Structure of chlorophyll molecule Long, non-polar hydrophobic “tail” anchors in lamellar membranes Bound to membranes “head” has a complex ring with Mg2+ at center Where light energy is trapped

Accessory Pigments Absorb wavelengths that chlorophyll can’t Pass along energy into photosynthetic process  plants more efficient this way Do not waste wavelengths Note that chlorphyll a, b, c each have slightly different spectrum, so each can use a different part of the light

Accessory Pigments Phycobilins Found in red algae & blue-green bacteria Phycoerythrin: (red) max absorption @ 550 nm Phycocyanin: (blue) max absorption @ 620 nm

Accessory Pigments Carotenoids Protect chlorophylls from damage by bright light Take in certain λ, transfer energy to chlorophyll a

Light Reactions Overview System I responds to λ of 700 nm Molecule is called P700 System II responds to λ of about 680 nm Molecule is called P680 BOTH reactions (both λ) are necessary for photosynthesis

Notice that Photosystem I is the SECOND molecule in the chain… This is only because that was the first one discovered, and they noticed that the electrons were being brought to P700 from somewhere else… the search begins!

Electron Transport Chain Light Reactions Photosystem I 700 nm light strikes P700 2e- released NADP+ reduced to NADPH (stored in stroma) NADPH contains energy NADP = nicotinamide adenine dinucleotide phosphate

Electron Transport Chain Light Reactions Photosystem II 680 nm light strikes P680 H2O splits, O2 released 2e- released e- goes to plastiquinone e- transferred to ETC Pumps H into lumen NADP = nicotinamide adenine dinucleotide phosphate

Photosystem I Light strikes P700 Electron released NADP reduced to NADPH (stored I stroma) Photosystem II Light strikes P680 Electron goes to plastiquinone (Pq), carrying H into lumen Electron goes into ETC then eventually back into Photosystem I

Electron Transport Chain Light Reactions Manganese (Mn)-complex give electrons back to P680 by splitting 2 water molecules 4 photons need to split 2 water molecules

Summary: Light Reactions NADPH stored in stroma H+ pumped into thylakoid reservoir (lumen) Creates electrochemical gradient for ATP synthesis

Chemiosmotic Theory of ATP formation (noncyclic flow) NADP+ PSII PSI NADPH lumen ATP synthase thylakoid membrane stroma ATP ADP

Cyclic flow Note that only PSI is used. The electrons cycle back from Fd to the cytochrome complex and from there continue on to a P700 chlorophyll. The purpose is to make more ATP