Photosynthesis is a fundamental biological process through which plants, algae, and some bacteria convert light energy into chemical energy. This process forms the basis of most food webs, providing organic compounds and oxygen necessary to sustain nearly all life.
The Basics of Photosynthesis
Photosynthesis primarily uses carbon dioxide from the atmosphere and water absorbed from the soil as its raw materials. Light energy drives reactions within specialized cellular structures called chloroplasts. The overall outcome is the production of glucose, a sugar molecule, and oxygen gas.
This process unfolds in two main stages: the light-dependent reactions and the light-independent reactions, often referred to as the Calvin cycle. During the light-dependent reactions, light energy is captured by pigments like chlorophyll, leading to the splitting of water molecules. This splitting releases electrons, protons, and oxygen as a byproduct. These reactions also generate energy-carrying molecules. The energy-carrying molecules then power the light-independent reactions. In this second stage, carbon dioxide is converted into glucose. While oxygen is an output of photosynthesis, its presence can also influence efficiency.
Oxygen: A Product, But Also a Competitor
Oxygen, a byproduct of the light-dependent reactions, can also inhibit photosynthetic efficiency. This dual role arises from the enzyme RuBisCO, short for ribulose-1,5-bisphosphate carboxylase/oxygenase. RuBisCO is the most abundant enzyme, playing a role in carbon dioxide fixation during the Calvin cycle.
RuBisCO can bind with two different gases: carbon dioxide and oxygen. When carbon dioxide concentrations are high, RuBisCO primarily binds with carbon dioxide, initiating efficient sugar production. However, when oxygen concentrations are high compared to carbon dioxide, RuBisCO can bind with oxygen instead. This binding of oxygen initiates a different metabolic pathway that diverts resources away from sugar production. The competition between oxygen and carbon dioxide for RuBisCO’s active site means oxygen can directly reduce the enzyme’s efficiency in fixing carbon dioxide. This alternative reaction leads to photorespiration, which can impact a plant’s growth and productivity.
Understanding Photorespiration’s Impact
When RuBisCO binds with oxygen, it initiates photorespiration. Instead of producing two molecules of 3-phosphoglycerate, oxygen binding forms one molecule of 3-phosphoglycerate and one molecule of phosphoglycolate. Phosphoglycolate is a two-carbon compound that cannot be directly used in the Calvin cycle.
The plant must expend energy to salvage carbon from phosphoglycolate through a series of reactions. This salvage pathway consumes ATP, a molecule that stores energy, and releases carbon dioxide back into the atmosphere. Consequently, photorespiration reduces net carbon fixation, meaning less sugar is produced for a given amount of light energy, and energy resources are wasted.
Photorespiration becomes more pronounced under environmental conditions, particularly in hot and dry environments. High temperatures decrease the solubility of carbon dioxide relative to oxygen, increasing the likelihood of RuBisCO binding oxygen. In dry conditions, plants tend to close their stomata, the small pores on their leaves, to conserve water. This stomatal closure restricts fresh carbon dioxide entry while oxygen accumulates within the leaf, promoting photorespiration.
How Plants Cope with Oxygen’s Effects
Plants have evolved strategies to mitigate the negative effects of photorespiration and optimize their photosynthetic efficiency. Two adaptations are C4 photosynthesis and Crassulacean Acid Metabolism (CAM). These pathways allow plants to maintain higher carbon dioxide concentrations around RuBisCO, favoring carbon fixation over photorespiration.
C4 plants, such as maize and sugarcane, spatially separate initial carbon fixation from the Calvin cycle. They use an enzyme called PEP carboxylase in their mesophyll cells, which has a high affinity for carbon dioxide and no affinity for oxygen. This enzyme initially fixes carbon dioxide into a four-carbon compound. This compound is then transported to specialized bundle sheath cells, where it releases carbon dioxide, creating a localized high concentration around RuBisCO and minimizing photorespiration.
CAM plants, like cacti and succulents, employ a temporal separation strategy. They open their stomata and fix carbon dioxide during cooler night hours, storing it as a four-carbon acid. During the day, when light is available for photosynthesis but temperatures are high, they close their stomata to conserve water. The stored carbon dioxide is then released internally, providing a concentrated supply for the Calvin cycle and RuBisCO, effectively reducing photorespiration under arid conditions.