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Photosynthesis: Capturing Energy

Photosynthesis: Capturing Energy. Chapter 8. Light. Composed of photons – packets of energy Visible light is a small part of the electromagnetic spectrum All energy travels as waves Wavelength is the distance from 1 wave peak to the next

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Photosynthesis: Capturing Energy

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  1. Photosynthesis: Capturing Energy Chapter 8

  2. Light • Composed of photons – packets of energy • Visible light is a small part of the electromagnetic spectrum • All energy travels as waves • Wavelength is the distance from 1 wave peak to the next • Shorter wavelengths have more energy than longer wavelengths

  3. One wavelength Longer wavelength 760 nm TV and radio waves Red 700 nm Micro- waves Orange Infrared 600 nm Color spectrum of visible light Visible Yellow UV X-rays Green 500 nm Blue Gamma rays Violet 400 nm 380 nm Electromagnetic spectrum Shorter wavelength • Visible light: 380-760 nm • Energy from visible light is used in photosynthesis • Why? • Longer wavelengths don’t have enough energy; higher wavelengths have too much

  4. Chloroplasts • Organelles enclosed by a double membrane • Located mainly within mesophyll cells inside a leaf • Each cell contains 20-100 chloroplasts • This portion of the leaf has many air spaces and a high water vapor content

  5. Palisade mesophyll Vein Spongy mesophyll Stoma

  6. Thylakoids Outer membrane Inner membrane Stroma Thylakoid membrane Granum (stack of thylakoids) Thylakoid lumen Intermembrane space

  7. Chlorophyll • Main photosynthetic pigment: • chlorophyll a (initiates the light-dependent reactions) • Accessory pigments: chlorophyll b, carotenoids (these absorb different wavelengths of light and pass the energy on to chlorophyll a) • These pigments are found in the thylakoid membranes of chloroplasts • Pigments reflect the color of light we see; absorb the other colors • Is green light used during photosynthesis? Why or why not?

  8. Overview of photosynthesis • Hydrogens from water reduce carbon dioxide; oxygen from water is oxidized • Photosynthesis is a redox reaction: 6 CO2 + 6 H2O  C6H12O6 + 6 O2 reduction oxidation

  9. Overview… • Two phases: 1st: Light-dependent (‘photo’) Occurs in the thylakoids of the chloroplasts H2O is split and molecular oxygen is released Electrons energized by light produce ATP and NADPH which are both needed for the endergonic next phase 2nd: Carbon-fixation (‘synthesis’) Occurs in the stroma of the chloroplasts ATP and NADPH provide the energy needed for the formation of carbohydrates

  10. Overview… Light reactions Carbon-fixation reactions ATP ADP Light reactions Calvin cycle NADPH NADP+ C02 carbohydrates H20 02

  11. Light-dependent Reactions • Occur in the thylakoids • Energy is absorbed from light and converted to chemical energy stored in ATP and NADPH • Oxygen is released • Photosystem I and II both involved – these have similar pigments but different roles

  12. Photosystems I and II • Each system includes: • Chlorophyll a molecules and associated proteins • Multiple antenna complexes • Photosystem I = P700 - wavelength absorbed • Photosystem II = P680 - wavelength absorbed

  13. Thylakoid Primary electron acceptor e– Chloroplast Reaction center Photon Photosystem Antenna complexes

  14. Light reactions: Step #1 • Light energy forces e- to a higher energy level in 2 chlorophyll a molecules of PS II (e- is excited) • e- leave chlorophyll a (oxidation) • a replacement e- is donated by H2O: • 2H2O  4 H+ + 4 e- + O2 • This is noncyclic electron transport

  15. Light reactions: Step #2 • e- goes to a primary electron acceptor in the thylakoid membrane (reduction)

  16. Light reactions: Step #3 • e- donated from the primary electron acceptor to a series of molecules in the thylakoid membrane – the electron transport chain • e- lose energy as they move through the chain – this energy moves H+ into the thylakoid lumen • this H+ gradient will be used to produce ATP from ADP and Pi using ATP synthase

  17. Light reactions: Step #4 • Light is absorbed by PS I • e- leave chlorophyll a and go to another primary electron acceptor • these e- are replaced by e- from the electron transport chain • This is cyclic electron transport

  18. Light reactions: Step #5 • e- from the primary electron acceptors in PS I go to another electron transport chain on the stroma side of the thylakoid membrane • e- with H+ and NADP+ NADPH • ATP + NADPH made during the light reactions are both needed to power the carbon-fixation reactions

  19. Primary electron acceptor Electron transport chain A0 Primary electron acceptor A1 FeSx Electron transport chain FeSB FeSA Plastiquinone Ferredoxin ADP Pi Cytochrome complex H+ (from medium) Production of ATP by chemiosmosis NADPH ATP NADP+ Plastocyanin O2 H+ 1/2 + 2 2 Photosystem I(P700) H2O 1 Photosystem II(P680)

  20. ATP Synthesis and electron transport… the main ideas • Electrons (e-) move down the electron transport chain and release energy as they go • Protons (H+) move from the stroma to the thylakoid lumen, creating a proton gradient • The greater concentration of H+ lowers the pH • The thylakoid membrane is impermeable to H+ except through ATP synthase • The flow of H+ through ATP synthase generates ATP

  21. Carbon-fixation reactions • Also known as the Calvin Cycle or the light-independent reactions • Occur in the stroma • CO2 + chemical energy from ATP and NADPH are used to make organic compounds – carbon is ‘fixed’

  22. Three phases of the Calvin Cycle • CO2 uptake • Carbon reduction • RuBP regeneration

  23. Carbon fixation: Step #1 CO2 uptake: CO2 + RuBP*  unstable 6-C molecule, which splits  2 3-C molecules + 3 PGA** *RuBP = ribulose biphosphate (5 C) **PGA = 3 phosphoglycerate

  24. Carbon fixation: Step #2 Carbon reduction: 2 molecules of 3-PGA are converted to 2 molecules of G3P* in a 2-part process, using the energy from ATP and the H+ from NADPH from the light reactions * G3P = glyceraldehdye 3-phosphate

  25. Carbon fixation: Step #3 RuBP regeneration: One molecule of G3P leaves the Calvin cycle to be converted into carbohydrates such as glucose or starch The other G3P molecule uses the energy from ATP and is converted back to RuBP The RuBP is returned back to the Calvin cycle

  26. 6 molecules of CO2 CO 2 molecules are captured by RuBP, resulting in an unstable intermediate that is immediately broken apart into 2 PGA 6 molecules of ribulose bisphosphate (RuBP) P P 12 molecules of phosphoglycerate (PGA) 1 CO2 uptake phase 6 ADP P ATP 12 ATP 6 molecules of ribulose phosphate (RP) CALVIN CYCLE 12 ADP P 3 2 RuBP regeneration phase Carbon reduction phase 12 NADPH 10 molecules of G3P P 12 NADP+ Glucose and other carbohydrate synthesis P PGA is phosphorylated by ATP and reduced by NADPH. Removal of a phosphate results in formation of G3P. P 12 molecules of glyceraldehyde-3- phosphate (G3P) P Through a series of reactions G3P is rearranged into new RuBP molecules or another sugar 2 molecules of glyceraldehyde-3- phosphate (G3P)

  27. Adjustments based on weather… • C3 plants – use the C3 pathway – the initial carbon fixation product is a 3-C sugar • These plants must close their stomata during hot, dry weather to reduce water loss • This reduces carb production • Adaptations for hot, dry environments: • C4 plants – 1st fix CO2 into a 4-C compound • CAM plants – fix CO2 at night (cactus)

  28. Table 8-2 p. 167

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