Osmosis is the diffusion of water from a dilute to a more concentrated solution through a partially permeable membrane that allows water to pass through.
Differences in the concentration of solutions inside and outside a cell cause water to move into and out of the cell by osmosis.
1.2 Active transport
Substances are sometimes absorbed against a concentration gradient by active transport.
Active transport uses energy from respiration.
Cells can absorb ions from very dilute solutions and actively absorb substances such as sugar and salt against a concentration gradient using active transport.
1.3 The sports drink dilemma
Most sports drinks contain:
- water to replace water lost when sweating
- sugar to replace sugar used for energy release in exercise
- mineral ions to replace those lost in sweating
For normal levels of exercise, water is likely to be as effective as a sports drink.
1.4 Exchanging materials - the lungs
Large surface area, short diffusion paths and steep concentration gradients increase the effectiveness of an exchange surface.
The alveoli are the air sacs in the lungs.
The lungs are adapted to make gaseous exchange as efficient as possible.
They have many alveoli, which provide a large surface area with a good blood supply and short diffusion distances.
The lungs are ventilated to maintain steep diffusion gradients.
Diagram of alveoli:
1.5 Ventilating the lungs
The lungs are in your thorax protected by your ribcage and separated from your abdomen by the diaphragm.
The intercostal muscles contract to move your ribs up and out and flatten the diaphragm, increasing the volume of your thorax. The pressure decreases and air enters lungs.
The intercostal muscles relax and the ribs move down and in, and the diaphragm domes up, decreasing the volume of your thorax. The pressure increases and air is forced out.
1.7 Exchange in the gut
The food we eat is digested in the gut into small, soluble molecules.
In the small intestine these solutes are absorbed into the blood.
The villi line the inner surface of the small intestine and are the exchange surface for food molecules.
They provide a large surface area with an extensive network of blood capillaries.
This makes villi well adapted to absorb the products of digestion by diffusion and active transport.
1.8 Exchange in plants
Plants have stomata that allow them to obtain carbon dioxide from the atmosphere.
Carbon dioxide enters the leaf by diffusion. Leaves have a flat, thin shape and internal air spaces to increase the surface area available for diffusion.
Most of the water and mineral ions needed by a plant are absorbed by the root hair cells, which increase the surface area of the roots.
Plants need to let carbon dioxide diffuse into the leaves for photosynthesis.
At the same time the plant is losing water vapour through the stomata and could wilt if the roots are not absorbing water fast enough.
CROSS-SECTION OF A LEAF:
The loss of water vapour from the surface of plant leaves is known as transpiration.
Water vapour is lost through the stomata, which are opened and closed to let in carbon dioxide for photosynthesis.
Transpiration is more rapid in hot, windy dry or bright conditions.
Conditions which increase the rate of evaporation will increase the rate of transpiration.
Wilting can protect the leaf from excessive water loss, as do the guard cells.