6.4 and 6.5 Gas exchange in the leaf of a plant and reducing water loss

Question 1

What is the gas exchange for respiration of a plant cell

What about for photosynthesis

Answer 1

They take in oxygen and release carbon dioxide

The cells that are doing photosynthesis take in carbon dioxide and release oxygen

Question 2

Why is it good that photosynthesis and respiration occur at the same time

Answer 2

The oxygen released from photosynthesis can be used for respiration.

This reduces gas exchange with the environment

Question 3

How is gas exchange in plants similar to gas exchange in insects

(short diffusion pathway)

Answer 3

. No living cell is far from the external air so is therefore close to 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

So there is a short diffusion pathway

Question 4

How do leaves have a large surface area to volume ratio

Answer 4

The air spaces inside leaves have a large surface area compared to the volume of living tissue

Question 5

Where does most gas exchange occur in plants

Answer 5

In leaves

Question 6

How do gases move in and out of the plant

Answer 6

There is no specific transport system so the gases move in and out by diffusion directly
They enter and leave through the stomata

Question 7

What are the adaptations of leaves for gas exchange

Answer 7

. They contain many small pores called stomata where gases can diffuse in and out.
No cell is far from a stomata so the diffusion pathway is short.

. Large surface area of mesophyll cells for rapid diffusion

. Numerous interconnecting air spaces that occur throughout the mesophyll so gases can readily come in contact with mesophyll cells that could be photosynthesising or respiring

Question 8

What are stomata and what are they controlled by

Answer 8

Mini pores that are mainly found in leaves, especially on the underside of them
Two guard cells are on either side of each stoma and they can control when to open and close each stomatal pore

Question 9

Why is it good for plants to have stomata

Answer 9

They can control the rate of water loss by evaporation in terrestrial organisms.

They help to balance the conflicting needs of gas exchange and water loss by being able to close at times when water loss would be excessive

Question 10

Describe the layers in leaves

Answer 10

. Upper epidermis
. Palisade mesophyll containing chloroplasts for photosynthesis
. Spongy mesophyll where gas exchange can occur through all the gaps in it
. Lower epidermis containing stomata and guard cells

Question 11

Majority of water loss occurs through leaves
Name the 5 adaptations do leaves have to help reduce this

Answer 11

. Thick waxy cuticle layer

. Rolling up leaves

. Stomata in pits or grooves

. Hairy leaves

. A reduced surface area to volume ratio in leaves

Question 12

How does having a thick waxy cuticle layer reduce water loss in plants

Answer 12

Thick waxy cuticle layer can form a waterproof barrier, however up to 10% of water loss can occur through this route
Thicker the cuticle, the less water that can escape. Eg in Holly

Question 13

How does rolling up leaves reduce water loss in plants

Answer 13

. Most leaves have their stomata mainly on the lower epidermis.
Leaves can roll up to protect these stomata from the outside which can trap a region of still air within the rolled up leaf
. This region can become very saturated with water vapour so has a high water potential

This means there is no water potential gradient between the inside and outside of leaf so therefore there is no water loss

Eg Marram leaves

Question 14

How do hairy leaves reduce water loss of plants

Answer 14

Thick layer of hairs, especially on lower epidermis can trap a layer of moist air next to the leaf surface

This reduces the water potential gradient between the inside and outside of leaf so there is much less loss of water

Eg heather plants

Question 15

How do the stomata in pits and grooves of leaves reduce water loss

Answer 15

These trap still, moist air next to them which reduces the water potential gradient between the inside and outside of leaf so less water is loss by evaporation

Eg pine trees

Question 16

How does a reduced surface area to volume ratio in the leaves reduce water loss

Name examples of plants

Answer 16

The smaller the surface area to volume ratio, the lower the rate of diffusion

So having small and circular leaves eg pine needles, rather than big and flat leaves can reduce the rate of water loss

You need to balance this with the need in surface area for sunlight for photosynthesis

Question 17

What are xerophytes

Answer 17

Plants that are adapted to live in areas where water is in short supply, they may show a large range of desert adaptations eg in hot and dry conditions

Question 18

Why should water loss limited in insects

Answer 18

Most insects are terrestrial so live on land which means there is a problem of water evaporating from their surface and they can become dehydrated

Insects have to balance the conflicting needs for a good gas exchange system, whilst reducing water loss

Question 19

What adaptations do insects have that reduce water loss

And what does this mean for gas exchange of insects

Answer 19

• Small surface area to volume ratio to minimise the area over which water is lost
• Waterproof coverings over the body surfaces eg having a rigid outer skeleton of chitin that is covered with a waterproof cuticle
• Spiracles are the openings to the trachea at the body surfaces and they can be closed to reduce water loss. This conflicts with the need for oxygen so occurs largely when the insect is at rest

This means that gas exchange by diffusion can’t occur so they have to use their trachea to carry oxygen to the tissues

Question 20

Why do plants not usually have a small surface area to volume ratio

Answer 20

They rely on photosynthesis for energy which means there has to be a large surface area for the chloroplasts to absorb the maximum amount of sunlight