The cell is the smallest unit of all living organisms. Living organisms exists from cells and as such they are the very foundations of life. No living organism which exists today is not composed of cells. Cells can not be seen by the unaided eyes except through the help of an electron microscope.
There are two processes that brings about the exchange of materials in cells and they are
1. DIFFUSION: this is the movement of dissolved or gaseous substances from a region of higher concentration to a region of lower concentration until they are evenly distributed.
Examples of diffusion in cells’ activities are as follows;
1. Absorption of nutrients. E.g. mineral salt uptake in roots.
2. Exchange of gases in the cells or tissues. E.g; oxygen and carbon (IV) oxide exchange through the stomata.
3. Transport of materials within the cell.
4. Exchange of waste products. E.g. diffusion of carbon (IV) oxide into the alveoli.
1. Hormones are poured into the bloodstream directly by diffusion.
2. Food and oxygen diffuse from the maternal blood to the fetus through placenta.
3. Gaseous exchange in one-celled aquatic organisms is by diffusion.
4. Absorption of amino acids, glucose, fatty acids and glycerol by the villi of the small intestine occur by the process of diffusion.
SIGNIFICANCE OF DIFFUSION
When molecules are unevenly distributed in the cell, diffusion tends to equalise the distribution.
This is the movement of water molecules into substances that swell or increase in volume as a result of interaction between them and water molecules.
This is the movement of water molecules from a region of lower concentration gradient to a region of higher concentration gradient through a deferentially or semi permeable membrane in order to maintain an ionic equilibrium. Osmosis is therefore a special form of diffusion.
Thus osmosis may be defined as the net movement of water from an area of low diffusion pressure deficient or higher water concentration to a region of low water concentration through a semi permeable membrane.
ROLES OF OSMOSIS IN CELLS’ ACTIVITIES
1. Osmosis helps in the absorption of nutrients or uptake of nutrients or water.
2. Plasmolysis of cells or cells shrinkage in hypertonic solutions.
3. It helps in the turgidity of cells.
4. Haemolysis of red blood cells also known as bursting of animal cells in hypotonic solutions.
EXAMPLES OF OSMOSIS IN PLANTS
1. Water absorption by the roots.
2. Plasmolysis of plant’s cells.
3. Movement of water from one cell to another in plants.
4. Maintenance of turgor pressure in plant cells.
5. Opening and closing of stomata.
EXAMPLES OF OSMOSIS IN ANIMALS CELLS
1.Re-absorption of water by kidney tubule.
2. Haemolysis and crenation of red blood cells.
3. Osmo-regulation in Amoeba and Paramecium.
4. Maintenance of shape of mammalian cells in the surrounding tissue fluid.
Osmosis is significantly concerned with the transport of materials between cells.
Active transport is the movement of a substance across a cell membrane from a region of low concentration to a region of high concentration. This requires energy in form of ATP and it takes place only in actively respiring cells. Examples are the re-absorption of materials such as amino acids, glucose and sodium takes place INSIDE the kidney INTO the kidney tubules and mineral salts absorption by plants.
Distention of the wall of a plant cell by the accumulation of water within the cell is known as Turgor while a cell that is firm due to water uptake is said to be Turgid. When a cell becomes turgid as it absorbs so much water to attain its maximum sizein known as Turgidity.
The factors influencing turgidity of cells are as follows;
1. Formation of osmotically active substances inside the cells.
2. An adequate supply of water , and
3. A semi permeable membrane.
IMPORTANCE OF TURGIDITY IN CELLS
1. Turgidity helps in the transfer of nutrients from cell to cell
2. It helps in growth through the full expansion of tissues and organs.
3. It’s responsible for various movements of different organs of plants.
4. It helps in the transport of water from the cortex of the root into the xylem vessels and
5. It gives support and rigidity to plants.
When plants loose more water to the atmosphere, then what they absorb from the soil is said to be insufficient to maintain them. In such a situation, the reserved water from the cell vacuole is withdrawn and used. The plants become weak, limp and soft. The condition is known as Flaccidity.
HAEMOLYSIS AND CRENATION
Haemolysis is the swelling and bursting of red blood cells if placed in a weaker solution. Haemolysis is an example of osmolysis.
Crenation is the shrinking of red blood cells when placed in a stronger solution. Crenation may also be described as Laking.
This is the shrinking away of the cytoplasm continually until the sap vacuole is no longer visible from the cell wall when a cell is in a hypertonic solution. Plasmolysis is an example of ex-osmosis.