Leaf Structure and Function

Question 1

What do leaves typically consist of?

Answer 1

• a narrow stalk (petiole)
• a thin flat expanded blade (lamina)
• attached to stems at nodes

Question 2

What do the structure and orientation of leaves aim to maximise and minimise?

Answer 2

• maximise interception of sunlight

- minimise water loss

Question 3

What are leaves typically modified and adapted for?

Answer 3

• photosynthetic function in their particular environment

- an entirely different function (e.g storage or defence)

Question 4

What are simple leaves?

What are compound leaves?

Answer 4

Simple leaves: leaflets with individual lamina

Compound leaves: many leaflets joined to a single lamina

Question 5

What are dorsiventral leaves?

Answer 5

Leaves that hang horizontally with upper (dorsal) and lower (ventral) surfaces that are easily distinguishable from each other due to being different in structure.

Question 6

What are isobilateral/bifacial leaves?

Answer 6

Held vertically. Both sides have the same anatomy (possess palisade mesophyll only) and epidermal features.

Question 7

What are dorsiventral and isobilateral leaves both adaptations for?

Answer 7

They are both adaptations to maintain equal illumination on both sides of the blade, while at the same time maintaining an orientation that reduces heat absorption.

Question 8

Describe the structure of a transverse section of a dorsiventral leaf. Compare this to a isobilateral leaf.

Answer 8

• cuticle on top of upper epidermis
• beneath upper epidermis is the palisade mesophyll
• vascular bundle containing xylem and phloem
• beneath palisade mesophyll is the spongy mesophyll containing lots of air spaces
• beneath the spongy mesophyll is the lower epidermis which contains stomata

Isobilateral leaves only contain palisade mesophyll, and the epidermis structures are identical on both sides with an equally thick cuticle on both sides also.

Question 9

Where are stomata located?

Answer 9

In the epidermis of the leaf.

Question 10

Why does the cuticle arch over the stomata?

Answer 10

The cuticle arches over the stomata so that they are sunk beneath the epidermis, which reduces water loss.

Question 11

What is the function of the stomata?

Answer 11

Allows intake of CO2 from the atmosphere and release of oxygen and water (transpiration).

Question 12

What do stomata consist of?

Answer 12

Two kidney shaped guard cells. These guard cells contain chloroplasts whereas other epidermal cells do not.

Question 13

Describe the properties and function of the cuticle.

Answer 13

• waxy and hydrophobic
• protects plant from water loss, UV radiation, pathogens
• is also impermeable to gas so gas must enter through the stomata only.

Question 14

Name and describe two examples of specialised epidermal cells.

Answer 14

1. Non-glandular trichomes
   - common in plants in arid (dry) and alpine areas
   - trichomes aka epidermal hair trap a boundary layer of air against the leaf surface, preventing water and heat loss
   - trichomes also modulate light absorption and deter herbivores
2. Glandular trichomes
   - trichomes contain compounds that deter herbivores (plant eaters)

Question 15

Describe the leaf vascular system and its purpose.

Answer 15

• leaf veins contain xylem and phloem
• veins are interconnected and branching
• the veins supply the mesophyll cells with water (xylem) and translocate the sugars made in photosynthesis (phloem)

Question 16

What is transpiration?

Answer 16

Loss of water from leaves through evaporation.

Question 17

What prevents leaves from wilting?

Answer 17

Water provides turgor pressure to prevent leaves from wilting

Question 18

When and how does water evaporate?

Answer 18

Water evaporates when the stomata are open and water vapour moves out of the leaf.

Question 19

Describe the moistness of the interior of the leaf and its mesophyll cells.

Answer 19

Interior of the leaf is very moist, outside of mesophyll cells is wet and provides surface for CO2 diffusion into cells.

Question 20

Describe the mechanism of how stomata open and close.

Answer 20

• H+ ions pumped out of guard cells and K+ ions pumped in
• the 20x increase in the concentration of K+ ions causes water to flow into the guard cells via osmosis (water moves from region of high to low water potential), thereby making the guard cells turgid
• the guard cell walls adjacent to the pore are thickened and rigid, and their inability to expand with the rest of the guard cell causes the pore to open

Question 21

Describe what happens in hot, arid conditions to the leaf stomata.

Answer 21

• Leaves lose water by evaporation through stomata.
• stomata close to conserve water as a result, however this leads to a buildup in O2 from photosynthesis within the leaf and CO2 is unable to enter the leaf.

Question 22

Describe the modifications in plants with C4 and CAM photosynthesis.

Answer 22

• spatial separation in C4 plants

- temporal (time) separation in CAM plants

Question 23

Why don’t all plants perform CAM or C4 photosynthesis?

Answer 23

• trade off in energy costs
• when water is not limiting, C3 plants can deal with photorespiration
• C4 photosynthesis is energy consuming, but in hot and dry conditions that energy trade off is worth it

Question 24

Describe the changes in leaf nutrition at elevated CO2 levels.

Answer 24

• C3 plants will fix carbon more efficiently, because the oxygenation reaction catalysed by RUBISCO will occur less frequently.
• more sucrose/starch produced, plants will accumulate more biomass and grow faster.
• nutrient composition changes in some crop species e.g decreases in protein, zinc, ion
• may not be a problem when fertiliser added to crops, but plants can end up being less nutritious to herbivores.

Question 25

Describe an adaptation to arid environments.

Answer 25

Sclerophylly: heavily lignified leaf tissues, causing them to be tough and rigid. This prevents them from wilting when they are water stressed, and allows them to maintain photosynthesis and avoid tissue damage in a dry environment

Question 26

Describe modifications in leaf structure to:

• shadier environments
• sunnier environments

Answer 26

Shadier environments:

• more chlorophyll (dark green)
• larger in size to capture as much light as possible
• thin leaves

Sunnier environments:

• lighter in colour, less chlorophyll
• smaller in size
• more palisade mesophyll, thicker leaves