Exchange of materials

Osmosis is a special case of diffusion, it is only the movement of water molecules, from a dilute to a more concentrated solution through a partially permeable membrane that allowes water to pass through. So osmosis is diffusion through a partially permeable membrane.
Osmosis in animals: osmosis helps keep the balance of water right from both sides, in cells. But osmosis can also cause problems in the animal cells. If the solution outside the cell is more dilute than the cell contents, water will move in the cell. It will then swell and then burst. In the opposite case, water will move out of the cell. It will then shrivel up. Then it can no longer survive.
Osmosis in plants: plants rely on osmosis to support their stems and leaves. Water moves into the cells, this causes the vacuole to swell and press the cytoplasm against the plant cell walls. The pressure builds up until no more water can physically enter the cell. This makes the cell hard and rigid. The swollen state keeps the leaves and stems firm. So the fluid surrounding the cells needs to have a higher concentration of water. This keeps the osmosis moving in the right direction.

Active transport is when substances are absorbed against a concentration gradient. Unlike osmosis and diffusion, it requires energy from respiration. This is because the particles are being absorbed against a concentration gradient.

Moving substances by active transport: The movement is against a concentration gradient, so cells can absorb ions from very dilute solutions. Also it enables them to move substances like sugars and ions, from one place to another. The molecule is carried across a membrane by a transport protein. It then returns to its original position.

The importance of active transport: It is widely used in cells, but there are some situations which it is partially important. For example mineral ions in the soil are usually found in very dilute solutions. These are more dilute than the solution in the plant cells. By using active transport the plants can absorb these mineral ions.

When you exercise your muscles respire, to release energy. When you get hot you sweat. But when you sweat you lose mineral ions and water. If you sweat a lot your body cells can become dehydrated. Sports can replace what you, and help rehydrate your cells.

The drinks are designed to help balance the concentration of body fluids and the concentrations inside the cells. If the drink concentration matches the body fluids the solution is called isotonic.

The evidence for the benefits of the sports drinks is varied. Some scientists think it is just as good to have water for short periods of exercise. But the drinks may help to replace mineral ions and sugar as well as water.

As living organisms get bigger and more complex, their SA:V ratio gets smaller. Thai makes it more difficult to exchange materials quick enough. So many in larger organisms there are special surfaces where gas and solute exchange take place.
Adaptions for exchanging materials: these include; larger SA, being thin (shorter diffusion path), having efficient blood supply and being ventilated. An example is that the musk turtle had a specially adapted tongue. It is covered in tiny bud, that increase the SA. It also has a good blood supply. The tongue is used for gaseous exchange, the buds absorb oxygen from water that passes over them.
Exchange of gases in th lungs: the lungs are made up of clusters of alveoli. These tiny air sacks give a very large SA. Also they have short diffussion distances.They also have a rich blood supply, this maintains a concentration gradient in both directions. Oxygen is constantly moved from the air to the lungs into the blood. CO2 is constantly delivered from the blood to lungs as well. As a result gas exchange takes place along the steepest concentration gradient. Making it rapid and effective. The layer of cells between the air in the lungs and the blood in the capillaries is also very thin.

For a gas exchange system to work efficiently you need a steep concentration gradient. Humans maintain a steep concentration gradient of both oxygen in and carbon dioxide out. This is known as ventilating e lungs or breathing. It takes place in your specially adapted breathing system.
The breathing system: your lungs are found in your thorax and are protected by your ribcage. They are separated from your digestive organs by your diaphragm. This is a strong sheet of muscle.the lungs provide an efficient surface for gas exchange in the alveoli.
When you breathe in: the intercostal muscles and the diaphragm contract. The ribcage moves up and out and the diaphragm flattens. The volume of the thorax increases. The pressure of the thorax decreases and air is drawn in.
When you breathe out: the intercostal musicals and the diaphragm relax. The ribcage moves down and in and the diaphragm becomes domed. The volume of the thorax decreases. The pressure increases and the air is forced out.

People sometimes struggle to breathe; the tubes leading to the lungs are very narrow, the structure of the alveoli can break down and some people are paralysed in an accident.
The 'iron lung', negative pressure: this involves the person being put into a cylinder, with a tight around the neck. Air is pumped out, lowering the pressure. As a result the chest wall of the patient moves up. Air moves from higher pressure outside, to lower in the body. When the vacuum is turned off, air moves into the chamber increasing the pressure, so air is forced out the lungs.
Positive breathing pressure: this forces a carefully measured amount of air into the lungs. When the lungs are inflated, air pressure drops, and the lungs deflate. it can be used quickly I'm an emergency, using a hand held ventilator bag. It can be given as a simple mask or by a tube going into the trachea.

Absorption in the gut: for digested food molecules to reach your cells they must move from inside your small intestine into your bloodstream. They do the by a combination if diffusion and active transport. The food molecules move through the walls of the small intestine to the blood vessels. They move in this direction because there is a very high concentration of the food molecules and much lower in the gut. The lining of the is folded into thousands of villi. They provide a large SA, with an extensive network of blood capillaries. They greatly increase the uptake of digested food, there is more room for diffusion. They have a good blood supply, so a steep concentration is maintained.
Active transport in the small intestine: as the time since your last meal gets longer you have more dissolved food molecules in your blood than in your digestive system. Glucose and other dissolved food molecules are then moved from the small intestine into the blood by active transport. This makes sure that no digested food is wasted in the faeces.

Gases diffuse in and out of leaves through tiny holes called 'stomata', the size is controlled by guard cells. These gases are; oxygen, needed for respiration and waste product of photosynthesis and carbon dioxide, needed for respiration and waste product of respiration. Plants also lose water vapour through the stomata due to evaporation in the leaves. The stomata allow carbon dioxide to diffuse into the leaf for the air.

Leaves are flat and very thin, so the gases do need not need to diffuse very far. There are also internal air spaces. These increase the SA for the diffusion of gases.

Water and mineral ions are taken up by the roots. Root hair cells increase the SA, for the absorption of water and mineral ions.

If plants lose water faster than it is replaced by the roots, the stomata can close to prevent wilting.

Water loss from the leaves: the stomata can be opened and closed by the guard cells. As water evaporates, more water is pulled up through the xylem to take its place. This is known as transpiration stream. So anything that affects the rate of evaporation will affect transpiration.

The effect of the environment on transpiration: evaporation is more rapid in hot, dry, windy or bright conditions.

Controlling water loss: most leaves have a waxy, waterproof layer (the cuticle). In very hot conditions the cuticle may be very thick and shiny. Most of the stomata are found on the underside of the leave, this protects them from direct sunlight, and reduces the time they are open. If it loses water faster than it is replaced, the whole plant may wilt. The stomata can close and prevent most water lose and further wilting.