Chapter 10 Photosynthesis

Importance of Photosynthesis Plant: Glucose for respiration Cellulose Global: O2 Production Food source

Photosynthesis Overview Energy for all life on Earth ultimately comes from photosynthesis. 6CO2 + 12H2O C6H12O6 + 6H2O + 6O2 Photoautotrophs: use light energy to make organic molecules Heterotrophs: consume organic molecules from other organisms for energy and carbon Redox Reaction: water is split  e- transferred with H+ to CO2  sugar Remember: OILRIG Oxidation: lose e- Reduction: gain e-

Photoautotrophs Oxygenic photosynthesis is carried out by: cyanobacteria, 7 groups of algae, all land plants

Chloroplasts: sites of photosynthesis in plants thylakoid membrane – internal membrane arranged in flattened sacs contain chlorophyll and other pigments grana – stacks of thylakoid membranes stroma – semiliquid substance surrounding thylakoid membranes

Sites of Photosynthesis mesophyll: chloroplasts mainly found in these cells of leaf stomata: pores in leaf (CO2 enter/O2 exits) chlorophyll: green pigment in thylakoid membranes of chloroplasts

Discovery of Photosynthesis Jan Baptista van Helmont – mass not completely from soil Joseph Priestly – plants “renew” air Jan Ingenhousz – leaves use sunlight to restore air, give off oxygen F.F. Blackman – multistage process

C. B. van Niel, 1930’s -proposed a general formula: CO2+H2A + light energy CH2O + H2O + 2A where H2A is the electron donor -van Niel identified water as the source of the O2 released from photosynthesis -Robin Hill confirmed van Niel’s proposal that energy from the light reactions fuels carbon fixation

Photosynthesis = Light Reactions + Calvin Cycle

photon: a particle of light Nature of sunlight Light = Energy = electromagnetic radiation Shorter wavelength (λ): higher energy Visible light - detected by human eye Light: reflected, transmitted or absorbed photon: a particle of light acts as a discrete bundle of energy energy content of a photon is inversely proportional to the wavelength of the light

Electromagnetic Spectrum

Pigments Pigments absorb different λ of light Each pigment has a characteristic absorption spectrum, the range and efficiency of photons it is capable of absorbing. photoelectric effect: removal of an electron from a molecule by light occurs when photons transfer energy to electrons Interaction of light with chloroplasts

Chlorophyll Pigments Chlorophyll a Chlorophyll b The main photosynthetic pigment Primarily absorb red and blue light (reflect green light) Chlorophyll b The accessory photosynthetic pigment

Photosynthetic pigments chlorophyll – absorb violet-blue/red light, reflect green chlorophyll a (blue-green): light reaction, converts solar to chemical E chlorophyll b (yellow-green): conveys E to chlorophyll a carotenoids (yellow, orange): photoprotection, broaden color spectrum for photosynthesis Types: xanthophyll (yellow) & carotenes (orange) anthocyanin (red, purple, blue): photoprotection, antioxidants

Electrons in chlorophyll molecules are excited by absorption of light

Photosynthesis Broken into 2 phases Phase I : light-dependent reaction Light energy splits H2O to O2 releasing high energy electrons (e-) Movement of e- used to generate ATP Electrons end up on NADP+, reducing it to NADPH Phase II: light-independent reactions ATP and NADPH that were formed in phase I are used to make glucose

Photosystem: reaction center & light-harvesting complexes (pigment + protein)

In chloroplasts, two linked photosystems are used in noncyclic (linear) photophosphorylation 1. photosystem I -reaction center pigment (P700) with a peak absorption at 700nm 2. photosystem II -reaction center pigment (P680) has a peak absorption at 680nm

Light Dependent Reactions Photosystem II acts first Reaction center of PSII absorbs a photon Electron in P680 is excited which transfers this electron to an acceptor Electrons are removed from oxygen in water molecules (water is split) Produces 1 O2 in the process Electrons replace those lost when excited by light

