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 circumstances 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 (C3 pathway) to form sugars and pyruvic acid, which is transported back to a mesophyll cell where it is converted back into PEP.

    8.7: Photorespiration and Photosynthetic Pathways (3)

    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 C4 pathway to prevent photorespiration, they do not have to open their stomata to the same extent as C3 plants 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 (C3 pathway).

    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)

    8.7: Photorespiration and Photosynthetic Pathways (2024)

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