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