Photosynthesis Chapter 6

Autotrophs Sustain themselves without eating other organisms Produce organic molecules Photoautotrophs Light Photosynthesis Plants, protists and prokaryotes Chemoautotrophs Chemical

Chloroplasts Found in leaves, stems, unripen fruit Chlorophyll Green pigment inside chloroplasts Absorbs energy Drives synthesis of food Found in mesophyll Tissue part of the inside of leaves

Nutrient pathway Stomata Veins Openings on leaves Water absorbed Allows O2 to enter and CO2 to exit Veins Water absorbed Export sugar to roots and other parts of the leaves

Mesophyll cell 30 – 40 chloroplasts Stroma Thylakoid Dense fluid Interconnected system of compartments Encloses lumen Stacked into columns Called grana

Leaf cross section Chloroplasts Vein Mesophyll Stomata CO2 O2 Chloroplast Mesophyll cell Outer membrane Thylakoid Intermembrane space Stroma Granum 20 m Thylakoid space Inner membrane 1 m

Pathway of photosynthesis 6CO2 + 12H2O → C6H12O6 + 6O2 + 6H2O Two stages

Light Reaction Solar energy into chemical energy Electrons and hydrogen from H2O to NADP+ Water is split Gives off O2 Reduced NADP to NADPH addition of an electron pair and H+ Photophosphorylation ADP + phosphate group > ATP Light energy initially transformed to chemical energy Occurs in thylakoids

Calvin Cycle Carbon Fixation Reduces fixed carbon into carbohydrate Incorporates Co2 into organic molecules Reduces fixed carbon into carbohydrate Produces sugar Reduced by NADPH Energizes electrons Needs both NADPH and ATP Don’t need light directly Mostly occurs in daylight Light reaction needed for NADPH and ATP production Occurs in the stroma

Sunlight Electromagnetic energy Wavelength Spectrum 380nm – 750 nm = visible light Spectrum

Light Reaction Photon Atmosphere only allows visible light through Interactions of light with matter Light particles Quantity of energy Atmosphere only allows visible light through Drives photosynthesis

Light Absorption Can only use absorbed light wavelengths Light seen is light reflected Pigments differ in absorption spectrum

Engelmann’s experiment Action spectrum Rate of photosynthesis at different wavelengths Not = absorption spectrum Only chlorophyll directly participates Accessory pigments can give energy collected from different wavelengths to Ca Chlorophyll b Carotenoids Photo protection Absorb and dissipate excess potentially harmful energy

Excited electrons Absorbing photons Electron excited to next orbital More potential energy Return to original orbital Release light or heat Excited state Heat e Photon (fluorescence) Ground state Photon Chlorophyll molecule Energy of electron Excitation of isolated chlorophyll molecule

Photosystem Light harvesting unit Antenna complex Reaction center Collection of chloroplast a, chloroplast b and carotenoids Deliver energy to chloroplast a Reaction center Location of specialized chlorophyll a Only functions properly within a photosystem Starts light reaction Primary electron acceptor Reduced by chlorophyll a

Chloro- phyll a Chlorophyll b Absorption of light by chloroplast pigments Carotenoids (a) Absorption spectra 400 500 600 700 Wavelength of light (nm) Rate of photosynthesis (measured by O2 release) (b) Action spectrum 400 500 600 700 Aerobic bacteria Filament of alga Engelmann’s experiment (c) 400 500 600 700

Types of photosystems Photosystem I Photosystem II P700 center Photosystem II P680 center Associated with different proteins

THYLAKOID SPACE (INTERIOR OF THYLAKOID) Photosystem STROMA Photon Light- harvesting complexes Reaction- center complex Primary electron acceptor Chlorophyll STROMA e Thylakoid membrane Thylakoid membrane Transfer of energy Special pair of chlorophyll a molecules Pigment molecules Protein subunits THYLAKOID SPACE (INTERIOR OF THYLAKOID) THYLAKOID SPACE (a) How a photosystem harvests light (b) Structure of photosystem II

Noncyclic electron flow Uses photosystems I and II Generates ATP and NADPH Equal amounts Electrons pass from H2O to NADPH

Electron transport chain Primary acceptor Primary acceptor 4 7 Electron transport chain Fd Pq e 2 e 8 e e H2O NADP 2 H Cytochrome complex NADP reductase + H + 3 1/2 O2 NADPH Pc e e P700 5 P680 Light 1 Light 6 ATP Pigment molecules Photosystem I (PS I) Photosystem II (PS II)

Steps Photosystem II absorbs light Chlorophyll a oxidized Enzyme used electrons from water to replace Ca’s e- Splits water and adds O together > O2 Electron transferred to photosystem I Electron transport chain Noncyclic photophosphorylation Chemiosmosis Uses energy produced by ETC to power ADP > ATP Powers the Calvin Cycle

Steps Electron is supplied to chlorophyll a in Photo I Electron collected by primary electron acceptor undergoes the ETC NADP+ reductase transfers electron to NADP+ NADPH used in Calvin Cycle Reducing agent

Cyclic electron flow Simpler pathway Only uses photosystem I Chemiosmosis Cyclic photophosphorylation Generates ATP Calvin cycle uses more ATP than NADPH NADPH concentration might regulate which pathway is used

Chemiosmosis in chloroplasts H+ pumped into Thylakoid space Proton gradient H+ used by ATP synthase to drive combination of ADP and a phosphate group Pumped back into the stroma

Calvin Cycle Uses ATP and NADPH to convert CO2 to sugar In stroma 3 CO2 > sugar Glyceraldehyde – 3 – phosphate Cycles 3 times to fix carbon thrice

Calvin Cycle Carbon fixation Reduction Regeneration of Rubisco Enzyme rubisco attaches CO2 to 5 carbon sugar 6 carbon intermediate > 3 phosphoglycerates Reduction Phosphoglycerate + phosphate group from ATP > bisphosphoglycerate Reduced by NADPH 3 CO2 makes 6 G3P Regeneration of Rubisco 5 G3P rearranged into 3 RuBP Uses 3 ATP Uses CO2

(Entering one at a time) Input 3 (Entering one at a time) CO2 Phase 1: Carbon fixation Rubisco 3 P P Short-lived intermediate 3 P P 6 P Ribulose bisphosphate (RuBP) 3-Phosphoglycerate 6 ATP 6 ADP 3 ADP Calvin Cycle 6 P P 3 ATP 1,3-Bisphosphoglycerate 6 NADPH Phase 3: Regeneration of the CO2 acceptor (RuBP) 6 NADP 6 P i 5 P G3P 6 P Glyceraldehyde 3-phosphate (G3P) Phase 2: Reduction 1 P G3P (a sugar) Glucose and other organic compounds Output

Net gain 3 CO2, 9 ATP and 6 NADPH > G3P Transferred into other metabolic pathways glucose + other carbs Both light and dark reactions needed for sugar production

Labs Leaf photosynthesis Sodium bicarbonate = carbon source With photosynthesis the leaves would produce oxygen within its mesophyll Decrease density 30 cm away Float Dark for half of the exposure time Fewer disks floating Water/soap control = no carbon source All sink 50 cm away