SA to vol ratio

Question 1

Why is simple diffusion inadequate for relatively large animals

Answer 1

As organisms get larger, their volume increases more rapidly than the surface area, meaning simple diffusion of substances across the outer surface can only meet the needs of relativley inactive organisms, and even if the outer surface could suplly enough substance, it would still take too long for the substance to reach the middle of the organism

Question 2

How are organisms adapted to overcome the challanges imposed by SA to vol ratio

Answer 2

A flattened shape so that no cell is ever too far from the surface
Specialised exchange surfaces with large areas to increase the surface area to vol ratio

Question 3

Describe the adaptations of a specialised exchange system

Answer 3

Large sa to vol ratio increasing rate of exchange
Very thin so that the diffusion distance is short and therefore materials cross the exchange surface rapidly
Selectivley permeable membrane to allow selected materials to cross
Movement of the enviromental medium , to maintain a concentration gradient

Question 4

Describe the structure of gas exchange in an insect

Answer 4

They have an internal network of tubes called tracheae and these are supported by strengthend rings to prevent them from collapsing. These trachea then divide into smaller dead end tubes called tracheoles and these extend throughout all the body tissues in the insect. This means atmospheric air with the oxygen it contains can be brought directly to respiring tissues as there is a short diffusion pathway from a tracheole to any body cell

Question 5

Describe how respiratory gases move throughout an insect down a conc gradient

Answer 5

When cells are respiring oxygen is used up meaning there is a lower concentration of oxygen near the cells at the end of the tracheoles creating a diffusion gradient causing gaseous oxygen to diffuse from the atmosphere along the trachea and tracheoles to the cells. Carbon dioxide is produced from the cells during respiration creating a diffusion gradient in the other direction causing gaseous carbon dioxide to diffuse along the tracheoles and trachea from the cells to the atmosphere

Question 6

How does mass transport happen to exchange gases in an insect

Answer 6

The contraction of muscles in an insect can squeeze the trachea enabling the mass movement of air in and out, this speeds up the exchange of respitory gases

Question 7

How does the ends of the tracheoles being filled with water support an insect during gas exchange?

Answer 7

During periods of major activity the muscle cells around the tracheoles do anaerobic respiration producing lactate which is soluble and lowers the water potential of the muscle cells. Water then moves into the tracheoles by osmosis. The water in the ends of the tracheoles decrease in volume and pull air into them meaning the final diffusion phase is gaseous making the diffusion alot more rapid

Question 8

Explain how the tracheal system limits the size of insects

Answer 8

It relies mostly on diffusion to exchange gases between the environment and cells. For diffusion to be effective, there needs to be a short diffusion pathway which is why insects are a small size. This means the length of the diffusion pathway limits the size the insect can attain

Question 9

What is the function of spiracles?

Answer 9

Gases enter and leave the tracheae through these tiny pores called spiracles and these may be opened or closed by a valve. When these spiracles are opened, water vapour can evaporate from the insect. Most of the time the insect keeps these spiracles closed to prevent this water loss

Question 10

Explain why there is a conflict between gas exchange and conserving water in terrestrial insects

Answer 10

Terrestrial insects have evolved mechanisms that have adapted to conserve water, and the increased surface area required for gas exchange conflicts with conserving water because water will evaporate from it

Question 11

What is the specialised gas exchange system in fish

Answer 11

Gills

Question 12

Describe the structure of gills

Answer 12

They are located in the body of the fish behind the head and they are made up of gill filaments, they are stacked into a pile. At right angles to this pile there are gill lamellae which increase the surface area of the gills. Water is taken in through the mouth and is forced over the gills and out through an opening on each side of the body

Question 13

Describe the process of counter current flow

Answer 13

This is where the blood and water in a fish flow in opposite directions on the gill lamellae which means that blood that is already well loaded with oxygen meets water which already has its maximum concentration of oxygen. Therefore diffusion of oxygen from the water to the blood takes place. This also means that blood with very little oxygen in it meets water which has had most if not all of its water removed, and again diffusion of oxygen from the water to the blood happens again. As a result a diffusion gradient for oxygen uptake is maintained across the gill lamellae, 80% of the oxygen avainable in the water is absorbed into the blood of the fish

Question 14

Why is counter-current flow an efficient means of exchanging gases across the gills of fish

Answer 14

Because a steady diffusion gradient is maintained over the whole length of the gill lamellae. Therefore more oxygen diffuses from the water into the blood

Question 15

Why is a one way flow of water over fish gills an advantage?

