B4- gas exchange in fish, insects ✅ and plants

explain how the body of a unicellular organism is adapted for gas exchange

1. thin flat shape → large SA:V ratio

2. thin → short diffusion distance to all parts of cells → rapid diffusion

why can a unicellular organism rely on gas exchange via diffusion but a multicellular organism can’t?

• unicellular → large SA:V ratio → short diffusion distances for substances to travel → can rely on diffusion

• multicellular → would take too long for gases to diffuse and reach all cells

describe the tracheal system of an insect

1. spiracles = pores on the surface that can open / close to allow diffusion

2. tracheae = large tubes full of air that allow diffusion

3. tracheoles = smaller branches from tracheae, permeable to allow gas exchange with cells

adaptations of insects tracheal system for gas exchange

• Spiracles- pores on the surface which can open and close

• Tracheoles have thin walls → so short diffusion distance to cells

• high numbers of highly branched tracheoles→ so short diffusion distance to cells and 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 and out → maintains concentration gradient for diffusion

• fluid in the end of tracheoles drawn into tissue by osmosis during exercise→ as fluid is removed, air fills tracheoles, so rate of diffusion to gas exchange surface increases as diffusion is faster through air

• The tracheal system also replenishes storage air sacs, which are important when the insect needs to conserve water.

explain structural and functional compromises in terrestrial insect that allow effect gas exchange while limiting water loss

1. thick waxy cuticle / exoskeleton → increases diffusion distance so less water loss via evaporation

2. spiracles can open to allow gas exchange and close to reduce water loss (evaporation)

3. hairs around spiracles → trap moist air, reducing water potential gradient so less water loss (evaporation)

describe and explain 3 ways gases are exchanges in a tracheal system

1. down a diffusion gradient

• respiration uses oxygen, creating low conc. oxygen at end of tracheoles

• oxygen diffuses from spiracles down conc grad to the cells (opposite occurs for CO₂)

2. abdominal pumping → down a pressure grad→ speeds up diffusion

• contraction of muscles in abdomen squeezes tracheae

• enabling mass transport of air in and out

• CO₂ forced out down pressure grad

• pumps more oxygen in

• also helps maintains greater diffusion grad which speeds up exchange of gases after exercise

3. water collects at end of tracheoles (due to respiration)

• during eg flying, muscle cells around tracheoles respire anaerobically and produce lactate

• lowers water potential of cells so water moves into cells by osmosis

• causes rate of gas exchange to increase bc: gases diffuse faster in air than through water, greater SA exposed to air, decreases pressure in tracheoles and as a result more air from atmosphere is drawn in

why do larger insects need abdominal pumping but smaller ones don’t

larger insects:

• speeds up diffusion of oxygen into body

• also helps removal of carbon dioxide

smaller insects:

• tiny size = large SA:V and short diff distances

• allowing enough oxygen to reach cells via diffusion down conc grads through tracheal system