bio edpuzzle 7.1

Overview

This lecture explains the process of photosynthesis, detailing the light-dependent reactions and the Calvin Cycle, and highlights how plants convert sunlight, water, and carbon dioxide into glucose and oxygen.

Basics of Photosynthesis

• Photosynthesis converts sunlight, carbon dioxide (CO₂), and water (H₂O) into glucose and oxygen (O₂).
• Occurs in plant cells via organelles called chloroplasts.
• Two main stages: light-dependent reactions and light-independent reactions (Calvin Cycle).

Inputs and Structures Involved

• Water is absorbed by roots and transported to leaves via xylem tissues.
• Carbon dioxide enters and oxygen exits through leaf pores called stomata.
• Sunlight is absorbed by chlorophyll, a pigment stored in thylakoid membranes inside chloroplasts.
• Thylakoids are stacked into grana within chloroplasts; their membrane creates concentration gradients.

Light-Dependent Reactions

• Occur in thylakoid membranes and require sunlight.
• Chlorophyll absorbs photons, causing electrons to become "excited" (photoexcitation).
• Photosystem II (PSII) captures light energy, starts electron transport chain.
• Splitting water in PSII releases electrons, hydrogen ions (protons), and oxygen as a byproduct.
• Electrons move to the Cytochrome Complex, which pumps protons to build a gradient.
• Protons flow through ATP Synthase, forming ATP from ADP and inorganic phosphate.
• Electrons re-energized in Photosystem I (PSI), used to convert NADP+ into NADPH.
• End products: ATP, NADPH, and oxygen.

The Calvin Cycle (Light-Independent Reactions)

• Occurs in the stroma of chloroplasts; does not require light directly, but depends on ATP and NADPH.
• Begins with carbon fixation: CO₂ attached to Ribulose Bisphosphate (RuBP) by the enzyme RuBisCo.
• Unstable 6-carbon intermediate splits into two molecules of 3-Phosphoglycerate (3-PGA).
• ATP and NADPH used to convert 3-PGA into Glyceraldehyde 3-Phosphate (G3P), a high-energy compound.
• Some G3P exits the cycle to form sugars; most G3P recycled to regenerate RuBP.
• Cycle uses 9 ATP and 6 NADPH to produce one G3P for carbohydrate synthesis per 3 CO₂.

Key Terms & Definitions

• Photosynthesis — Conversion of sunlight, CO₂, and H₂O into glucose and O₂.
• Chloroplast — Organelle where photosynthesis occurs.
• Thylakoid — Membrane sac inside chloroplast, site of light-dependent reactions.
• Chlorophyll — Green pigment that absorbs light energy.
• Photosystems I & II (PSI & PSII) — Protein complexes that drive electron movement in light-dependent reactions.
• ATP — Energy currency molecule.
• NADPH — Electron carrier molecule used in biosynthesis.
• Calvin Cycle — Series of reactions making sugars from CO₂ using ATP and NADPH.
• RuBisCo — Enzyme catalyzing CO₂ fixation to RuBP.
• G3P (Glyceraldehyde 3-Phosphate) — Sugar product of the Calvin Cycle.

Action Items / Next Steps

• Review diagrams of chloroplast structure and the photosynthetic process.
• Memorize the stages and main products of light-dependent reactions and the Calvin Cycle.
• Prepare answers for likely test questions about photosynthesis steps and key terms.