[6.2-3] gas exchange in unicellular organisms, insects and fish

Question 1

describe the unicellular organism gas exchange system

Answer 1

• eg. amoeba, euglena, paramecium
• have a very high SA:V ratio so don’t need a specialised gas exchange surface
• their cell membrane is their gas exchange surface and it is enough to bring in oxygen / get rid of carbon dioxide by simple diffusion

Question 2

what features would you expect to see in the insect gas exchange system?

Answer 2

• large cumulative SA
• steep concentration gradient
• short diffusion pathway / thin exchange surface

Question 3

what structural adaptation means there is a large cumulative SA in insects?

Answer 3

• large network of tracheae and tracheoles distribute air directly to tissues, supplying oxygen for respiration
• air sacs for storage of air internally

Question 4

what structural adaptation means there is a steep concentration gradient in insects?

Answer 4

• multiple spiracles (air holes) + pumping mechanism of abdomen muscles
• this enables fresh air to be forced into the insect, thus maintaining a steep concentration gradient which allows oxygen to enter muscle tissues

Question 5

what structural adaptation means there are short diffusion pathways and thin exchange surfaces in insects?

Answer 5

• multiple spiracles and dense tracheole network means air is widely distributed internally
• point of exchange between tracheoles and muscles is only a single layer of cells thick

Question 6

why are the ends of tracheoles (in insects) filled with water? [4]

Answer 6

1. end of tracheoles next to muscle tissue are fluid filled
2. any lactic acid produced in the muscle via anaerobic respiration will make muscle’s water potential more negative
3. therefore, fluid in tracheoles will move into muscle tissue by osmosis
4. therefore, air behind fluid in tracheole will also be forced into the muscle tissue down the resulting pressure gradient

Question 7

describe the structure of the gills

Answer 7

• located within the body of the fish, behind the head
• made up of gill filaments, which are stacked up in a pile like the pages of a book
• perpendicular to the filaments are gill lamellae, which increase the SA of the gills
• water is taken in through the mouth and forced over the gills and out through an opening on each side of the body
• flow of water over gill lamellae and flow of blood within them are in opposite directions

Question 8

what structural adaptation means there is a steep concentration gradient in fish?

Answer 8

• dense network of blood vessels in the gill arches
• blood capillaries in gill lamellae

Question 9

what structural adaptation means there is a large cumulative SA in fish?

Answer 9

• each gill filament is elongated and thin in shape and each gill filament contains many gill lamellae, which is the actual gas exchange surface
• two sets of gills overall, each made of multiple gill arches, each made of multiple gill filaments, each covered in multiple gill lamellae

Question 10

what structural adaptation means there is a short diffusion pathway / thin exchange surface in fish?

Answer 10

• gill lamellae contain blood capillaries
• lamellae are very thin in surface
• this means blood and water are very close

Question 11

what is a countercurrent flow?

Answer 11

it describe how the flow of water over the gill lamellae and the flow of blood within them are in opposite directions (in a fish)

Question 12

why does the countercurrent exchange system work?

Answer 12

• the concentration of oxygen is always higher in water than in blood
• therefore, oxygen always diffuses from blood to water
• this is the case along the whole lamellae, maximising useful SA for diffusion

> draw graphs to show oxygen concentration along the lamellae with and without the countercurrent exchange system