Photosynthesis is the fundamental process by which plants, algae, and some bacteria convert light energy into chemical energy. This process is crucial for nearly all life on Earth, forming the basis of food chains and producing the oxygen necessary for many organisms’ survival. While sunlight is a primary requirement, the timing of its various stages presents more complexity than often recognized.
Photosynthesis: A Daytime Process
The initial phase of photosynthesis, the light-dependent reactions, relies on light energy. These reactions occur within the chloroplasts of plant cells, specifically on thylakoid membranes. Chlorophyll and other pigments within these membranes absorb light.
The absorbed light energy drives the splitting of water molecules, releasing oxygen as a byproduct. This process also generates energy-carrying molecules: adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide phosphate (NADPH). These molecules capture light energy in chemical form.
ATP and NADPH serve as the energy currency and reducing power for subsequent photosynthesis stages. Their production is linked to the presence of light, making this phase a daytime occurrence. Without sufficient light, these energy carriers cannot be generated, halting the process.
The Calvin Cycle: A Continuous Stage
Following the light-dependent reactions, the next phase is the Calvin cycle, also called the light-independent reactions. This stage does not directly require light, but depends on the ATP and NADPH produced during the light-dependent reactions.
The Calvin cycle takes place in the chloroplast’s fluid-filled stroma. Atmospheric carbon dioxide is incorporated into organic molecules via carbon fixation. This fixed carbon, utilizing ATP and NADPH, is then used to synthesize the plant’s food (glucose).
Although the Calvin cycle is termed “light-independent,” it typically occurs during the day. This is because ATP and NADPH, quickly consumed, are continuously supplied by the light-dependent reactions in daylight. If light is absent, the supply of these energy carriers diminishes, effectively stopping the Calvin cycle.
Nighttime Photosynthesis: The CAM Exception
Crassulacean Acid Metabolism (CAM) plants are an exception to daytime photosynthesis. They thrive in arid environments by conserving water. Examples include cacti, pineapples, and succulents.
CAM plants open their stomata, tiny leaf pores, only at night to absorb carbon dioxide. This nocturnal opening minimizes water loss through transpiration, significant during hot, dry daytime. Collected carbon dioxide is stored in cells, typically as malic acid, until daylight.
During the day, with closed stomata to conserve water, CAM plants convert stored malic acid back into carbon dioxide. This internally released carbon dioxide enters the Calvin cycle, utilizing ATP and NADPH from light-dependent reactions occurring in sunlight. This temporal separation of gas exchange and carbon fixation allows CAM plants to photosynthesize, significantly reducing water evaporation.