Organisms Exchange Substances with their Environment: Surface Area to Volume Ratio

Question 1

What is the relationship between the size of an organism and its surface area:volume ratio?

Answer 1

• Smaller organisms have higher surface area:volume ratios
• Larger organisms have lower surface area:volume ratio
• As an organism becomes larger, their volume increases at a larger rate than their surface area

Question 2

How do you calculate the surface area and volume of a cylinder?

Answer 2

• Surface area
- 2πrh + 2πr^2

• Volume
- πr^2h

Question 3

How do you calculate the surface area and volume of a sphere?

Answer 3

• Surface area
- 4πr^2

• Volume
- 4/3πr^3

Question 4

How do you calculate the surface area and volume of a cube?

Answer 4

• Surface area
- 6a^2

• Volume
- a^3

Question 5

How does the size of an organism affect exchange across the body surface?

Answer 5

• Small organisms have a high enough surface area:volume to allow efficient exchange across the body surface
• As organisms become larger, the volume increases at a larger rate than their surface area - simple diffusion would be too slow

Question 6

Can single-celled and multicellular organisms exchange substances through their body surfaces?

Answer 6

• Single-celled organisms

• Substances diffuse directly into or out of cell across cell-surface membrane
• Diffusion rate is quick as substances have small distances to travel

• Multicellular organism

• Cannot diffuse across outer membrane
• Some cells are deep within body - big distance between cells and outside environment
• Larger animals have lower SA:V - difficult to exchange enough substances to supply a large volume of animal through a relatively small outer surface

Question 7

How do large organisms overcome their low surface area:volume to exchange substances?

Answer 7

• To overcome their low surface area:volume ratios large organisms have evolved to have specialised exchange surfaces

• Specialised exchange surfaces:

• Have large surface area
• Are thin (so distance is short)
• Selectively permeable to allow selected material to cross without obstruction
• Needs movement of environmental medium (air, water) to maintain a steep concentration gradient
• Needs movement of internal medium (blood) to maintain a steep concentration gradient

Question 8

Describe how the surface area:volume ratio affects the metabolic rate in organisms

Answer 8

• The metabolic activity inside cells releases heat
• The rate of heat loss from an organism depends on its surface area:volume ratio
• The larger the SA:V, the easier it is to lose heat
• Smaller organisms need a relatively high metabolic rate, to generate enough heat to stay warm

Question 9

Which physical characteristics affect SA:V and therefore metabolic rate?

Answer 9

• Body size

• Larger organisms have lower SA:V
• Harder to lose heat from body
• Lower metabolic rate needed to keep warm - less food

• Body shape

• Animals with compact shape have a smaller SA:V
• Reducing heat loss and metabolic rate
• Animals can have body parts that make SA larger than V to increase heat loss