Table of Contents
C4 Plants Definition
The C4 pathway is also known as the Hatch–Slack pathway. It is one of three photosynthetic pathways of carbon fixation in plants that have been discovered. The C4 cycle was discovered by Marshall Davidson Hatch and Charles Roger Slack in the 1960s, thus, the name. According to this, some plants, when provided with 14CO2, aggregate the 14C label into four-carbon molecules first.
What is a C4 plant? In the mesophyll cell, CO2 is coupled to the primary carbon dioxide acceptor in C4 plants phosphoenolpyruvate, resulting in the creation of a four-carbon molecule (oxaloacetate), which is then shuttled to the bundle sheath cell, where it is decarboxylated to free carbon dioxide for usage in the C3 pathway.
In plants, there are two types of respiration: dark plant respiration and photorespiration. Photorespiration is a process that occurs in plants when they are exposed to light and results in the loss of fixed carbon as carbon dioxide. It is sometimes referred to as the C2 cycle. Photorespiration makes use of three organelles, particularly, chloroplasts, peroxisomes, and mitochondria.
The oxidative photosynthetic carbon cycle is often referred to as photorespiration (or C2 cycle). It is a metabolic process in plants in which the enzyme RuBisCO oxygenates RuBP (Ribulose 1,5-bisphosphate), absorbing part of the energy produced during photosynthesis. RuBisCO should react with RuBP to produce carbon dioxide (carboxylation), which is an essential step in the Calvin–Benson cycle.
Roughly 25% of Rubisco reaction (by the addition of oxygen to RuBP; oxygenation) results in a product that cannot be utilized in the Calvin–Benson cycle. As a result of this process, photosynthesis efficiency is lowered, potentially by up to 25% in C3 plants.
Other types: C3 plants and CAM plant
The Calvin cycle begins with the fixation of carbon dioxide by rubisco, and plants that rely only on this “conventional” approach are referred to as C3 plants, after the three-carbon molecule (3-PGA) generated as a consequence of the process. Rice, wheat, soybeans, and all trees are classed as C3 plants, accounting for nearly 85 % of all plant species in the world.
Cacti and pineapples, for example, employ the crassulacean acid metabolism (CAM) pathway to save water and reduce photorespiration while growing in dry areas. The name comes from the Crassulaceae family of plants, which is the first group of plants when scientists discovered the pathway. (Academy, 2022)
A C4 plant is a plant that utilizes the C4 carbon fixation pathway,i.e., the CO2 is first bound to phosphoenolpyruvate in the mesophyll cell resulting in the formation of a four-carbon compound (oxaloacetate). Oxaloacetate is, then, shuttled to the bundle sheath cell where it will be decarboxylated to liberate the CO2 to be utilized in the C3 pathway. Most C4 plants have special leaf anatomy (called Kranz anatomy) in which the vascular bundles are surrounded by bundle sheath cells. Upon fixation of CO2into a 4-carbon compound in the mesophyll cells, this compound is transported to the bundle sheath cells where it will be decarboxylated. The CO2 is re-fixed via the C3 pathway. The enzyme involved in this process is PEP carboxylase. In this mechanism, the tendency of rubisco (the first enzyme in the Calvin cycle) to photorespire, or waste energy by using oxygen to break down carbon compounds to CO2, is minimized. C4 plants would therefore be better adapted than C3 plants in an environment with high daytime temperatures, intense sunlight, drought, or nitrogen or CO2 limitation. Examples of C4 plants include sugarcane, maize, sorghum, amaranth, etc. Compare: C3 plant, CAM plant. See also: C4 carbon fixation pathway, Calvin cycle.
Types of Photosynthesis
Anoxygenic photosynthesis and oxygenic photosynthesis are the two forms of photosynthesis in plants that occur in the environment. Although they both operate on very similar principles, oxygenic photosynthesis is the more prevalent type and may be found in plants, algae, and cyanobacteria. Anoxygenic photosynthesis, on the other hand, makes use of electron donors that are not water and does not result in the production of oxygen. Bacteria such as green sulfur bacteria and phototrophic purple bacteria are examples of organisms that undergo this procedure. (Dobrijevic, 2021)
Table 1: Difference between C3 and C4 plants
|Character||C3 plants||C4 plants|
|Definition||The dark reaction of photosynthesis is carried out in C3 plants via the C3 pathway, which is also known as the Calvin cycle.||Photosynthesis in C4 plants takes place via the C4 route, also known as the Hatch-Slack Pathway, which is responsible for the dark reaction.|
|Season||These are cool-season plants that like moist, balmy environments.||Typically, they are warm-season plants that thrive in arid environments.|
|Product||The result of the C3 cycle is phosphoglyceric acid, which is a three-carbon molecule.||The result of the C4 cycle is Oxaloacetic acid, which is a four-carbon molecule.|
|Presence||Approximately 95% of all green plants on the planet are C3 species.||C4 plants make up about 5% of all plants on earth.|
|Conditions||C3 plants are more commonly found in temperate climates.||C4 plants are abundant in tropical climates.|
