Practice Question

1.Multicellular organisms need exchange surfaces and transport systems in order to survive. Explain why.

Diffusion is too slow in multicellular organisms to meet their needs in terms of exchanging materials with the environment. This is due to 

Small surface area to volume ratio

Longer diffusion pathway, due to multiple cell layers deep

To solve this problem an exchnage surface and transport system is used to increase the rate of diffusion, so diffusion is fast enough to meet the needs of the multicellular organism.

2. The villi in the intestine absorb broken down food molecules into the blood. Describe the adaptations they have to fulfil this role.

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. 

3. Explain how the alveoli are adapted for their role in 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.