Photosynthesis Stages and Impact

Overview

This lecture explains the two main stages of photosynthesis—light-dependent and light-independent (Calvin cycle)—detailing their processes, importance, locations, reactants, products, and the impact of removing reactants on the plant and ecosystem.

Stages of Photosynthesis

• Photosynthesis occurs in two stages: the light-dependent (light reaction) and the light-independent (Calvin cycle or dark reaction).
• The light reaction requires light and occurs in the thylakoid membranes of the chloroplast.
• The dark reaction does not require light and takes place in the stroma of the chloroplast.

Light-Dependent Reaction

• Uses light energy and water to produce ATP, NADPH, and oxygen as a byproduct.
• Chlorophyll absorbs light, excites electrons, and initiates an electron transport chain (ETC).
• Water is split by light (photolysis) to replenish electrons, releasing oxygen.
• Electron movement through ETC creates a hydrogen ion gradient, powering ATP synthase to form ATP.
• Electrons are re-energized in photosystem I and used to form NADPH.

Calvin Cycle (Light-Independent Reaction)

• Occurs in the stroma and uses ATP and NADPH to convert carbon dioxide into glucose.
• Four main stages: carbon fixation (CO₂ is attached to RuBP), reduction (uses ATP/NADPH to make G3P), carbohydrate formation (some G3P makes sugar), regeneration (remaining G3P regenerates RuBP).
• Does not require light but depends on ATP and NADPH from the light reaction.

Reactants and Products

• Light reaction: Reactants are water and light; products are ATP, NADPH, and O₂.
• Calvin cycle: Reactants are CO₂, ATP, and NADPH; product is glucose (sugar).

Importance & What Happens if Reactants Are Removed

• Without light, the light reaction stops; no ATP/NADPH are made, halting the Calvin cycle.
• Without water, no electrons are replenished; O₂ and energy carriers cannot be produced.
• Without CO₂, the Calvin cycle cannot proceed, so no sugar is formed.
• Plants and ecosystems rely on photosynthesis for energy and oxygen; removing reactants disrupts this balance.

Key Terms & Definitions

• Photosynthesis — process by which plants make food (glucose) from H₂O, CO₂, and sunlight, releasing O₂.
• Chloroplast — plant cell organelle where photosynthesis occurs.
• Thylakoid — membrane structure in chloroplast where light-dependent reaction happens.
• Stroma — fluid inside chloroplast where Calvin cycle occurs.
• Chlorophyll — pigment that absorbs light for photosynthesis.
• ATP (Adenosine Triphosphate) — energy carrier molecule.
• NADPH — high-energy molecule produced in light reactions.
• Photolysis — splitting of water by light.
• Photosystem I & II — protein complexes in thylakoid that absorb light and energize electrons.
• Electron Transport Chain (ETC) — series of proteins transferring electrons to generate ATP and NADPH.
• RuBP (Ribulose 1,5-bisphosphate) — molecule that fixes CO₂ in Calvin cycle.
• G3P (Glyceraldehyde-3-phosphate) — 3-carbon sugar formed in Calvin cycle.

Action Items / Next Steps

• Review the flowchart/sequence of steps for both light and dark reactions.
• Be able to match reactants and products to each stage of photosynthesis.
• Prepare to answer questions on effects of removing light, water, or CO₂ from the process.