TRANSPORT- DIFFUSION AND OSMOSIS - PART 2

 MOVEMENT OF SUBSTANCES IN AND OUT OF CELLS

Multi-cellular organisms have cells which allow substances in solution to pass into and out of them. This happens in two major ways – diffusion and osmosis. 

DIFFUSION

Diffusion is the movement of molecules from a region of high molecular concentration to the region of low molecular concentration until the molecules are evenly distributed. 

The difference in concentration is called concentration gradient. The higher the gradient the faster the rate of diffusion. In other words diffusion stops when the concentration gradient of the two regions is the same. 

 Heating and stirring increase the movement of molecules, and therefore, can speed up the rate of diffusion. Diffusion is very prominent in gases because the molecules in gases have wide spaces between them. 

Diffusion is slower in liquids because the molecules are closely packed together than they are in gases. The molecules in solids are tightly packed and therefore the molecules do not have enough rooms to move around, hence diffusion is absent in solids. 

 Fig. 11.1: Diffusion in a liquid Examples of diffusion in living organisms 

1. Diffusion of oxygen into the blood and carbon dioxide out of the blood in the lung of mammals. 

2. Diffusion of hormones out of the endocrine glands. 

3. Absorption of end products of digestion, e.g. glucose, amino acids in the ileum (small intestines) of mammals. 

4. Diffusion of carbon dioxide into the blood and oxygen out of the blood in the tissue of vertebrates. 

5. Diffusion of oxygen into the cells and carbon dioxide out of the cells of unicellular organisms such as amoeba. 

6. Diffusion of carbon dioxide into the leaves and oxygen out of the leaf through the stomata. 

7. Diffusion of mineral salt from the soil water into root hair cells. 

8. Diffusion of oxygen into the leaves and carbon dioxide out of the leaves of plants at night. 

OSMOSIS

Osmosis is the movement of water molecules from a dilute solution to a more concentrated solution across a semi-permeable membrane. 

The higher the difference in concentration between the two solutions, the faster the rate of osmosis. A rise in temperature also speeds up the rate of osmosis. A semi-permeable membrane is a membrane that allows certain substances to pass through and blocks the passage of some other substance depending on the nature and size of the substance. 

Demonstrating Osmosis in a Semi Permeable Membrane 

 Peel off the skin of a yam tuber 

 Cut it into two

 Make a cavity at the centre of each half and flatten the bottom 

 Label one A and fill it with known volume of a concentrated salt solution 

 Label the other one B and fill it with water 

  Fill the two vessel with water to about 1/3 full and record the volumes 

 Sit the yam cups into each of the vessels 

 Leave the whole experimental setup for 24 hours. Observation After 24 hours, it would be observed that the level of concentrated salt solution in yam cup A has increased in volume whiles the water in the vessel has decreased. 

 Compared to B, the volume of water in both yam cup and the vessel will remain the same.

 Conclusion Water molecules in the vessel of setup A have moved from a low concentration to a high concentration (salt solution) through the yam which acts as a semi-permeable membrane. 

In setup B, water molecules in both the vessel and the yam cavity have the same (low) concentration. Therefore, there was no movement of molecules from one region to the other.

Demonstrating Osmosis in Non-Living Tissue

 Cover the ends of two thistle funnels, labelled A and B, with cellophane paper. 

 Fill funnel A with sugar solution and B with water. 

 Clamp them both upside down into a beaker of water. 

 Leave the setup for 24 hours. Observation After 24 hours, it would be observed that the level of liquid in test tube A would rise but that of test tube B would remain the same. 

Conclusion Water molecules have moved from a lower concentration to a higher concentration through the cellophane paper which acts as a semi-permeable membrane. 

 Examples of Osmosis 

1. Absorption of water from the soil by roots of plant. 

2. Movement of water across the cell of plants. 

3. Movement of water from the root hair into the cortex. 

4. Absorption of water in the colon (large intestines) of mammals. 

5. Absorption of water in the nephron of the kidney of mammals. 

6. Entering of water into the cells of amoeba. 

Differences between diffusion and osmosis 

Diffusion                                                 Osmosis           

Occurs in both gases and liquids          Occurs in liquids only 

Does not require partial membrane      Requires a partial membrane

Diffusion Involves movement of any substance (both solvent and solute)                                                                   

Osmosis Involves movement of only solvent particles (e.g. water molecules)