Surface area to volume ratio

Question 1

What is the relationship between the size of an organism/structure and the surface area to volume ratio?

Answer 1

• Smaller organisms have a larger surface area to volume ratio
• Larger organisms have a smaller surface area to volume ratio

Question 2

Why do organisms need to exchange substances with the environment?

Answer 2

1. Cells need oxygen and glucose for aerobic respiration (and nutrients)
2. They also need to excrete waste products like carbon dioxide and urea
3. Heat also needs to be exchanged as most organisms need to stay at roughly the same temperature

Question 3

Why can multicellular organisms (animals/plants) not perform gas exchange via their surfaces?

Answer 3

Diffusion is too slow

• have a small surface area to volume ratio
• multicellular (large diffusion distance and high demand)
• impermeable surface (prevent pathogens entering and reduce water loss)
• therefore, require specialised Exchange & Transport systems

(exchange system = increases rate of diffusion of nutrients in and wastes out)
(transport system = deliver nutrients and remove waste from all cells)

Question 4

What is an exchange system?

Answer 4

increases rate of diffusion of nutrients in and wastes out

Question 5

What is a transport system?

Answer 5

deliver nutrients and remove waste from all cells

Question 6

How does body size affect heat loss?

Answer 6

The rate of heat loss depends on an organisms surface area

• if an organism has a large volume its surface area is relatively small - this makes it harder for it to lose heat from its body
• if an organism is small its relative surface area is large so heat is lost more easily

Therefore, this mean smaller organisms need a relatively high metabolic rate, in order to generate enough heat to stay warm

Question 7

How does body shape affect heat loss?

Answer 7

1. Animals with a compact shape have a small surface area relative to their volume - minimising heat loss from their surface.
2. Animals with a less compact shape (gangly/have sticky out bits) have a larger surface area relative to their volume - this increases heat loss from their surface

(whether an animal is compact or not depends on the temperature of its environment- animals body shape is adapted to suit its environment)

Question 8

What are some behavioural and physiological adaptations to aid exchange?

Answer 8

1. Animals with a high surface area: volume ratio tend to lose more water as it evaporates from their surface. (This is a problem in hot regions) Some small desert animals have kidney structure adaptions so that they produce less urine to compensate
2. To support their high metabolic rates, small mammals living in cold regions need to eat large amounts of high energy foods such as seeds and nuts
3. Smaller mammals may have thick layers of fur or hibernate when the weather gets really cold
4. Larger organisms living in hot regions (such as elephants and hippos) find it hard to keep cool as their heat loss is relatively slow. Elephants have developed large flat ears which increase their surface area, allowing them to lose more heat. Hippos spend much of the day in the water - a behavioural adaption to help them lose heat