Exchange Surface

Question 1

Exchange surface

Answer 1

All living organisms need O2 and nutrients to survive, as well as remove waste products so these don’t build up. In large organisms which have 2+ layers of cells diffusion are no longer efficient enough, and a specialised exchange system is needed.

Question 2

Factors that affect the need for an exchange system

Answer 2

Size
Surface area to volume ratio
Level of metabolic activity

Question 3

size:

Answer 3

• In very small organisms (single-celled) the cytoplasm is close enough to the environment they live in
  for diffusion to supply enough O2 and nutrients to keep cells alive and active.
• However multicellular organisms have several layers of cells, the O2 and nutrients have a longer
  diffusion pathway, making diffusion too slow to enable a sufficient supply to the innermost cells.

Question 4

Surface area to volume ratio (note: expressed as a ratio, x:1):

Answer 4

• Small organisms have a small surface area, and a small volume. Their SA is relatively large when
  compared to their volume, therefore, they have a large SA:V ratio. The surface area is large enough
  to supply all their cells with sufficient oxygen by diffusion.
• Larger organisms have a large SA and volume, but their volume is relatively large compared to their
  SA. Therefore, they have a small SA:V ratio.
• Some animals increase their surface area to increase their SA:V ratio (e.g. flatworms).

Question 5

Level of metabolic activity:

Answer 5

• Some organisms are more active than others, metabolic activity uses energy from food and oxygen
  to release energy in aerobic respiration.
• The cells of active organisms need good O2 and nutrient supply for energy and movement,
  (mammals need this energy to keep warm).
• Multicellular organisms are more metabolically active than single-celled organisms and so diffusion
  isn’t sufficient.

Question 6

Features of good exchange surfaces:
large surface area

Answer 6

• Large surface area – increases diffusion rate and provides space for molecules to pass through. Often
achieved by folding walls and membranes involved (e.g. root hair cells – increases osmosis of water
and active transport of mineral ions rate).

Question 7

Features of good exchange surfaces:
Thin barrier

Answer 7

• Thin barrier – to reduce diffusion distance, increasing diffusion rate, also must be permeable to the
substances being exchanged (e.g. alveolar epithelium is 1 cell thick).

Question 8

Features of good exchange surfaces: Good blood supply/ventilation

Answer 8

• Good blood supply/ventilation – brings fresh supply of molecules to supply side, keeping the
concentration high, or removing molecules from the demand side to keep concentration low. This is important to maintain steep concentration gradient to keep high rate of diffusion (e.g. fish gills and alveolus).