Exchange In Single Celled Organisms, Insects,Fish and Plants

Question 1

Describe how gas exchange occurs in single celled organisms

Answer 1

Single called organisms are small and therefore have a large surface area to volume ratio. Oxygen is absorbed by diffusion across their body surface, which is covered by a cell surface membrane. In the same way, carbon dioxide from respiration diffuses out across their body surface.

Question 2

Describe the tracheal system by which gases are transported in insects

Answer 2

• Insects have tiny spores called spiracles on their body surface by which gases enter/leave.
• Once the gases enter via the spiracles, they are transported by a network of tubes called tracheae.
• The tracheae are supported by strengthened rings to prevent them from collapsing.
• The tracheae divide into smaller dead end tubes called tracheoles. The tracheoles extend throughout all of the body tissue of an insect.
• In this way, the oxygen in the air is brought directly to the respiring tissues as there is a short diffusion pathway from a tracheole to any body cell.

Question 3

Name the three key ways by which respiratory gases move in and out of the tracheal system

Answer 3

1) Along a diffusion gradient
2) Mass transport
3) Via The end of the tracheoles which are filled with water.

Question 4

Describe how oxygen moves in and out of the tracheal system along a concentration gradient.

Answer 4

• When cells are respiring, oxygen is used up and so its concentration towards the end of the tracheoles falls.
• This creates a diffusion gradient that causes gaseous oxygen to diffuse from the atmosphere along the tracheae and tracheoles to the cells.

Question 5

Describe how carbon dioxide move in and out of the tracheal system along a concentration gradient.

Answer 5

• Carbon dioxide is produced by cells during respiration.
• This creates a diffusion gradient in the opposite direction to that of oxygen.
• This causes gaseous carbon dioxide to diffuse along the tracheoles and tracheae from the cells to the atmosphere.

Question 6

Describe how respiratory gases move in and out of the tracheal system by mass transport

Answer 6

The contraction of muscles in insects can squeeze the trachea which enables the mass movement of air in and out. This further speeds up the exchange of respiratory gases.

Question 7

Describe how respiratory gases move in and out of the tracheal system due to the ends of the tracheoles being filled with water

Answer 7

• During periods of major activity, the muscle cells around the tracheoles carry out some anaerobic respiration.
• This produces lactate which is soluble and lowers the water potential of muscle cells.
• Water therefore moves into the cells from the 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 that the final diffusion pathway is in a gas rather than a liquid phase, and therefore diffusion is more rapid.
• This increases the rate at which air is moved into the tracheoles but leads to greater water evaporation.

Question 8

What limitations does the tracheal system of gas exchange in insects have

Answer 8

• When the spiracles are open, water vapour can evaporate from the insect.
• It relies on diffusion and therefore insects must be small so that the diffusion pathway is short.

Question 9

What specialised internal gas exchange system have fish evolved

Answer 9

The Gills

Question 10

Describe the structure of the gills

Answer 10

• the gills are made up of gill filaments which are stacked up in a pile
• At right angles to the gill filaments are gill lamellae which increase the surface area 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.
• The flow of water over the gill lamellae and the flow of blood are in opposite directions which is known as countercurrent flow.

Question 11

What does the countercurrent exchange between the blood and water in a fish mean:

Answer 11

1) That blood that is already well loaded with oxygen meets water, and diffusion of oxygen occurs from that water (that has the maximum concentration of oxygen) to the blood.
2) blood with little oxygen meets water that has had most, but not all of its oxygen removed. Again, here, diffusion of oxygen from the water to the blood takes place.

Question 12

Why is the countercurrent exchange system useful in fish gills

Answer 12

It maintains the diffusion gradient for oxygen uptake for the entire length of the gill lamellae. If the flow of water and blood had been in the same direction (parallel flow), the diffusion gradient would only be maintained across part of the length of the gill lamellae.

Question 13

What gas exchange occurs when a plant is photosynthesising

Answer 13

When photosynthesis takes place, although some carbon dioxide comes from respiration in cells, most of it is obtained from the external air. In the same way, some oxygen from photosynthesis is used in respiration but most of it diffuses out of the plant.

Question 14

What gas exchange occurs when a plant is not photosynthesising

Answer 14

When photosynthesis is not occurring, for example in the dark, oxygen diffuses into the leaf because it is constantly being used by cells in respiration. In the same way, the carbon dioxide produced during respiration diffuses out.

Question 15

In what ways is gas exchange in plants similiar to gas exchange in insects

Answer 15

• No living cell is far from the external air, and therefore a source of oxygen and carbon dioxide.
• diffusion takes place in the gas phase (air) which makes it more rapid than if it were in water.

Question 16

What adaptations do leaves have for rapid gas exchange

Answer 16

• They have many small pores called stomata, so no cell is far away form a stomata and this makes the diffusion pathway short.
• they have numerous interconnecting air spaces that occur throughout the mesophyll so that gases can readily come into contact with mesophyll cells.
• the mesophyll cells have a large surface area for rapid diffusion.

Question 17

Why is the presence of guard cells important

Answer 17

• They control when the stomata are open and the rate of gaseous exchange.
• This is important because organisms lose water when gas exchange occurs.
• Plants balance the conflicting needs of gas exchange and prevention of water loss by closing the stomata at times when water loss would be excessive.

Question 18

What adaptations have insects evolved to minimise water loss

Answer 18

1) The spiracles can be closed to minimise water loss. This conflicts with the need for oxygen and so occurs mainly when the insect is at rest.
2) Insects often have waterproof coverings over their body surfaces. In insects this covering is a rigid outer skeleton of chitin that is covered with a waterproof cuticle.
3) Small surface area to volume ratio (only in some cases)

Question 19

What are xerophytes

Answer 19

Plants which have adapted to live in areas where water is in short supply. They evolve a range of adaptations to limit water loss via transpiration.

Question 20

List the five key modifications that plants may develop to limit water loss

Answer 20

1) A thick cuticle
2) Rolling up of leaves
3) hairy leaves
4) Stomata in pits or grooves
5) A reduced surface area to volume ratio

Question 21

Explain how having a thick cuticle helps leaves to limit water loss

Answer 21

The waxy cuticle on leaves forms a waterproof barrier. However it does not completely prevent water loss and so the thicker the cuticle, the less water loss occurs.

Question 22

Describe how rolling up leaves helps plants limit water loss

Answer 22

• Most leaves have their stomata mainly on the lower epidermis.
• Some plants roll their leaves in a way that protects the lower epidermis from the outside and helps to trap a region of still air within the rolled leaf.
• This region becomes saturated with water vapour and so has a high water potential.
• This means that there is no water potential gradient between the inside and outside of the leaf and so water loss is limited.

Question 23

Explain how having hairy leaves helps plant to minimise water loss

Answer 23

• A thick layer of hairs on leaves, especially on the lower epidermis, traps still, moist air next to the leaf surface.
• The water potential gradient between the inside and outside of the leaf is reduced and so less water loss occurs.

Question 24

Explain how having stomata in pits or grooves helps a plant to limit water loss

Answer 24

These again trap still, moist air next to the leaf and reduce the water potential gradient.