Photosynthesis and ATP Synthesis – Detailed Study Notes

Introduction & Context

  • Living organisms are divided into:

    • Autotrophs – self-sufficient; build all organic molecules from inorganic CO$2$, H$2$O and ions (e.g. NO<em>3−NO<em>3^-, PO</em>43−PO</em>4^{3-}, Mg2+Mg^{2+}) using sunlight.

    • Heterotrophs – depend on ready-made organic molecules; energy source is food (carbohydrates, fats, proteins).

  • Animals & fungi are heterotrophs; plants (plus many bacteria & phytoplankton) are autotrophs.

  • Energy captured by plants becomes the ultimate source of food for virtually every food chain.

  • Respiration (Chapter 2) releases stored energy by oxidising organics to make ATP; every cell must synthesise its own ATP.

Overview of Photosynthesis

  • Occurs mainly in chloroplasts of palisade & spongy mesophyll in leaves.

  • Overall balanced equation:
    6CO<em>2+6H</em>2O  →light/ chlorophyll  C<em>6H</em>12O<em>6+6O</em>26\text{CO}<em>2 + 6\text{H}</em>2O \;\xrightarrow{\text{light/ chlorophyll}}\; C<em>6H</em>{12}O<em>6 + 6\text{O}</em>2

  • Two major stages:

    • Light-dependent reactions – on thylakoid membranes; convert light energy → chemical energy (ATP & reduced NADP).

    • Light-independent reactions (Calvin cycle) – in stroma; use ATP & reduced NADP to fix CO$_2$ into carbohydrates.

Leaf Structure & Function

  • Macroscopic features

    • Broad, thin lamina → large surface area & short diffusion path.

    • Midrib + network of veins (xylem & phloem) for support & transport.

    • Petiole may orientate lamina for optimal light.

  • Positioning (Leaf Mosaic) – arrangement minimises shading, maximising light interception.

  • Epidermis & Cuticle

    • Upper epidermis: transparent, flat cells; secretes waxy cuticle → waterproof, reduces uncontrolled water loss, yet transmits light.

    • Lower epidermis: similar, but usually bears numerous stomata (pores) bounded by two guard cells.

  • Guard Cells & Stomatal Regulation

    • Uneven cell-wall thickness: inner (pore) wall thick; outer wall thin.

    • Cellulose microfibrils arranged like hoops → turgor changes cause lengthwise expansion & bending.

    • Opening mechanism: active transport of H$^+$ out → K$^+$ influx → lowered ψ (water potential) → water enters osmotically → turgid → pore opens; reverse closes.

  • Mesophyll Layers

    • Palisade mesophyll – main photosynthetic site; elongated cylinders at right angles to epidermis, densely packed with chloroplasts.

    • Adaptations:

      • Fewer cross-walls in upper leaf → maximum light penetration.

      • Large central vacuole pushes chloroplasts to periphery for light access.

      • Chloroplasts can relocate within cytoplasm (cytoskeletal proteins) to optimise capture or avoid photodamage.

    • Spongy mesophyll – loosely packed with large air spaces; fewer chloroplasts; primary role in gaseous exchange.

  • Vascular Bundles

    • Xylem – brings water & dissolved ions for photosynthesis & turgor.

    • Phloem – exports sucrose & other assimilates to the rest of the plant.

Chloroplast Structure & Function

  • Typical photosynthesising cell contains 10–100 chloroplasts.

  • Envelope: double membrane (outer & inner) encloses stroma.

  • Internal membrane system:

    • Thylakoids – flattened sacs; internal compartment = thylakoid lumen.

    • Grana – stacks of thylakoids; interconnected by stroma lamellae.

  • Stroma – fluid matrix containing enzymes (notably Rubisco), starch grains, lipid droplets, 70S ribosomes & circular DNA (evidence of endosymbiotic origin).

  • Functional relevance:

    • Large membrane surface area anchors photosynthetic pigment–protein complexes; maximises light capture.

    • Thylakoid lumen enables H$^+$ accumulation → proton-motive force for ATP synthesis.

    • Starch grains store immediate products of photosynthesis.

Photosynthetic Pigments

  • Primary pigments – Chlorophyll a & b (porphyrin ring + hydrophobic tail).

    • Chlorophyll a absorbs slightly longer wavelengths than chlorophyll b.

  • Accessory pigments – Carotenoids (e.g. carotene, xanthophylls).

    • Extend absorption spectrum (especially blue-green light) & protect chlorophyll from photo-oxidative damage.

  • Absorption spectra (Fig. 1.8): peaks around 430 nm430\,\text{nm} & 660 nm660\,\text{nm} for chlorophyll a; carotene absorbs mainly 400–500 nm400–500\,\text{nm} → pigments appear green or yellow-orange due to reflected wavelengths.

Light-Dependent Reactions (Photophosphorylation)

  • Carried out by pigment–protein complexes called photosystems embedded in thylakoid membranes.

    • Photosystem I (PSI) – reaction centre P700 (chlorophyll a).

    • Photosystem II (PSII) – reaction centre P680.

  • When light excites chlorophyll, high-energy electrons are emitted from reaction centres.

Cyclic Photophosphorylation

  • Involves PSI only.

  • Electron flow: PSI → series of electron carriers → returns to PSI.

  • Energy released phosphorylates ADP:
    ADP+Pi→ATP\text{ADP} + P_i \rightarrow \text{ATP}

  • No photolysis, no O$_2$ evolution, no NADP reduction.

Non-Cyclic Photophosphorylation (Z-Scheme)

  • Involves PSII & PSI sequentially.

  1. Light excites PSII → electrons expelled.

  2. Photolysis of water at PSII replaces these electrons: 2H<em>2O→4H++4e−+O</em>22\text{H}<em>2\text{O} \rightarrow 4\text{H}^+ + 4e^- + O</em>2 (oxygen released as a by-product).

  3. Electrons pass through an electron transport chain (cytochromes, plastoquinone, etc.) → energy used to pump H$^+$ into thylakoid lumen → ATP synthesis via ATP synthase.

  4. Electrons reach PSI; light boosts them to higher energy again.

  5. Electrons + H$^+$ reduce NADP$^+$: NADP++2e−+H+→reduced NADP\text{NADP}^+ + 2e^- + H^+ \rightarrow \text{reduced NADP}

  • Products: ATP, reduced NADP, and O$_2$.

Light-Independent Reactions (Calvin Cycle)

  • Occur in stroma; enzyme: Rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase).

  • Cycle steps (for each CO$_2$ fixed):

    1. Carboxylation – CO$_2$ combines with RuBP (5C) → unstable 6C intermediate → splits into 2 × GP (3C).

    2. Reduction – GP + ATP + reduced NADP → TP (triose phosphate, 3C).

    3. Regeneration – 5/6 of TP + ATP → RuBP.

  • Stoichiometry:

    • 6 CO$2$ + 12 reduced NADP + 18 ATP → 1 glucose (after TP condensation) + 12 NADP$^+$ + 18 ADP + 18 P$i$.

  • Fates of TP:

    • Hexose phosphates → glucose, fructose, sucrose, starch, cellulose.

    • Precursor for lipids & amino acids (requires N from NO<em>3−NO<em>3^-/NH</em>4+NH</em>4^+).

  • ADP, P$_i$, NADP$^+$ recycled to light-dependent stage.

Factors Affecting Photosynthetic Rate

  • Process needs light energy, CO$2$, H$2$O, suitable temperature, and sufficient chlorophyll & enzymes.

Light Intensity

  • Rate ∝ light intensity up to light saturation point; beyond that, another factor (often CO$_2$) becomes limiting (curve Fig. 1.13).

CO$_2$ Concentration

  • Atmospheric level ≈ 0.04 % often limiting.

  • Increasing [CO$2$] boosts Calvin cycle until CO$2$ saturation point; thereafter limitation shifts to light intensity or enzyme capacity (curves Fig. 1.14 & 1.15).

Temperature (mentioned implicitly)

  • Affects enzymatic kinetics (Rubisco & Calvin cycle enzymes); low T slows reactions, high T may denature enzymes or increase photorespiration.

Limiting Factors & Crop Productivity (Preview)

  • Concept: The rate of a physiological process is controlled by the factor in least favourable concentration (Liebig’s Law of the Minimum).

  • Manipulation in agriculture (e.g. glasshouses) – supplement light, CO$_2$, temperature to maximise yield; economic cost-benefit analysis determines optimum levels.

Connections & Relevance

  • ATP synthesis mechanism parallels oxidative phosphorylation in mitochondria (Chapter 2).

  • Photolysis releases atmospheric O$_2$; evolutionary significance – enabled aerobic respiration & complex life.

  • Excessive light → photoinhibition; accessory pigments & chloroplast movement guard against oxidative damage.

  • Ethical/environmental: Artificial CO$_2$ enrichment or high-energy lighting in greenhouses impacts energy consumption & carbon footprint.

Key Terminology

  • Autotroph / Heterotroph

  • Mesophyll (palisade vs spongy)

  • Stomata / Guard cells / Turgor

  • Thylakoid / Grana / Stroma

  • Photosystem I & II (P700, P680)

  • Photophosphorylation (cyclic, non-cyclic)

  • Photolysis

  • Rubisco, RuBP, GP, TP (GALP)

  • Light saturation / CO$_2$ saturation / Limiting factor

Numerical & Equation Summary

  • Overall photosynthesis: 6CO<em>2+6H</em>2O→C<em>6H</em>12O<em>6+6O</em>26\text{CO}<em>2 + 6\text{H}</em>2O \rightarrow C<em>6H</em>{12}O<em>6 + 6\text{O}</em>2

  • Photolysis: 2H<em>2O→4H++4e−+O</em>22\text{H}<em>2\text{O} \rightarrow 4\text{H}^+ + 4e^- + O</em>2

  • ATP formation: ADP+Pi→ATP\text{ADP} + P_i \rightarrow \text{ATP} (photophosphorylation)

  • NADP reduction: NADP++2e−+H+→reduced NADP\text{NADP}^+ + 2e^- + H^+ \rightarrow \text{reduced NADP}

  • Calvin cycle stoichiometry for 1 glucose: 6CO<em>2+18ATP+12reduced NADP→C</em>6H<em>12O</em>6+18ADP+18Pi+12NADP+6\text{CO}<em>2 + 18\text{ATP} + 12\text{reduced NADP} \rightarrow C</em>6H<em>{12}O</em>6 + 18\text{ADP} + 18P_i + 12\text{NADP}^+