C3 photosynthesis

During photosynthesis, the photosynthetic pigments of plants, algae, or bacteria absorb light and transfer their energy to electrons. Subsequently, the energy carried by these electrons is used to synthesize ATP and NADPH. This process generally requires complex photosystems and only occurs in the light. Perhaps it comes as no surprise that these reactions are commonly referred to as “light reactions.” These molecules, in turn, serve as donors of electrons and energy for the synthesis of sugars. This second stage also generally occurs in the light, though is not directly dependent upon light. These reactions go by a number of names, including the Calvin Cycle reactions. Through these two sets of reactions, photosynthetic organisms convert the electromagnetic energy of sunlight into energy-rich organic molecules. Within photosynthetic organisms, different biochemical pathways carry out this energy conversion. Biologists often speak of photosynthesis as “carbon fixation,” which refers to the reactions in which CO2 becomes incorporated into a carbon-containing acid. In the photosynthetic pathway used by most plants and all algae, the CO2 first combines with a five-carbon compound called ribulose bisphosphate, or RuBP. The product of this initial reaction, which is catalyzed by the enzyme RuBP carboxylase (RUBISCO), is phosphoglyceric acid, or PGA, a three-carbon acid. This photosynthetic pathway is usually called C3 photosynthesis and the plants that employ it are called C3 plants.  To fix carbon, plants must open their stomata to let CO2 into their leaves, but as CO2 enters, water exits. Water vapour flows out faster than CO2 flows in. The movement of water is more rapid because the gradient in water concentration from the leaf to the atmosphere is much steeper than the gradient in CO2 concentration from the atmosphere to the leaf. In C3 plants, there is another factor that contributes to a low rate of CO2 uptake: RUBISCO has a low affinity for CO2. Relatively high rates of water loss are generally not a problem for plants that live in cool, moist conditions but in hot, dry climates, high rates of water loss can close the stomata and shut down photosynthesis.