1. Capturing Light Energy:
* Photosystems: Photosynthesis occurs in chloroplasts, specifically within structures called thylakoids. Thylakoid membranes contain clusters of pigments called photosystems (Photosystem I and Photosystem II).
* Chlorophyll: The primary pigment in photosystems is chlorophyll. Chlorophyll absorbs light energy, primarily in the red and blue wavelengths, and reflects green light (which is why plants appear green).
* Excitation: When light hits chlorophyll, it excites electrons within the pigment molecule, raising them to a higher energy level.
2. Electron Transport Chain:
* Photosystem II: The excited electrons from chlorophyll are passed along a series of electron carriers in the thylakoid membrane. This is called the electron transport chain.
* Water Splitting: As electrons move, water molecules are split, releasing electrons (to replace those lost from chlorophyll), protons (H+), and oxygen (O2) as a byproduct.
* Proton Gradient: The movement of electrons across the membrane pumps protons (H+) into the thylakoid lumen, creating a concentration gradient.
3. ATP Production:
* Chemiosmosis: The proton gradient drives ATP synthase, an enzyme embedded in the thylakoid membrane. This enzyme uses the potential energy stored in the gradient to synthesize ATP (adenosine triphosphate), the primary energy currency of cells.
4. NADPH Production:
* Photosystem I: After passing through Photosystem II, electrons are passed to Photosystem I. Here, they are re-energized by light and then transferred to a molecule called NADP+ (nicotinamide adenine dinucleotide phosphate).
* NADPH Formation: NADP+ accepts the electrons and a proton (H+), becoming NADPH. NADPH is a reducing agent, meaning it can donate electrons to other molecules.
Summary:
Light energy is absorbed by chlorophyll and used to excite electrons, which then flow through an electron transport chain. This flow generates a proton gradient that drives ATP production. Electrons are also used to reduce NADP+ to NADPH.
Key Products:
* ATP: Energy currency for the cell.
* NADPH: Electron carrier used in the Calvin cycle.
* Oxygen (O2): Released as a byproduct.
The Calvin Cycle
The ATP and NADPH produced in the light-dependent reactions are then used in the Calvin cycle, a series of biochemical reactions that occur in the stroma (the fluid inside the chloroplast). The Calvin cycle uses these energy sources to convert carbon dioxide (CO2) into glucose, the sugar that fuels the plant.
