photosynthesis

• Photosynthesis equation: 6CO2 + 6H2O → C2H12O6 + 6O2
• Plant pigments include chlorophyll and carotenoids
• The light-dependent reaction ends when energy is absorbed by P680 and P700, which are chlorophyll a molecules Note the PSII and PSI, which are the same as P680 and P700, respectively
• P700 forms PS1
• P680 forms PSII
• Antenna pigments capture pigments that chlorophyll a does not They pass energy to chlorophyll a, where a direct light reaction occurs
• Antenna pigments include chlorophyll b, carotenoids, and phycobilins Phycobilins are a red algae pigment
• Chlorophyll a has double bonds, which are critical for light reactions Chlorophyll a contains a porphyrin ring that has double bonds for light reactions and complexes with Mg:
• Noncyclic photophosphorylation is ADP + Pi + light → ATP, also known as the light-dependent reaction Pi is an inorganic phosphate (highlighted in yellow):
• In chloroplasts, H+ accumulates in the thylakoid lumen
• Stroma is fluid inside the chloroplast
• The Calvin cycle occurs in the stroma
• Noncyclic photophosphorylation takes place on the thylakoid membrane
• Cyclic photophosphorylation takes place on stroma lamellae
• Photolysis takes place in the thylakoid lumen
• Non-cyclic photophosphorylation Step 1: Electrons trapped by P680 in PSII are energized by light
• Non-cyclic photophosphorylation Step 2: Excited electrons passed to primary electron acceptor Image by Somepics, CC BY-SA 4.0
• Non-cyclic photophosphorylation Step 3: ETC, similar to oxidative phosphorylation, produces 1.5 ATP per 2 electrons
• Non-cyclic photophosphorylation Step 4: ETC terminates at PS1 or P700, and electrons are re-energized by light
• Non-cyclic photophosphorylation After electrons are re-energized in Step 4, electrons can enter non-cyclic or cyclic pathways Image by Somepics, CC BY-SA 4.0
• Non-cyclic photophosphorylation Step 5: If electrons go into the non-cyclic path, they go through another ETC and produce NADPH from NADP+ and H+
• Non-cyclic photophosphorylation Step 6: Electrons lost in Step 2 are replaced by water splitting into H+ and O2 Note that this would take 2 molecules of H2O
• Non-cyclic photophosphorylation takes place on the thylakoid membranes Image by Somepics, CC BY-SA 4.0
• Photolysis takes place inside the thylakoid lumen From there, electrons go to the membrane for noncyclic photophosphorylation
• The Calvin cycle takes place in the stroma
• Chemiosmosis takes place across the thylakoid membrane Chemiosmosis is using the H+ gradient to generate ATP
• Thylakoid membranes absorb light, not the chloroplast membranes!
• Cyclic photophosphorylation replenishes ATP when the Calvin cycle consumes it
• In cyclic phosphorylation, two excited electrons from PSI join with protein carriers in the first ETC to generate 1 ATP
• In cyclic phosphorylation, after the electrons help generate ATP, they are recycled into PSI and continue to have an option between cyclic and non-cyclic pathways
• The Calvin cycle is a dark reaction that produces glucose out of CO2
• The Calvin cycle Step 1: 6 CO2 join with 6 RuBP to form 12 PGA
• Calvin cycle carboxylation is catalyzed by RuBisCO
• In the reduction step of the Calvin cycle, 12 ATP + 12 NADPH is required to convert 12 PGA into 12 G3P (PGAL)
• NADP+ and ADP produced from the reduction step of the Calvin cycle end up in non-cyclic photophosphorylation
• The regeneration step of the Calvin cycle involves 6 ATP converting 10 G3P into 6 RuBP, allowing the cycle to repeat
• Carbohydrate synthesis in the Calvin cycle uses the 2 remaining G3P from reduction to build glucose
• The overall equation for photosynthesis is: 6 CO2 + 18 ATP + 12 NADPH + H+ → 18 ADP + 18 Pi + 12 NADP++ 1 glucose (via 2 G3P) Recall that Pi means inorganic phosphate
• The Calvin cycle is also known as the light-independent reactions
• Indirectly, the Calvin cycle depends on light because ATP and NADPH (high-energy molecules) produced in the light-dependent reactions end up in the Calvin cycle Note the input of ATP and NADPH in the Calvin cycle: