Photosynthesis

light-dependent reactions overview

• take place in thylakoid membrane, converting light energy to chemical energy in the form of ATP and NADPH

• photoactivation and photolysis (oxidation) in photosystem II

• electron transport chain and proton pump

• reduction in photosystem I

• chemiosmosis

• net production of O2, NADPH, and ATP

photosystem structure

• light-harvesting protein complex with accessory pigments (e.g. chlorophyll b, carotenoids, xantophylls, pheophytins)

• reaction center - two chlorophyll a molecules

• found in thylakoid membrane of chloroplasts of plants, algae, or cyanobacteria

advantage of antenna pigment molecules being close and in precise orientation to one another

photons of light avoid traveling large distances and disappearing, instead allowing for the seamless transition of energy

advantage of different pigment molecules

different pigments absorb different ranges of wavelengths and surround one chlorophyll pigment reaction center, ensuring the most efficient absorption of energy at any time of the day/season

photosystem function

• accessory pigments in photosystem absorb light, exciting electrons

• electrons drop back down to original state and release energy, exciting electrons in adjacent pigment molecules

• process of excitation energy transfer repeated across antenna pigments until reaction center chlorophyll is reached

• in the reaction center, electrons transferred to electron acceptor (main electron acceptor NADP+)

photosystem II (PSII)

wavelength absorption peak at 680 nm, strongest biological oxidizing agent (photolysis)

photolysis of water

H2O molecules split by light, creating H+ protons and waste product O2

electron transport chain

passes excited electrons from primary acceptor chlorophyll a of PSII along several electron carriers (intermembrane proteins) to the primary acceptor of PSI

proton pump as a result of the electron transport chain

energy from photoactivated electrons used to pump protons across thylakoid membrane from stroma into lumen, causing the accumulation of H+ ions within the thylakoid (concentration gradient)

photosystem I (PSI)

wavelength absorption peak at 700nm, strongest biological reducing agent

• activated electrons received by carrier ferredoxin, reducing NADP+ → NADPH to be used in light independent reaction

chemiosmosis

• high concentration of H+ ions in thylakoid lumen

• diffusion of H+ ions through ATP synthase to stroma generates ATP

alternative path of photoexcited electrons

• occurs when light is not the limiting factor - there is an accumulation of NADPH in the chloroplast

• photoexcited electrons become cyclic, synthesize ATP more rapidly through chemiosmosis, but no NADPH

light independent/carbon fixation reaction/Calvin cycle

take place in stroma of chloroplasts, converting CO2 into sugars using energy synthesized in light dependent reactions

1. carbon fixation

2. synthesis of triose phosphate

3. regeneration of RuBP

carbon fixation

enzyme RuBisCo attaches ribulose bisphosphate (RuBP, 5C) with CO2 to fix carbon and produce intermediate products 2 glycerate 3-phosphate (3C each, 6C total)

synthesis of triose phosphate

• 2 glycerate 3-phosphate (3C each) —(2ATP→2ADP)→ 2 bisphosphoglycerate (3C each)

• 2 bisphosphoglyceratae (3C each) —(2NADPH→2NADP+)→ 2 triose phosphate (3C each

regeneration of RuBP

2 triose phosphate (3C each) —(ATP→ADP)→ RuBP (5C)

Calvin Cycle math

• oxidized NADP+ and ADP molecules per CO2 molecule return to thylakoids to participate in light dependent reactions

• 6 cycles needed to fix 6CO2 from C6H12O6

starch granules in the chloroplast under electron microscope

large, transparent blobs

lipids in the chloroplasts under electron microscope

oil droplets appearing as dark spheres