Photosynthesis and Plant Hormones

Photosynthesis

  • Light Absorption & Electron Transport

    • Chlorophylls aa and bb absorb blue and red light.

    • Z-scheme: H2Osplittinginthylakoidsreleaseselectrons(splitting in thylakoids releases electrons (e^-),protons(), protons (H^+),and), andO2.</p></li><li><p>.</p></li><li><p>H^+gradientinlumendrivesATPsynthesis(photophosphorylation).</p></li></ul></li><li><p><strong>C3Pathway</strong></p><ul><li><p>Rubiscofixesgradient in lumen drives ATP synthesis (photophosphorylation).</p></li></ul></li><li><p><strong>C3 Pathway</strong></p><ul><li><p>Rubisco fixesCO_2intoRuBP,formingtwo3−PGA.</p></li><li><p>Mostcommonpathway,butinefficientinhighinto RuBP, forming two 3-PGA.</p></li><li><p>Most common pathway, but inefficient in highO_2/hightemperatureduetophotorespiration.</p></li></ul></li><li><p><strong>C4Pathway</strong></p><ul><li><p>/high temperature due to photorespiration.</p></li></ul></li><li><p><strong>C4 Pathway</strong></p><ul><li><p>CO_2fixedinto4−COAAinmesophyllcells,thentransportedtobundlesheathcells.</p></li><li><p>Bundlesheathcellsconcentratefixed into 4-C OAA in mesophyll cells, then transported to bundle sheath cells.</p></li><li><p>Bundle sheath cells concentrateCO_2((\approx 10\times)forRubisco,minimizingphotorespiration.</p></li><li><p>Advantageousinhot,high−lightenvironments,butenergy−intensive.</p></li></ul></li><li><p><strong>CAMPathway(CrassulaceanAcidMetabolism)</strong></p><ul><li><p>Temporalseparation:) for Rubisco, minimizing photorespiration.</p></li><li><p>Advantageous in hot, high-light environments, but energy-intensive.</p></li></ul></li><li><p><strong>CAM Pathway (Crassulacean Acid Metabolism)</strong></p><ul><li><p>Temporal separation:CO_2uptakeandfixationinto4−COAAatnight(stomataopen).</p></li><li><p>Malatestoredinvacuole,releasesuptake and fixation into 4-C OAA at night (stomata open).</p></li><li><p>Malate stored in vacuole, releasesCO_2duringtheday(stomataclosedforwaterconservation).</p></li><li><p>Energyinefficientinduring the day (stomata closed for water conservation).</p></li><li><p>Energy inefficient inCO_2$$ fixation but adapted for arid conditions.

Photoassimilate Distribution

  • Phloem Transport: Sugars (photoassimilates) are actively loaded from source cells (leaves) into sieve tubes via companion cells.

  • Mass Flow Hypothesis: Osmotic water movement into sieve cells creates turgor pressure, driving mass flow of sugars and water from source to sink.

  • Source-Sink Dynamics: Young leaves are sinks, mature leaves are sources. Sinks compete for assimilates.

  • Phloem Sap Composition: Rich in non-reducing sugars (sucrose), amino acids, organic acids, and mobile nutrients (e.g., N, P, K).

Plant Hormones

  • General Characteristics

    • Small, mobile molecules, synthesized in multiple locations, with multiple functions.

    • Perceived by receptors, often on the plasma membrane.

  • Classic Types & Roles

    • Auxins (IAA): Produced in shoot tips, young leaves. Transported basipetally. Inhibits axillary bud growth (apical dominance).

    • Cytokinins: Produced in roots, seeds. Transported acropetally. Stimulates lateral bud growth.

    • Gibberellins (GAs): Produced in developing stems, leaves, seeds. Promotes stem elongation.

    • Abscisic Acid (ABA): Produced in water-stressed roots/leaves, seeds. Transported acropetally. Triggers stomatal closure (K+ efflux from guard cells).

    • Ethylene: Gas, produced in response to stress/damage, ripening, senescence. Transported by diffusion.