3. Plant Gas Exchange

Question 1

What happens to CO2 in photosynthesis?

Answer 1

Carbon is “fixed” from CO2 and used to produce glucose

Question 2

What happens to water in photosynthesis?

Answer 2

It is split: the H is used to help in the production of glucose, but the oxygen is excreted as a waste gas

Question 3

Energy transfer in photosynthesis

Answer 3

Light energy is transferred to chemical energy stored in the glucose molecule

Question 4

What is glucose used for (as a product of photosynthesis)?

Answer 4

1. Respiration
2. Stored as starch
3. Building cell walls out of cellulose

Question 5

Function of waxy cuticle

Answer 5

Reduces water loss via transpiration

Question 6

Main feature & function of upper epidermis

Answer 6

Transparent: allows light through

Question 7

Adaptations/features of palisade cells

Answer 7

1. Lots of chloroplasts: high rate of photosynthesis
2. Cells arranged vertically: more light energy being trapped, more cells exposed, more efficient photosynthesis
3. Cells are very compact

Question 8

Adaptations/features of spongy mesophyll cells

Answer 8

Cells loosely packed with lots of air spaces: efficient gas exchange, gases can reach all the cells, more space for diffusion

Question 9

Function of xylem

Answer 9

Brings water & mineral ions from the soil

Photosynthesis & ensures cells are turgid

Question 10

Function of phloem

Answer 10

Takes away products of photosynthesis (sucrose)

Question 11

What maintains the concentration gradient of gases for photosynthesis?

Answer 11

Wind!

Oxygen gets swept away so that there is a high CO2 concentration outside the leaf

Question 12

What do cotyledons contain?

Answer 12

Food reserves for the developing embryo, usually in the form of starch

Question 13

What are monocotyledons/dicotyledons?

Answer 13

Monocotyledons have one embryo leaf, dicotyledons have 2

Question 14

Leaf adaptations for efficient gas exchange

Answer 14

1. Large SA: wide & flat leaves, long & thin palisade cells, spongy mesophyll spaced apart
2. Short diffusion distance: flat leaves, air spaces in spongy mesophyll layer, stomata
3. Steep conc gradient: CO2 used in photosynthesis

Question 15

How does evaporation in the leaf tissue happen?

Answer 15

Some of the light energy absorbed by leaves is converted into heat, which evaporates water within the spongy mesophyll. This vapour diffuses out of the leaf via the stomata, creating a negative pressure gradient within the leaf.

Question 16

How does evaporation within the leaf cause water to be drawn up from the xylem?

Answer 16

The negative pressure gradient creates a tension force in leaf cell walls, drawing water from the xylem (transpiration pul). The water is pulled from the xylem under tension due to the adhesive attraction between water and the leaf cell walls.

Question 17

What is transpiration rate, and what is it regulated by?

Answer 17

Amount of water lost from the leaves

Regulated by the opening & closing of the stomata

Question 18

What happens when guard cells become flaccid?

Answer 18

Block the opening/stomata, so stomatal pore closes due to loss of turgor

Question 19

Why do levels of photosynthesis affect transpiration?

Answer 19

Because stomatal pores are responsible for gas exchange in the leaf

Question 20

Factors affecting transpiration: humidity

Answer 20

High humidity = shallow diffusion gradient inside & outside of leaf = less transpiration

Question 21

Factors affecting transpiration: temperature

Answer 21

High temp = higher rate of evaporation = faster transpiration

Question 22

Factors affecting transpiration: light intensity

Answer 22

Higher light intensity = open stomata = higher rate of transpiration

Question 23

Xerophytes

Answer 23

Plants that have adapted to thrive under dry conditions

Question 24

Halophytes

Answer 24

Plants that have adapted to cope with high-salt environments (e.g. living in or near the sea)

Question 25

How do xerophytes survive (in a nutshell)?

Answer 25

By reducing transpiration!

Question 26

Environments that xerophytes live in

Answer 26

Deserts, sand dunes

Question 27

Xerophytes life cycle adaptations

Answer 27

1. Perennial plants bloom in wet seasons

2. Dormant seeds can survive for many years until conditions are ideal for growth

Question 28

Xerophytes metabolic adaptations

Answer 28

CO2 absorbed at night & stored as a C4 compound. During the day, photosynthesis can occur with the stomata closed by using these carbon stores.

Question 29

Xerophytes physical adaptations

Answer 29

1. Fewer leaves/stomata
2. Rolled leaves/spines
3. Stomata in pits with hairs
4. Deeper roots to reach water
5. Waxy cuticle reduces evaporation

Question 30

Cross section of marram grass adaptations

Answer 30

1. Waxy cuticle
2. Rolled leaf protects boundary layer from wind & reduces outward SA (like a little humid pocket!)
3. Hairs trap a layer of water vapour
4. Stomata in pits maintain boundary layer

Question 31

Explaining general adaptations of xerophytes: thick cuticle

Answer 31

Stops uncontrolled evaporation from leaf cells

Question 32

Explaining general adaptations of xerophytes: small leaf SA to volume ratio

Answer 32

Reduces SA for evaporation

Question 33

Explaining general adaptations of xerophytes: stomata sunken in pits

Answer 33

Maintains humid air around stomata so shallower concentration gradient

Question 34

Explaining general adaptations of xerophytes: leaf hairs

Answer 34

Maintains humid air around stomata so shallower concentration gradient

Question 35

Explaining general adaptations of xerophytes: rolled leaves

Answer 35

Maintains humid air around stomata so shallower concentration gradient

Question 36

Explaining general adaptations of xerophytes: deep roots

Answer 36

Access deep water sources

Question 37

Explaining general adaptations of xerophytes: widespread shallow roots

Answer 37

As soon as it rains water can be quickly absorbed into the roots (rapid access to rainwater)

Question 38

Explain how the waxy cuticle reduces water loss

Answer 38

Impermeable to water/waterproof