Rucete ✏ AP Biology In a Nutshell
6. Photosynthesis
This chapter explains the process of photosynthesis in autotrophs, focusing on the two main stages: the light-dependent reactions and the light-independent reactions (Calvin cycle). It also explores the organelles and structures involved in capturing light energy and converting it into chemical energy.
Overview of Photosynthesis
• Heterotrophs obtain organic molecules by consuming others; autotrophs make their own from inorganic sources.
• Photoautotrophs use light energy to make organic molecules through photosynthesis.
• Photosynthesis involves redox reactions: – Carbon in CO₂ is reduced (gains H). – Oxygen in H₂O is oxidized (loses H).
• Mnemonic: OILRIG – Oxidation Is Losing, Reduction Is Gaining (hydrogens/electrons).
Photosynthesis Overview
• Two main stages: – Light-dependent reactions: occur in the thylakoid membranes, use sunlight to split water, release O₂, and produce ATP and NADPH. – Light-independent reactions (Calvin cycle): occur in the stroma, use ATP, NADPH, and CO₂ to produce sugars.
• The two stages are interdependent—products of one are used by the other.
Chloroplast Structure
• Chloroplast has: – Outer membrane – Inner stroma (liquid) – Thylakoid membranes (stacks called grana)
• Light-dependent reactions happen in the thylakoids; Calvin cycle occurs in the stroma.
• In photosynthetic prokaryotes: – Light-dependent reactions occur on plasma membrane infoldings. – Calvin cycle happens in cytosol.
Light-Dependent Reactions
• Light energy powers photophosphorylation (ATP production).
• Light excites electrons in chlorophyll → passed through ETC → energy released.
• NADP⁺ accepts electrons at the end → forms NADPH (reducing power).
Photosystems and Chlorophyll
• Chlorophyll is a pigment that absorbs light energy; found in PSI and PSII.
• PSI (P700) and PSII (P680) are protein–pigment complexes in thylakoid membranes.
• Photosystems are linked by an electron transport chain (ETC).
Electron Flow and Photolysis
• PSII absorbs photons → excites electrons → passed through ETC.
• ETC energy pumps H⁺ across thylakoid membrane → forms proton gradient.
• Water is split (photolysis) to replace lost electrons: – Produces electrons (to PSII), protons (for gradient), and O₂ (released).
ATP and Chemiosmosis
• H⁺ ions flow back through ATP synthase, powering ATP formation (chemiosmosis).
• Similar to mitochondrial ATP production.
Production of NADPH
• Electrons reach PSI → re-energized by light → passed to NADP⁺ by NADP⁺ reductase.
• Forms NADPH for the Calvin cycle.
The Calvin Cycle (Light-Independent Reactions)
• Occurs in the stroma of the chloroplast.
• Uses ATP and NADPH from light-dependent reactions to fix carbon from CO₂.
• Main goal: produce glucose and other organic molecules.
• Three phases: 1. **Carbon fixation**: – Enzyme RuBisCO fixes CO₂ to RuBP (5-carbon sugar). – Forms an unstable 6-carbon compound → splits into two 3-carbon molecules (3-PGA).
2. **Reduction**: – 3-PGA is phosphorylated by ATP and reduced by NADPH. – Produces G3P (glyceraldehyde-3-phosphate), a 3-carbon sugar.
3. **Regeneration**: – Some G3P exits to build glucose. – The rest is used to regenerate RuBP using ATP.
• For 1 G3P to exit: – 3 CO₂ – 9 ATP – 6 NADPH
• For 1 glucose (2 G3P): – 6 CO₂ – 18 ATP – 12 NADPH
Summary of Photosynthesis
• Light reactions (thylakoids): – Inputs: light, H₂O, NADP⁺, ADP – Outputs: O₂, ATP, NADPH
• Calvin cycle (stroma): – Inputs: CO₂, ATP, NADPH – Outputs: G3P (sugar), NADP⁺, ADP
Adaptations in Photosynthesis
• C₃ Plants: standard Calvin cycle; efficient in cool, moist climates.
• C₄ Plants: spatial separation of steps; CO₂ fixed in mesophyll, Calvin cycle in bundle sheath cells (e.g., corn, sugarcane).
• CAM Plants: temporal separation; open stomata at night to fix CO₂, Calvin cycle during day (e.g., cacti, succulents).
• These adaptations reduce photorespiration and water loss in hot, dry environments.
In a Nutshell
Photosynthesis converts solar energy into chemical energy in two stages: the light-dependent reactions produce ATP and NADPH, while the Calvin cycle uses them to fix carbon into sugars. Chloroplast structure supports this process, and specialized adaptations allow plants to optimize photosynthesis in different environments.