exchange of substances

Question 1

what is the equation for diffusion rate?

Answer 1

SA x conc. gradient/ diffusion pathway

Question 2

how can you maximise diffusion rate using the equation?

Answer 2

1) maximise SA and conc. gradient
2) minimise diffusion pathway

Question 3

how are fish adapted for gas exchange? (6)

Answer 3

1) need a specialised gas exchange surface as they have a small SA:V ratio
2) have four layers of gills on each side of the head, which are supported by the gill arch
3) along each arch there are multiple projections called gill filaments, with lamellae on them where gas exchange occurs and provide a large SA
4) blood and water flow across the lamellae in a counter current direction meaning they flow in the opposite direction to one another
5) this ensures that a steep diffusion gradient is maintained so that the maximum amount of oxygen is diffusing into the deoxygenated blood from the water
6) short diffusion pathway as gas exchange only happens on gill lamellae which are very thin

Question 4

what is the counter-current principle?

Answer 4

1) blood and water flow across the lamellae low in the opposite direction to one another
2) this ensures that a steep diffusion gradient is maintained across the entire width of the gill lamellae so that the maximum amount of oxygen is diffusing into the deoxygenated blood from the water
3) ensures equilibrium is not reached

Question 5

describe the movement of water in fish (3)

Answer 5

1) water flows into the mouth and into the buccal cavity
2) water rushes in and over the gills
3) passes out through the gill lists

Question 6

why does the volume of water passing over the gills increase if the temperature of the water increases?

Answer 6

At higher temperatures, less oxygen dissolves in the water so fish need to respire faster by absorbing more oxygen from the water

Question 7

why is counter current flow useful to a fish? (2)

Answer 7

1) faster respiration rate/ respiration rate is maximised
2) faster swimming speed as more O2 is absorbed

Question 8

what is the relationship between the size of an organism and
its surface area to volume ratio? (2)

Answer 8

1) as the size of an organism increases, its surface area to volume ratio decreases.
2) because its volume increases faster than its surface area, meaning the no. of cells respiring and using the O2 becomes greater than that of the area available to diffuse O2

Question 9

what is the physiological significance of surface area to volume ratio? (3)

Answer 9

1) small organisms with a higher SA:V lose heat energy more quickly per kg of bodyweight
2) so they have to respire fast to replace the body heat they are losing
3) large organisms with a lower SA:V have specialised gas exchange surfaces (gills, lungs, leaves) to increase the surface area for O2 diffusion/

Question 10

what adaptations do single-celled organisms have for gas exchange? (3)

Answer 10

1) small and have a large SA:V ratio
2) oxygen is absorbed by diffusion across their body surface which is only covered by a cell-surface membrane
3) Carbon dioxide from respiration diffuses out across their body surface
4) if organism has a cell wall, it doesn’t act as an additional barrier to the diffusion of gases

Question 11

what adaptations do insects have for gas exchange? (6)

Answer 11

1) tracheal system made up of tracheae which branch out to form additional tracheoles which increase surface area.
2) large number of tracheoles which have thin walls so there is a short diffusion distance to cells
3) O2 and CO2 conc. gradient is maintained by body movement and muscle contraction which moves air
4) tracheae provide tubes full of air so fast diffusion into insect tissues
5) fluid in the ends of the tracheoles moves out into tissues during exercise so faster diffusion through the air to the gas exchange surface

Question 12

how does the production of lactic acid during exercise speed up oxygen diffusion? (3)

Answer 12

1) ends of the tracheoles are filled with water- during intense activity, muscles around the tracheoles carry out anaerobic respiration producing lactic acid.
2) This lowers the water potential of muscle cells so water moves into them from the tracheoles by osmosis
3) The volume of water in the ends of the tracheoles decreases and in doing so draws air further into them, speeding up diffusion of oxygen.

