Gas Exchange

Question 1

Relationship of SA:Vol ratio with organism size

Answer 1

SA:Vol ratio decreases as an organism gets larger

Question 2

Single celled organisms SA:Vol

Answer 2

Single celled organisms and some small multicellular organisms have large enough surface area to volume ratios to meet their gas exchange needs by diffusion across their surface

Question 3

What makes a good exchange surface

Answer 3

• large SA
• large conc. gradient
• moist (O2/CO2 diffuse in solution)
• thin exchange surface
• permeable (entry+exit)

Question 4

Explain the advantage for larger animals of having a specialised system that facilitates oxygen uptake

Answer 4

1. Large(r) organisms have a small(er) surface area:volume (ratio)
2. Faster rate of diffusion;

Question 5

How is water loss limited in insects?

Answer 5

• waterproof covering
• relatively small SA:Vol ratio

Question 6

Movement of O2 in insects

Answer 6

1. Oxygen enters the insect through spiracles and into the tracheae. Spiracles close.
2. Oxygen diffuses through the tracheae into the tracheoles
3. Oxygen is delivered directly to the respiring tissues.
   (CO2 produced moves in opposite direction and leaves when spiracles open)

Question 7

Explain how an insect’s tracheal system is adapted for efficient gas exchange.

Answer 7

1.Tracheoles have thin walls so short diffusion distance to cells;
2. Highly branched so short diffusion distance to cells;
3. Highly branched so large surface area (for gas exchange);
4.Tracheae provide tubes full of air so fast diffusion

Question 8

Ventilation

Answer 8

Movement of the insects muscles creates a mass movement of air in and out the trachea, speeding up the rate of gaseous exchange. They also have small air sacs in their trachea. Muscles around the trachea contract and pumps the air in the sacs deeper into the tracheoles.

Question 9

Getting additional Oxygen during flight

Answer 9

During flight, the insect may partly respire anaerobically and produce some lactate (lactic acid).
This lowers the water potential of the muscle cells. As the lactate builds up, water passes via osmosis from the tracheoles into the muscle cells.
- adaptation reduces the diffusion distance

Question 10

A fish uses its gills to absorb oxygen from water. Explain how the gills of a fish are adapted for efficient gas exchange.

Answer 10

1 Large surface area provided by many lamellae over many gill filaments;
2 Increases diffusion
3 Thin epithelium between water and blood;
4 Water and blood flow in opposite directions
5 maintains concentration gradient
6 As water always next to blood with lower concentration of oxygen;
7 Circulation replaces blood saturated with oxygen;
8 Ventilation replaces water

Question 11

Concurrent Flow

Answer 11

• water and blood flow in opposite directions
• maintaining a conc. gradient across whole length of gill lamellae
• as water always next to blood with lower conc. O2
• circulation replaces blood saturated with oxygen
• ventilation replaces water

Question 12

Adaptations of leaf for gaseous exchange

Answer 12

1. Flat – gives larger surface area to volume ratio
2. Many Stomata – pores to allow air to move in and out of leaf.
3. Air spaces in leaf so short distance between mesophyll cells and air
4. At night, guard cells close stomata to prevent water loss
5. Upper/lower surfaces have waxy cuticle
6. Many stomata found on lower surface

(4-5 reduce water loss)

Question 13

Diffusion of CO2 for photosynthesis

Answer 13

1. Mesophyll cells photosynthesise and this reduces the concentration of CO2 in the cells.
2. CO2 diffuses from the air spaces into the cells.
3. This in turn reduces the CO2 concentration in the air spaces causing CO2 to move into the air spaces from the air outside the leaf, through the stomata.

Question 14

Diffusion of O2

Answer 14

1. Mesophyll cells produce O2 as a result of photosynthesis.
2. O2 diffuses into the air spaces from the cells
3. This increases the concentration of O2 in the air spaces, causing O2 to move from the air spaces to outside the leaf via the stomata.

Question 15

Xerophytic plants (reducing water loss)

Answer 15

1. Reduced number of stomata -> Less Surface Area for water loss
2. Stomata in pits -> Reduced concentration gradient
3. Hairs to trap water vapour -> Reduced concentration gradient
4. Rolled leaves -> Reduced concentration gradient
5. Leaves reduced to spines -> Less surface area for water loss
6. Thick waxy cuticles -> Increased diffusion distance

Question 16

Describe how carbon dioxide in the air outside a leaf reaches mesophyll cells inside the leaf

Answer 16

1. (Carbon dioxide enters) via stomata;
2. (Stomata opened by) guard cells;
3. Diffuses through air spaces;
4. Down diffusion gradient;

Question 17

Explain why plants grown in soil with very little water grow only slowly

Answer 17

1. Stomata close;
2. Less carbon dioxide (uptake) for less photosynthesis