9.3 Photorespiration and C4/CAM pathways

Photorespiration and Its Impact on Photosynthesis

Photorespiration is a wasteful process where RuBisCO, the enzyme responsible for carbon fixation, grabs $O_2$ instead of $CO_2$. This mistake costs the plant energy and releases carbon it already fixed, cutting into photosynthetic efficiency. Understanding photorespiration explains why plants evolved alternative pathways like C4 and CAM to survive hot, dry environments.

How photorespiration works

RuBisCO has a dual nature: it can act as either a carboxylase (fixing $CO_2$) or an oxygenase (fixing $O_2$). When conditions shift in favor of oxygenase activity, photorespiration kicks in.

Three conditions favor photorespiration:

• High temperatures increase $O_2$ solubility relative to $CO_2$ and speed up the oxygenase reaction
• Low $CO_2$ concentration around RuBisCO (often caused by closed stomata during drought)
• High light intensity, which drives $O_2$ production from the light reactions

When RuBisCO fixes $O_2$, it produces a 2-carbon compound (phosphoglycolate) instead of the useful 3-carbon 3-PGA. The cell then has to spend ATP to salvage some of that carbon, and $CO_2$ is lost in the process. In C3 plants under hot, dry conditions, photorespiration can reduce photosynthetic efficiency by up to 50%.

Comparison of C3, C4, and CAM Photosynthetic Pathways

Plants use three main photosynthetic strategies. C3 is the default and most common. C4 and CAM are adaptations that concentrate $CO_2$ around RuBisCO to suppress photorespiration, but they do it in fundamentally different ways: C4 separates the process across space (different cell types), while CAM separates it across time (night vs. day).

C3 pathway

C3 photosynthesis is used by the majority of plant species, including wheat, rice, and soybeans. RuBisCO directly fixes $CO_2$ into a 3-carbon compound called 3-phosphoglycerate (3-PGA) in the mesophyll cells. This is the simplest arrangement, but it leaves RuBisCO fully exposed to atmospheric $O_2$, making C3 plants vulnerable to photorespiration whenever temperatures rise or $CO_2$ drops.

C4 pathway

C4 plants (maize, sugarcane, sorghum) use spatial separation across two cell types to concentrate $CO_2$:

1. In mesophyll cells, $CO_2$ is first fixed by PEP carboxylase (PEPC) into a 4-carbon compound (oxaloacetate, then converted to malate). PEPC has no affinity for $O_2$, so photorespiration can't happen at this step.
2. The 4-carbon compound is transported to bundle sheath cells, which are tightly packed around the leaf veins.
3. In bundle sheath cells, the 4-carbon compound is decarboxylated, releasing $CO_2$ directly to RuBisCO at a high concentration.
4. RuBisCO runs the Calvin cycle in this $CO_2$-rich environment, where its oxygenase activity is effectively suppressed.

The tradeoff: C4 fixation costs extra ATP to regenerate PEP. But in hot environments, the energy saved by avoiding photorespiration more than makes up for it.

CAM pathway

CAM plants (cacti, agaves, many succulents) use temporal separation within the same cell:

1. At night, stomata open and $CO_2$ enters the leaf. PEPC fixes it into a 4-carbon compound, which is stored as malic acid in the cell's vacuoles.
2. During the day, stomata close to prevent water loss. The stored malic acid is released from the vacuoles and decarboxylated, flooding the chloroplast with $CO_2$.
3. RuBisCO fixes this $CO_2$ through the Calvin cycle in a high-$CO_2$ environment, suppressing photorespiration.

By only opening stomata at night (when it's cooler and humidity is higher), CAM plants lose far less water than C3 or C4 plants. The tradeoff is slower growth, since $CO_2$ uptake is limited to nighttime storage capacity.

Adaptations for harsh environments

Ecological roles of C4 and CAM plants

• C4 plants dominate grasslands, savannas, and subtropical regions with high temperatures and moderate rainfall. Crops like maize, sugarcane, and sorghum are all C4, and these plants contribute a disproportionately large share of global primary productivity relative to their species count.
• CAM plants thrive in arid and semi-arid environments like deserts and rock outcrops. Cacti and agaves are classic examples. They're critical for ecosystem function in water-limited habitats, providing food and shelter for desert animals.
• Both pathways give plants a competitive edge as temperatures rise and water becomes scarcer, which makes them increasingly relevant under climate change scenarios.