Photosynthesis is the biological process by which plants, algae, and certain bacteria convert light energy into chemical energy, creating their own food. This reaction involves taking in carbon dioxide (CO2) from the air and water (H2O) to synthesize sugars, primarily glucose, while releasing oxygen (O2) as a byproduct. This process forms the base of nearly all food webs and generates the oxygen content in the atmosphere. Since plants cannot ingest food, photosynthesis is how they acquire the energy necessary for survival and growth. Understanding the timing requires separating it into two distinct, interconnected stages.
The Stage That Requires Sunlight
The initial phase, known as the light-dependent reactions, occurs strictly during the day because it requires light energy (photons) to begin. This stage takes place within the thylakoid membranes inside the plant’s chloroplasts, where specialized pigment molecules like chlorophyll absorb specific wavelengths of sunlight. This absorption excites electrons and initiates an electron transport chain.
The captured light energy is used to split water molecules in a process called photolysis, which provides the necessary electrons and releases oxygen. The flow of these electrons and the resulting chemical gradients power the production of two short-lived energy-carrying molecules: adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide phosphate (NADPH). These molecules store the captured energy for use in the next stage of sugar production. If the sun sets, the supply of photons stops, and the production of ATP and NADPH halts immediately.
The Stage That Does Not Require Direct Light
The second major phase is the light-independent reaction, often called the Calvin cycle. This stage does not directly utilize sunlight, but it is entirely dependent on the ATP and NADPH generated during the light-dependent stage. The reactions occur in the stroma, the fluid-filled space surrounding the thylakoids in the chloroplast.
During this cycle, the stored chemical energy from ATP and NADPH is used to “fix” carbon dioxide (CO2), meaning the inorganic carbon from the atmosphere is converted into an organic molecule. The enzyme RuBisCO facilitates the combination of CO2 with an existing five-carbon sugar, leading to the creation of three-carbon sugar molecules. These smaller sugars are then used to synthesize glucose and other complex carbohydrates for long-term energy storage and growth. Because ATP and NADPH are highly unstable and break down quickly, the light-independent reactions typically occur almost simultaneously with the light-dependent reactions during daylight hours.
Specialized Timing: Nocturnal Carbon Fixation
Most plants perform both stages of photosynthesis during the day, but a specialized group, known as Crassulacean Acid Metabolism (CAM) plants, temporally separates the two phases. CAM plants, such as cacti, succulents, and pineapples, thrive in arid environments where water loss is a serious threat. Their adaptation involves shifting gas exchange to the night when temperatures are cooler and humidity is higher, which minimizes water evaporation.
At night, CAM plants open tiny pores on their leaves, called stomata, to take in atmospheric carbon dioxide (CO2). They cannot run the entire photosynthetic process at this time because they lack the necessary light energy. Instead, the CO2 is rapidly fixed and stored as a four-carbon acid, typically malic acid, within the plant’s vacuoles.
When daylight arrives, the CAM plant closes its stomata tightly to conserve water, effectively sealing off its interior from the dry air. The stored malic acid is transported out of the vacuole and broken down, releasing a concentrated supply of CO2 internally. This internally released CO2 is immediately fed into the Calvin cycle, using the ATP and NADPH that the plant’s light-dependent reactions are actively producing from the sun’s energy. This unique mechanism allows CAM plants to collect carbon at night and produce sugars during the day, effectively splitting the overall process across a full 24-hour cycle.