Carbon dioxide and plants

Question 1

Describe stomata?

Answer 1

Transpiration is largely controlled by the stomatal aperture when the boundary layer effects are small (moving, turbulent air), but not when the boundary layer is large (still air).

Water vapour evaporates and leaves. Carbon dioxide enters the leaf by diffusion via a vapour, dissolves into solution once in the cell. Heat gets lost from the leaf as well.

Water and carbon dioxide are mostly affected by Calvin cycle and light dependent stage.

Question 2

How many water molecules get lost for every carbon dioxide fixed?

Answer 2

500 water moecules lost for every 700 carbon dioxide molecules fixed.

Question 3

Stomatal responses and signalling?

Answer 3

Direct light or humidity effects are signals.

Carbon dioxide feedback.

ABA is the most common hormone associated, and stomata respond to very low levels of this hormone.

Question 4

Evolution of leaves in Earth’s history?

Answer 4

Early Devonian - microphyll structure. Low stomatal densities

Late Devonian - megaphyll structures. High stomatal densities. Oxygen levels rose and CO2 dropped.

Question 5

How did the megaphylls evolve?

Answer 5

There is a well-established inverse relationship between CO2 and stomatal density

As a result of lowered CO2 and increased stomatal densities, transpirational cooling would have become more effective

This may have permitted the evolution of megaphylls without tissue temperatures reaching lethal values

Question 6

Costs and benefits of transpiration vs photosynthesis?

Answer 6

Maximal water use efficiency.

Question 7

What energy enters leaves?

Answer 7

Absorbed solar irradiation

Absorbed infrared irradiation from surroundings

Question 8

What energy leaves leaves?

Answer 8

Emitted infrared radiation
Heat loss by conduction and convection
Heat loss by water evaporation

Question 9

Morphological characters of shoot which help to minimise transpiration?

Answer 9

Small leaves reducing total transpiring surface

High leaf reflectance

Leaf hairs

Low cuticular conductance

Stomata are few, as well as small and sunken

Question 10

How do plants survive in water limited environments?

Answer 10

Short life cycle with a dormancy period.

Small leaves.
Surface characteristics
Stomata
Extensive/deep root systems

Turgor maintenance
Protective solutes in cytoplasm

Question 11

Physiological plasticity in plants?

Answer 11

Shaded leaves generally have a thinner leaf lamina, more thylakoid lamellae in granal stacks in chloroplasts.

More chlorophyll per reaction centre.

Higher ratio of chlorophyll b to a.

Higher ratio of PSII to PSI

Question 12

What is photorespiration, and other issues with Rubisco?

Answer 12

Very large enzyme so heavy investment in protein nitrogen.

Has a low turnover rate, so it has to be a very abundant protein (50% of soluble protein in photosynthetic plant tissue)

Relatively high Km so only 50% of the Carbon Dioxide in C3 plants gets processed under present day conditions.

Relatively poor selectivity for CO2 vs O2 under typical physiological conditions.

Question 13

How does photorespiration vary with temperature?

Answer 13

Increases steeply with temperature.

Oxygenase activity of rubisco increases more than carboxylase activity.

Question 14

What is photorespiration?

Answer 14

A respiratory process in many higher plants by which they take up oxygen in the light and give out some carbon dioxide, contrary to the general pattern of photosynthesis.

Question 15

How did photorespiration evolve?

Answer 15

Wasn’t a massive problem in the past, as photosynthesis began when there was a very high CO2 concentration, and a low O2 concentration.

The oxygenase activity of Rubisco can also act as a safety valve, helping to prevent the electron transport chain from bieng over reduced.

Question 16

Describe C4 plants?

Answer 16

C4 plants minimise photorespiration by separating initial CO2 fixation and the Calvin cycle in space, performing these steps in different cell types.

Light dependent reactions occur in mesophyll cells. Calvin cycle occurs in bundle-sheath cells around the leaf veins.

Although ATP must be used to transfer malate to the bundle sheath cell, the constant presence of malate means there is always a high concentration of CO2 relative to O2 around rubisco. Minimises photorespiration.

3% of vascular plants.

Sugarcane and corn.

Common in hot habitats, as the benefits of reduced photorespiration exceeds the ATP cost.

Question 17

Describe CAM plants?

Answer 17

Minimise photorespiration and save water by separating these steps in time (day and night).

At night, CAM plants open their stomata, allowing CO2 to diffuse into the leaves. This is converted into malate.
CO2 + PEP -> malic acid

Stored inside vacuoles until the next day. In daylight, don’t open stomata but can break down the organic acids to release carbon dioxide, which enters the Calvin cycle. Maintains a high concentration of CO2 around rubisco.
Malic acid -> CO2

Requires ATP at multiple steps. But avoid photo-respiration, and are very water efficient as only open their stomata at night when temperatures are cooler.

Common in dry environments.

Cacti and pineapples

Question 18

CO2 starvation hypothesis for the origin of C4 plants?

Answer 18

How to explain long delay between timing of minimum CO2 concentrations (~ 25 Ma) and emergence of C4 -dominated ecosystems (~ 6 Ma)?

Declining CO2 during Eocene was associated with progressive cooling (as CO2 is a greenhouse gas). Further major cooling at end of Miocene and in Pliocene was associated with gradual aridification, which restricted tree cover, allowing C4 grasses to become dominant

Also, drier ecosystems are more fire-prone, which would favour grasses over trees

So there may be significant positive feedbacks:

• grasslands are more fire-prone, further restricting tree cover
• smoke plumes cause drier air and prevent cloud formation, further reducing rainfall and favouring grasses over trees

Ecosystem reached a tipping point where it switched from C3 to C4 dominated biome.