Exchange of Materials

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Osmosis is the diffusion of water across a partially permeable membrane.  It requires no energy from the cell because the water molecules move at random.  The water moves from a region of high concentration (dilute) to a region of low concentration (concentrated) along a concentration gradient.


Cells may need to absorb substances that are in short supply, against the concentration gradient. Cells use active transport to absorb substances across partially permeable membranes against a concentration gradient.  Active transport requires energy that is released in respiration.  Active transport takes place through transport proteins.  They rotate and release the molecule inside the cell using energy released from respiration.  The protein rotates back again.


Glucose + oxygen ----> carbon dioxide + water + energy

When you exercise, your muscles respire to release energy.  Glucose, a sugar, is used in respiration.  After a while, the body gets hot and you sweat more in order to cool down.  Sweat contains water and mineral ions. If you sweat a lot,the cells in your body may become dehydrated due to the water leaving the cells.  If you exercise for long periods of time you may need to replace the sugar, mineral ions and water which have been used or lost.

Sports drinks are solutions of sugar and mineral ions.  They also contain water, which helps to rehydrate the body cells. The drinks are designed to help balance the concentration of body fluids and the concentration inside cells.  If the drink concentration matches those of the body fluids, the drink is said to be isotonic.  The evidence for the benefits of sports drinks is varied.  Some sports scientists suggest that having a drink of water is just as effective.


Large, complex organisms have special exchange surfaces to obtain all the food and oxygen they need. Solutes are absorbed by the intestine.  Oxygen is absorbed by the lungs and carbon dioxide is removed from them.  Efficient exchange surfaces have a large surface area, thin walls or a short diffusion path.  They also have an efficient transport system - the blood supply in humans.

The lungs contain the gaseous exchange surface.  The surface area of the lungs is increased by the alveoli.  They have thin walls, a large surface area and a good blood supply via the capillary network.  The lungs are ventilated to maintain a steep concentration gradient.  Oxygen diffuses into the many capillaries surrounding the capillaries and carbon dioxide diffuses back into the lungs to be breathed out.


The lungs contain…


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