Transmission Electron Microscopy (TEM) and Thylakoid Structure

Transmission Electron Microscopy (TEM) and Thylakoid Structure • Chloroplast Structure: o Enclosed by double membranes (outer and inner envelope). o Thylakoid membranes contain integral membrane proteins ▪ • essential for photosynthesis. o stroma lamellae contain PSI o Grana lamellae (stacked thylakoids) contain PSII. TEM Imaging of Chloroplasts: o TEM images show stacked grana and interconnect thylakoid membranes. o The lumen of thylakoids is positively charged compared to the stroma ▪ essential for proton gradient formation. Arrangement of Photosystems in the Thylakoid Membrane • PSII predominantly in stacked regions (grana lamellae) • PSI and ATP synthase in unstacked regions protruding into the stroma. • Cytochrome b6f complexes are evenly distributed, facilitating electron transport between PSII and PSI. • • Electron carriers: o Plastocyanin (PC) in the thylakoid lumen. o Plastoquinone (PQH2) in the membrane. Chlorophyll-protein complexes optimize energy transfer and minimize waste. Photosynthetic Electron Transport (PET): Linear and Cyclic Pathways • Linear Electron Transport (Z-Scheme): o PSII absorbs red light (P680) → Excited electrons move through the electron transport chain (ETC). o Water splitting in PSII: Produces O₂ and protons in the thylakoid lumen. o Electrons transferred through cytochrome b6f complex → plastocyanin → PSI. • o PSI absorbs far-red light (P700) → Reduces NADP⁺ to NADPH. o Proton gradient formed → Drives ATP synthesis by ATP synthase. Cyclic Electron Transport (CET): o Electrons cycle between PSI and cytochrome b6f instead of reducing NADP⁺. o No NADPH formation, only ATP synthesis. o Prevents over-reduction of NADP⁺ and provides additional ATP for the Calvin cycle. Effects of Light on Photosynthesis • Light-response curves show PPFD (photosynthetic photon flux density) where photosynthesis is limited by light or carboxylation. • • Sun vs. Shade Plants: o Shade plants: Higher PSII/PSI ratio, increased antenna chlorophyll to PSII. o Sun plants: Lower PSII/PSI ratio, optimized for high light conditions. Photoinhibition: o Dynamic photoinhibition: Temporary excess light absorption dissipated as heat. o • Chronic photoinhibition: Permanent damage due to excessive light. Xanthophyll cycle: Converts excess absorbed light into heat, preventing oxidative damage. Effects of Temperature on Photosynthesis • Temperature ranges: o Below 30°C: C3 plants have higher quantum yield than C4 plants. o Above 30°C: C4 plants outperform C3 plants due to reduced photorespiration. • Temperature stress: o High temperatures decrease rubisco activity and enhance photorespiration. o Low temperatures can limit phosphate availability and triose phosphate metabolism. Effects of CO₂ on Photosynthesis • • • CO₂ diffusion: o Enters leaf through stomata, diffuses through air spaces to chloroplasts. o Stomatal resistance is the major limiting factor under most conditions. CO₂ enrichment: o Higher atmospheric CO₂ increases photosynthesis and plant productivity. o C3 plants are more responsive to elevated CO₂ than C4 plants. CO₂ compensation point: o C3 plants have a higher CO₂ compensation point due to higher photorespiration. o C4 plants have a lower CO₂ compensation point, more efficient at CO₂ fixation. Stable Isotopes and Photosynthesis • • Carbon isotope ratios (¹³C/¹²C) distinguish C3 and C4 plants: o C3 plants discriminate more against ¹³C due to rubisco affinity for CO₂. o C4 plants have a less negative carbon isotope signature. CAM plants: o Can switch between C3 and C4 pathways depending on water availability. o CAM mode at night to conserve water; shifts to C3 when well-watered.