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Biology Notes Class Essay

Cells were first discovered by Robert Hooke in 1665.

Leeuwenhoek (1674), with the improved microscope, discovered the free living cell in pond water for first time. Robert Brown in 1831 discovered the nucleus in the cell. Purkinje in 1839 coined the term ‘protoplasm’ for the fluid substance of the cell. The cell theory that all plants and animals are composed of cells and that the cell is the basic unit of life, was presented by two biologists, Schleiden (1838) and Schwann (1839). The cell theory was further expanded by Virchow (1855) by suggesting that all cells arise from pre-existing cells. With the discovery of electron microscope in 1940, it was possible to observe and understand the complex structure of the cell and its various organelles. The cells that are made up of a single cell are called unicellular organisms. Amoeba, Chlamydomonas, Paramoecium and bacteria are examples of unicellular organisms.{ uni = single}

The cells that are made up of more than thousands of cells are called multicellular organisms. Animal and plants are examples of multicellular organisms. { multi = many} Plasma membrane is the outer most covering of the cell that separates the contents of the cell from its external environment. It allows the entry or exit of some materials in and out of the cell. It also prevents the entry of unwanted materials. Plasma membrane is also known as selectively permeable membrane. Substances like CO2 or O2 can move across the cell membrane by a process of diffusion. The spontaneous movement of a substance from higher concentration to lower concentration is the same that happens with a cell. Water also obeys the laws of diffusion. The movement of water molecules through such a selectively permeable membrane is called osmosis. The movement of water across the plasma membrane is also affected by the amount of substance dissolved in water.

Important:

If the medium surrounding the cell has a higher water concentration than the cell, meaning that the outside solution is very dilute, the cell will gain water by osmosis. Such a solution is known as a hypotonic solution. Water molecules are free to pass across the cell membrane in both directions, but more water will come into the cell than water enters the cell. The cell is likely to swell up. If the medium has exactly the same water concentration as the cell, there will be no net movement of water across the cell membrane. Such a solution is known as an isotonic solution. Water crosses the cell membrane in both directions, but the amount going in is the same as the amount going out, so there is no overall movement of water.

The cell will stay of the same size. If the medium has a lower concentration of water than the cell, meaning that it is a very concentrated solution, the cell will lose water by osmosis. Such a solution is called hypertonic solution. Again, water crosses the cell membrane in both directions, but this time more water leaves the cell than enters it. Therefore the cell will shrink. The plasma membrane is flexible and is made up of organic molecules called lipids and proteins.


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