Photorespiration, C4, and CAM

C4 grasses such as crabgrass can gain a growth advantage over C3 grasses in hot conditions. Their carbon-concentrating pathway reduces the cost of photorespiration, a process initiated when rubisco reacts with oxygen. CAM plants address a related water-conservation problem by taking in carbon dioxide at night. Both strategies have energy costs, so their advantages depend on the environment.

Watch the process

C-4 Photosynthesis

Rubisco catalyzes both carboxylation and oxygenation. Its active site does not discriminate perfectly between carbon dioxide and oxygen, and when it binds \(\text{O}_2\) instead of \(\text{CO}_2\) it starts a pathway called photorespiration. That pathway consumes ATP, releases previously fixed \(\text{CO}_2\), and produces no sugar. It reduces net carbon gain, although the salvage pathway is necessary to recover carbon from products of rubisco oxygenation and can have protective roles.

Which gas rubisco binds depends on their relative concentrations inside the leaf, and that ratio is what environmental stress changes. On a hot, dry day a plant closes its stomata to conserve water, which stops gas exchange. The Calvin cycle keeps consuming \(\text{CO}_2\) and the light reactions keep producing \(\text{O}_2\), so internal \(\text{CO}_2\) falls, \(\text{O}_2\) rises, and rubisco increasingly grabs the wrong molecule. Photorespiration is a heat-and-drought problem created by the plant’s own water conservation, and C3 plants, named for the three-carbon 3-PGA they make first, can lose a large share of their fixed carbon this way on a hot afternoon.

Two adaptations reduce the loss, and both work the same way: concentrate \(\text{CO}_2\) around rubisco so oxygenation becomes less frequent. They differ in whether the separation is spatial or temporal.

C4 plants such as corn, sugarcane, and crabgrass separate the steps in space. In mesophyll cells, the enzyme PEP carboxylase fixes \(\text{CO}_2\) onto a three-carbon acceptor to make a four-carbon acid, which is the origin of the name. PEP carboxylase has no affinity for oxygen at all, so it works well even when internal \(\text{CO}_2\) is low. The four-carbon acid is shipped to bundle-sheath cells that surround the veins, where it is decarboxylated. That releases \(\text{CO}_2\) at high local concentration right next to rubisco, which runs a normal Calvin cycle in an atmosphere it can handle.

CAM plants such as cacti, agaves, and pineapple separate the steps in time. They open stomata at night, when the air is cooler and more humid and water loss is lower, and fix \(\text{CO}_2\) into organic acids stored in vacuoles. By day the stomata close, the acids are decarboxylated, and the released \(\text{CO}_2\) feeds the Calvin cycle behind sealed stomata. Both use initial carbon capture and later carbon-dioxide release, with different cellular arrangements and timing.

Neither solution is free. Both spend extra ATP on the preliminary fixation and transport, which is why C4 grasses outgrow C3 grasses through a hot summer and lose that edge in cool, moist conditions where photorespiration is minor. CAM plants pay a further cost in growth rate, since a night’s storage capacity caps the day’s carbon. The ATP cost and the reduction in photorespiration help explain why the advantage changes with climate.

Why Heat Makes the Problem Worse Twice Over

A C3 plant loses a larger share of its fixed carbon to photorespiration at \(35^{\circ}\text{C}\) than at \(20^{\circ}\text{C}\), even when its stomata stay open. Give two independent reasons, and identify which one the stomatal response adds on top.

First reason: the enzyme’s preference shifts. Rubisco’s relative affinity for carbon dioxide over oxygen is not fixed across temperature. As temperature rises, the oxygenation reaction speeds up more than the carboxylation reaction does, so a larger fraction of rubisco’s activity goes to the wrong gas even if the gases themselves were unchanged.

Second reason: the gases themselves change. Both gases become less soluble in water as it warms, but carbon dioxide’s solubility falls faster than oxygen’s. Inside a leaf’s wet cell walls, the dissolved \(\text{CO}_2\) to \(\text{O}_2\) ratio therefore drops with heat, handing rubisco a worse mixture to choose from.

