BIOL 205 Photosynthesis Lecture 1 Introduction and the

BIOL 205 : : Photosynthesis Lecture 1 Introduction and the light reactions

What is photosynthesis? • The process of converting solar energy into chemical energy. • Can use only water and carbon dioxide to create sugars = chemical energy. • Responsible for removal of ~ 200 billion tons of C from the atmosphere yearly.

What is photosynthesis? 6 CO 2 + 12 H 2 O + hv C 6 H 12 O 6 +6 O 2 +6 H 2 O • hv designates light • you should memorize this equation!! you will see it again O 2 = oxygenic photosynthesis

What is photosynthesis? • 2 stages of photosynthesis – Light reactions and Dark reactions • Light reactions convert sunlight into chemical energy (ATP + NADPH) • Dark reactions use those products to form sugars (stored chemical energy)

Overview of Photosynthesis CO 2 Dark Reactions ATP NADPH H 2 O + CO 2 Sugars Light Reactions O 2

Where does photosynthesis take place? • Prokaryotes have both anoxygenic and oxygenic • Cyanobacteria have oxygenic -Photosynthesis on thylakoids (from plasma membrane) -Cyanobacteria are source of eukaryotic photosynthesis • Eukaryotes • Oxygenic only • Takes place in chloroplasts

Chloroplast Morphology

Chloroplast Morphology-Terms • • • Inner Membrane Outer Membrane Thylakoid lumen Stroma Granum

How can light provide energy for plants? • Light is composed of particles: photons • Light behaves like a wave – Can e described w/ wavelength & frequency • Only a small portion of the electromagnetic spectrum.

The electromagnetic spectrum PAR = photosynthetically available radiation

Pigments • Pigment = a light absorbing molecule • Associated with the thylakoid membranes • Chlorophyll – Chl a and Chl b (Chl c in some algae) • Xanthophylls • Carotenoids – ß-carotene

Chlorophyll • Chl a has a methyl group • Chl b has a carbonyl group Porphyrin ring delocalized e- Phytol tail

Different pigments absorb light differently

Different pigments absorb light differently

2 -minute quiz 1. What is the relationship between the structure of chlorophyll and its location in the chloroplast? 2. Why are plants green?

Light Reactions 1: Light capture and redox • 2 spatially & functionally distinct units = Photosystems • Photosystem II = 1 st stage • Photosystem I = 2 nd stage • Named after order of discovery

Light capture 1 1. Most Chlorophyll is located in the Light Harvesting Complex 2. Sunlight is absorbed in the LHC and is passed from pigment to pigment Chl a Chl b ß-carotene LHC Remember the porphyrin ring? Reaction Center

Light capture - 2 3. Energy finally ends up in a pair of special chlorophyll a molecules: P 680 e -* 4. e- in P 680 Chl a goes to excited state and is shed = Charge separation LHC * Reaction Center P 680 chlorophylls Optimal @ <680 nm

Light capture - 3 e -* Q + LHC Reaction Center 5. High-energy eaccepted by quinone Q = primary e- acceptor e- transport chain 6. Q has been reduced; P 680 Chl a has been 7. The excited e- is oxidized shunted into the electron transport Oxidation by light = chain photo-oxidation

Light capture - 4 8. The O 2 evolving complex + Chl a+ strip efrom H 2 O and reduce Chl a+ = most powerful biological oxidizing agent + O 2 evolving complex e. H 2 O 1/2 O 2 4 H+

Light capture - 5 9. The reaction center is reset and ready to go again

Light Capture Movie

PSII electron transport chain H+ H+ 1. Q accepts 2 e- from Q P 680 and removes 2 H+ e- efrom the stroma e- e- cytochrome b/f 2. Q passes the e- to cytochrome b/f complex & pumps the 2 H+ into thylakoid lumen complex 3. As the e- moves through b/f more H+ are pumped into lumen

The PS II Electron transport chain H+ H+ Q e- ecytochrome b/f complex e- 2 H+ H+ plastocyanin

PS II to PS I eplastocyanin 4. e- end up on plastocyanin: a soluble electron carrier in the lumen 5. Plastocyanin serves as e- donor for PSI reaction center Chl a eplastocyanin

Photosystem I e-* Optimal wavelength = 700 nm + eplastocyanin 1. Charge separation and photo-oxidation are similar to PSII P 700 2. Plastocyanin acts as reducing agent on P 700 Chl a

PSI electron transport chain Ferredoxin Fe/S e- 1. Ferredoxin recieves e- from P 700* 2. e- moves throuh FAD (flavin adenine dinucleotide) FAD-NAPD Reductase H+ e. NADP+ NADPH 3. e- plus stromal H+ are used to reduce NADP+ to NADPH = FINAL PRODUCT!

PSI electron transport chain e- Ferredoxin Fe/S ee. FAD-NAPD Reductase H+ e. NADP+ NADPH

ATP synthesis H+ stroma H+ H+ H+ 2. Q and Cyt b/f pump H+ from stroma into lumen H+ 1. O 2 evolving complex liberates H+ into lumen from water H+ H+ 3. NADP+ scavenges protons from the stroma p. H separation across membrane = Proton Motive Force!

ATP synthesis ADP + Pi F 0 F 1 complex ATP stroma H+ H+ lumen H+ H+ H+ The energy released as protons travel down their concentration gradient is used to fuel an ATP synthase

Light Reactions Products In: CO 2, H 2 O, sunlight Out: O 2, ATP, NADPH