PHOTOSYNTHESIS IN HIGHER PLANTS - QUICK REVISION.
OVERVIEW: Photosynthesis is a physico-chemical process by which green plants (autotrophs) use light energy to synthesise organic compounds from CO2 and water, releasing O2. Overall equation: 6CO2 + 12H2O -> C6H12O6 + 6H2O + 6O2 (light). The O2 released comes from water (van Niel; proved by radioisotopes). It occurs mainly in the green leaves, in chloroplasts.
EARLY EXPERIMENTS: Priestley (1770) - plants restore air fouled by candles/animals; Ingenhousz - sunlight essential, only green parts release O2 (bubbles); von Sachs (1854) - glucose made in green parts, stored as starch; Engelmann - action spectrum (O2 max in blue and red light) using Cladophora and aerobic bacteria.
CHLOROPLAST: Membrane system (grana + stroma lamellae) does the light reactions (trapping light, making ATP and NADPH); stroma does the dark/biosynthetic reactions (sugar synthesis).
PIGMENTS (4): chlorophyll a (blue green, chief pigment), chlorophyll b (yellow green), xanthophylls (yellow), carotenoids (yellow to yellow-orange). Accessory pigments absorb light, transfer energy to chlorophyll a, widen usable wavelengths and protect chlorophyll a from photo-oxidation. Maximum photosynthesis is in blue and red regions.
LIGHT REACTION: Pigments organised into LHC within PS I (reaction centre P700) and PS II (reaction centre P680). Named in order of discovery, not function. Photochemical phase = light absorption, water splitting, O2 release, ATP and NADPH formation.
ELECTRON TRANSPORT / Z SCHEME: PS II P680 excited -> primary acceptor -> cytochromes (downhill) -> PS I P700 excited -> acceptor -> NADP+ reduced to NADPH + H+. Water splitting (2H2O -> 4H+ + O2 + 4e-) at PS II replaces electrons and produces O2. The path traced on a redox scale gives the characteristic Z shape.
PHOTOPHOSPHORYLATION: Non-cyclic (PS II then PS I in series) makes both ATP and NADPH. Cyclic (only PS I; in stroma lamellae which lack PS II and NADP reductase) makes only ATP; also occurs when only wavelengths beyond 680 nm are available.
CHEMIOSMOSIS: ATP synthesis linked to a proton gradient across the thylakoid membrane; protons accumulate in the lumen because (a) water splitting releases protons in lumen, (b) H carrier moves protons stroma->lumen, (c) NADP reductase removes protons from stroma. Protons diffuse back through CF0 of ATP synthase; CF1 makes ATP. Needs membrane, proton pump, gradient and ATP synthase.
CALVIN CYCLE (C3): Primary acceptor RuBP (5C); enzyme RuBisCO. Three stages: carboxylation (RuBP + CO2 -> 2 x 3-PGA), reduction (forms triose phosphate using ATP and NADPH), regeneration (of RuBP, needs 1 ATP). Per CO2: 3 ATP + 2 NADPH. Per glucose (6 turns): 18 ATP + 12 NADPH.
C4 PATHWAY (Hatch and Slack): For dry tropical plants (maize, sorghum). Kranz anatomy (large bundle sheath cells, many chloroplasts, no intercellular spaces). Primary acceptor PEP (3C) in mesophyll; enzyme PEPcase; first product OAA (4C). C4 acid moves to bundle sheath, decarboxylated to release CO2 (raising CO2 at RuBisCO); 3C molecule returns to mesophyll. Calvin cycle runs only in bundle sheath cells. Mesophyll lacks RuBisCO; bundle sheath lacks PEPcase.
PHOTORESPIRATION: In C3 plants, RuBisCO active site binds both CO2 and O2 (competitive). When O2 binds, RuBP forms phosphoglycerate + 2C phosphoglycolate; no sugar, ATP or NADPH made; CO2 released. C4 plants avoid it by concentrating CO2 at RuBisCO, giving higher productivity and high-temperature tolerance.
LIMITING FACTORS: Blackman's Law (1905) - rate set by the factor nearest its minimal value. Light: linear at low intensity; saturates at about 10 percent of full sunlight; rarely limiting in nature. CO2: major limiting factor (atmosphere 0.03-0.04 percent); C4 saturates near 360 microlitre per litre, C3 beyond 450. Temperature: C4 plants favour higher temperature optima than C3. Water: acts indirectly by closing stomata (reducing CO2) and causing wilting.