A brief overview of C4 photosynthesis - Marietta College
Phosphoenolpyruvate carboxylase - Wikipedia
The C3 and C4 refer to how these classes of plants assimilate carbon dioxide into their systems. During the first steps in CO2 assimilation, C3 plants form a pair of three carbon-atom molecules. C4 plants, on the other hand, initially form four carbon-atom molecules. In C3 plants CO2 enters the leaf through the stomata, which are microscopic pores found on the under-surface of leaves and on stems. They occur in the epidermal tissue. The CO2 then diffuses into the mesophyll cells where a bifunctional enzyme called Rubisco fixes carbon dioxide or molecular oxygen, which leads to photosynthesis or photorespiration. The Rubisco catalyzes the CO2 of and forms two Phosphoglycerate (PGA) molecules, which is a three carbon compound. This PGA is converted to sugars and transported to the growing leaves, roots and reproductive structures. This form of photosynthesis is found in all major plant families or in about 300,000 species and make up 95% of all plants. Typical C3 plants include: barley, sunflower, rice, tomatoes, wheat, peanuts, cotton, sugar beet, oats, and most trees and are found in typically cooler and wetter environments. C4 plants on the other hand the CO2 enters through the stomata again, but goes into the mesophyll tissue, where it is fixed by PepCarboxylase to form , which unlike Rubisco does not have the ability to fix oxygen, which contributes to lower photorespiratory carbon losses in C4 plants. This Oxaloacetate is then converted to malate, which is a four carbon molecule and transported to the bundle sheath cells. From this CO2 is released and forms sucrose and starch. These plants have a special mechanism within their leaves by which they are able to increase CO2 concentration several times higher than ambient levels. These plants tend to be found in warmer and water-limited environments. Typical examples include many tropical grasses and agricultural crops such as maize (corn), sugarcane, and sorghum. Characteristically, C4-plants have higher rates of photosynthesis than C3-plants. Photosynthesis in C4 plants does not saturate but increases at high light intensities and can continue at very low CO2 concentrations. Subsequently, these plants have rapid growth rates and higher biomass and economic yields than C3-plants.
Oxaloacetate and aspartate are easily inter-convertible through a …
Fig. 1 Major types of C4 photosynthesis. C4 photosynthesis can be categorized into three major classes by the primary decarboxylation enzymes utilized. The most common type of C4 uses NADP-ME localized in bundle sheath (BS) chloroplasts to release CO2 to the Benson–Calvin cycle. Here, malate is the major C4 acid that is transported from mesophyll (ME) to BS cells. NAD-ME-type C4 uses mitochondrion-localized NAD-ME to decarboxylate malate; aspartate is the major shuttle for carbon transport from ME to BS cells and is converted to malate in BS. Plants that utilize PEPCK to decarboxylate oxaloacetate in the BS cytosol transport phosphoenolpyruvate back to ME cells. It is important to note that many plants may utilize more than one pathway to transport carbon between BS and ME cells. Ala, alanine; ASP, aspartate; CA, carbonic anhydrase; Mal, malate; MDH, malate dehydrogenase; NAD-ME, NAD-dependent malic enzyme; NADP-ME, NADP-dependent malic enzyme; OAA, oxaloacetate; PEP, phosphoenolpyruvate; PEPCase, phosphoenolpyruvate carboxylase; PEPCK, phosphoenolpyruvate carboxykinase; PPDK, pyruvate orthophosphate dikinase.