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Diffusion is the passive random movement of molecules or particles in a fluid (liquid or gas) from a region in which they are in higher concentration to regions of lower concentration.
Diffusion stops when a uniform concentration is established.
Some major examples of diffusion in biology:
1. Gas exchange at the alveoli in the lungs — oxygen from air to blood, carbon dioxide from blood to air.
2. Gas exchange at the leaves during photosynthesis — carbon dioxide from air to leaf, oxygen from leaf to air.
3. Gas exchange at the capillaries for respiration — oxygen from blood to tissue cells, carbon dioxide in opposite direction.
4. Transfer of transmitter substance e.g. acetylcholine, from presynaptic membrane to postsynaptic membrane of neurons.
5. Absorption of mineral nutrients by roots.
6. Absorption of nutrients by the alimentary canal.
7. Osmosis — diffusion of water through a semi permeable membrane.
Shortening the distance, increasing the concentration difference and raising the temperature increase the rate of diffusion. Diffusion is much, much faster in a gas (air) than in a liquid (water).
Osmosis — a special example of diffusion.
Osmosis is the diffusion of water through a semi permeable membrane from a more dilute solution to a more concentrated solution.
Note: osmosis is passive i.e. energy from ATP is not used.
A semi permeable membrane is a barrier that permits the passage of the solvent molecules only but not the solutes (dissolved substances).
Cell membranes are described as selectively permeable because not only do they allow the passage of water but also allow the passage of certain solutes. The presence of particular solutes stimulates the membrane to open specific channels or active transport mechanisms to allow the passage of those chemicals across the membrane.
Some major examples of osmosis
1. Absorption of water by plant roots. Absorption of water by seeds for germination.
2. Re-absorption of water by the proximal and distal convoluted tubules of the nephron (renal tubule).
3. Re-absorption of tissue fluid into the venule ends of the blood capillaries.
4. The turgor of plant cells gives support to soft non-woody tissue.
6. Opening and closing of the stomata due to the change in shape of guard cells brought about by osmosis.
7. Absorption of water by the alimentary canal — stomach, small intestine and in particular the colon.
Osmosis and animal cells
(a) Lysis: the bursting of animal cells due to the excessive uptake of water by osmosis.
Cell membranes are not strong and are not elastic - they will not stretch, so they rupture.
(b) Crenation: the shrinkage of animal cells due to the loss of water by osmosis to a more concentrated external solution.
Animal cells can only function efficiently if they are surrounded by a solution of a particular concentration.
Osmoregulation: maintaining the concentration of cell cytoplasm or blood at a suitable concentration.
(a) Amoeba, living in freshwater, uses a contractile vacuole to expel the excess water from its cytoplasm.
(b) The kidneys are the osmoregulatory organs in the human body maintaining the blood at the correct concentration.
Osmosis and plant cells
(a) Plasmolysis: the shrinkage of the protoplasm plant cells from their cell wall due to the loss of water by osmosis to an external solution of higher concentration. The shrinkage stops when the vacuole cell sap is at the same concentration of the external solution.
Plasmolysis causes wilting of soft non-woody tissue e.g. drooping leaves and petals.
Plasmolysis is used in food preservation. Water is withdrawn by osmosis from contaminating bacteria and fungi inhibiting their growth and preventing their reproduction.
High sugar concentration is used in jam making to preserve fruit.
High salt concentration is used in the preservation of fish or meat.
(b) Turgor: the swollen rigid state of a plant cells due to the pressure of their protoplasm against the cell wall - the force is generated by the difference in concentration between the cell sap and the external solution.
Turgor supplies mechanical support for soft non-woody tissue, causes the change in shape of the guard cells to open the stoma, brings about the elongation of growing plant cells and maintains the protoplasm of the plant cell at the correct concentration.
Plant cells do not undergo lysis because a rigid wall of cellulose surrounds them.
Demonstration of Osmosis
1. Visking tubing is an artificial semi permeable membrane.
2. Tie a knot at one end of a short length of visking tubing.
3. Half fill the tubing with a strong glucose solution.
4. Tie a knot to seal the open end.
5. Place the glucose solution tube into a beaker of pure water.
6. Allow to stand for 30 minutes to one hour.
7. Test the liquid inside and outside the tubing for glucose using Benedict’s reagent.
Result: the tubing fills and expands plus the volume of water outside the tubing decreases; glucose is present inside the tubing but not outside the tubing.
Conclusion: only water passed through the tubing; water passed from the beaker through the tubing into the glucose solution; therefore osmosis has taken place.
Active transport
Active transport is the energy demanding transfer of a substance across a cell membrane against its concentration gradient, i.e., from a region where the substance is in lower concentration to where it is in higher concentration.
Special proteins within the cell membrane act as specific ‘carriers’.
ATP generated by respiration supplies the energy for active transport.
Some major examples of active transport
1. Re-absorption of glucose, amino acids and salts by the proximal convoluted tubule in the kidney.
2. The absorption of mineral nutrients by plant roots.
3. The pumping out of sodium ions by animal cells.
4. The pumping out of hydrogen ions by plant cells.
The syllabus also requires you to know
- A description of the application of high salt or sugar concentration in food preservation.
Practical activity: Conduct any activity to demonstrate osmosis.
Links
This provides a good overview of the diffusion process as well as providing interactivity for users.
http://www.bbc.co.uk/schools/gcsebitesize/biology/processes/diffusionandosmosisrev1.shtml
An overview of osmosis as well as some simple experiments.
http://www.dc.peachnet.edu/~ddonald/biolab/celtrns.htm
A brief description of the process of osmosis with a good diagram.
http://www.bbc.co.uk/scotland/revision/biology/investigating_cells/cells_and_diffusion_rev.shtml#osmosis
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