Gas exchange in plants ( Component 3 )

Question 1

Draw a diagram of a dicotyledonous leaf and label all the structures ( cuticle, upper epidermis, palisade and spongy mesophyll (with chloroplasts), vascular bundle (xylem, phloem and bundle sheath parenchyma), air spaces, lower epidermis, stomata, guard cells )

Answer 1

…

Question 2

Describe the function of the waxy cuticle

Answer 2

Reduces water loss from the leaf surface which also reduces gaseous exchange

Question 3

Describe how the upper epidermis is adapted for photosynthesis

Answer 3

Layers of transparent cells allow light to strike the mesophyll tissue
Epidermal cells also synthesise the waxy cuticle, reducing water loss

Question 4

Where is the palisade mesophyll layer located ?

Answer 4

Directly below the upper epidermis

Question 5

How is the palisade mesophyll layer adapted for photosynthesis ?

Answer 5

It receives the most light so contains the greatest concentration of chloroplasts

Question 6

How is the spongy mesophyll layer adapted for photosynthesis ?

Answer 6

Contains the air spaces which reduce the diffusion distance for carbon dioxide to reach the chloroplasts in the palisade layer
Contains some chloroplasts

Question 7

What is a vascular bundle ?

Answer 7

The vascular system in dicotyledonous plants.
It contains two types of transport vessel - xylem and phloem

Question 8

Why are vascular bundles important in photosynthesis ?

Answer 8

They form a large network to deliver water and nutrients to photosynthetic tissues and remove glucose

Question 9

Describe how the lower epidermis is adapted for photosynthesis

Answer 9

Contains many stomata which enable the evaporation of water and inward diffusion of carbon dioxide

Question 10

What are stomata ?

Answer 10

Small holes found on leaves that can be opened or closed by guard cells to control gas exchange and water loss

Question 11

Summarise the “ malate “ theory

Answer 11

The malate theory states that the accumulation or loss of malate and K+ ions by guard cells results in changes in turgor pressure that open or close the stomata

Question 12

By what mechanism do K+ ions enter the guard cell ?

Answer 12

Active transport

Question 13

How does the accumulation of K+ ions and malate ions affect guard cells ?

Answer 13

Lowers water potential of guard cells
Water moves in by osmosis down water potential gradient
Guard cells become turgid, opening the stomata

Question 14

Why is starch important for stomatal opening ?

Answer 14

Starch is converted into malate ions

Question 15

What do the adaptations in leaf structures allow

Answer 15

harvesting light energy, inward diffusion of CO2, provision of water and removal of products of photosynthesis

Question 16

Key features of palisade cells

Answer 16

elongated and densely arranged in a layer, or layers
contain many chloroplasts which arrange themselves according to the light intensity.

Question 17

What can pass through to the spongy mesophyll ?

Answer 17

Light

Question 18

What do the spaces between mesophyll cells allow ?

Answer 18

The spaces between mesophyll cells allow carbon dioxide to diffuse to the cells and oxygen can diffuse away

Question 19

Why are the ( mesophyll ) cells moist ?

Answer 19

So gases can dissolve

Question 20

What structures allow gases and water through ?

Answer 20

Pores and stomata

Question 21

What are the two main functions of the stomata ?

Answer 21

Control water loss
Allow for exchange of gases for photosynthesis and respiration

Question 22

What is the role of guard cells in the stomata ?

Answer 22

Guard cells around the stomata can change shape to open and close the stomata so helping to control gas exchange and water loss.

Question 23

Why can guard cells change shape ?

Answer 23

because of changes in turgor; in the light, water flows in by osmosis so the cells expand.

Question 24

Why is the inner wall elastic ?

Answer 24

So the pairs of cells curve away from each other and the pore opens.

Question 25

Which is thicker the inner cell wall or outer cell wall and why ?

Answer 25

Inner cell wall is thicker than outer cell wall Causes cells to curve away as water enters the cell and pore opens

Question 26

" malate theory " summary from spec

Answer 26

Mechanism by which stomata open and close. In the light, chloroplasts in the guard cells produce ATP through photosynthesis. ATP is used for active transport of K+ into the guard cells. Starch is converted to malate ions. K+ and malate ions lower water potential to below that of surrounding cells, so water moves by osmosis. Due to the uneven thickening of the inner walls of the guard cells, as the cell increases in size, the stomatal pore opens. The reverse occurs in dark conditions.

Question 27

What does the opening and closing of the stomatal pores alter ?

Answer 27

Water loss through transpiration

Question 28

What are plants classified on the basis of ?

Answer 28

Structure in relation to the prevailing water supply

Question 29

What are the three classifications of plants ?

Answer 29

Hydrophytes Mesophytes Xerophytes

Question 30

Describe the roots and leaves in hydrophytes ( and give an example of one )

Answer 30

eg water lily Live with their roots submerged in the mud at the bottom of the pond Have floating leaves on the surface

Question 31

In hydrophytes, describe : the need for support or transport tissues cuticle and stomata air spaces

Answer 31

Have little need for support or transport tissues Little or no cuticle only on the upper surface of leaves Stomata absent or reduced on submerging leaves Large air spaces present in both stem and leaf tissue

Question 32

What conditions have xerophytes adapted to ?

Answer 32

Living under conditions of low water availability

Question 33

What do xerophytes have to prevent excessive water loss ?

Answer 33

Modified structures eg : Succculent stems for water storage Leaves reduced to spines Close stomata during daylight Needle like leaves to reduce SA over which water can be lost Can survive on hot dry deserts or cold regions where soil is frozen for much of the year

Question 34

What type of plant does marram grass demonstrate ?

Answer 34

Role of a xerophyte

Question 35

How does marram grass demonstrate the role of a xerophyte and why ?

Answer 35

With its leaf shape, sunken stomata - humid air trapped outside of stomata so reduces water potential grad between leaf and atm so less transpiration thick cuticle - lowers rate of cuticular transpiration hairs - trap water vapour and reduce water potential grad rolled leaves - large thin walled epidermal cells shrink when they lose water by transpiration causing leaf to roll in on itself - reduces leaf area over which transpiration can occur to reduce water loss

Question 36

What are mesophytes ?

Answer 36

Plants of temperate regions that flourish in habitats with adequate blood supply

Question 37

What conditions do mesophytes need to survive and how do they do this ?

Answer 37

They need to survive unfavourable times of the year They do this by shedding their leaves, surviving underground or as dormant seeds.