C3 Photosynthesis

Plants which use only the Calvin cycle for fixing the carbon dioxide from the air are known as C3 plants. In the first step of the cycle CO₂ reacts with RuBP to produce two 3-carbon molecules of 3-phosphoglyceric acid (3-PGA). This is the origin of the designation C3 or C₃ in the literature for the cycle and for the plants that use this cycle.

The entire process, from light energy capture to sugar production occurs within the chloroplast. The light energy is captured by the non-cyclic electron transport process which uses the thylakoid membranes for the required electron transport.

About 85% of plant species are C3 plants. They include the cereal grains: wheat, rice, barley, oats. Peanuts, cotton, sugar beets, tobacco, spinach, soybeans, and most trees are C3 plants. Most lawn grasses such as rye and fescue are C3 plants.

C3 plants have the disadvantage that in hot dry conditions their photosynthetic efficiency suffers because of a process called photorespiration. When the CO₂ concentration in the chloroplasts drops below about 50 ppm, the catalyst rubisco that helps to fix carbon begins to fix oxygen instead. This is highly wasteful of the energy that has been collected from the light, and causes the rubisco to operate at perhaps a quarter of its maximal rate.

The problem of photorespiration is overcome in C4 plants by a two-stage strategy that keeps CO₂ high and oxygen low in the chloroplast where the Calvin cycle operates. The class of plants called C3-C4 intermediates and the CAM plants also have better strategies than C3 plants for the avoidance of photorespiration.

C4 Photosynthesis

Sugarcane is a champion at photosynthesis under the right conditions and is a prime example of a C4 plant, one which uses C4 photosynthesis. Sugarcane has been recorded at 7% photosynthetic efficiency.

C4 plants almost never saturate with light and under hot, dry conditions much outperform C3 plants. They use a two-stage process were CO₂ is fixed in thin-walled mesophyll cells to form a 4-carbon intermediate, typically malate (malic acid). The reaction involves phosphoenol pyruvate (PEP) which fixes CO2 in a reaction catalyzed by PEP-carboxylate. It forms oxaloacetic acid (OAA) which is quickly converted to malic acid. The 4-carbon acid is actively pumped across the cell membrane into a thick-walled bundle sheath cell where it is split to CO₂ and a 3-carbon compound.

This CO₂ then enters the Calvin cycle in a chloroplast of the bundle sheath cell and produces G3P and subsequently sucrose, starch and other carbohydrates that enter the cells energy transport system.

The advantage that comes from this two-stage process is that the active pumping of carbon into the bundle sheath cell and the blocking of oxygen produce an environment with 10-120x as much CO₂ available to the Calvin cycle and the rubisco tends to be optimally utilized. The high CO₂ concentration and the absence of oxygen implies that the system never experiences the detractive effects of photorespiration.

The drawback to C4 photosynthesis is the extra energy in the form of ATP that is used to pump the 4-carbon acids to the bundle sheath cell and the pumping of the 3-carbon compound back to the mesophyll cell for conversion to PEP. This loss to the system is why C3 plants will outperform C4 plants if there is a lot of water and sun. The C4 plants make some of that energy back in the fact that the rubisco is optimally used and the plant has to spend less energy synthesizing rubisco.

Moore, et al. say that only about 0.4% of the 260,000 known species of plants are C4 plants. But that small percentage includes the important food crops corn, sorghum, sugarcane and millet. Also inluded are crabgrass and bermuda. Many tropical grasses and sedges are C4 plants.

C3-C4 Intermediate Photosynthesis

Moore, et al. point to Flaveria (Asteraceae), Panicum (Poaceae) and Alternanthera (Amarantheceae) as genera that contain species that are intermediates between C3 and C4 photosynthesis. These plants have intermediate leaf anatomies that contain bundle sheath cells that are less distinct and developed than the C4 plants.

These intermediates are characterized by their resistance to photorespiration so that they can operate in higher temperatures and dryer environments than C3 plants. At right, the ranges of CO2 compensation points for the three types of plants are shown. These compensation points are the values at which the plants cease to provide net photosynthesis.

The connection to hot and dry conditions comes from the fact that all the plants will close their stomata in hot and dry weather to conserve moisture, and the continuing fixation of carbon from the air drops the CO₂ dramatically from the atmospheric concentration of nominally 38,000 ppm. If the CO₂ compensation point is lower on the above scale, the plant can operate in hotter and dryer conditions. The limits are placed by the fact that rubisco begins to fix oxygen rather than CO₂, undoing the work of photosynthesis. C4 plants shield their rubisco from the oxygen, so can operate all the way down to essentially zero CO₂ without the onset of photorespiration.

Crassulacean Acid Metabolism (CAM)

The CAM plants represent a metabolic strategy adapted to extremely hot and dry environments. They represent about 10% of the plant species and include cacti, orchids, maternity plant, wax plant, pineapple, Spanish moss, and some ferns. The only agriculturally significant CAM plants are the pineapple and an Agave species used to make tequila and as a source of fiber.

The sketch below of the day-night cycle of the CAM plants is patterned after Moore, et al. The name Crassulacean Acid Metabolism came from the fact that this strategy was discovered in a member of the Crassulaceae which was observed to become very acidic at night and progressively more basic during the day.

The acidity was found to arise from the opening of their stomata at night to take in CO₂ and fix it into malic acid for storage in the large vacuoles of their photosynthetic cells. It could drop the pH to 4 with a malic acid concentration up to 0.3M . Then in the heat of the day, the stomata close tightly to conserve water and the malic acid is decarboxylated to release the CO₂ for fixing by the Calvin cycle. PEP is used for the initial short-term carbon fixation as in the C4 plants, but the entire chain of reactions occurs in the same cell rather than handing off to a separate cell as with the C4 plants. In the CAM strategy, the processes are separated temporally, the initial CO₂ fixation at night, and the malic acid to Calvin cycle part taking place during the day.

With stomata open only at night when the temperature is lower and the relative humidity higher, the CAM plants use much less water than either C3 plants or C4 plants. Some varieties convert to C3 plants at the end of the day when their acid stores are depleted if they have adequate water, and even at other times when water is abundant.

Photorespiration

Respiration refers to the metabolism of oxygen and the release of carbon dioxide. In cellular respiration it is a positive term, a process vital to life. But photorespiration is an entirely negative term because it represents a severe loss to the process of using light energy in photosynthetic organisms to fix carbon for subsequent carbohydrate synthesis. By leading to the loss of up to half of the carbon that has been fixed at the expense of light energy, photorespiration undoes the work of photosynthesis.

Photorespiration happens in C3 plants when the CO₂ concentration drops to about 50 ppm. The key enzyme that accomplishes the fixing of carbon is rubisco, and at low concentrations of CO₂ it begins to fix oxygen instead.

Under moderate temperature conditions when C3 plants have sufficient water, the supply of carbon dioxide is abundant and photorespiration is not a problem. The CO₂ concentration of the atmosphere as of 2004 was about 38,000 ppm and this CO₂ freely diffuses through the stomata of leaves and across the membranes of the chloroplasts while water diffuses out through the stomata. But during hot and dry conditions, the stomata close to prevent excessive water loss and the continuing fixation of carbon in the Calvin cycle dramatically reduces the relative concentration of CO₂. When it reaches a critical level of about 50 ppm the rubisco stops fixing CO₂ and begins to fix O₂ instead. Even though the detoured process feeds some PGA back into the cycle, the photorespiration process causes rubisco to operate at only about 25% of its optimal rate.

The C4 plants and CAM plants avoid photorespiration and therefore operate at much higher efficiencies in hot and dry climates.