surrounded by cell wall.
- The electron microscope has shown that the cell wall is formed by three main layers:
(a) Middle lamella
(b) Primary cell wall
(c) Secondary cell wall
- First of all Middle Lamella is formed between the primary walls of the neighboring cells. It is not the true wall and is composed of pectin or calcium pectate.
- Primary wall is a true wall and develops in newly growing cells. It is mainly made up of cellulose with some deposition of pectin and hemicellulose.
- In the primary cell wall the cellulose fibres are arranged in a criss-cross manner which give strength to the cell wall.
- Secondary cell wall is formed on inner surface of primary wall. It is thick and rigid than primary wall. Chemically it is composed of inorganic salts, silica, wax, cutin and lignin etc.
- Cell wall protects the cell from osmotic lysis.