4.1.7: Photorespiration and Photosynthetic Pathways
Learning Objectives
- Define photorespiration.
- Explain how C3, C4, and CAM plants reduce photorespiration.
- Outline the C 4 pathway and compare its use by C 4 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 C 3 , C 4 , 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 C 3 pathway : the compound formed during fixation (3-PGA) has three carbon atoms. Before discussing the details of the C 4 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 CO 2 to ribulose-1,5- bisphosphate (RuBP) — the carboxylase activity. The second is adding O 2 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 O 2 to form intermediates that enter mitochondria where they are broken down to CO 2 . So this process uses O 2 and liberates CO 2 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 O 2 and CO 2 with high CO 2 , low O 2 favoring the carboxylase action and high O 2 , low CO 2 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.
C 3 Plants
One solution to photorespiration is for plants to open their stomata to release O 2 and obtain CO 2 . However, if conditions are hot or dry, this will result in too much water loss ( transpiration ). For this reason, C 3 plants, which only do the C 3 pathway and do not use the C 4 pathway to prevent photorespiration (see below), do best in cool, moist areas. Rice and potatoes are examples of C 3 plants.
C 4 Plants
Many angiosperms have developed adaptations which minimize the losses to photorespiration. They all use a supplementary method of CO 2 uptake which initially forms a four-carbon molecule compared to the two three-carbon molecules that are initially formed in the C 3 pathway. Hence, these plants are called C 4 plants . Note that C 4 plants will eventually conduct the light-independent reactions (C 3 pathway), but they form a four-carbon molecule first.
C 4 plants have structural changes in their leaf anatomy so that synthesizing the four-carbon sugar (the C 4 pathway) and resuming the light-independent reactions (C 3 pathways) are separated in different parts of the leaf with RuBisCO sequestered where the CO 2 level is high and the O 2 level low. After entering through stomata, CO 2 diffuses into a mesophyll cell (Figure \(\PageIndex{1}\)). Being close to the leaf surface, these cells are exposed to high levels of O 2 , but they have no RuBisCO so cannot start photorespiration (nor the light-independent reactions).
Instead, the CO 2 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 O 2 ). 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 (C 3 pathway) to form sugars and pyruvic acid, which is transported back to a mesophyll cell where it is converted back into PEP .
These C 4 plants are well adapted to (and likely to be found in) habitats with high daytime temperatures and intense sunlight. Because they use the C 4 pathway to prevent photorespiration, they do not have to open their stomata to the same extent as C 3 plants and can thus conserve water. Some examples crabgrass, corn (maize), sugarcane, and sorghum. Although comprising only ~3% of the angiosperms by species, C 4 plants are responsible for ~25% of all the photosynthesis on land.
CAM Plants
CAM stands for c rassulacean a cid m etabolism because it was first studied in members of the plant family Crassulaceae. CAM plants also do the C 4 pathway. However, instead of segregating the C 4 and C 3 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.
| Night | Morning |
|---|---|
|
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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.
Attribution
Curated and authored by Melissa Ha using 16.2E Photorespiration and C4 Plants from Biology by John W. Kimball (licensed CC-BY )