EFB530 Plant Physiology

Photosynthesis: CO₂ concentrating mechanisms

Mechanisms have evolved to reduce the wasteful influence of the oxygenation reaction

• systems to concentrate CO₂ in the locale of Rubisco

• active when grown in low CO₂ (as in air) ~0.03% CO₂
• can concentrate CO₂ up to 1000X inside the cells

Hal Hatch & Roger Slack (mid '60's) in Australia

• repeated the Calvin & Benson approach using ¹⁴CO₂ to label sugarcane
• first labeled products of photosynthesis in sugarcane was malate (not PGA)
• label in C4 of malate went to C1 of PGA
• enzymes involved in carbon fixation are divided between two cell types
• split between mesophyll cells and bundle sheath cells
• Kranz anatomy (Kranz=wreath in German)

In different species, three variants of this cycle have evolved:

• we will only talk about one: the NADP malic enzyme type
• includes maize, sugarcane, sorghum

Four steps:

1) carbon assimilation (in mesophyll)
2) transport to bundle sheath cell
3) decarboxylation
4) transport & regeneration in mesophyll

The initial carboxylation reaction is performed by phosphoenolpyruvate carboxylase
in mesophyll cells

• PEP carboxylase has a high affinity for HCO₃^-
• PEP + HCO₃^- -> oxaloacetate (OAA) + Pi
• OAA + NADPH + H⁺ -> malate + NADP⁺

malate is transported to bundle sheath cells

• malate + NADP⁺ -> pyruvate + CO₂ + NADPH + H⁺
• CO₂ used by Rubsico for C3-type carboxylation

pyruvate transported back to mesophyll cells

• pyruvate + Pi + ATP -> PEP + AMP + PPi
• (this is equivalent to 2 ATP->2 ADP)

There is an energetic cost: +2 ATP for each CO₂ fixed (but essentially NO oxygenation)

Many of the C4 pathway enzymes are regulated by light (active in light)
C4 pathway allows more efficient carbon fixation at high temperatures

1) PEP carboxylase binds CO₂ at very low conc, so stomata can partially close
2) photorespiration is very low, so no effect of higher [O₂] at higher temps

The C4 pathway is mostly found among tropical grasses (growing in warm, sunny environments). C4 plants cannot compete with C3 plants in moist, colder, and less sunny environments. Less than 1% of plant species use C4 photosynthesis.

3) Crassulacean acid metabolism (CAM)
=separation of Rubisco from O₂ over time

• found in many desert plants (cacti, in hot, arid environment)

these plants assimilate CO₂ at night, when their stomata are open

• use PEP carboxylase to fix CO₂ into OAA, which is reduced to malate
• malate is stored in the vacuole overnight
• the PEP is supplied by starch breakdown

they close their stomata during the day, locking CO₂ inside the leaf

• this avoids water loss during the day
• malate is released from the vacuole and decarboxylated by NADP malic enzyme
• this concentrated CO₂ feeds Rubisco and the C3 pathway, driven by ATP and NADPH by the light reactions
• pyruvate derived from the decarboxylated malate is used for starch synthesis

CAM plants cannot compete with C3 plants in a moist, cool environment, because closing stomata during the day is much less efficient. Some plants are facultative CAM plants, though. They can switch between C3 and CAM photosynthesis, depending on environmental conditions.

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