7.1. SPECIALISED EXCHANGE SURFACES

Question 1

Why can some small organisms cope by diffusion alone to supply their needs?

Answer 1

• metabolic activity of single-celled organism is relatively low, so oxygen demands and carbon dioxide production of the cell are relatively low.
• surface area to volume ratio of organism is large.

Question 2

Why do many larger organisms need specialized exchange surfaces and can’t rely purely on diffusion?

Answer 2

• made up of many cells arranged in tissues, organs, and organ systems.
• metabolic activity is much higher than most single-celled organisms.
• small SA:V ratios (larger distances for substances to travel from outside to the cells in the centre of the body).
• so gases can’t be exchanged fast enough or in large enough amounts for the organism to survive.

Question 3

What are the main specialised exchange surfaces?

Answer 3

• increased surface area
• thin layers
• good blood supply
• ventilation to maintain diffusion gradient

Question 4

How is good blood supply help with exchange of substances?

Answer 4

• the steeper the concentration gradient, the faster diffusion takes place
• a good blood supply ensures substances are constantly delivered to and removed from the exchange surface, this maintains a steep concentration gradient for diffusion.
• for example, the alveoli of the lungs

Question 5

how does ventilation help with exchange of substances?

Answer 5

• for gases, a good ventilation system helps maintain the concentration gradients
• for example, alveoli and gills of a fish