Electron Acceptors 3. Primary initial acceptor is quinone molecule Turns into plastiquinone – an electron donor 4. Transfers electrons to b6-f complex embedded within thylakoid membrane Resembles bc1 from respiration Series of electron carriers 5. b6-f complex pumps a proton into the thylakoid space 6. Plastocyanin then transfers electron to photosystem I

7. e- passed to Photosystem I via ETC Ferredoxin is used to transfer electron (iron sulfur compound) 8. Electrons are used to change NADP+ reduced to NADPH NADP reductase catalyzes reaction MAIN IDEA: Use solar energy to generate ATP & NADPH to provide energy for Calvin cycle

Light-Dependent Reactions ATP is produced via chemiosmosis. ATP synthase is embedded in the thylakoid membrane protons have accumulated in the thylakoid space protons move into the stroma only through ATP synthase ATP is produced from ADP + Pi

Cyclic Electron Flow: uses PS I only; produces ATP for Calvin Cycle (no O2 or NADPH produced) - electron creates ATP instead of NADPH - goes into the b6-f complex

Both respiration and photosynthesis use chemiosmosis to generate ATP

Chemiosmosis Protons accumulate in the thylakoid space, creating a concentration gradient Electrons flow down the concentration gradient to produce ATP The breakdown of water to start PHOTOSYSTEM II  is essential for providing the e- that initiate the ETC and providing the protons necessary to drive the ATP synthesis for chemiosmosis

Proton motive force generated by: H+ from water H+ pumped across by cytochrome Removal of H+ from stroma when NADP+ is reduced

Calvin Cycle: Uses ATP and NADPH to convert CO2 to sugar Occurs in the stroma Uses ATP, NADPH, CO2 Produces 3-C sugar G3P (glyceraldehyde-3-phosphate) Three phases: Carbon fixation Reduction Regeneration of RuBP (CO2 acceptor)

Phase 1: 3 CO2 + RuBP (5-C sugar ribulose bisphosphate) Catalyzed by enzyme rubisco (RuBP carboxylase)

Phase 2: Use 6 ATP and 6 NADPH to produce 1 net G3P

Phase 3: Use 3 ATP to regenerate RuBP

Alternative mechanisms of carbon fixation have evolved in hot, arid climates Photorespiration Metabolic pathway which: Uses O2 & produces CO2 Uses ATP No sugar production (rubisco binds O2  breakdown of RuBP) Occurs on hot, dry bright days when stomata close (conserve H2O) Why? Early atmosphere: low O2, high CO2?

Evolutionary Adaptations Problem with C3 Plants: CO2 fixed to 3-C compound in Calvin cycle Ex. Rice, wheat, soybeans Hot, dry days: partially close stomata, ↓CO2 Photorespiration ↓ photosynthetic output (no sugars made)

C4 Plants: CO2 fixed to 4-C compound Ex. corn, sugarcane, grass Hot, dry days  stomata close 2 cell types = mesophyll & bundle sheath cells mesophyll : PEP carboxylase fixes CO2 (4-C), pump CO2 to bundle sheath bundle sheath: CO2 used in Calvin cycle ↓photorespiration, ↑sugar production WHY? Advantage in hot, sunny areas

C4 Leaf Anatomy

CAM Plants: Crassulacean acid metabolism (CAM) NIGHT: stomata open  CO2 enters  converts to organic acid, stored in mesophyll cells DAY: stomata closed  light reactions supply ATP, NADPH; CO2 released from organic acids for Calvin cycle Ex. cacti, pineapples, succulent (H2O-storing) plants WHY? Advantage in arid conditions

Comparison C3 C4 CAM C fixation & Calvin together C fixation & Calvin in different cells C fixation & Calvin at different TIMES Rubisco PEP carboxylase Organic acid

Review of Photosynthesis

Comparison RESPIRATION PHOTOSYNTHESIS Plants + Animals Needs O2 and food Produces CO2, H2O and ATP, NADH Occurs in mitochondria membrane & matrix Oxidative phosphorylation Proton gradient across membrane Plants Needs CO2, H2O, sunlight Produces glucose, O2 and ATP, NADPH Occurs in chloroplast thylakoid membrane & stroma Photorespiration Proton gradient across membrane