Chapter 5 Photosynthesis http://www.youtube.com/watch?v=sQK3Yr4Sc_k

Photosynthesis 6CO2 + 6H2O → C6H12O6 + 6O2 Converting light energy into chemical energy to assemble organic molecules Two stages Light-dependant reactions Absorption of photons of light PI and PII Light-independent reactions (Calvin Cycle) Does not require light 6CO2 + 6H2O → C6H12O6 + 6O2

Photosynthesis Photosynthesis takes place in the green portions of plants Leaf of flowering plant contains mesophyll tissue Cells containing chloroplasts Specialized to carry on photosynthesis CO2 enters leaf through stomata Diffuses into chloroplasts in mesophyll cells In stroma, CO2 fixed to C6H12O6 (sugar) Energy supplied by light

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

Photosynthetic Stages Light-dependant reactions Occur in the thylakoid membranes capture energy from sunlight make ATP and reduce NADP+ to NADPH Carbon fixation reactions (light-independent reactions) Occurs in the stroma use ATP and NADPH to synthesize organic molecules from CO2

Photosynthesis

Capturing Light Energy Pigments Absorb photon Excited electron moves to a high energy state Electron is transferred to an electron accepting molecule – primary electron acceptor

Pigments Chlorophyll a Accessory pigments A pigment molecule does not absorb all wavelengths of light Chlorophyll a Donates electron to PEA Accessory pigments Chlorophyll b Carotenoids Known as antenna complex Transfers light energy to chlorophyll a

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 moelcules and PEA PI - Contains p700 PII - Contains p680

How Photosystem I and II Work Trap photons of light Energy trapped used to energize chlorophyll a molecule in reaction centre Chlorophyll a is oxidized (loses electrons) Transfers electrons to PEA Water molecule donates electron chlorophyll a lost Transported through electron transport chain High energy electrons are used to drive ATP synthesis and assemble glucose molecules http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120072/bio13.swf::Photosynthetic%20Electron%20Transport%20and%20ATP%20Synthesis

How Photosystem I and II Work

Light Dependant Reactions Photosystem II

Linear Electron Transport and ATP Synthesis

The Role of Light Energy Z scheme Two photons of light needed for production of NADPH

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

Cyclic Electron Transport PI can function independently from PII Produces additional ATP molecules 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: C₃ Metabolism CO2 is attached to 5-carbon RuBP molecule Result in a 6-carbon molecule This splits into two 3-carbon molecules (3PG) Reaction accelerated by RuBP Carboxylase (Rubisco) CO2 now “fixed” because it is part of a 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

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% Stomata Hot dry climates Low levels of CO₂

C₄ Metabolism

C₄ Plants 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

C₄ vs C₃ C₄ Can open stomata less Require 1/3 to 1/6 as much rubisco Lower nitrogen demand Run Calvin cycle and C₄ cycles simultaneously

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