OPTIMIZING PHOTOSYNTHESIS

How leaf structure helps optimize photosynthesis

--photosynthesis takes place in chloroplasts of mesophyll

cells in pallisade and spongy tissues

--according to formula:

12 H2O + 6 CO2 + light --->

C6H12O6 + 6 O2 + 6 H2O + heat

--thus, the process depends on water, CO2, light (also, the

leaf must avoid buildup of products, especially O2 and

heat)

Dependence of photosynthesis on light

Adaptations to adjust light intensity

Sun vs. shade leaves: shade leaves thinner (developmental plasticity to optimize thickness, absorption of light)

Sun tracking: pulvinus--turgor movements point leaves toward light (or parallel to excess light)--e.g., lupine

Anthocyanins in epidermis--many spring leaves have

these pigments, which absorb light, limit

photoinhibition

Xanthophyll cycle: zeaxanthin (a carotenoid) absorbs

excess energy from excited chlorophyll, prevents

destruction of antenna, reaction center

Dependence of photosynthesis on CO2

CO2 is the usual limiting factor for photosynthesis

--normal [CO2] 0.03% -> 0.0388% and rising (from fossil

fuel burning and deforestation) --gases diffuse quickly in air, but slowly in H2O and

cytoplasm

Adaptations to optimize gas flow

1. Flat shape: high surface/volume ratio

2. Air spaces between cells

3. Stomata (holes in epidermis)--open with PS is active,

closed otherwise

Adaptations to optimize gas flow

1. Flat shape: high surface/volume ratio

2. Air spaces between cells

3. Stomata (holes in epidermis)--open with PS is active,

closed otherwise

Adaptations to optimize gas flow

1. Flat shape: high surface/volume ratio

2. Air spaces between cells

3. Stomata (holes in epidermis)--open with PS is active,

closed otherwise

Light

Dark

Low CO2

High CO2

Leaves of aquatic plants

Elodea, Anacharis, Potomogeton)

Aquatic plants get carbon for

photosynthesis in the form of dissolved CO2 or HCO3-,

depending on pH

HCO3- is a problem, because...

--rate of dissolution and hydration

is slow --HCO3- does not cross cell

membrane easily (- inside)

Thus, leaves need different structure to take up HCO3-

--no epidermis, photosynthetic

cells in contact with water --ion (H+) pumps reform CO2

Summary

Optimizing light

•Sun vs. shade leaves•Pigments•Sun tracking

Optimizing CO2

•Thin shape•Air spaces•Stomatal control•Proton pumps in aquatic plants