Exchange across mass surfaces

Question 1

What is tissue fluid?

Answer 1

The environment around the cells of multicellular organisms

Question 2

What does there need to be for exchange to be effective?

Answer 2

The exchange surface must be large compared with its organism

Question 3

What happens as organisms become larger?

Answer 3

Their volume increases at a faster rate than their surface area

Question 4

What kind of organisms can simple diffusion meet the needs of?

Answer 4

Relatively inactive organisms
Single celled organisms

Question 5

What kind of features have organisms evolved?

Answer 5

• A flattened shape so that no cell is ever far from the surface (e.g. leaf)
• Specialised exchange surfaces with large areas to increase the surface area ti volume ratio

Question 6

Give one characteristic of exchange surfaces

Answer 6

A large surface area to volume ratio

Question 7

Give one characteristic of exchange surfaces

Answer 7

Very thin so that the diffusion distance is short and so materials cross the exchange surface rapidly

Question 8

Give one characteristic of exchange surfaces

Answer 8

Selectively permeable to allow selected materials to cross

Question 9

Give one characteristic of exchange surfaces

Answer 9

Movement of the environmental medium(e.g. air) to maintain a diffusion gradient

Question 10

Give one characteristic of exchange surfaces

Answer 10

A transport system to ensure the movement of the internal medium (e.g. blood) to maintain a diffusion gradient

Question 11

What is diffusion proportional to?

Answer 11

(Surface area x difference in concentration) / length of diffusion path

Question 12

What can happen to specialised exchange surfaces, given that they are thin?

Answer 12

They are easily damaged and dehydrated

Question 13

How are specialised exchange surfaces prevented from getting damaged and dehydrated?

Answer 13

They are located inside an organism

Question 14

Where an exchange surface is located in the body, what does the organism need to have a means of?

Answer 14

A means of moving the external medium over the surface (e.g. a means of ventilating the lungs in a mammal)

Question 15

Why is diffusion in plants quicker than in water?

Answer 15

Diffusion takes place in the gas phase (air) which makes it more rapid than if in water

Question 16

In plants, do all living cells have a source of CO2 and O2?

Answer 16

Yes as no living cell is far from the external air

Question 17

Inside plants, how are air spaces adapted for quick diffusion?

Answer 17

They have a very large surface area compared with the volume of living tissue

Question 18

Is there a specific transport system for gas?

Answer 18

No - gases simply move in and out of the plant through diffusion

Question 19

What adaptations do leaves show for rapid diffusion/gaseous exchange?

Answer 19

• Many small pores, stomata, and so no cell is far from a stoma so diffusion pathway short
• Numerous interconnecting air spaces throughout the mesophyll so gases can readily come in contact with mesophyll cells
• Large surface area of mesophyll cells for rapid diffusion

Question 20

What are stomata?

Answer 20

Minute pores that occur mainly (but not exclusively) on the leaves, especially the underside

Question 21

What is each stoma surrounded by?

Answer 21

Guard cells
- These can open and close the stomatal pore
- In this way, they control the rate of gaseous exchange

Question 22

How do terrestrial organisms lose water?

Answer 22

By evaporation

Question 23

How have plants evolved to balance the conflicting needs of gas exchange and control of water loss?

Answer 23

They do this by closing stomata at times when water loss would be excessive

Question 24

How do terrestrial plants reduce water loss?

Answer 24

They have a waterproof covering over parts of the leaves and the ability to close stomata when necessary

Question 25

What are xerophytes?

Answer 25

• Plants that are adapted to living in areas where water is in short supply
• They have evolved a range of other adaptations

Question 26

What kind of modifications do leaves have to reduce water loss?

Answer 26

• A thick cuticle
• Rolling up of leaves
• Hairy leaves
• Stomata in pits or grooves
• A reduced surface area to volume ratio of the leaves

Question 27

How does a thick cuticle reduce water loss in leaves?

Answer 27

• Although the waxy cuticle on leaves forms a waterproof barrier, up to 10% of water loss can still occur via this route
• Thicker the cuticle, less water can escape by this means

Question 28

How does the rolling up of leaves reduce water loss?

Answer 28

• Most leaves have their stomata largely on the lower epidermis
• The rolling of leaves protects the lower epidermis from the outside
• This helps to trap a region of still air within the rolled leaf
• This region becomes saturated with water vapour and also has a high water potential
• No water potential gradient between the inside and outside of leaf so no water loss

Question 29

How do hairy leaves reduce water loss?

Answer 29

• A thick layer of hairs, especially on lower epidermis, traps still, moist air next to leaf surface
• Water potential gradient between inside and outside of leaf is reduces so less water lost by evaporation

Question 30

How do stomata in pits or grooves reduce water loss?

Answer 30

These trap still, moist air next to leaf and reduce water potential gradient

Question 31

How does a reduced surface area to volume ratio of leaves reduce water loss?

Answer 31

• By having leaves that are small and roughly circular in cross section, rate of water loss can be considerably reduced
• This reduction in SA is balanced against need for a sufficient area of photosynthesis to meet requirements of plants