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

Question 32

What have insects evolved for gas exchange?

Answer 32

An internal network of tubes called tracheae

Question 33

What are tracheae supported by?

Answer 33

Strengthened rings - to prevent from collapsing

Question 34

What do tracheae divide into?

Answer 34

Smaller dead-end tubes called tracheoles

Question 35

What do tracheoles extend throughout?

Answer 35

All the body tissues of the insect

Question 36

Is atmospheric air brought directly to the respiring tissues in an insect?

Answer 36

Yes as there is a short diffusion pathway from a tracheal to any body cell?

Question 37

How do respiratory gases move in and out of the tracheal system?

Answer 37

Along a diffusion gradient
Mass transport
The ends of the tracheoles are filled with water

Question 38

How do respiratory gases move in and out of the tracheal system via a diffusion gradient?

Answer 38

• As cells respire, O2 is used up to its concentration towards the end of the tracheoles fall
• This creates a diffusion gradient
• Gaseous oxygen from atmosphere moves in along gradient
• CO2 is the same but in the opposite way

Question 39

How do respiratory gases move in and out of the tracheal system via mass transport?

Answer 39

• The contraction of muscles in insects can squeeze the trachea enabling mass movements of air in and out
• This further speeds up the exchange of respiratory gases

Question 40

How do respiratory gases move in and out of the tracheal system via (the ends of the tracheole are filled with water)?

Answer 40

• During times of major activity, muscle cells around tracheole carry out anaerobic respiration
• This produces lactae, which is soluble and lowers water potential of cells
• Water moves into cells from tracheoles by osmosis
• Water in ends of tracheoles decrease in volume which draws air further into them
• So final diffusion pathway is in a gas rather than liquid phase so diffusion is quicker

Question 41

What does (the ends of the tracheole are filled with water) mean for the rate of exchange of respiratory gases?

Answer 41

It increases the rate at which air is moved in to the tracheoles but leads to greater water evaporation

Question 42

How do gases enter and leave the tracheae?

Answer 42

Through tiny pores, spiracles, on the body surface

Question 43

How are spiracles opened and closed?

Answer 43

By a valve

Question 44

What happens when the spiracles are open in terms of water?

Answer 44

Water vapour can evaporate from the insect

Question 45

What is the opening and closing of spiracles like in insects?

Answer 45

• For most of the time, spiracles are closed to prevent water loss
• Periodically, insects open their spiracles to allow gas exchange

Question 46

What are the limitations of the tracheal system?

Answer 46

Relies mostly on diffusion to exchange gases between environment and cells

Question 47

How does exchange of gases in insects limit their size?

Answer 47

For exchange to be effective, diffusion pathway needs to be short so insects are a small size
As a result, the length of the diffusion pathway limits the size that insects can attain

Question 48

What adaptations of efficient gas exchange conflict with the need to conserve water in insects?

Answer 48

A thin, permeable surface with a large area

Question 49

Give one adaptation insects have evolved that reduce water loss in terms of size?

Answer 49

Small surface area to volume to minimise area over which water is lost

Question 50

Give one adaptation insects have evolved that reduce water loss in terms of body surfaces?

Answer 50

• Waterproof coverings over their body surface
• This covering is a rigid outer Skeleton of chitin that is covered with a waterproof cuticle

Question 51

Give one adaptation insects have evolved that reduce water loss in terms of gas exchange?

Answer 51

• Spiracles can be closed to reduce water loss
• This conflicts with the need for oxygen and so occurs largely when the insect is at rest

Question 52

Do fish have an outer covering?

Answer 52

Yes

Question 53

What kind of outer covering do fish have?

Answer 53

A waterproof, gas-tight one

Question 54

Why do fish need a specialised gas exchange system?

Answer 54

They have a small SA to volume ratio so their body surface is not adequate enough to supply and remove their respiratory gases

Question 55

Where are gills located in a fish?

Answer 55

Within the body, behind the head

Question 56

What are gills made up of?

Answer 56

Gill filaments

Question 57

How are the gill filaments arranged?

Answer 57

They are stacked up in a pile

Question 58

What are at right angles to the gill filaments and how do they increase the rate of gas exchange?

Answer 58

Gill lamellae - they increase the SA of the gills
They are thin and so there is a short diffusion distance
They have an extensive network of capillaries

Question 59

How does water travel through a fish?

Answer 59

• Taken in through the mouth
• Forced over the gills and out through an opening on each side of the body

Question 60

What is the countercurrent flow?

Answer 60

The flow of water over the gill lamellae and the flow of blood within them are in opposite directions

Question 61

What would happen if water and blood flowed in the same direction?

Answer 61

Far less gas exchange would take place

Question 62

How does oxygenated blood interact with the water flowing over it?

Answer 62

• Oxygenated blood meets water, which has its maximum concentration of oxygen
• So diffusion of oxygen from the water to the blood takes place

Question 63

How does deoxygenated blood interact with the water flowing over it?

Answer 63

• Blood with little oxygen in it meets water which has had almost all of its oxygen removed
• So diffusion of the remaining oxygen from the water to the blood takes place

Question 64

What happens as a result of the countercurrent flow?

Answer 64

A diffusion gradient for oxygen uptake is maintained across the entire width of the gill lamellae

Question 65

How much of the oxygen available in the water is absorbed into the blood of the fish?

Answer 65

About 80%

Question 66

How much of the oxygen available in the water is absorbed into the blood of the fish if they flow in the same direction?

Answer 66

About 50%