C3 Pathways: How Plants Convert CO2 Into Sugar

The C3 pathway is the most widespread method plants use to convert atmospheric carbon dioxide into the sugars they need for energy. It is a fundamental assembly line for photosynthesis, taking CO2 and creating glucose through a series of chemical reactions. This process is the foundation for most of the planet’s biomass. The name C3 comes from the first stable carbon compound produced during the process, which contains three carbon atoms.

The Process of Carbon Fixation

The C3 pathway, also known as the Calvin Cycle, unfolds in three main stages within a plant’s chloroplasts. The first stage is carbon fixation. A molecule of carbon dioxide (CO2) enters the leaf and is captured by the enzyme RuBisCO. RuBisCO attaches the CO2 to a five-carbon molecule called ribulose-1,5-bisphosphate (RuBP), creating a temporary six-carbon compound that immediately splits into two molecules of 3-phosphoglycerate (3-PGA).

Following fixation, the reduction stage begins. The plant uses energy harvested from sunlight, stored in the molecules ATP and NADPH, to modify the 3-PGA molecules. Each molecule of 3-PGA receives a phosphate group from ATP and then gains electrons from NADPH. This conversion transforms 3-PGA into a three-carbon sugar known as glyceraldehyde-3-phosphate (G3P), storing light energy as chemical energy.

The final stage is regeneration. For every six molecules of G3P created, only one exits the cycle to be used by the plant to synthesize glucose and other organic molecules. The remaining five G3P molecules stay within the cycle. Through a series of reactions that consume additional ATP, these five molecules are rearranged to reform three molecules of the initial RuBP. This regeneration ensures the cycle is prepared to capture more CO2.

Environmental Conditions and Efficiency

The C3 pathway’s effectiveness is closely tied to environmental conditions. It operates best in cool, moist, and temperate climates. Under these conditions, plants can keep their stomata—small pores on the leaf surface—open for extended periods. This allows for a steady influx of carbon dioxide for the Calvin Cycle.

A limitation of the C3 pathway arises in hot, dry conditions, leading to a wasteful process called photorespiration. To conserve water, plants close their stomata, which restricts the entry of CO2. This closure causes the concentration of oxygen, a byproduct of photosynthesis, to increase inside the leaf.

This buildup of oxygen creates a problem for RuBisCO, which can bind with oxygen just as it does with CO2. When RuBisCO captures an oxygen molecule instead of CO2, it initiates photorespiration. This process consumes energy and releases previously fixed carbon as CO2 without producing sugar, reducing photosynthetic efficiency by as much as 25%.

Comparison with C4 and CAM Pathways

In response to the inefficiencies from photorespiration, some plants evolved alternative pathways known as C4 and CAM. The C4 pathway is a spatial solution. C4 plants, like corn and sugarcane, have a specialized leaf anatomy that physically separates the initial CO2 capture from the Calvin Cycle. They use an enzyme called PEP carboxylase to fix CO2 into a four-carbon compound, which gives the pathway its name.

This four-carbon molecule is then shuttled into deeper bundle-sheath cells, where it releases the CO2. This action creates a high concentration of carbon dioxide where the RuBisCO enzyme is located, preventing it from binding with oxygen. This adaptation makes C4 plants more efficient in hot, sunny environments.

The Crassulacean acid metabolism (CAM) pathway is a temporal solution used by plants in arid environments, like cacti and succulents. To minimize water loss, CAM plants open their stomata only at night to collect CO2. The CO2 is converted into an organic acid and stored overnight. During the day, with stomata closed, the plants release the stored CO2 into the Calvin Cycle to produce sugars.

Plants That Use the C3 Pathway

The C3 pathway is the most common form of photosynthesis, utilized by approximately 95% of the plant biomass on Earth. This includes a vast range of plants important to ecosystems and human agriculture. All woody trees, from massive oaks to pines, rely on this method for their growth.

Grain crops such as rice, wheat, and barley are C3 plants, forming the basis of diets for billions of people. Other staple crops including soybeans, potatoes, and cassava also use this pathway. The reliance of these food crops on the C3 pathway makes its water inefficiency in hot climates a concern for global food security.