B3.1.1 DISSOLVED SUBSTANCES
a) Dissolved substances move by diffusion and by active transport.
b) Water often moves across boundaries by OSMOSIS. Osmosis is the diffusion of water from a dilute to a more concentrated solution through a partially permeable membrane that allows the passage of water molecules.
c) Differences in the concentration of the solutions inside and outside a cell cause water to move into or out of the cell by osmosis.
d) Most soft drinks contain water, sugar and ions.
e) Sports drinks contain sugars to replace the sugar used in energy release during the activity. They also contain water and ions to replace the water and ions lost during sweating.
f) If water and ions are not replaced, the ion/water balance of the body is distrubed and the cells do not work as efficiently.
B3.1.1 DISSOLVED SUBSTANCES CONTINUED...
g) Substances are sometimes absorbed agaisnt a concentration gradient. This requires the use of energy from respiration. The process is called active transport. Active transport enables cells to absorb ions from very dilute solutions.
h) Many organ systems are specialised for exchanging materials. The effectiveness of an exchange surface is increased by: having a large surface area, being thin to provide a short diffusion path, (in animals) having an efficient blood supply and being ventilated.
i) Gas and solute exchange surfaces in humans and other organisms are adapted to maximuse effectiveness.
j) The size and complexitiy of an organism increases the difficulty of exchanging materials.
k) In humans: the surface area of the lungs is increased by the alveoli and the surface area of the small intestine is increased by villi.
l) The villi provide a large surface area with an extensive network of capillaries to absorb the products of digestion by diffusion and active transport.
B3.1.2 GASEOUS EXCHANGE
a) The lungs are in the upper part of the body (thorax) protected by the ribcage and separated from the lower part of the body (abdomen) by the diaphragm.
b) The breathing system takes air into and out of the body so that oxygen from the air can diffuse into the bloodstream and carbon dioxide can diffuse out of the bloodstream into the air.
c) To make air move into the lungs the ribcage moves out and up and the diaphragm becomes flatter. These changes are reversed to make air move out of the lungs. The movement of air into and out of the lungs is known as ventilation.
B3.1.3 EXCHANGE SYSTEMS IN PLANTS
a) In plants:carbon dioxide enters leaves by diffusion. Most of the water and mineral ions are absorbed by roots.
b) The surface area of the roots is increased by roots hairs and the surface area of leaves is increased by the flattened shape and internal air spaces.
c) Plants have stomata to obtain carbon dioxide from the atmosphere and to remove oxygen produced in photosynthesis.
d) Plants mainly lose water vapour from their leaves. Most of the loss of water vapour takes place through the stomata. Evaporation is more rapid in hot, dry and windy conditions. If plants lose water faster than it is replaced by the roots, the stomata can close to prevent wilting.
e) The size of stomata is controlled by guard cells, which surround them.