Capturing Sunlight to Make Food
Photosynthesis is the process used by plants, algae, and some bacteria to convert light energy into chemical energy, stored in the bonds of glucose. It is the primary way energy enters most ecosystems. The overall balanced equation is:
6CO₂ + 6H₂O + Light Energy → C₆H₁₂O₆ + 6O₂
This process takes place in a specialized organelle called the chloroplast.
The Chloroplast Structure
Thylakoids: A system of interconnected membranous sacs. A stack of thylakoids is called a granum.
Stroma: The dense fluid-filled space surrounding the thylakoids.
Stage 1: The Light-Dependent Reactions
This stage directly uses light energy to produce energy-carrying molecules.
Location: Thylakoid membranes.
Key Player: Chlorophyll, the green pigment that absorbs light energy.
Inputs: Light, Water (H₂O), ADP, NADP⁺.
Process:
1.Light energy excites electrons in chlorophyll molecules within protein complexes called photosystems.
2.Water is split (photolysis) to replace these electrons, releasing oxygen (O₂) as a byproduct and protons (H⁺).
3.The excited electrons are passed down an electron transport chain, similar to cellular respiration. This pumps protons into the thylakoid space, creating a proton gradient.
4.ATP Synthase uses this gradient to produce ATP.
5.The electrons, at the end of the chain, are used to reduce NADP⁺ to NADPH, another high-energy electron carrier.
Outputs: ATP, NADPH, Oxygen (O₂).
The ATP and NADPH produced here are not used to power the cell directly; they are sent to the next stage.
Stage 2: The Calvin Cycle (Light-Independent Reactions)
This stage uses the chemical energy from the light reactions to synthesize sugar. It does not directly require light but depends on the products of the light reactions.
Location: Stroma of the chloroplast.
Inputs: Carbon Dioxide (CO₂), ATP, NADPH.
Process:
1.Carbon Fixation: The enzyme RuBisCO attaches one molecule of CO₂ to a five-carbon sugar called RuBP.
2.Reduction: The resulting molecule is converted into a three-carbon sugar (G3P) using energy from ATP and reducing power from NADPH.
3.Regeneration: For every six G3P molecules created, one exits the cycle to be used by the plant to make glucose and other organic compounds. The other five are used, with more ATP, to regenerate the starting RuBP molecule.
Outputs: Glucose (via G3P), ADP, NADP⁺.
The ADP and NADP⁺ are recycled back to the light-dependent reactions to be re-energized.