8.7: Photorespiration and Photosynthetic Pathways (2024)

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Learning Objectives

Define photorespiration.
Explain how C3, C4, and CAM plants reduce photorespiration.
Outline the C₄ pathway and compare its use by C₄ plants and CAM plants.

Different plant species have adaptations that allow them to do different variations of the light-independent reactions. These are called photosynthetic pathways. Plants are classified as C₃, C₄, or CAM depending on their use of these pathways, but note that some plants can switch photosynthetic pathways depending on environmental conditions. The process for light-independent reactions described in the previous section was the C₃ pathway: the compound formed during fixation (3-PGA) has three carbon atoms. Before discussing the details of the C₄ pathway, it is important to understand the circ*mstances that led to these adaptations.

Photorespiration

As its name suggests, ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) catalyzes two different reactions. The first is adding CO₂ to ribulose-1,5- bisphosphate (RuBP) — the carboxylase activity. The second is adding O₂ to RuBP — the oxygenase activity.

The oxygenase activity of RuBisCO forms the three-carbon molecule 3-phosphoglycerate (3-PGA), just as in the light-independent reactions, and the two-carbon molecule glycolate. The glycolate enters peroxisomes, where it uses O₂ to form intermediates that enter mitochondria where they are broken down to CO₂. So this process uses O₂ and liberates CO₂ as aerobic cellular respiration does, which is why it is called photorespiration. It undoes the work of photosynthesis, which is to build sugars.

Which action of RuBisCO predominates depends on the relative concentrations of O₂ and CO₂ with high CO₂, low O₂ favoring the carboxylase action and high O₂, low CO₂ favoring the oxygenase action. The light reactions of photosynthesis liberate oxygen, and more oxygen dissolves in the cytosol of the cell at higher temperatures. Therefore, high light intensities and high temperatures (above ~ 30°C) favor the second reaction and result in photorespiration.

C₃ Plants

One solution to photorespiration is for plants to open their stomata to release O₂ and obtain CO₂. However, if conditions are hot or dry, this will result in too much water loss (transpiration). For this reason, C₃ plants, which only do the C₃ pathway and do not use the C₄ pathway to prevent photorespiration (see below), do best in cool, moist areas. Rice and potatoes are examples of C₃ plants.

C₄ Plants

Many angiosperms have developed adaptations which minimize the losses to photorespiration. They all use a supplementary method of CO₂ uptake which initially forms a four-carbon molecule compared to the two three-carbon molecules that are initially formed in the C₃ pathway. Hence, these plants are called C₄ plants. Note that C₄ plants will eventually conduct the light-independent reactions (C₃ pathway), but they form a four-carbon molecule first.

C₄ plants have structural changes in their leaf anatomy so that synthesizing the four-carbon sugar (the C₄ pathway) and resuming the light-independent reactions (C₃ pathways) are separated in different parts of the leaf with RuBisCO sequestered where the CO₂ level is high and the O₂ level low. After entering through stomata, CO₂ diffuses into a mesophyll cell (Figure \(\PageIndex{1}\)). Being close to the leaf surface, these cells are exposed to high levels of O₂, but they have no RuBisCO so cannot start photorespiration (nor the light-independent reactions).

8.7: Photorespiration and Photosynthetic Pathways (2)

Instead, the CO₂ is inserted into a three-carbon compound called phosphoenolpyruvic acid (PEP) forming the four-carbon compound oxaloacetic acid. Oxaloacetic acid is converted into malic acid or aspartic acid (both have 4 carbons), which is transported through plasmodesmata into a bundle sheath cell. Bundle sheath cells are deep in the leaf, so atmospheric oxygen cannot diffuse easily to them (Figure \(\PageIndex{2}\)). Additionally, they often have thylakoids with reduced photosystem II complexes (the one that produces O₂). Both of these features keep oxygen levels low in bundle sheath cells, which is where the four-carbon compound is broken down into carbon dioxide, which enters the light-independent reactions (C₃pathway) to form sugars and pyruvic acid, which is transported back to a mesophyll cell where it is converted back into PEP.

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These C₄ plants are well adapted to (and likely to be found in) habitats with high daytime temperatures and intense sunlight. Because they use the C₄pathway to prevent photorespiration, they do not have to open their stomata to the same extent as C₃plants and can thus conserve water. Some examples crabgrass, corn (maize), sugarcane, and sorghum. Although comprising only ~3% of the angiosperms by species, C₄ plants are responsible for ~25% of all the photosynthesis on land.

CAM Plants

CAM stands for crassulacean acid metabolism because it was first studied in members of the plant family Crassulaceae. CAM plants also do the C₄ pathway. However, instead of segregating the C₄ and C₃ pathways in different parts of the leaf, CAM plants separate them in time instead (Table \(\PageIndex{1}\)). As a result, CAM plants do not need to open their stomata in the daytime to reduce photorespiration because they have already formed a four-carbon molecule at night that can be broken down to release carbon dioxide during the day.

Table \(\PageIndex{1}\): Activities of CAM plants at night and in the morning.
Night	Morning
CAM plants take in CO₂ through their open stomata (they tend to have reduced numbers of them). The CO₂ joins with PEP to form the four-carbon oxaloacetic acid. This is converted to four-carbon malic acid that accumulates during the night in the central vacuole of the cells.	The stomata close (thus conserving moisture as well as reducing the inward diffusion of oxygen). The accumulated malic acid leaves the vacuole and is broken down to release CO₂. The CO₂ is taken up into the light-independent reactions (C₃pathway).

CAM plants thus thrive in conditions of high daytime temperatures, intense sunlight, and low soil moisture. Some examples of CAM plants include cacti (Figure \(\PageIndex{3}\)), pineapples, all epiphytic bromeliads, sedums, and the "ice plant" that invade the California coast line.

8.7: Photorespiration and Photosynthetic Pathways (4)

Attribution

Curated and authored by Melissa Ha using 16.2E Photorespiration and C4 Plants from Biology by John W. Kimball (licensed CC-BY)