Chapter 5 Photosynthesis 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 + 6H2OC6H12O6 + 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 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 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 HO 4 H + 4 e + O Cyclic Electron Transport
PI can function independently from PII Produces additional ATP molecules Reduction of COrequires ATP Light-Independent Reactions
Carbon Fixation Series of 11 enzyme-catalyzed reactions NADPH reduces CO into sugars Overall process is endergonic ATP is hydrolyzed to supply energy of reactions Divided into three phases Fixation Reduction Regeneration Calvin Cycle: Fixation: C Metabolism
CO2 is attached to 5-carbon RuBPmolecule Result in a 6-carbon molecule This splits into two 3-carbonmolecules (3PG) Reaction accelerated by RuBPCarboxylase (Rubisco) CO2 now fixed because it is part of acarbohydrate 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) 3CO 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 CO % of atmosphere Rubisco can also catalyze O Slows Calvin Cycle, consumes ATP, releases carbon photorespiration Decrease carbon fixation up to 50% Stomata Hot dry climates Low levels of CO C Metabolism C Plants Minimize photorespiration Calvin Cycle C Cycle
Performed by bundle-sheath cells Separates exposure of Rubisco to O C Cycle CO 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 CO C vs C C Can open stomata less Require 1/3 to 1/6 as much rubisco
Lower nitrogen demand Run Calvin cycle and C cycles simultaneously CAM Plants Crassulacean Acid Metabolism
Run Calvin Cycle and C at different time of the day C - night Calvin Cycle day