What Happens During the Dark Reaction of Photosynthesis?
Learn how plants use energy captured from sunlight in a separate chemical process to transform carbon dioxide from the air into essential sugars for growth.
The dark reaction of photosynthesis, also known as the Calvin cycle or light-independent reactions, is a biochemical process that converts carbon dioxide into sugar. It occurs in the stroma, the fluid-filled space within a plant’s chloroplasts. The primary function of this cycle is to take the energy captured during the light-dependent reactions and use it to create long-term energy storage in the form of glucose.
Essential Inputs for the Dark Reaction
The dark reaction depends on specific molecules to function. Carbon dioxide (CO₂) from the atmosphere is a primary input, entering the plant through small pores called stomata. This CO₂ provides the carbon atoms needed to build carbohydrate molecules. Without a continuous supply of atmospheric carbon dioxide, the process cannot proceed.
Energy to drive the reactions is provided by adenosine triphosphate (ATP). ATP is an energy-carrying molecule produced during the light-dependent stage of photosynthesis. It releases energy when one of its phosphate groups is broken off, powering the conversion of CO₂ into sugar.
The third required input is nicotinamide adenine dinucleotide phosphate (NADPH). Also generated during the light-dependent reactions, NADPH functions as a reducing agent, which means it donates high-energy electrons. These electrons are necessary to transform the low-energy carbon from CO₂ into the high-energy bonds of a sugar molecule. The availability of both ATP and NADPH is directly tied to the plant’s exposure to light.
The Calvin Cycle: How Plants Make Sugar
The conversion of carbon dioxide into sugar occurs through a series of enzyme-mediated steps known as the Calvin cycle. The cycle begins with a stage called carbon fixation. During this step, a molecule of CO₂ is attached to a five-carbon organic molecule named ribulose-1,5-bisphosphate (RuBP). This reaction is catalyzed by the enzyme RuBisCO, resulting in an unstable six-carbon compound that immediately splits into two three-carbon molecules.
Following fixation, the reduction stage begins. In this phase, the three-carbon molecules are energized and chemically reduced. ATP provides the necessary energy, while NADPH donates its high-energy electrons to the molecules. This transforms the molecules into a three-carbon sugar called glyceraldehyde-3-phosphate (G3P). This step is a direct conversion of the light energy, now stored in ATP and NADPH, into the chemical energy of a carbohydrate.
The final stage of the cycle is regeneration. For every six molecules of G3P created, only one exits the cycle to be used by the plant. The remaining five G3P molecules, along with additional energy from ATP, are used to regenerate the initial three molecules of RuBP. This regeneration ensures the cycle can continue to fix more carbon dioxide.
What the Dark Reaction Produces
The primary product of the dark reaction is glyceraldehyde-3-phosphate (G3P). Two G3P molecules can be combined to form one molecule of glucose, which serves as a central energy source for the plant. This glucose can be used immediately for cellular respiration, converted into starch for long-term storage, or used to build cellulose for cell walls.
The process also regenerates the molecules that were consumed from the light-dependent reactions. When ATP gives up its energy, it becomes adenosine diphosphate (ADP) and an inorganic phosphate group. Similarly, when NADPH donates its electrons, it reverts to its oxidized form, NADP+. These “spent” molecules are not waste products.
Instead, ADP and NADP+ are sent back to the thylakoid membranes where the light-dependent reactions take place. There, they are recharged using light energy, converting them back into ATP and NADPH. This recycling creates a continuous loop, linking the light and dark reactions of photosynthesis.
Why Is It Called the “Dark” Reaction?
The term “dark reaction” can be misleading. It does not imply that these reactions must occur in darkness. Instead, the name signifies that the reactions are light-independent, meaning they do not directly use photons from sunlight to proceed. The enzymes and chemical steps of the Calvin cycle can operate without light.
However, the dark reactions are indirectly but heavily dependent on light. They require a constant supply of ATP and NADPH, which are products of the light-dependent reactions.
As a result, the dark reactions typically happen during the day, concurrently with the light reactions. If the light source is removed, the production of ATP and NADPH ceases, and the dark reactions will quickly halt due to a lack of necessary inputs.