C3, C4, and CAM plants
How the C4 and CAM pathways help minimize photorespiration.
- Photorespiration is a wasteful pathway that occurs when the Calvin cycle enzyme rubisco acts on oxygen rather than carbon dioxide.
- The majority of plants are plants, which have no special features to combat photorespiration.
- plants minimize photorespiration by separating initial fixation and the Calvin cycle in space, performing these steps in different cell types.
- Crassulacean acid metabolism (CAM) plants minimize photorespiration and save water by separating these steps in time, between night and day.
High crop yields are pretty important—for keeping people fed, and also for keeping economies running. If you heard there was a single factor that reduced the yield of wheat by and the yield of soybeans by in the United States, for instance, you might be curious to know what it was.
As it turns out, the factor behind those (real-life) numbers is photorespiration. This wasteful metabolic pathway begins when rubisco, the carbon-fixing enzyme of the Calvin cycle, grabs rather than . It uses up fixed carbon, wastes energy, and tends to happens when plants close their stomata (leaf pores) to reduce water loss. High temperatures make it even worse.
Some plants, unlike wheat and soybean, can escape the worst effects of photorespiration. The and CAM pathways are two adaptations—beneficial features arising by natural selection—that allow certain species to minimize photorespiration. These pathways work by ensuring that Rubisco always encounters high concentrations of , making it unlikely to bind to .
In the rest of this article, we'll take a closer look at the and CAM pathways and see how they reduce photorespiration.
A "normal" plant—one that doesn't have photosynthetic adaptations to reduce photorespiration—is called a 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 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.
Image of the C3 pathway. Carbon dioxide enters a mesophyll cell and is fixed immediately by rubisco, leading to the formation of 3-PGA molecules, which contain three carbons.
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. These cells are called bundle-sheath cells.
To see how this division helps, let's look at an example of photosynthesis in action. First, atmospheric is fixed in the mesophyll cells to form a simple, -carbon organic acid (oxaloacetate). This step is carried out by a non-rubisco enzyme, PEP carboxylase, that has no tendency to bind . Oxaloacetate is then converted to a similar molecule, malate, that can be transported in to the bundle-sheath cells. Inside the bundle sheath, malate breaks down, releasing a molecule of . The is then fixed by rubisco and made into sugars via the Calvin cycle, exactly as in photosynthesis.
In the C4 pathway, initial carbon fixation takes place in mesophyll cells and the Calvin cycle takes place in bundle-sheath cells. PEP carboxylase attaches an incoming carbon dioxide molecul to the three-carbon molecule PEP, producing oxaloacetate (a four-carbon molecule). The oxaloacetate is converted to malate, which travels out of the mesophyll cell and into a neighboring bundle-sheath. Inside the bundle sheath cell, malate is broken down to release CO, which then enters the Calvin cycle. Pyruvate is also produced in this step and moves back into the mesophyll cell, where it is converted into PEP (a reaction that converts ATP and Pi into AMP and PPi).
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 . However, because the mesophyll cells constantly pump into neighboring bundle-sheath cells in the form of malate, there’s always a high concentration of relative to right around rubisco. This strategy minimizes photorespiration.
The pathway is used in about of all vascular plants; some examples are crabgrass, sugarcane and corn. 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 from the mesophyll cell to the bundle-sheath cell.
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.
Image of a succulent.
Instead of separating the light-dependent reactions and the use of in the Calvin cycle in space, CAM plants separate these processes in time. At night, CAM plants open their stomata, allowing to diffuse into the leaves. This is fixed into oxaloacetate by PEP carboxylase (the same step used by 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 , which enters the Calvin cycle. This controlled release maintains a high concentration of around rubisco.
CAM plants temporally separate carbon fixation and the Calvin cycle. Carbon dioxide diffuses into leaves during the night (when stomata are open) and is fixed into oxaloacetate by PEP carboxylase, which attaches the carbon dioxide to the three-carbon molecule PEP. The oxaloacetate is converted to another organic acid, such as malate. The organic acid is stored until the next day and is then broken down, releasing carbon dioxide that can be fixed by rubisco and enter the Calvin cycle to make sugars.
The CAM pathway requires ATP at multiple steps (not shown above), so like 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.
