6.2 - Gas exchange in single-celled organisms and insects Flashcards
Describe gas exchange in single-celled organisms
- single-celled organisms are small and therefore have a large SA:Vol
- oxygen is absorbed by diffusion across their body surface, which is only covered by a CSM
- in the same way, CO2 from respiration diffuses out across their body surface
What is the amount of oxygen needed by an organism determined by
- the amount of living cells
- the rate they need to respire
—> requirement is related to volume and the rate depends on the SA (as well as metabolic rate)
What is the problem for gas exchange in insects
- cells need to be exposed to air in order for the oxygen to diffuse into the organism
- terrestrial organism’s bodies are made of a high % of water
- when living cells are exposed to the air = water molecules evaporate and the cell dehydrates
How do insects overcome this problem
- Small SA:Vol ratio = minimises the area over which water is lost
- water proof covering over their body surfaces. It is a rigid outer skeleton of chitin that is covered with a waterproof cuticle
- Spiracles are the openings of the tracheae at the body surface and these can be closed to reduce water loss. Because this conflicts with the need for oxygen —> it’ll occur when the insect is at rest
What have insects evolved to have in order to perform gas exchange
- internal network of tubes called tracheae (supported by rings of chitin to prevent them collapsing)
- the tracheae then divide into smaller deadend tubes called tracheoles
Describe what tracheae
- tubes that penetrate inside the body carrying air to every tissue
- they carry air directly to cells for gas exchange
- they’re held open by rings of chitin
Describe tracheoles
- have ends filled with water
- during periods of intense activity, e.g. flight, production of lactate lowers the water potential in the muscle cells
- so water can move from the tracheoles into the cells via osmosis
- gases dissolved in the water also move into the cell
- But the disadvantage is that this increases evaporation and therefore desiccation of the insect, however the spiricles can combat this by closing to reduce water loss
What are the 3 ways that respiratory gases move in and out of the tracheal system
- Along a diffusion gradient
- Mass transport
- The ends of the tracheoles are filled with water
How does ‘along a diffusion gradient’ help gases move in and out of the tracheal system
- when cells are respiring, oxygen is used up and so its concentration towards the ends of the tracheoles falls
- this creates a diffusion gradient that causes gaseous oxygen to diffuse from the atmosphere along the trachea and tracheoles to the cells
- CO2 is produced by cells during respiration
- creates a diffusion gradient in the opposite direction
- this causes gaseous CO2 to diffuse along the tracheoles and tracheae from the cells to the atmosphere
- as diffusion in air is much more rapid than in water, respiratory gases are exchanged quickly by this method
How does ‘mass transport’ help gases move in and out of the tracheal system
- the contraction of muscles in insects can squeeze the trachea enabling mass movement of air in and out
- this further speeds up the exchange of respiratory gases
How does ‘the ends of the tracheoles are filled with water’ help gases move in and out of the tracheal system
- During periods of major activity, muscle cells around the tracheoles respire carry out some anaerobic respiration
- this produces lactate, which is soluble and lowers the water potential of the muscle cells
- water therefore moves into the cells from tracheoles by osmosis
- the water in the ends of the tracheoles decreases in volume and in doing so draws air further into them
- This means the final diffusion is more rapid
- this increases the rate at which air is moved in the tracheoles but leads to greater water evaporation
How does gases enter and leave the tracheae
- Through tiny pores called spiracles on the body surface
- the spiracles may be opened/closed by a valve
- when open: water vapour can evaporate from the insect
- when closed: prevents water loss
- periodically they will open to allow gas exchange
What are some of the limitations to the tracheal system
- 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 of a small size
- as a result the length of the diffusion pathway limits the size that insects can attain
How come a prehistoric insect could have a wingspan of 75cm
- the atmosphere had a much higher concentration of oxygen (35%)
- therefore it could get enough oxygen to help it with respiration without having to sacrifice it’s size
Describe what happens to the concentration of oxygen in the tracheae when the spiracles are closed
- It falls steadily and then remains at the same level
Suggest an explanation for this change in the concentration of oxygen when the spiracles are closed
- cells use up oxygen during respiration and so it diffuses out of the tracheae and into these cells
- with the spiracles closed, no oxygen can diffuse in from the outside to replace it
- ultimately, all the oxygen is used up and so the level ceases to fall
Use the information provided by the graph to suggest what causes the spiracles to open
The increasing level of carbon dioxide
