The two stages of photosynthesis are the light-dependent reactions and the light-independent reactions (also known as the Calvin cycle).
- Light-Dependent Reactions: The light-dependent reactions occur in the thylakoid membranes of the chloroplasts. These reactions require light energy and take place in the presence of chlorophyll and other pigments. The main steps involved in the light-dependent reactions are as follows:
a. Light Absorption: Chlorophyll and other pigments in the chloroplasts absorb light energy from the sun. This energy is captured by the chlorophyll molecules in the photosystems, which are protein complexes embedded in the thylakoid membrane.
b. Electron Transport: The absorbed light energy excites electrons in the chlorophyll molecules, causing them to move to higher energy levels. These energized electrons are then passed through a series of electron carrier molecules in an electron transport chain (ETC) within the thylakoid membrane. As the electrons move through the ETC, their energy is harnessed to create ATP (adenosine triphosphate), which is a molecule that stores and transports energy.
c. Oxygen Release: During the electron transport chain, water molecules are split into hydrogen ions (H+), electrons, and molecular oxygen (O2). The release of oxygen is an important byproduct of the light-dependent reactions, and it is essential for supporting aerobic respiration and sustaining life on Earth.
d. Formation of NADPH: In addition to ATP production, another molecule called NADPH (nicotinamide adenine dinucleotide phosphate) is formed. NADPH acts as a carrier of high-energy electrons and hydrogen ions, which are used in the next stage of photosynthesis.
- Light-Independent Reactions (Calvin Cycle): The light-independent reactions, or the Calvin cycle, occur in the stroma of the chloroplasts. Unlike the light-dependent reactions, the Calvin cycle does not directly require light energy. Instead, it relies on the products generated in the light-dependent reactions, ATP and NADPH, as well as carbon dioxide (CO2) from the atmosphere. The steps involved in the Calvin cycle are as follows:
a. Carbon Fixation: Carbon dioxide molecules from the atmosphere are incorporated into an organic molecule called ribulose-1,5-bisphosphate (RuBP). This reaction is catalyzed by the enzyme rubisco, resulting in the formation of a six-carbon molecule that quickly breaks down into two molecules of three-carbon compound called 3-phosphoglycerate (3-PGA).
b. Reduction and Sugar Production: ATP and NADPH produced in the light-dependent reactions are used to convert the 3-PGA molecules into a three-carbon sugar called glyceraldehyde-3-phosphate (G3P). Some G3P molecules are used to regenerate RuBP, while others can be combined to form glucose and other carbohydrates.
c. Regeneration of RuBP: To sustain the Calvin cycle, some G3P molecules are converted back into RuBP. This step requires additional ATP generated by the light-dependent reactions.
The overall outcome of photosynthesis is the conversion of light energy into chemical energy stored in the form of glucose and other organic molecules. These carbohydrates serve as the primary energy source for plants and are vital for the sustenance of various organisms in the food chain.