3.1.1 Exchange surfaces

Question 1

What do cells need to survive?

Answer 1

Oxygen and nutrients, they need to remove waste products so they do not become toxic.

Question 2

How do small organisms exchange substances?

Answer 2

Exchange through diffusion over the surface of the body.

Question 3

How do large organisms exchange substances?

Answer 3

They need a specialised surface for the exchange of substances with their environment.

Question 4

What are the three factors that affect the need for an exchange system?

Answer 4

Size,
Surface area: ratio
Level of activity

Question 5

How does size affect the need for an exchange system?

Answer 5

-Small organisms have a very small diffusion pathway.
-This means diffusion will supply enough oxygen and nutrients to keep cells alive.
-Larger organisms have a longer diffusion pathway
-diffusion is too slow to enable sufficient supply to cells

Question 6

How does SA:V ratio affect the need for an exchange system?

Answer 6

-Small organisms have a large SA:V ratio
-This means their SA is large enough to supply their cells with sufficient oxygen
-Larger organisms have a small SA:V ratio
-Their SA is small compared to their V
-Some organisms increase their surface area by adopting a different shape e.g flatworm
-Large organisms need a range of tissues to give body support.
-Their volume increases as their body gets thicker but the surface are does not increase as much

Question 7

How does level of activity affect the need for an exchange system?

Answer 7

-Metabolic activity uses energy from food and requires oxygen to release energy from aerobic respiration
-Cells of active organism need good supplies of nutrients and oxygen to supply energy for movement.
-This need is increased in animals e.g mammals to keep themselves warm.

Question 8

What are the features of a good exchange system?

Answer 8

-A large surface area to provide more space for molecules to pass through e.g root hairs in plants
- A thin barrier to reduce diffusion distance , barrier must be permeable to substances being exchanged e.g alveoli of lungs
-A good blood supply. This can bring fresh supplies of molecules to one side (supply side) keeping concentration high or it may remove molecules from demand side to keep concentration low. Important in maintaining a steep conc gradient so that diffusion can occur rapidly. E.g Gills in fish

Question 9

What is the equation for surface area?

Answer 9

4 x π x r2

Question 10

What is the equation for volume?

Answer 10

4/3 x π x r3

Question 11

Calculate the surface area to volume ratio of a sphere with a radius of 10mm

Answer 11

SA= 4 x π x 10(2) = 1257
V= 4/3 x π x 10(3) = 1489
1257:1489 = 0.30:1

Question 12

Calculate the surface are to volume ratio of a human, given that the typical skin surface area is 1.8m and the volume is 0.07m

Answer 12

25:1

Question 13

Calculate the ratio of lung surface area to body volume of a human using lungs with a surface area of 70m

Answer 13

1000:1

Question 14

Comment on the need of lungs in a human.

Answer 14

-To exchange gases in the body as oxygen and co2 cannot efficiently diffuse to all cells through the skin’s surface area
-This is because humans have a small SA:V ratio, and need an exchange system to keep oxygen supplies to cells for aerobic respiration.