The light and dark reactions (also known as the light-dependent and light-independent reactions, or the Calvin cycle) are the two main stages of photosynthesis in plants, each with distinct functions and locations within the chloroplast.
Key Differences Summarized
Here's a table summarizing the core differences:
| Feature | Light Reactions | Dark Reactions (Calvin Cycle) |
|---|---|---|
| Primary Goal | Capture light energy and convert it into chemical energy (ATP and NADPH) | Use chemical energy (ATP and NADPH) to fix CO2 into sugars (carbohydrates) |
| Location | Thylakoid membrane of chloroplast | Stroma of chloroplast |
| Input | Light, Water, ADP, NADP+ | CO2, ATP, NADPH |
| Output | Oxygen, ATP, NADPH | Glucose (or other sugars), ADP, NADP+ |
| Energy Source | Light energy | Chemical energy (ATP and NADPH) |
| Key Processes | Photophosphorylation, electron transport chain, water splitting | Carbon fixation, reduction, regeneration of RuBP |
| Alternative Names | Light-dependent reactions | Light-independent reactions, Calvin-Benson cycle |
Detailed Explanation
Light Reactions
The light reactions occur in the thylakoid membranes inside the chloroplast. Their primary purpose is to convert light energy into chemical energy in the form of ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate). Here's a breakdown of the processes:
- Light Absorption: Chlorophyll and other pigments within photosystems II and I absorb light energy.
- Electron Transport Chain: The absorbed light energy excites electrons in chlorophyll, initiating an electron transport chain. This chain passes electrons from molecule to molecule, releasing energy along the way.
- Water Splitting (Photolysis): To replenish the electrons lost by chlorophyll in photosystem II, water molecules are split, releasing oxygen (O2) as a byproduct, protons (H+), and electrons. This is where the oxygen we breathe originates.
- ATP Synthesis (Photophosphorylation): The energy released during electron transport is used to pump protons (H+) across the thylakoid membrane, creating a proton gradient. This gradient drives ATP synthase, an enzyme that phosphorylates ADP to produce ATP.
- NADPH Formation: At the end of the electron transport chain in photosystem I, electrons are used to reduce NADP+ to NADPH, another energy-carrying molecule.
Dark Reactions (Calvin Cycle)
The dark reactions, also known as the Calvin cycle, take place in the stroma, the fluid-filled space surrounding the thylakoids within the chloroplast. They use the ATP and NADPH generated during the light reactions to fix carbon dioxide (CO2) and produce sugars. The Calvin cycle involves three main phases:
- Carbon Fixation: CO2 from the atmosphere is incorporated into an organic molecule called ribulose-1,5-bisphosphate (RuBP), catalyzed by the enzyme RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase).
- Reduction: The resulting unstable six-carbon molecule is immediately split into two molecules of 3-phosphoglycerate (3-PGA). ATP and NADPH are then used to convert 3-PGA into glyceraldehyde-3-phosphate (G3P), a three-carbon sugar.
- Regeneration: Some of the G3P molecules are used to regenerate RuBP, the initial CO2 acceptor, allowing the cycle to continue. This regeneration step also requires ATP.
The G3P produced during the Calvin cycle can then be used to synthesize glucose and other carbohydrates, providing the plant with the energy and building blocks it needs to grow and function. The ADP and NADP+ generated during the Calvin cycle are then recycled back to the light reactions.
