The exchange of materials is vital in the life of all organisms. Throughout life, an organ- ism continually exchange materials between itself and its surroundings, and between the cells and fluids within its body.

1. For cells to obtain nutrient.


2. Allow cells to excrete waste substances.


3. Allows cells to excrete useful substance.


4. To generate ionic gradient for muscular and nervous activity.


5. To maintain suitable pH and ionic concentration for enzyme activity.

Molecules move in and out of a cell through the cell membrane, which forms the boundary of each cell. The cell membrane is selectively permeable to substances, which means that it permits entry and exit of certain molecules only. The movement of molecules takes place by diffusion, osmosis, and active transport.

1. Permeable membrane

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It is a membrane through which molecules of both solute and solvent can pass e.g. cell wall of plants



2. Semi-permeable membrane (or selectively permeable membrane)

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It is a membrane which allows some types of substance only to pass through it. A semi-permeable membrane or partially-permeable membrane allows only solvent mole- cules but not solute molecule (dissolved solid) to pass through it. Goat skin, pigs blad- der, cellophane, toads skin, parchment paper, visking tube are all semi-permeable membrane. Cellophane, parchment paper and visking tube are non-ling semi-permeable membranes

Diffusion occurs only when a concentration gradient exists. The concentration gradient is the difference in concentration of molecules between two regions. The steeper the concentration gradient, the faster the rate of diffusion. The rate of diffusions affected by the following factors:

1. Temperature - Increase in temperature increases diffusion and decrease in temperature decreases diffusion.

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2. Concentration gradient - The greater the difference between the concentrations of the two regions, the higher the rate of diffusion and the lesser the difference the lower the rate of diffusion.

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3. Size of the molecules-When the molecules are smaller the rate of diffusion is high and when they are big, the rate is slower.

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4. Permeability of surface membrane - A lower permeability will impede molecular movement and thus it will lead to slower movement.

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5. The state of molecules - gaseous molecules diffuse much faster than liquid molecules.

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6. Agitation or shaken

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1. Plants absorb some mineral salts by diffusion from the soil through root hair cells.


2. Oxygen diffuses out and carbon dioxide diffuses in through the stomata in leaves during the day.


3. Unicellular organisms take in oxygen and get rid of carbon dioxide by diffusion across the general body surface.


4. Oxygen diffuses into the blood and carbon dioxide diffuses out of the blood in the lungs of mammals.


5. Gaseous exchange in the gills e.g. in fish.

• Open a bottle containing perfume in the corner of a room


• Move some distance away from the bottle of perfume


• Wait for a few minutes

Obtain a bottle of highly scented perfume. Stand at one corner of a classroom and after closing all the doors and windows. Put two drops of the perfume on the floor and move to the opposite end of the classroom. The scent of the perfume will be perceived throughout the room which means that the particles of the perfume have moved from a region of higher concentration to a region of lower concentration

• A beaker is filled with water


• It is left undisturbed for a few minute to get it settled


• A crystal of potassium permanganate is carefully dropped into the water


• The beaker is left on a bench for at least 30 minutes

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

Osmosis is the movement of water molecules from a region of high water concentratio to a region of low water concentration across a semi-permeable membrane

Osmosis can be explained by the fact that less concentrated solution contains more free energy, so solvent molecules tend to diffuse to a place of lower free energy in order to equalize free energy.

1. Concentration gradient - the greater the difference in concentration between the two solution, the faster the rate of osmosis


2. Semi-permeable membrane or permeability of the membrane


3. Temperature- the higher the temperature, the faster the rate of osmosis

1. In humans


• Reabsorption of water in the tubules of the nephrons of the kidney


• Reabsorption of water from the large intestines into the blood stream


• Haemolysis and crenation of red blood cells. Saltwater from the ocean is hypertonic to the cells of the human body; the drinking of ocean water dehydrates body tissues instead of quenching thirst.


2. In plants

• Absorption of water from soil into the vacuole of root hair


• Water uptake through root hairs to the cortex or xylem


• Opening and closing of the stomata


• Gives turgidity to plant cell


• Germinating seeds imbibe water through osmosis.


• Continuous loss of water from the plant will lead to death or wilting

1. The mouth of the thistle funnel is tied with visking tube.


2. The thistle funnel is filled half-way the stem with a strong sugar solution (30% sugar solution)


3. The initial level of the sugar solution is marked


4. The thistle funnel is placed into a beaker containing water with the covered end inside the water


5. A control experiment is set up in which water is used to fill both the beaker and the thistle funnel


6. The level of the sugar solution is observed at regular intervals for about six hours