Calvin cycle

The Calvin cycle is a vital series of chemical reactions used by photosynthetic organisms. It is often called carbon fixation because it turns inorganic carbon dioxide into organic compounds. These compounds, such as sugars, provide energy for the plant and the animals that eat it.

This cycle occurs in all photosynthetic eukaryotes and many photosynthetic bacteria. In plants, these reactions take place in the stroma. The stroma is the fluid-filled region inside a chloroplast, located outside the thylakoid membranes.

The cycle functions through a sequence of reduction-oxidation, or redox, reactions. It does not convert molecules into sugar in one single step. Instead, it uses a step-wise process to build complex molecules. The cycle relies on two specific products from the light-dependent reactions: ATP and NADPH. ATP provides chemical energy, while NADPH provides reducing power. These substrates drive the transformation of carbon dioxide into three-carbon sugar molecules.

Scientists divide the Calvin cycle into three distinct functional stages. The first stage is carboxylation. During this phase, the enzyme RuBisCO catalyzes the addition of carbon dioxide to a five-carbon molecule called ribulose 1,5-bisphosphate, or RuBP. This creates an unstable six-carbon compound that immediately splits. The second stage is the reduction reaction. Here, the molecules are modified using ATP and NADPH to produce glyceraldehyde-3-phosphate, known as G3P. The third stage is RuBP regeneration. This stage uses more ATP to rearrange G3P molecules back into RuBP so the cycle can continue.

In 1950, researchers discovered the specific mechanics of this cycle. Melvin Calvin, James Bassham, and Andrew Benson conducted their work at the University of California, Berkeley. They used a radioactive isotope called carbon-14 to track the movement of carbon atoms. This method allowed them to map the complex biochemical pathways of the cycle. Their discovery provided a clear view of how plants build organic matter from carbon dioxide.

The mathematical precision of the cycle is quite remarkable. To produce one net molecule of G3P, the cycle must turn three times. This process requires exactly nine molecules of ATP and six molecules of NADPH. Because G3P is a three-carbon sugar, it takes six turns of the cycle to produce one six-carbon glucose molecule.

While many texts focus on glucose, the immediate carbohydrate products are actually triose phosphates like G3P. Plants can then use this surplus G3P to create starch, sucrose, or cellulose depending on their needs.

It is a common misconception that the Calvin cycle occurs only in the dark. While it is sometimes called the "dark reaction," it actually requires light to function. This is because the cycle depends on NADPH, which is a short-lived molecule. NADPH is produced during the light-dependent reactions and is used up quickly. If there is no light, the plant must rely on starch reserves to provide energy. Furthermore, the enzymes in the cycle are often activated by light or by the products of light-dependent reactions. This regulation prevents the plant from wasting energy when it cannot produce new sugars.

The Calvin cycle is also closely connected to other biological processes. One such connection is photorespiration, also known as the C2 cycle. This occurs when the RuBisCO enzyme reacts with oxygen instead of carbon dioxide. Photorespiration can be a negative consequence for the plant because it leads to a loss of carbon. Some plants, like corn, have evolved C4 carbon fixation to help circumvent this problem. Other plants, known as CAM plants, use a different method by storing malic acid at night to make the cycle work during the day.