Comparisons of , , and CAM plants
, and CAM plants all use the Calvin cycle to make sugars from . These pathways for fixing have different advantages and disadvantages and make plants suited for different habitats. The mechanism works well in cool environments, while and CAM plants are adapted to hot, dry areas.
Both the and CAM pathways have evolved independently over two dozen times, which suggests they may give plant species in hot climates a significant evolutionary advantage.
|Type||Separation of initial fixation and Calvin cycle||Stomata open||Best adapted to|
|No separation||Day||Cool, wet environments|
|Between mesophyll and bundle-sheath cells (in space)||Day||Hot, sunny environments|
|CAM||Between night and day (in time)||Night||Very hot, dry environments|
Want to join the conversation?
- What are the average acidity levels on the PH scale of C3, C4, and CAM?(6 votes)
- Not always true. How much deuterium is in the plant determines its pH. This is related to soil, latitude, and the rain water consumption by photosynthesis. Deuterium content inside the tropics in rain water is massively different and this has huge implications to plant food webs and acidity More deuterium means more acidity and inflammatory effect due to the kinetic isotope effect of the extra atomic mass in deuterium compared to H+.(0 votes)
- In CAM cycle, doesn't the oxygen produced in light dependent reaction cause photorespiration (the stomata is closed)? When almost all the CO2 is utilized there will be much higher quantity of O2 compared to CO2.
Thanking you for your help.(7 votes)
- no, because it uses PEP and not Rubisco, which cannot take O2. CAM is just a variation of C4 where the first fixation is done at night and day separation rather than mesophyll and bundle sheath separation in C4(3 votes)
- what role does temperature have in this(2 votes)
- The high temperature will make the plant close its stomata to reduce water loss by evaporation. If the stomata is closed, the oxygen from photosynthesis will build up inside the leaf while the carbon dioxide will not get into the leaf.
This situation will make the concentration of oxygen inside the leaf higher than carbon dioxide. The rubisco will more likely bind the oxygen. So, the photorespiration happens.
For more details, you could visit the photorespiration article
- This might be a bad question, but is there a difference in the equations for each path of photosynthesis?(4 votes)
- This is not a bad question! The pathway and reaction is all the same but when/how it occurs varies.(2 votes)
- Why is it that C3 plants are more abundant in nature than C4 plants? Afterall C4 are more efficient than C3 as they lack photorespiration(3 votes)
- A likely explanation for this is that C4 plants arose later on the evolutionary timeline than C3 plants. If C4 plants indeed happen to be more efficient than C3 plants, we can expect the proportion of C4 plants in nature to gradually rise over time, over millions of years...(1 vote)
- Which enzyme is involved in the breakdown of malic acid to CO2 and pyruvate (in C4 pathway)?(1 vote)
- Malate dehydrogenase — you can start learning more about two different forms of this enzyme in the following wikipedia links:
- The plant in which water use efficiency is higher is(1 vote)
- C4 have higher water efficiency than C3 plants. C3 grasses release 833 molecules of water per co2 molecule while C4 grasses release 277 molecules of water.(5 votes)
- What would be the result of a competition between aCAM plant and a C4 plant in a hot, very wet, environment?(2 votes)
- Many plants will not grow well in soils that are constantly moist or wet. However, several plants are tolerant of and have adapted to perform well under these conditions.
both CAM and C4 plants are dominant in very hot, dry environments like deserts so thriving in very wet soil would not be an advantage for them.
However, there are epiphytic CAM plants in very moist forests during the rainy season.
- Which type of plants are most efficient(1 vote)
- It could be argued that CAM plants are the most efficient because they create the most amount of energy from the least amount of water. However, these plants won't do very well in a very wet environment because they're used to opening their stomata for only small amounts of water. It really all depends on the environment, each plant adapts to be the most efficent for where they are!(4 votes)
- Does the Calvin Cycle in C4 photosynthesis occur in the cytoplasm of the bundle sheath cells? I see no reason for the bundle sheath cells to contain chloroplasts... but the Calvin Cycle does occur in the stroma of the chloroplasts in C3 photosynthesis.(3 votes)
- The Calvin Cycle in bundle sheath cells is just the same as the Calvin Cycle in C3 plants, so I think the Calvin Cycle does occur in chloroplasts in the bundle sheath cells. The diagrams in the following website suggest this: