LV

Photosynthesis & Related Concepts

Photosynthesis vs. Cellular Respiration

  • Photosynthesis and cellular respiration are opposite, yet inter-linked processes
    • Photosynthesis converts electromagnetic (light) energy → chemical energy (stored in organic molecules)
    • Cellular respiration converts chemical energy (glucose) → usable ATP, releasing heat & waste
  • Overall balanced photosynthesis reaction (write, memorize):
    • 6\,CO2 + 6\,H2O + \text{light} \;\longrightarrow\; C6H{12}O6 + 6\,O2
    • Glucose ((C6H{12}O6)) then fuels respiration: C6H{12}O6 + 6\,O2 \;\longrightarrow\; 6\,CO2 + 6\,H_2O + \text{energy (ATP + heat)}
  • Energy profile:
    • Photosynthesis = endergonic (requires energy, products have ↑ potential energy)
    • Cellular respiration = exergonic (releases energy)
    • “Gear analogy”: exergonic reaction can drive an endergonic one
  • Redox perspective
    • CO₂ is reduced to carbohydrate (e⁻ pulled toward C–H bonds)
    • H₂O is oxidized to O₂ (e⁻ removed from H, O left as waste)

Why Plants Need Water, Light & CO₂

  • CO₂ (atmospheric) supplies carbon skeleton of sugars
  • H₂O supplies electrons; splitting water extracts e⁻ and H⁺, releases O₂ as by-product
  • Light provides photon energy that excites pigment electrons → drives electron transport chain (ETC)
  • Practical gardening link: watering = giving the plant an electron source

Two Phases of Photosynthesis

  • Light-dependent reactions (aka “light reactions”)
    • Location: thylakoid membrane
    • Convert light energy → chemical energy (ATP) and reducing power (NADPH)
    • Process is overall exergonic
  • Calvin cycle (“dark” / light-independent reactions)
    • Location: stroma
    • Uses ATP + NADPH to reduce CO₂ → carbohydrate (G3P → glucose, starch, etc.)
    • Overall endergonic (driven by products of light reactions)

Chloroplast Structure

  • Double-membrane organelle (outer & inner envelopes)
  • Inner membrane folds into flattened thylakoids
    • A stack = granum (plural grana)
    • Interior space of each thylakoid = lumen (proton reservoir)
  • Fluid outside thylakoids but inside chloroplast = stroma (Calvin cycle site)
  • A single leaf cell contains ~40–50 chloroplasts → photosynthetic capacity scales with leaf area
  • Metaphor: chloroplast = a green "Gusher" candy; thylakoids = fruity pockets, stroma = syrup in-between

Pigments & Light Absorption

  • Pigment = molecule that absorbs specific wavelengths; appears colored because un-absorbed light is reflected/transmitted
  • Two main pigment classes
    • Chlorophylls (a & b) – primary; absorb violet-blue (~450 nm) & red (~650–700 nm); reflect green
    • Carotenoids – accessory; absorb blue-green (~450–550 nm); reflect yellow, orange, red
  • Seasonal color change: when chlorophyll degrades (fall), carotenoids dominate → leaves appear orange/brown; plants now also absorb some green they formerly reflected
  • Photon energy relation: E \propto \frac1{\lambda}
    • Shorter wavelength (violet/UV) = higher energy; longer (red) = lower
    • Plants harvest extremes (blue & red) because they carry most usable energy, ignore mid-range (green)

Why Plants Look Green – Color & Perception Sidebar

  • Green light is mostly reflected; our eyes detect that reflection
  • Color = brain’s interpretation of specific electromagnetic vibrations; visible spectrum is only a tiny slice (~400–700 nm) of the total EM spectrum (radio–gamma)
  • Thought experiment on subjective color (“my green = your blue”) – possible only if cone/rod biology differed; typically human color perception is consistent unless color-blind mutations exist

Photochemistry – Exciting Electrons

  • Photon absorbed by chlorophyll raises an electron to an excited state
    • Red photon → +1 energy level
    • Blue photon → +2 energy levels (higher energy)
    • Green photon → energy mismatch, not absorbed
  • Four fates of an excited electron (illustration):
    1. Fluorescence + heat release (isolated pigment glows)
    2. Resonance energy transfer among antenna pigments (no e⁻ moved, only energy)
    3. Photochemistry at reaction center – e⁻ actually transferred to primary acceptor (pheophytin)
    4. Energy lost as heat without fluorescence (minor)

Photosystems & Antenna Complex

  • Photosystem II (PSII)
    • Harvesting complex (~200–300 chlorophyll & accessory pigments) funnels energy to a special pair of chlorophylls in the reaction center
    • Primary electron acceptor = pheophytin (Pheo)
    • After e⁻ leaves, PSII splits H₂O to replace electron; releases O_2 + 4H^+ into lumen
  • Photosystem I (PSI) (not detailed deeply in transcript but implied) further boosts e⁻ energy → NADP⁺ reduction

Electron Transport Chain (ETC) – Thylakoid Membrane

  • Components: quinones (e.g., plastoquinone – PQ), cytochromes (protein complexes), plastocyanin, etc.
    • Quinone = lipid-soluble shuttle ferrying e⁻ between large complexes
    • Cytochromes = membrane-embedded proteins passing e⁻ sequentially
  • As e⁻ flow downhill energetically, H⁺ are pumped from stroma → lumen (against gradient)
    • Creates electrochemical proton motive force (PMF)
    • Analogy: filling a water balloon inside thylakoid
  • ATP synthase spans membrane; has a proton channel + rotary motor
    • H⁺ diffuse down gradient through enzyme → rotor spins → enzymatic site phosphorylates ADP + P_i \to ATP
  • Net light reaction outputs per water split: O_2 (waste), ATP, NADPH

Energy Coupling Summary (Gear Analogy Revisited)

  • Light reactions (exergonic) = gear #1; release usable energy (ATP/NADPH)
  • Calvin cycle (endergonic) = gear #2; uses those products to fix CO₂ → sugar
  • Remove water or light → first gear stops → second gear stalls

Key Chemical Carriers

  • NADP⁺ / NADPH
    • NADP⁺ = oxidized (no extra e⁻)
    • NADPH = reduced (loaded with high-energy e⁻)
    • Memory tip: the P in NADP(H) = Photosynthesis
  • In respiration, analogous carriers = NAD⁺/NADH & FAD/FADH₂; ETC logic is conserved between chloroplasts & mitochondria

Practical / Ethical / Real-World Tie-Ins

  • Oxygen we breathe is a waste product of photosynthesis; our respiration waste (CO₂) feeds plants → global carbon cycle
  • Watering plants is not just “thirst” – you’re supplying electron fuel for photolysis
  • Excessive high-energy radiation (UV, X-ray, γ) is dangerous; medical imaging uses lead shielding (dentist apron example) to block ionizing radiation that can cause cancer
  • Ecological relevance: leaf area index, chloroplast count per cell, and pigment composition affect ecosystem productivity & global carbon sequestration

Numerical / Statistical Nuggets & Terms

  • Chloroplast per leaf cell: ~40–50
  • Thylakoid stack (granum) = dozens of individual thylakoids
  • Wavelength ranges (approx.):
    • Blue/violet: \sim 400!\text{–}!480\,\text{nm}
    • Green: \sim 500!\text{–}!570\,\text{nm} (mostly reflected)
    • Red: \sim 650!\text{–}!700\,\text{nm}
  • Proton gradient ΔpH in lumen can reach ≈3 units (not directly in transcript but foundational)

Metaphors & Classroom Analogies

  • Diamond ≠ organic (pure C, lacks many C–H bonds); organic = “high proportion of C–H bonds”
  • Gusher candy visual for chloroplast: thylakoid stacks suspended in syrupy stroma
  • Water balloon for lumen proton buildup that later bursts through ATP synthase
  • Gear train: exergonic & endergonic reactions interlock
  • Hopscotch/resonance: energy jumps pigment-to-pigment toward reaction center

Study Reminders / Must-Know List

  • Write & balance photosynthesis vs. respiration equations
  • Define: endergonic, exergonic, oxidation, reduction, resonance energy transfer
  • Diagram chloroplast & label: outer membrane, inner membrane, stroma, thylakoid, lumen, granum
  • Trace electron path: H₂O → PSII → PQ → cytochrome complex → PC → PSI → ferredoxin → NADP⁺ → NADPH → Calvin cycle
  • Know pigment absorption peaks & seasonal color logic
  • Explain why watering & lighting conditions affect photosynthetic “gears”
  • Recognize that ETC & chemiosmosis operate in both chloroplasts and mitochondria (big integrative theme!)