Question 13

what is the function of spiracles in insects? (2)

Answer 13

1) gases enter and leave the tracheae via tiny pores called spiracles which can open and close by a valve
2) periodically open to allow gas exchange

Question 14

how are the leaves of dicotyledonous plants adapted for gas exchange? (4)

Answer 14

1) large number of stomata which allow gas exchange found in the leaf’s bottom surface means no cell is far from the stomata, reducing the diffusion distance.
2) air spaces in the spongy mesophyll tissue provide a large surface area to allow gases to enter and move around the leaf and easily come into contact with photosynthesising mesophyll cells
3) CO2 concentration gradient is maintained from the air to the leaf cells as there is a low conc at the palisade cells due to it being used in photosynthesis
4) short diffusion pathway is also provided through the thin flat surface of the leaf

Question 15

what are the structural and functional compromises between the opposing needs for efficient gas exchange and the limitation of water loss shown by terrestrial insects? (3)

Answer 15

1) gas exchange causes water loss because when spiracles open to allow gas exchange, water evaporates out
2) If insects are losing too much water, they close their spiracles using muscles.
3) insects have a waterproof, waxy cuticle all over their body and tiny hairs around their spiracles, both of which reduce evaporation.

Question 16

what are structural and functional compromises between the opposing needs for efficient gas exchange and the limitation of water loss shown by xerophytic plants? (4)

Answer 16

1) gas exchange causes water loss because when stomata open to allow gas exchange, water evaporates out
2) plant stomata open in day to allow gas exchange
3) waxy cuticle reduces evaporation
4) guard cells fill with water to open stomata and lose water to close them

Question 17

how does gas exchange in leaves occur? (3)

Answer 17

1) carbon dioxide/oxygen diffuse through the stomata, which are opened by guard cells becoming turgid
2) carbon dioxide/oxygen diffuse into mesophyll layer into air spaces
3) carbon dioxide/oxygen then diffuse down a concentration gradient.

Question 18

how do stomata open?

Answer 18

1) guard cells absorb water by osmosis and becomes turgid
2) causes stomata to open during the day to allow diffusion of CO2

Question 19

how do stomata close?

Answer 19

1) guard cells lose water by osmosis and becomes flaccid
2) causes stomata to close at night as no photosynthesis is happening

Question 20

why is there a net CO2 uptake during the day? (3)

Answer 20

1) photosynthesis rate is higher than respiration rate
2) more CO2 is used for photosynthesis than is made by respiration
3) CO2 diffuses in through open stomata

Question 21

why is there a net uptake of O2 at night? (3)

Answer 21

1) respiration rate is higher than photosynthesis rate/ photosynthesis is not occurring
2) more O2 is being used for respiration than is being made by photosynthesis
3) O2 diffuses in through closed stomata

Question 22

what is the ‘compensation point’ in photosynthesis? (2)

Answer 22

1) photosynthesis and respiration rates are equal
2) all CO2 made by respiration is used for photosynthesis

Question 23

why are the stomata usually open in the daytime and closed at night? (3)

Answer 23

1) allow CO2 to diffuse in for photosynthesis
2) closed at night to prevent loss of water through evaporation/transpiration
3) no photosynthesis occurs at night

Question 24

why would a herbicide that closes the plant’s stomata cause the death of that plant? (3)

Answer 24

1) CO2 cannot diffuse in through stomata
2) less photosynthesis will happen
3) no glucose/sugars produce so plant will die

Question 25

describe the similarities and differences between plant leaves and insect tracheal system as gas exchange systems (5)

Answer 25

1) both have a large surface area (air spaces in the spongy mesophyll layer and lots of branched tracheoles)
2) both have a short diffusion pathway as insects are small and thin tracheole walls and leaves are thin and flat.
3) both have pores to allow gas exchange (stomata and spiracles) which can be closed to reduce water loss through evaporation
4) insects have the ability to ventilate trachea by contracting muscles whereas leaves cannot
5) they differ in which gases diffuse in and out. During the day, CO2 diffuses into the leaves and O2 diffuses out. Whereas in insects, O2 diffuses in and CO2 diffuses out.