Photosynthesis Conversion of light energy from the sun into stored chemical energy in the form of glucose and other organic molecules

Site of Photosynthesis Photosynthesis takes place in mesophyll tissue Cells containing chloroplasts Specialized to carry out photosynthesis CO2 enters leaf through stomata (pore) Exchange of gases occurs here Controlled by guard cells (opening/closing) CO2 diffuses into chloroplasts CO2 fixed to C6H12O6 (sugar) Energy supplied by light

Chloroplasts Site of Photosynthesis Consists of Stroma Aqueous environment Houses enzymes used for reactions Thylakoid membranes Form stacks of flattened disks called grana Contains chlorophyll and other pigments

Photosynthesis 2 stages Light-dependant reactions Photosystem II and I Occurs in the thylakoid membrane of chloroplasts capture energy from sunlight make ATP and reduce NADP+ to NADPH 2. Calvin Cycle (light-independent reactions) Occurs in stroma of chloroplast use ATP and NADPH to synthesize organic molecules from CO2

Capturing Light Energy Pigments Absorb photon (wave of light) Excited electron moves to a high energy state Electron is transferred to an electron accepting molecule (primary electron acceptor) Chloryphyll a donates electrons to PEA

Accessory Pigments Chlorophyll b and carotenoids Known as antenna complex Transfers light energy to chlorophyll a Chloryphyll donates electrons to PEA A pigment molecule does not absorb all wavelengths of light

Pigments Photosynthesis depends on the absorption of light by chlorophylls and carotenoids

Pigments and Photosystems Chlorophylls and carotenoids do not float freely within thylakoid Bound by proteins Proteins are organized into photosystems Two types Photosystem I Photosystem II

Photosystem I and II Composed of Reaction Centre PI - Contains p700 Large antenna complex 250-400 pigment molecules surrounding reaction centre Reaction Centre Small number of proteins bound to chlorophyll a molecules and PEA PI - Contains p700 PII - Contains p680

Photosystem II Oxidation of p680 Oxidation-reduction of platiquinone Photon absorbed excites p680 Transfers e⁻ to PEA e⁻ supplied by splitting of a water molecule inside lumen Oxidation-reduction of platiquinone PEA transfers e⁻ to plastiquinone Plastiquinone shuttles electrons between PII and cytochrome complex responsible for increase proton concentration in thylakoid lumen 3. Electron transfer to PI Cytochrome complex transfers e⁻ to plastocyanin Plastocyanin Shuttles electrons from cytochrome complex to PI

Photosystem I Oxidation-reduction of p700 Photon absorbed excites p700 p700 transfers electron to PEA P700⁺ forms ready to accept another e⁻ from plastocyanin Electron transfer to NADP⁺ by ferredoxin PEA transfer e⁻ to ferredoxin Ferredoxin Iron-sulfur protein Oxidation of ferredoxin reduces NADP⁺ to NADP Formation of NADPH Ferredoxin transfers second e⁻ and H⁺ NADP⁺ reductase reduces NADP to NADPH

Linear Electron Transport and ATP Synthesis

The Role of Light Energy Z scheme Two photons of light needed for production of NADPH p700 molecule too electronegative to give up e⁻ Second photon needed to move e⁻ further away from nucleus of p700 so it can transfer to NADP⁺

Oxygen How many photons of light are needed to produce a single molecule of oxygen? 2 H₂O → 4 H⁺ + 4 e⁻ + O₂

Chemiosmosis and ATP Synthesis Proton gradient inside lumen increases e⁻ transfer by plastoquinone between PII and cytochrome complex Water molecule splitting inside lumen Removal of H⁺ from stroma for each NADPH molecule produced Proton-motive force created inside thylakoid lumen ATP synthase uses proton-motive force to synthesize ATP molecule

Cyclic Electron Transport PI can function independently from PII Ferredoxin does not transfer e⁻ to NADP⁺ Ferredoxin transfers e⁻ back to plastoquinone Plastoquinone continually moves protons into thylakoid lumen Splitting of water molecule not needed Produces additional ATP molecules (photophosphorylation) Reduction of CO₂ requires ATP

Light-Independent Reactions Carbon Fixation Series of 11 enzyme-catalyzed reactions NADPH reduces CO₂ into sugars Overall process is endergonic ATP is hydrolyzed to supply energy of reactions Divided into three phases Fixation Reduction Regeneration

Calvin Cycle: Fixation CO₂ is attached to 5C RuBP molecule 6C molecule is produced 6C splits into 2 3C molecules (3PG) RuBisco RuBP carboxylase Most abundant protein on earth Involvd in first major step of carbon fixation CO₂ is now fixed Becomes part of carbohydrate

Calvin Cycle: Reduction Two 3PG is phosphorylated ATP is used Molecule is reduced by NADPH Two G3P are produced

Calvin Cycle: Regeneration RuBP is regenerated for cycle to continue Takes 3 cycles Produces 3 RuBP molecules Process (3 turns of cycle) 3CO₂ combine with 3 molecules of RuBP 6 molecules of 3PG are formed 6 3PG converted to 6 G3P 5 G3P used to regenerate 3 RuBP molecules 1 G3P left over

Glyceraldehyde-3-phosphate (G3P) Ultimate goal of photosynthesis Raw material used to synthesize all other organic plant compounds (glucose, sucrose, starch, cellulose) What is required to make 1 molecule of G3P? 9 ATP 6 NADPH What is required to make 1 molecule of glucose? 18 ATP 12 NADPH 2 G3P

Alternate Mechanisms of Carbon Fixation Problems with photosynthesis Not enough CO₂ - 0.04% of atmosphere Rubisco can also catalyze O₂ Slows Calvin Cycle, consumes ATP, releases carbon (photorespiration) Decrease carbon fixation up to 50% Wasteful to cell Costs 1 ATP and 1 NADPH Stomata Hot dry climates – closes to prevent water loss Low levels of CO₂

C₄ Cycle Minimize photorespiration Calvin Cycle C₄ Cycle Performed by bundle-sheath cells Separates exposure of Rubisco to O₂ C₄ Cycle CO₂ combines with PEP (3 carbon molecule) Produces oxaloacetate (4 carbon molecule) Oxaloacetate reduced to malate Malate diffuses into bundle-sheath cells and enters chloroplast Malate oxidized to pyruvate releasing CO₂

Benefits of C4 Plants Can open stomata less Require 1/3 to 1/6 as much rubisco Lower nitrogen demand Run C3 and C4 cycles simultaneously Corn

CAM Plants Cactus Crassulacean Acid Metabolism Run Calvin Cycle and C4 at different time of the day C4 - night Calvin Cycle – day Cactus