Now add the stomatal effect. If the day is also dry, the plant closes its stomata to limit water loss. Gas exchange stops while the Calvin cycle keeps consuming \(\text{CO}_2\) and the light reactions keep making \(\text{O}_2\), so the internal ratio falls further and faster than temperature alone would cause. This is the third effect, and it is the one under the plant’s control, which is why it is the one C4 and CAM anatomy is built to work around.

Connect the environmental change to the reaction mechanism. Heat and drought together lower the ratio of carbon dioxide to oxygen at rubisco’s active site, and because rubisco cannot discriminate perfectly, a lower ratio means more oxygenation, more ATP spent on salvage, and less net carbon fixed.

Answer

Rubisco’s oxygenation rate rises faster with temperature than its carboxylation rate, and carbon dioxide’s solubility falls faster than oxygen’s; stomatal closure in dry heat then lowers the internal ratio further.

Comparing a C3 and a C4 Grass Across Two Climates

Two grasses are grown in growth chambers at two temperatures with identical light, water, and nutrients. Net carbon fixation is reported in micromol of \(\text{CO}_2\) per square meter of leaf per second.

Species	\(20^{\circ}\text{C}\)	\(35^{\circ}\text{C}\)
Species 1 (C3)	22	14
Species 2 (C4)	18	33

Use the stated pathway identities to explain the observed reversal in rank; decide what the two-temperature data alone can establish.

Claim. Species 1 is the C3 grass and Species 2 is the C4 grass, and the C4 pathway pays for itself only at the higher temperature.

Evidence. Compute the change for each species across the fifteen-degree rise. \[\text{Species 1: } 14-22 =-8, \qquad \text{Species 2: } 33-18 = +15 .\] As a percentage of each species’ own cool-chamber rate, Species 1 falls by \(8/22 = 36\) percent while Species 2 rises by \(15/18 = 83\) percent. The ranking reverses between chambers.

Reasoning. A C3 plant’s net fixation falls with heat because photorespiration claims an increasing share of rubisco’s activity. A C4 plant concentrates carbon dioxide in the bundle-sheath cells, so rubisco keeps working in a favorable gas mixture despite lower carbon dioxide in the leaf’s air spaces, within the pathway’s capacity, and its rate follows the ordinary temperature response of enzymes upward. At \(20^{\circ}\text{C}\) photorespiration is minor, so the C4 plant gains little while still paying the extra ATP for its shuttle, and it falls behind. That trade-off is consistent with the reversal in the table; measurements of photorespiration and energy use would help test its contribution.

What the data do not establish. Two chamber temperatures do not locate the crossover point, and nothing here separates the temperature effect from a possible difference in water use, since water was not varied. A follow-up varying humidity would be needed before claiming anything about drought tolerance.

Conclusion

Species 1 is C3 and Species 2 is C4. The C3 rate falls 36 percent with heat while the C4 rate rises 83 percent, because concentrating carbon dioxide around rubisco pays off only when photorespiration would otherwise be high.

Distinctions to keep clear

The name photorespiration invites the wrong association. It is not cellular respiration and it makes no ATP; it consumes ATP. It shares with respiration only that it takes in oxygen and gives off carbon dioxide, and that superficial similarity is where the name came from. An item offering “photorespiration produces ATP for the plant” is offering a distractor built on the name.

C4 and CAM separate carbon capture from the Calvin cycle in different ways. Both concentrate carbon dioxide around rubisco using PEP carboxylase, and both still run an ordinary Calvin cycle with rubisco. The difference is only where the two steps are kept apart: C4 separates them in space, in different cells, and CAM separates them in time, at night and by day. Use location and timing to distinguish the pathways.

Neither adaptation eliminates rubisco or photorespiration entirely. They reduce the oxygenation rate by improving the gas ratio at the active site. Answers claiming a C4 plant “does not use rubisco” are wrong.

Review: Photorespiration and the C4 and CAM Adaptations

Rubisco cannot fully distinguish \(\text{CO}_2\) from \(\text{O}_2\), and binding oxygen starts a pathway that costs ATP and releases fixed carbon without making sugar.

Hot, dry conditions close stomata and lower the internal \(\text{CO}_2\)-to-\(\text{O}_2\) ratio, which raises photorespiration in C3 plants.

C4 and CAM both concentrate carbon dioxide around rubisco and both cost extra ATP; the difference is spatial versus temporal separation, not the presence of rubisco.