AQA AS Biology - Gas Exchange

For gas exchange surfaces to be successful, they must:

• have a large surface area
• be very thin
• maintain a steep concentration gradient of gases
  

Both of these factors maximise diffusion

Single-celled organisms can simply diffuse the gases they need across their cell membrane, but other organisms require a gas exchange system instead... 

Because water contains oxygen, fish can use the oxygen for respiration. They oxygen enters the fish through its mouth, and passes out through the gills.

• Each gill is made up of lots of small gill filaments, which increase the surface area
• The gill filaments are covered in even smaller structures called lamellae, which further increase surface area

Blood flows through the capillaries of the lamellae, as water flows in the opposite direction. This means, along the lamellae, there is always a steep concentration gradient between the water and the blood - allowing for maximum amount of oxygen to diffuse into the blood. This is called the counter-current principle.

Insects have microscopic air-filled pipes called tracheae which are used for gas exchange. Air moves into the tracheae through pores on the surface of insects called spiracles.

Oxygen moves into the insect along a concentration gradient, towards the cells. At the same time, carbon dioxide moves out of the cells, towards the spiracles.

The tracheae branch off into smaller tubes called tracheoles, which go into individual cells, where oxygen enters them. Insects use rhythmic abdominal movements to move air in and out.

Inside the leaf of a plant there are mesophyll cells, which have a large surface area. This is where the gas exchange takes place. But plants do not only need oxygen for respiration, they also need carbon dioxide for photosynthesis.

Mainly on the bottom of leaves, there are special pores called stomata, where gases move in and out. For respiration, oxygen moves in through the stomata on the leaf, where it enters the mesophyll cells, where respiration takes place.

This can only happen as long as the stomata are open, but opening the stomata also allows plants to lose water to the air, so they must close stomata occasionally to conserve water, This closing is done by guard cells, which control the opening and closing.