Photosynthesis in Higher Plants

Story: Priestley 1770 bell jar experiment: Put candle alone in closed jar, dies. Mouse alone dies. Mint sprig alone grows? But mint + mouse + candle together both survive — mint restores air that mouse and candle spoil. He discovered plants restore air O2 but didn't know light needed.

Ingenhousz 1779: Showed light essential, only green parts in light restore air, in dark spoil. So photosynthesis needs light + green.

Mayer 1845: Light energy converted to chemical energy like solar cooker.

Sachs 1864: Green parts make glucose which makes starch, iodine test blue-black in light leaf, no colour dark leaf like you test for starch in potato.

Engelmann 1880 prism genius: Split white light through prism into VIBGYOR, put spirogyra spiral chloroplast algae in water with aerobic bacteria. Bacteria accumulate heavily at red and blue light areas where O2 max produced. Proved action spectrum red & blue best like plants love red blue light.

van Niel 1931: Studied purple sulphur bacteria H2S + CO2 -> CH2O + S + H2O, generalized 2H2A + CO2 -> CH2O + H2O + 2A light dependent.

Ruben & Kamen 1941 O18 experiment: Used heavy oxygen O18 in water H2O18 + CO2 normal -> O2 with O18, proved O2 comes from water not CO2. Like you track water source with colour. Overall: CO2 from air + H2O from soil makes food + O2 for us.

Chloroplast double membrane outer inner, stroma jelly, thylakoids stacked as grana coins like pile of coins, stroma lamella bridges connecting grana, thylakoid lumen inside. Contains chlorophyll, starch granule, circular DNA 70S ribosome semi-autonomous like bacteria earlier.

Pigments: Chlorophyll a main universal C55H72O5N4Mg with Mg at centre absorbs 430 nm blue and 662 nm red peaks like chef loves red blue, Chlorophyll b accessory 455 and 645, Carotenoids xanthophyll carotene accessory yellow orange absorb 400-500 blue green photoprotection like sunscreen protects chlorophyll from damage. Absorption spectrum graph absorption vs wavelength shows peaks, Action spectrum rate vs wavelength Engelmann proved red blue best matches absorption but not fully because accessory pigments help. Light Harvesting Complex LHC antenna chlorophylls funnel energy to reaction centre like workers collect light and give to boss P680 PSII and P700 PSI.

Photosystems: PSII P680 reaction centre at 680 nm and PSI P700 at 700 nm each has antenna LHC.

Z-scheme Non-cyclic: Most important. Water splitting photolysis in lumen 2H2O -> 4H+ + 4e- + O2 by oxygen evolving complex Mn Ca Cl cluster like water breaks gives electrons protons O2 we breathe. Electrons from water go to PSII P680 excited to P680* high energy -> plastoquinone PQ -> cytochrome b6f -> plastocyanin PC copper -> PSI P700 -> excited P700* -> ferredoxin Fd -> NADP reductase -> NADP+ + H+ -> NADPH. Shape like Z letter if you plot energy vs path. Along path protons pumped into lumen creates gradient for ATP.

Products: ATP via photophosphorylation + NADPH reducing power + O2.

Chemiosmosis Mitchell: Proton gradient 3 ways: water splitting inside lumen gives H+, PQ carries H+ from stroma to lumen, NADP reductase stroma uses H+ so stroma low H+ lumen high. High to low drives ATP synthase CF0 transmembrane channel + CF1 knob like turbine rotation 4H+ per ATP 14 H+ for 3 ATP. Like dam water flows turbine makes electricity.

Cyclic: Only PSI electrons from PSI -> Fd -> PQ -> Cyt b6f -> PC -> back to PSI cycle ATP only no NADPH no O2 no water splitting like short circuit. When NADPH high ATP low, plant switches to cyclic to make extra ATP. Non-cyclic both PSII PSI ATP NADPH O2, cyclic only PSI ATP only.

C3 cycle Calvin cycle: Most common wheat rice. 3 phases:

Carboxylation: CO2 + RuBP (5C) + RuBisCO enzyme most abundant enzyme on Earth -> 2 x 3PGA (3C) like 5+1=2x3.

Reduction: 3PGA + ATP + NADPH -> Triose phosphate G3P like using electricity to reduce. Uses 2 ATP 2 NADPH per 3PGA.

Regeneration: 5 G3P -> 3 RuBP uses 3 ATP to regenerate starting material like recycling.

Stoichiometry: 3 CO2 -> 1 G3P needs 9 ATP 6 NADPH, 6 CO2 -> 1 Glucose needs 18 ATP 12 NADPH gold numbers for NEET. RuBisCO is carboxylase + oxygenase dual nature. Calvin discovered using algae Chlorella with C14 radioactive tracer and paper chromatography.

Even dumb person remembers 3 phases Carboxylation Reduction Regeneration CRR like Car Ready.

Photorespiration: RuBisCO also acts as oxygenase at high O2 high temp. RuBP + O2 -> 3PGA + Phosphoglycolate (2C) waste no ATP no NADPH C2 cycle 3 organelles Chloroplast Peroxisome Mitochondria CO2 released NH3 released energy waste ~25% loss. Occurs in C3 plants. Like mistake RuBisCO picks O2 instead of CO2.

C4 Hatch Slack: Tropical plants maize sugarcane adapted to avoid photorespiration. Kranz anatomy German wreath bundle sheath cells thick walled around vascular bundle with many chloroplasts big, mesophyll thin around. Dimorphic chloroplasts: Mesophyll granal normal, Bundle sheath agranal or less grana.

Pathway shuttle: Mesophyll: CO2 + PEP (3C) + PEPcase high affinity even low CO2 -> OAA (4C) -> Malate (4C) -> transport to Bundle sheath -> decarboxylation CO2 released + Pyruvate -> CO2 + RuBP Calvin cycle makes glucose, Pyruvate back to mesophyll -> PEP via PPDK enzyme uses ATP. So CO2 concentrating mechanism high CO2 in bundle sheath no oxygenase activity. No photorespiration.

Efficiency: C4 needs 12 extra ATP total 30 ATP 12 NADPH per glucose vs C3 18 ATP 12 NADPH but no loss so better in tropical. First product OAA 4C so C4, first acceptor PEP not RuBP, enzyme PEPcase not RuBisCO for first fixation.

C3 vs C4 table NEET loves: Kranz No vs Yes, Cells mesophyll only vs both, First acceptor RuBP vs PEP, First enzyme RuBisCO vs PEPcase, First product 3PGA vs OAA, Calvin mesophyll vs bundle sheath, Photorespiration Yes vs No, ATP 18 vs 30, Optimum 20-25C vs 30-40C, Examples wheat rice vs maize sugarcane.

Factors Blackman limiting factor: Rate limited by slowest factor like queue slowest. Light quality red blue best intensity 10% full sunlight saturation, CO2 0.04% major limiting 0.05% increase growth, Temperature enzyme kinetics optimum 20-25 C3 30-40 C4 high temp denature, Water closes stomata reduces CO2. Internal chlorophyll age anatomy end product accumulation starch.