ATP is made in light-dependent reactions through a process called photophosphorylation, which is essentially using light energy to add a phosphate group to ADP, creating ATP. Here's a breakdown:
The Process of Photophosphorylation
Photophosphorylation occurs during the light-dependent reactions of photosynthesis and involves two main types:
- Non-cyclic Photophosphorylation: This is the more common pathway.
- Cyclic Photophosphorylation: This is a less common, alternative pathway.
We'll primarily focus on non-cyclic photophosphorylation, as it's the "standard" form.
Non-Cyclic Photophosphorylation:
- Light Absorption: Light energy is absorbed by chlorophyll in Photosystem II (PSII).
- Electron Excitation: This light energy excites electrons in PSII to a higher energy level.
- Water Splitting: To replace these electrons, water molecules are split (photolysis) into electrons, protons (H+), and oxygen (O2). Oxygen is released as a byproduct.
- Electron Transport Chain: The energized electrons from PSII are passed along an electron transport chain (ETC).
- Proton Pumping: As electrons move down the ETC, energy is released. This energy is used to pump protons (H+) from the stroma (the space outside the thylakoid) into the thylakoid lumen (the space inside the thylakoid).
- Creating a Proton Gradient: This pumping creates a high concentration of protons inside the thylakoid lumen and a low concentration in the stroma. This difference in proton concentration establishes an electrochemical gradient.
- ATP Synthase: Protons flow down this gradient, from the thylakoid lumen back into the stroma, through an enzyme called ATP synthase.
- ATP Production: The flow of protons through ATP synthase provides the energy needed to bind a phosphate group to ADP, generating ATP. This process is called chemiosmosis.
- Photosystem I (PSI): After passing through the electron transport chain, the electrons reach Photosystem I (PSI). PSI also absorbs light energy and re-energizes the electrons.
- NADPH Formation: These re-energized electrons are then passed to NADP+, reducing it to NADPH. NADPH, along with ATP, provides the energy needed for the Calvin cycle (light-independent reactions).
In summary, the light-dependent reactions use light energy to create a proton gradient and energized electrons. The proton gradient drives ATP synthesis via ATP synthase, while the energized electrons are used to create NADPH. Both ATP and NADPH are then used to power the Calvin cycle, where carbon dioxide is converted into glucose.
