AQA GCSE Diffusion for multicellular organisms (Biology)
Surface area to volume ratio.
Diffusion is slower in multicellular organisms as they have a smaller surface area to volume ratio. This is because less of the surface is exposed to the environment, also there are multiple cell layers deep which increases the diffusion distance, slowing down diffusion.
In the diagram below, when the gases only have to diffuse one cell this is easier, than diffusing across several cells to reach cells deeper into the tissue.
To solve these problems multicellular organisms have exchange surfaces and a transport system. Using both an exchange surface and transport system allows diffusion to be fast enough to meet the needs of the organism in terms of exchanging substances.
Without specialist exchange surfaces and transport system diffusion alone would not be sufficient to meet the needs of the multicellular organism
Exchange surfaces
In multicellular organisms, surfaces and organ systems are specialised for exchanging materials. This is to allow sufficient molecules to be transported into and out of cells to meet the organism’s needs.
Multicellular organisms have a specialist exchange surface to increase the rate of diffusion
A good exchange surface should have the following features:
1.Large Surface area
2.Thin to provide short diffusion pathway
3. Efficient blood supply (animals only)
4.Ventilation (animals only for gas exchange)
Exchange in the small intestine
The inside of the small intestine contains finger like projections called villi as shown below. Following digestion, the broken down nutrients such as glucose will diffuse from small intestine, into the blood.
Each villus provides a large surface area for diffusion to occur.
The villus has a thin wall, short diffusion distance for nutrients in the intestine to pass through.
There is a good blood supply to take away the absorbed nutrients, this helps to maintain the concentration gradient.
Villi have a brush border of microvilli, these are designed to further increase the surface area for diffusion, see the image below.
Exchange in lungs
The lungs are used to obtain oxygen for aerobic respiration and to remove carbon dioxide from the body.
Each lung contains about 150 million alveoli. Each alveolus is adapted for gas exchange.
Due to the high number of alveoli in the lungs they provide a large surface area for diffusion to occur, allowing efficient gas exchange.
Both the alveolus and the capillary wall are one cell thick each. This means that there is a short diffusion distance for the gases.
Ventilation constantly replenishes the air in the lungs. This ensures that the alveoli always have a high concentration of oxygen and low concentration of carbon dioxide to maintain concentration gradient.
Deoxygenated blood is supplied to the alveoli and oxygenated blood is taken away from the alveoli. This maintains the concentration gradient.
Gills in fish
As the fish moves forward, water enters through its mouth, passes over the gill filaments where gas exchange occurs. The water then leaves the fish, at the back of the head.
As water passes over the gill filaments oxygen diffuses from the water into the blood for aerobic respiration. Carbon dioxide produced by respiration diffuses from the blood into the water.
The gills are adapted for gas exchange.
Each gill is made up of many gill filaments. Each gill filament contains many gill lamellae which provide a large surface area for gas exchange
The gills have a good blood supply to bring deoxygenated blood and remove oxygenated blood, this maintains the concentration gradient.
Practice Question
1.Multicellular organisms need exchange surfaces and transport systems in order to survive. Explain why.
2. The villi in the intestine absorb broken down food molecules into the blood. Describe the adaptations they have to fulfil this role.
3. Explain how the alveoli are adapted for their role in gas exchange.