3.2 Gas exchange (Insects and Fish)

Question 1

What are the 3 key features of every gas exchange surface

Answer 1

1. Large surface area to volume ratio
2. Short diffusion distance
3. Maintained a conc grad

Question 2

What is the purpose of an exoskeleton in terrestrial insects

Answer 2

• For Protection
• Has a lipd layer to prevent water loss

The excoskeleton is made up of hard fibrous material (chitin)

Question 3

Explain how the body surface of a single celled organism is adapted for gas exchange

Answer 3

● Thin, flat shape and large surface area to volume ratio
● Short diffusion distance to all parts of cell → rapid diffusion eg. of O2 / CO2

Question 4

Describe the tracheal system of an insect (3)

Answer 4

1. Has spiracles-pores on surface that can open and close to allow diffusion
2. Tracheae-Large tubes full of air that allow diffusion
3. Tracheoles-Smaller branches from Tracheae, that are permeable to allow gas exchange with cells

Question 5

Explain how an insects tracheal system is adpated for gas exchange (12)

What do the trachea provide

;

What do the tracheoles have

Answer 5

● Tracheoles have thin walls
○ So short diffusion distance to cells
● High numbers of highly branched tracheoles
○ So short diffusion distance to cells
○ So large surface area
● Tracheae provide tubes full of air
○ So fast diffusion
● Contraction of abdominal muscles (abdominal
pumping) changes pressure in body, causing air to
move in / out
○ Maintains concentration gradient for diffusion
● Fluid in end of tracheoles drawn into tissues by
osmosis during exercise (lactate produced in
anaerobic respiration lowers ψ of cells)
○ As fluid is removed, air fills tracheoles
○ So rate of diffusion to gas exchange surface
increases as diffusion is faster through air

Question 6

Explain structural and functional compromises in terrestial insects that allow efficient gas exchange while limiting water loss (3)

Answer 6

• Thick waxy cuticles/exoskeletons–>increases diffusion distance so less water loss
• Spiracles can open to allow gas exchange and close to reduce water loss
• Hairs around spiracles–> trap moist air,reducing water potential gradient so less water loss

Question 7

Explain how the gills of fish are adapted for gas exchange (3)

Answer 7

● Gills made of many filaments covered with many lamellae
○ Increase surface area for diffusion
● Thin lamellae wall / epithelium
○ So short diffusion distance between water / blood
● Lamellae have a large number of capillaries
○ Remove O2 and bring CO2 quickly so maintains
concentration gradient

Question 8

What is counter current flow (4)

Answer 8

1. Blood and water flow in opposite directions through/ over lamallae
2. So oxygen conc always higher in water (than blood)
3. So maintains a conc gradient of O2 between water and blood
4. For diffusion along the** whole** length of lamellae

Countercurrent flow ensures Equilibrium is not reached

Question 9

What happens if there is parallel flow instead of counter current flow in Fish gills? (3)

Answer 9

• Equilibrium would be reached
• So oxygen would no longer be able to diffuse into blood
• Along the whole gill plate

Question 10

What happens when the spiracles are open

Answer 10

Water vapour can evaporate from the insect
This is why most of the time insects spiracles are closed(prevents water loss)

Question 11

What are the limitations of the tracheal system

Answer 11

• It relies mostly on diffusion to exchange gases between the environment and the cells
• For diffusion to be effective ,The diffusion pathway needs to be short which is why insects are a small size–> limits the size insects can attain

Question 12

What is Fick’s Law

Answer 12

Diffusion is directly proportional to: Surface area x difference in concentration/Length of diffusion path

Question 13

Why have fish evolved to have specialised internal gas exchange surfaces (Gills)

Answer 13

They have a small SA:V ratio Therefore their body surface is not adequate to supply and remove their respiratory gases

Question 14

Structure of the gills

Answer 14

• Gills are made up of gill filaments which are stacked
• At right angles to the filaments are the gill lamellae–> increase surface area of the gills

Question 15

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

fish

Answer 15

Far less gas exchange would take place

Question 16

What is the countercurrent exchange principle (5)

Answer 16

• The blood and water that flows over the gill lamellae do so in opposite directions.
  -This arrangement means that
• Blood that is already well loaded with oxygen meets water, which has its maxiumum concentration of oxygen therfore diffusion of oxygen from the water to the blood takes place
• Blood with little oxygen in it meets water which has had most, but not all, of its oxygen removed.
• Again, diffusion of oxygen from the water to blood takes plac
• As a result a diffusion gradient for oxygen uptake is maintained

Question 17

Outline why countercurrent flow is an efficient means of exchanging gases across the gills of a fish

Answer 17

A steady diffusion gradient is maintained over the whole length of the gill lamellae

Question 18

Why is a one way flow advantage to fish

Answer 18

Less energy is required because the flow does not have to be reversed (important as water is dense and difficult to move)

Question 18

Mackeral are active fast swimming fish while plaice spend most of their lives moving slowly on the sea. SUggest what the differneces in the gills could be

Answer 18

Mackeral have more gill lamellae/fillaments/larger SA compared to plaice

Question 19

Where do fish obtain there water from and why is this a limitation

Answer 19

They obtain thier oxygen from water
This is a limitation because there is less (30x) oxygen in water than air so they need gills to maintain a diff gradient