25 Chapter 25: Photosynthetic Pathways
Lisa Limeri
Learning Objectives
By the end of this section, students will be able to:
- Contrast C3, C4, and CAM photosynthesis with regard to timing of stomatal opening, efficiency of water use, use of stored energy, and location of primary carbon capture and the Calvin cycle.
Introduction
Different plant species have adaptations that allow them to do different variations of the light-independent reactions. These are called photosynthetic pathways. Most plants use just one of the three pathways, which are C3, C4, or CAM. However, there are some plants that have the ability to 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. The other two pathways are adaptations that have evolved in response to heat and drought stress. 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. Conditions with high CO2 concentration and low O2 concentration favor the carboxylase activity; in turn conditions with high O2 concentration and low CO2 concentration favor the oxygenase activity. 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) results in high oxygen concentrations and result in photorespiration.
To get rid of oxygen, plants open their stomata to allow gas exchange; excess oxygen is released and carbon dioxide is taken in. However, open stomata also allows the exchange of water vapor, and when the surrounding environment is dry, water tends to leave the plant through stomata. Photosynthesis in desert plants has evolved adaptations that conserve water (Fig. 25.1). In harsh, dry heat, every drop of water must be used to survive. Because stomata must open to allow for the uptake of CO2, water escapes from the leaf during active photosynthesis. Desert plants have evolved processes to conserve water and deal with harsh conditions. A more efficient use of CO2 allows plants to adapt to living with less water.
Some plants such as cacti can prepare materials for photosynthesis during the night by a temporary carbon fixation and storage process, because opening the stomata at night conserves water due to cooler temperatures. In addition, cacti have evolved the ability to carry out low levels of photosynthesis without opening stomata at all, a mechanism for surviving extremely dry periods. This process is called CAM photosynthesis (more on this below).
Reading Question #1
Photorespiration occurs when…
A. plants run out of oxygen.
B. rubisco is denatured by exposure to excessive sunlight.
C. plants conduct both photosynthesis and cellular respiration at the same time.
D. rubisco catalyzes a reaction with oxygen instead of carbon dioxide.
C3 photosynthesis
A “normal” plant—one that doesn’t have photosynthetic adaptations to reduce photorespiration—is called a C3 plant. The first step of the Calvin cycle is the fixation of carbon dioxide by rubisco, and plants that use only this “standard” mechanism of carbon fixation are called C3 plants, for the three-carbon compound (3-PGA) the reaction produces. About of the plant species on the planet are plants, including rice, wheat, soybeans and all trees.
Reading Question #2
Under what conditions is photorespiration most likely to occur for a C3 plant?
A. Hot, dry conditions.
B. Cool, wet conditions.
C. At nighttime.
D. Photorespiration is not a problem for C3 plants.
C4 Photosynthetic pathway
Both of the other photosynthetic pathways – C4 and CAM – have evolved to concentrate CO2 around RuBisCO, thereby increasing its efficiency. CAM concentrates it temporally, providing CO2 during the day and not at night, when respiration is the dominant reaction. C4 plants, in contrast, concentrate CO2 spatially, with a RuBisCO reaction centre in a bundle sheath cell that is inundated with CO2.
In plants, the light-dependent reactions and the Calvin cycle are physically separated, with the light-dependent reactions occurring in the mesophyll cells (spongy tissue in the middle of the leaf) and the Calvin cycle occurring in special cells around the leaf veins, the bundle sheath cells (Figure 25.3).
To see how this division helps, let’s look at an example of photosynthesis in action. First, atmospheric CO2 is fixed in the mesophyll cells to form a simple, -carbon organic acid (oxaloacetate). This step is carried out by an enzyme called PEP carboxylase. Unlike rubisco, PEP carboxylase has no tendency to bind O2, so the oxygen concentration does not affect its function. Oxaloacetate is then converted to a similar molecule, malate, that can be transported into the bundle-sheath cells. Inside the bundle sheath, malate breaks down, releasing a molecule of CO2. The CO2 is then fixed by rubisco and made into sugars via the Calvin cycle, exactly as in C3 photosynthesis. Because the mesophyll cells constantly pump CO2 into neighboring bundle-sheath cells in the form of malate, there’s always a high concentration of relative to O2 right around rubisco. This strategy minimizes photorespiration.
This process isn’t without its energetic price: ATP must be expended to return the three-carbon “ferry” molecule from the bundle sheath cell and get it ready to pick up another molecule of atmospheric CO2. The pathway is used in about of all vascular plants. plants are common in habitats that are hot, but are less abundant in areas that are cooler. In hot conditions, the benefits of reduced photorespiration likely exceed the ATP cost of moving CO2 from the mesophyll cell to the bundle-sheath cell.
Reading Question #3
Where do C4 plants conduct the Calvin Cycle?
A. Bundle sheath cells
B. Waxy cuticle
C. Stomata
D. Stroma of chloroplasts
CAM Photosynthetic Pathway
Some plants that are adapted to dry environments, such as cacti and pineapples, use the crassulacean acid metabolism (CAM) pathway to minimize photorespiration. This name comes from the family of plants, the Crassulaceae, in which scientists first discovered the pathway.
Instead of separating the light-dependent reactions and the use of CO2 in the Calvin cycle in space, CAM plants separate these processes in time (Figure 25.4). At night, CAM plants open their stomata, allowing CO2 to diffuse into the leaves. This CO2 is fixed into oxaloacetate by PEP carboxylase (the same step used by C4 plants), then converted to malate or another type of organic acid. The organic acid is stored inside vacuoles until the next day. In the daylight, the CAM plants do not open their stomata, but they can still photosynthesize. That’s because the organic acids are transported out of the vacuole and broken down to release CO2, which enters the Calvin cycle. This controlled release maintains a high concentration of CO2 around rubisco.
The CAM pathway requires ATP at multiple steps (not shown above), so like C4 photosynthesis, it is not an energetic “freebie.” However, plant species that use CAM photosynthesis not only avoid photorespiration, but are also very water-efficient. Their stomata only open at night, when humidity tends to be higher and temperatures are cooler, both factors that reduce water loss from leaves. CAM plants are typically dominant in very hot, dry areas, like deserts.
Reading Question #4
When do CAM plants tend to open their stomata and fix carbon?
A. At night.
B. During the day.
C. Both at night and during the day.
D. During a full moon.
Comparisons of C3, C4, and CAM plants
, C4, and CAM plants all use the Calvin cycle to make sugars from CO2. These pathways for fixing CO2 have different advantages and disadvantages and make plants suited for different habitats. The C3 mechanism works well in cool environments, while C4 and CAM plants are adapted to hot, dry areas. Both the C4 and CAM pathways have evolved independently over two dozen times, which suggests they may give plant species in hot climates a significant evolutionary advantage.
Reading Question #5
Which of the following describes how C4 and CAM plants separate the light-dependent reactions from the Calvin Cycle?
A. C4 temporally separates them whereas CAM spatially separates them
B. C4 spatially separates them whereas CAM temporally separates them
C. both temporally separate them
D. both spatially separate them
References
Text adapted from Kimball, J. W. (2023). Biology. LibreTexts. Retrieved from https://bio.libretexts.org/Bookshelves/Introductory_and_General_Biology/Biology_(Kimball)
Text adapted from “C3, C4, and CAM plants” Khan Academy. Accessed 2023 from https://www.khanacademy.org/science/biology/photosynthesis-in-plants/photorespiration–c3-c4-cam-plants/a/c3-c4-and-cam-plants-agriculture
