Lesson 51 - Exchange and SA:V

Question 1

What are the essential features of an exchange surface for rapid exchange?

Answer 1

Large surface area compared to the volume of the organism (SA:V ratio)
Thin - short diffusion pathways so diffusion is rapid
Selectively permeable - to allow select materials to cross
Maintenance of steep conc. gradients (ie. for diffusion) - may require a transport system to move the internal and external environments

Question 2

What is Surface Area to Volume ratio?

Answer 2

The surface area of an organism divided by its volume, expressed as a ratio or decimal

Question 3

What are the formulas for surface area and volume?

Answer 3

Sphere SA: 4πr2
Sphere V: 4/3 πr3
Cuboid SA: 2(lw + lh + wh)
Cuboid V: l x w x h
Cylinder SA: 2πrh+2πr2
Cylinder V: πr2h

Question 4

Why do unicellular organisms not need specialised exchange organs?

Answer 4

A single cell is so small that the distance from the cell-surface membrane to the centre of the cell and diffusion distances inside the cell are short. Therefore diffusion is rapid enough for the cell’s needs. So they can obtain nutrients and excrete waste products simply just by diffusion

Question 5

Metabolism definition?

Answer 5

All the chemical processes that occur within a living organism in order to maintain life. Includes building-up processes (anabolism) as well as breaking-down processes (catabolism). These processes need energy in order to proceed.

Question 6

Why do multicellular organisms need specialised exchange organs?

Answer 6

Diffusion across the outer membrane is too slow due to: some cells are deep within the body (big distance between them and the outside environment), as well as large animals having a low SA:V ratio, it’s difficult to exchange enough substances to supply a large volume of animal through a relatively small outer surface.

Question 7

How body size and shape relate to heat exchange?

Answer 7

Body size: Organism loses heat more easily if it has a larger SA:V ratio. Which means smaller organisms need a relatively high metabolic rate, in order to generate enough heat to stay warm.
Body shape: Organisms with a more compact shape have a small SA:V ratio - minimising heat loss from their surface.

Question 8

What are some examples of behavioural and physiological adaptations to aid exchange?

Answer 8

• To support their high metabolic rates, small animals living in cold regions need to eat large amounts of high energy foods, e.g. seeds/nuts
• Smaller mammals may have thick layers of fur or hibernate when the weather gets really cold
• Animals with a high SA:V ratio tend to lose more water as it evaporates through their surface, problematic for animals living in hot regions where water evaporates quickly. So have kidney structure to produce less urine to compensate
• Larger organisms living in hot regions, have slow relative heat loss. Elephants have developed large flat ears to increase SA to lose more heat, while hippos spend most of their time underwater.