What Is the Purpose of ATP in Photosynthesis?

Sunlight allows the production of ATP to fuel the reactions of photosynthesis.

Plants, algae, and some bacteria manufacture their own food through photosynthesis. The overall reaction for photosynthesis uses sunlight to convert six water molecules and six carbon dioxide molecules into six oxygen molecules and one glucose molecule. However, the actual process is quite complex with the sunlight and water being used to generate an energy molecule called adenosine triphosphate (ATP) that drives the synthesis of glucose from carbon dioxide. Without the ATP, the reactions that create glucose grind to a halt and food production ceases.

ATP

Cells use ATP to store the energy needed to drive many cellular processes, including the reactions that generate sugar during photosynthesis. The main structure of ATP is a ribose sugar attached to adenine, a purine molecule also used in DNA and RNA monomers. In addition to this structure are three phosphate groups attached to ribose. The removal of a phosphate group from ATP produces ADP (adenosine diphosphate) releasing energy that can be used to power other reactions in the cell. Conversely, the cell can store energy by adding a phosphate group to ADP to generate ATP.

Photosynthesis

All organisms need food to provide the energy to sustain life. Organisms such as plants make their own food through photosynthesis and consequently manufacture the food for the organisms that depend on the plants for their food. The reactions of photosynthesis may be grouped into the light reactions and the dark reactions. The light reactions generate energy from sunlight and water, while the dark reactions use carbon dioxide to synthesize food molecules, specifically glyceraldhyde-3-phosphate, which may later be converted into glucose by the cell.

Light Reactions

In the light reactions, also called photophosphorylation, the pigment chlorophyll uses sunlight to energize electrons, which are used to create a proton gradient across a membrane. The electron lost by the chlorophyll is replaced from a water molecule, eventually reducing the water molecule into protons (hydrogen ions) and diatomic oxygen. The protons flow back across the membrane through an enzyme called ATP synthase, which uses the energy to create ATP. This ATP is then used to power the dark reactions in photosynthesis.

Dark Reactions

In the dark reactions, also known as the Calvin cycle, carbon dioxide is attached to a molecule called ribulose bisphosphate to create glucose. ATP and another molecule called NADPH also generated in the light reactions are needed to power the enzymatic reactions. Three molecules of carbon dioxide are needed to produce a single molecule of glyceraldehyde 3-phosphate, consuming nine molecules of ATP in the process. The spent ATP becomes ADP, requiring the light reactions to convert back to ATP.

C-4 Pathway

Plants adapted to tropical or arid environments face a problem with the conventional pathway used by plants in temperate climates. This problem arises between the need for carbon dioxide during the dark reactions while at the same time reducing water loss during daylight hours when the light reactions are operating. These plants have managed to resolve this dilemma by storing carbon dioxide in other molecules during nighttime for use during daylight hours when sunlight is available to generate ATP. This allows the plants to close their stoma, openings in the leaf that allow gas exchange, during daylight and reduce water loss.

References

• Maricopa Community College: Photosynthesis
• Hyperphysics: Adenosine Triphosphate
• University of Massachusetts: Calvin Cycle