A cell is capable of independent existence and can carry out all the functions which are necessary for a living being. A cell carries out nutrition, respiration, excretion, transportation and reproduction, the way an individual organism does. Unicellular organisms are capable of independent existence which shows a cell's capability to exist independently. Due to this, a cell is called the functional unit of life.
All living beings are composed of the basic unit of life, i.e. cell. So, cell is called the structural unit of life. If you combine both the aspects, i.e. functional unit and structural unit, you can say that a cell is the fundamental unit of life.
Shape and Size of Cells: Cells come in all shapes and sizes. While most of the cells are spherical in shape, cells of various other shapes are also found. Most of the cells are microscopic in size, i.e. it is impossible to see them with naked eyes. Some cells are fairly large, e.g. a neuron in human body can be as long as 1 meter. The egg of an ostrich is the largest known cell of a living animal and an average egg of ostrich is 15 cm long and 13 cm wide.
A cell is enclosed in a membranous casing and is filled with a liquid substance which is called the cytoplasm. There are many cell organelles in a typical cell. Some of the main structures of a cell are as follows:
Plasma membrane: Plasma membrane is a semi-permeable membrane. It is composed of bilayer of lipid and protein.
Functions of Plasma Membrane: Plasma membrane provides a container to the cytoplasm. It facilitates passage of various substances in and out of the cell. Substances can move across the plasma membrane due to diffusion or osmosis.
Diffusion: Movement of molecules from region of high concentration to region of low concentration is called diffusion. You can witness diffusion in day to day life. When perfume is sprayed in a corner of a room, the fragrance fills the whole room. It happens because of diffusion of molecules which have fragrance. Smoke fills a room due to diffusion. Exchange of gases in living begins happens due to diffusion.
Osmosis: It is a type of diffusion but in case of osmosis there is movement of solvent molecules only. To understand this, let us take example of water which is a universal solvent. Let us assume that there is a beaker with two chambers in which the chambers are separated by a semi-permeable membrane. Salt solution is kept in one chamber and water is kept in another chamber. Water molecules will move towards the chamber which has salt solution. So, movement of water takes place from region of high water concentration to region of low water concentration. This happens in order to equalize the concentration on both sides of semi-permeable membrane. Such a movement of solvent is called osmosis. Water from soil enters roots because of osmosis.
When seeds are soaked in water for four to five hours or overnight, seeds take up water due to osmosis. As a result, seeds swell up in size.
Based on the relative concentration of water in a solution, there are three types of solution, viz. isotonic solution, hypotonic solution and hypertonic solution.
Cell wall: Cell wall is made of cellulose. It is somewhat hard but permeable to most of the substances. Cell wall is available in plant cells and in cells of bacteria and fungi. Cell wall in plant cells is composed of cellulose. Cell wall helps the cell to withstand the vagaries of nature. If a cell is present in very dilute medium, cell wall prevents it from bursting which may happen because of too much water entering the cell due to osmosis. In other words, cell wall helps in maintaining the osmotic pressure or turgor pressure inside the cell.
Nucleus: Nucleus is covered by double membrane, called nuclear membrane. The fluid which is inside the nucleus is called nucleoplasm. There is a dense network of thread-like structure inside the nucleus. These threads are called chromosomes. Chromosomes contain genes which are the carriers of genetic information. Nucleus plays an important role during cell division. Nucleus controls all the functions of the cell.
Cytoplasm: The fluid inside the cell is called cytoplasm.
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