specialised exchange surfaces (a,b)

Question 1

Why is diffusion sufficient to supply the needs of single celled organisms?

Answer 1

• metabolic activity of a single-celled organism is usually low, so oxygen demands and carbon dioxide production of the cell are relatively low
• SA:V of the organism is large

Question 2

Why is diffusion not sufficient to supply the needs of multicellular organisms?

Answer 2

• they have higher metabolic activity as they use lots of energy for movement etc
• as an organism increases in size, its SA:V ratio decreases
• distances that substances need to travel from the outside to reach the cells get longer

Question 3

Features of efficient exchange surfaces

Answer 3

• increased surface area - e.g. root hair cells
• thin layers - so short diffusion distances so fast and efficient process - e.g. alveoli
• good blood supply - gives steeper concentration gradient so faster diffusion - constant delivery of substances to and from the exchange surface e.g. alveoli or gills
• ventilation - helps maintain concentrations + make processes more efficient e.g. alveoli or gills