gas and solute exchange

life processes need gases or other dissolved substances before they can happen. For example, for photosynthesis to happen, carbon dioxide and water have to get into plant cells. And for respiration to take place, glucose and oxygen both have to get inside cells. Waste substances also need to move out of the cells so that the organism can get rid of them. These substances move to where they need to be by diffusion, osmosis and active transport. Diffusion is where particles move from an area of high concentration to an area of low concentration. For example, different gases can simply diffuse though one another, like whena weird smell spreads through a room. Alternatively, dissolved particles can diffuse in and out of cells through cell membranes. Osmosis is similar, but only refers to water. The water moves acroos a partially permeable membrane (e.g. a cell membrane) from an area of high concentration to an area of low concentration. Diffusion and osmosis both involve stuff moving from an area where theres a high concentration of it, to an area where there's a lower concentration of it. Sometimes substances need to move in the other direction which is where active transport comes in. in life processes, the gases and dissolved sunstances have to move through some sort of exchange surface. The exchange surface structures have to allow enough of the necessary substances to pass through.

Exchange surfaces are adapted to maximise effectiveness!

carbon dioxide diffuses into the air spaces within the leaf, then it diffuses into the cells where photosynthesis happens. The leaf's structure is adapted so that this can happen easily. The underneath of the leaf is an exchange surface. It's covered in tiny little holes called stomata which the carbon dioxide diffuses in through. Water vapour and oxygen also diffuse out through the stomata. (water vapour is actually lost from all over the leaf surface, but most of it is lost through the stomata). The size of the stomata are controlled by the guard cells. These close the stomata if the plant is loosing water faster than it is being replaced by the roots. Without these guard cells the plant would soon wilt. The flattened shape of the leaf increases the area of this exchange surface so that it's more effective. The walls of the cells inside the leaf form another exchange surface. The air spaces inside the leaf increases the area of this surface so there's more chance of carbon dioxide to get into the cells.

The water vapour escapes by diffusion because there's a lot of it inside the leaf and less of it in the air outside. This diffusion us called transpiration and it goes quicker when the air around the leaf is kept dry i.e. transpiration is quickest in hot, dry, windy conditions.

You need to get oxygen from the air into your bloodstream so that it can get to your cells for respiration. You also need to get rid of carbon dioxide in your blood. This all happens inside the lungs. Breathing is how the air gets in and out of your lungs.

The lungs are in the Thorax

The thorax is the top part of your "body". It's seperated from the lower part of the body by the diaphragm. The lungs are like big pink sponges and are protected by the ribcage. The air that you breath in goes through the trachea. This splits into two tubs called the "bronchi" (each one is "aa bronchus"), one going to each lung. The bronchi split into progressively smaller tubes called bronchioles. The bronchioles finally end at snall bags called alveoli whre the gas exchange takes place.

breathing in

intercostal muscles and diaghram contract.

Thorax volume increases.

This decreases the pressure, drawing air in.

breathing out

intercostal muscles and diaphragm relax.

thorax volume decreses.

air is forced out.