|Kranz anatomy||Kranz’s anatomy does not develop in leaves.||Kranz anatomy in c4 plants is present.|
|Chloroplast||The bundle sheath cells in C3 plants lack chloroplasts.||Chloroplasts are present in C4 plants’ bundle sheath cells.|
|CO2 acceptors||C3 plants have only one carbon dioxide acceptor.||Two carbon dioxide acceptors are present in C4 plants.|
|Secondary acceptor||C3 plants do not consist of secondary carbon dioxide acceptors.||C4 plants consist of secondary carbon dioxide acceptors.|
|Photosynthesis||C3 plants undertake photosynthesis only when their stomata are open.||Even when stomata are close around each other, C4 photosynthesis occurs.|
|Peripheral reticulum||The chloroplasts do not consist of the peripheral reticulum.||The chloroplasts consist of the peripheral reticulum.|
|Temperature||Photosynthesis occurs at a relatively low temperature.||Photosynthesis occurs at a high temperature.|
|Efficiency||C3 plants have a lower photosynthesis efficiency.||C4 plants have a higher efficiency of photosynthesis.|
|Photorespiration||The photorespiration rate is very high.||There is no photorespiration.|
|CO2 fixation||Carbon dioxide fixation is slow in C3 plants.||Carbon dioxide fixation is higher than the C3 plants.|
|Mesophyll Cell||In C3 plants, the dark reaction takes place only in the mesophyll cells.||Mesophyll cells in C4 plants will carry out only the very first steps of the C4 cycle. The majority of the work is completed on bundle sheath cells.|
|CO2 Composition||These plants have a high carbon dioxide composition point.||These plants have a low carbon dioxide composition point.|
|Growth||When the soil temperature is between 4 to 7 degrees, growth begins.||The growth occurs when the soil temperature is between 16 to 21 degrees.|
|Example||Wheat, Oats, Rice, Sunflower, Cotton.||Maize, Sugarcane, Amaranthus.|
C4 Plant Process
In C4 plants, light-dependent activities and the Calvin cycle occur in distinct cells. Calvin cycle reactions occur in particular cells found around the leaf veins. In the spongy tissue of mesophyll cells, light-dependent reactions take place. In c4 plants, the Calvin cycle operates in bundle-sheath cells. Take into consideration the C4 cycle steps to see how this division is advantageous.
At the start, in the mesophyll cells, carbon dioxide is fixed, which leads to the formation of simple 4-carbon organic acid (oxaloacetate). These reactions are catalyzed by PEP carboxylase, which is a non-rubisco enzyme with no attraction for the amino acid O.
After this, oxaloacetate is converted to malate, a structurally similar chemical capable of passing through the membrane of bundle-sheath cells. When malate decomposes, a molecule of CO2 is liberated from the bundle sheath. Rubisco, like C3 photosynthesis, fixes carbon dioxide and transforms it into sugars through the Calvin cycle. The following is a C4 cycle diagram:
From the bundle sheath cells, ATP must be converted into the three-carbon “ferry” molecule and prepared for the next stage, which is to carry out another molecule of ambient carbon dioxide from the cell membrane. As a result of the ongoing pumping of carbon dioxide into adjacent bundle-sheath cells in the form of malate, by the mesophyll cells, the level of carbon dioxide concentration remains high relative to oxygen, right around rubisco. Photorespiration is decreased as a consequence of this strategy.
About 3% of vascular plants use the C4 route; notable examples include sugarcane, crabgrass, and maize. While C4 plants can be found in abundance in hotter conditions, their numbers are dramatically reduced in cooler environments. According to studies, if the temperature is high enough, the advantages of decreased photorespiration will very certainly balance the ATP cost of carbon dioxide transport from the mesophyll cell into the bundle-sheath cell. (Academy, 2022) Nevertheless, since C4 morphological and biochemical adaptations demand more energy and resources from the plant than C3 photosynthesis, C3 plants are usually more photosynthetically effective and efficient under colder conditions. (Modules, 2018)
C4 Plants Examples
Is maize a c4 plant? Yes, it is a C4 plant. In fact, many crops are, such as sugarcane, maize, millet, sorghum, and switchgrass.
Try to answer the quiz below to check what you have learned so far about C4 plants.
- Academy, K. (2022). C3, C4, and CAM plants. Retrieved 26 March, 2022, from https://www.khanacademy.org/science/biology/photosynthesis-in-plants#photorespiration–c3-c4-cam-plants
- Dobrijevic, D. (2021). What is photosynthesis. Retrieved 26 March, 2022, from https://www.livescience.com/51720-photosynthesis.html
- Modules, I. (2018). C3 and C4 photosynthesis. Retrieved 26 March, 2022, from https://serc.carleton.edu/integrate/teaching_materials/food_supply/student_materials/1167#:~:text=Examples%20of%20C4%20plants%20include,more%20photosynthetically%20efficient%20and%20productive.
- Topper. (2022). Difference between C3 and C4 Plant. Retrieved 26 March, 2022, from https://www.toppr.com/guides/biology/difference-between/c3-and-c4-plants/#:~:text=C3%20plants%20use%20the%20C3,the%20dark%20reaction%20of%20photosynthesis.&text=These%20plants%20are%20cool%2Dseason,commonly%20found%20in%20dry%20areas.
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