Biology AS Cells Exchange and Transport Module 2 Exchange and Transport

An exchange surface is a specialised area that is adapted to make it easier for molecules to cross from one side of the surface to the other.

- oxygen for aerobic respiration
- glucose as a source of energy - proteins for growth and repair
- fats to make membranes and to be a store of energy
- water
- minerals to maintain their water potential and to help enzyme action and other aspects of metabolism

- large surface area to provide more space for molecules to pass through
- thin barrier to reduce diffusion distance
- fresh supply of molecules on one side to keep the concentration high
- removal of required molecules on the other side to keep the concentration low

1) small intestine (where nutrients are absorbed)
2) liver (where levels of sugars in the blood are adjusted)
3) root hairs of plants (where water and minerals are absorbed)
4) hyphae of fungi (where nutrients are absorbed)

Gaseous exchange is the movement of gases by diffusion between an organism and its environment across a barrier such as the alveolus wall.

Gases pass both ways through the thin walls of the alveoli. Oxygen passes from the air in the alveoli to the blood in the capillaries. Carbon dioxide passes from the blood to the air in the alveoli.

The large surface area provides more space for molecules to pass through.

The plasma membranes that surround the thin cytoplasm of the cells form the barrier to exchange. These readily allow the diffusion of oxygen and carbon dioxide.

- the alveolus wall is one cell thick
- the capillary wall is one cell thick
- both walls consist of squamous cells
- the capillaries are in close contact with the alveolus walls
- the capillaries are so narrow that the red blood cells are squeezed against the capillary wall, making them closer to the air in the alveoli and reducing the rate at which they flow past in the blood
- the total barrier to diffusion is only two flattened cells thick

To reduce the cohesive forces between the water molecules. Without the surfacant, the alveolus would collapse due to the cohesive forces between the water molecules lining the air sac.

For diffusion to be rapid.

This means having a high concentration of molecules on the supply side of the exchange surface and a low concentration on the demand side.

A fresh supply of molecules on one side is needed to keep the concentation there high, and a way of removing the molecules from the other side is needed to keep the concentration there low.

This is achieved by the action of the blood transport system and the ventilation movements. The blood brings carbon dioxide from the tissues to the lungs. This ensures that the concentration of carbon dioxide in the blood is higher than that in the air of the alveoli. It also carries oxygen away from the lungs, This ensures that the concentration of oxygen in the blood is kept lower than the concentration in the air inside the alveoli. The heart pumps the blood along the pulmonary artery to the lungs. In the lungs, the artery divides up to form finer and finer vessels. These eventually carry blood into tiny capillaries that are only just wide enough for a red blood cell to squeeze through. These capillaries lie over the surface of the alveoli. The breathing movements fo the lungs ventilate the lungs. They replace the air used with fresh air. This brings more oxygen into the lungs and ensures that the concentration of oxygen in the air of the alveolus remains higher than the concentration in the blood. Ventilation also removes air containing carbon dioxide from the alveoli. This ensures that the concentration of carbon dioxide in the alveoli remains lower than that in the blood.

- Diaphragm contacts to become flatter and pushes digestive organs down - External intercostal muscles contract to raise ribs
- Volume of chest cavity increases
- Pressure in chest cavity drops below atmospheric pressure - Air moves into lungs