Answer 15

Less energy is required because the flow does not have to be reversed, this is important because water is dense and difficult to move

Question 16

What would happen if the flow of oxygen and water was going in the same direction over the gill lamellae

Answer 16

The diffusion gradient would only be maintained across one length of the gill lamellae and only 50 % of the avaiable oxygen would be absorbed by the blood

Question 17

Describe the similarities between gas exchange in plants and in insects

Answer 17

No living cell is far from the external air and therefore a source of oxygen and carbon dioxide, diffusion takes place in the gas phase making it more rapid than if it was in water

Question 18

Describe the adaptations that leaves have for rapid gas exchange

Answer 18

Many small pores called stomata, no cell is far away from a stomata so diffusion pathway is short
Numerous interconnecting air spaces that occur within the mesophyll so that gases can come into contact with the mesophyll cells
Large surface area of mesophyll cells for rapid diffusion

Question 19

How are guard cells next to the stomata used to control the conflicting needs of gas exchange and water loss

Answer 19

Guard cells open and close the stomatas pores, and in doing this they can control the rate of gaseous exchange and this is important because terrestrial organisms lose water by evaporation. The guard cell will close the stomata at times when water loss would be excessive

Question 20

Give similarities between gas exchange in a plant leaf and gas exchange in a terrestrial insect

Answer 20

No living cell is ever far from the external air, diffusion takes place in the gas phase, they both need to avoid excessive water loss, air diffuses through pores in their outer covering

Question 21

Give differences between gas exchange in a plant leaf and gas exchange in a terrestrial insect

Answer 21

Insects may create a mass air flow whereas plants never do, insects have a smaller sa to vol ratio than plants, insects have special structures called tracheae along which gases can diffuse- plants do not. Insects do not interchange gases between respiration and photosynthesis whereas plants do

Question 22

Explain the advantage with plants being able to open and close their stomata

Answer 22

Helps to control water loss by evaporation/ transpiration

Question 23

What adaptations do insects have which helps them compromise between the need for an efficient gas exchange system and the need to conserve water?

Answer 23

They have a small surface area to volume ratio to minimise the area of which water is lost
Waterproof coverings over their body surfaces, in the case of insects this covering is a rigiid outer skeleton of chitin that is covered with a waterproof cuticle
They have spiracles which are the openings of the trachae at the body surface and these can be closed to reduce water loss, this happens when an insect is at rest

Question 24

Why cant plants have a small sa to volume ratio?

Answer 24

Because they carry out photosynthesis and for that they need a large surface area for the absorption of light and the exchange of gases

Question 25

How do plants compormise between the need for an efficient gas exchange system and the need to conserve water?

Answer 25

They have a waterproof covering and the ability to close the stomata when needed

Question 26

How are xerophytes adapted to limit water loss by having a thick cuticle

Answer 26

They reduce the rate at which water can be lost by evaporation
-A thick cuticle so less water can be lost by passing through it

Question 28

How does a xerophyte having hairy leaves conserve water?

Answer 28

A thick layer of leaves in the plant especially on the lower epidermis traps still moist air next to the leaf surface. The water potential gradient between the inside and outside of the leaves is reduced and therefore less water is lost by evaporation

Question 29

How does a xerophyte having stomata in pits or grooves conserve water?

Answer 29

These trap still moist air next to the leaf and reduce the water potential gradient e.g pine trees

Question 30

How does a xerophyte having a reduced surface area to volume ratio

Answer 30

By having leaves that are smaller and roughly circular in cross section, rather than broad and flat, the rate of diffusion out of the plant can be considerably reduced. This reduction in surface area is balanced against the need for a sufficient area for photosynthesis to meet the needs of the plant

Question 31

What do all the adaptations of xerophytes link to?

Answer 31

These adaptations mostly reduce a water potential graident and therefore slower diffusion, less water loss from air spaces and hence reduced evaporation of water

Question 32

What are problems that both insects and plants may face when it comes to living on land?

Answer 32

Efficient gas exchange requires a thin permeable surface with a large area, on land this may result in water loss by evaporation

Question 33

Describe modifications that both plants and insects use to reduce water loss

Answer 33

Waterproof covering the body, ability to open and close the openings of the gas exchange system ( stomata and spiracles)

Question 34

Why cant plants reduce their surface area to volume ratio in order to reduce water loss?

Answer 34

Plants photosynthesise and therefore need a larger surface area to volume ratio in order to absorb sunlight