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Photosynthesis

Photosynthesis. Photosynthesis. Plants capture light energy from the sun Energy is converted to chemical energy (sugars & organic molecule). Autotrophs. Photosynthesizers are autotrophs – organisms that produce organic molecules from CO 2 & inorganics from environment.

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Photosynthesis

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  1. Photosynthesis

  2. Photosynthesis • Plants capture light energy from the sun • Energy is converted to chemical energy (sugars & organic molecule)

  3. Autotrophs • Photosynthesizers are autotrophs– organisms that produce organic molecules from CO2 & inorganics from environment. • Photoautotrophs - plants, algae, some other protists, and some prokaryotes • Chemoautotrophs – oxidize inorganics (S, NH3). Unique to bacteria.

  4. Heterotrophs • Live on products of other organisms • Consumers • Decomposers • Completely dependent on autotrophs for byproducts of photosynthesis

  5. Location of Photosynthesis • Chloroplasts – any green part of plant, primarily leaves • ½ million chloroplasts/mm2 of leaf surface • Green color derived from pigment chlorophyll • Chlorophyll important in light absorption (more on that later)

  6. Chloroplasts in Elodea

  7. Location of Chloroplasts • Found mainly in mesophyllcells – interior of leaf • O2 exits and CO2 enters leaf throughstomata • Stomata in close proximity to chloroplasts – WHY? • Veins deliver H20 from roots and carry off sugar to other areas where needed

  8. Typical mesophyll cell has 30-40 chloroplasts • Chloroplast structure – remember? • Thylakoids/grana, stroma

  9. Leaf cross section

  10. Photosynthesis – Redox Rxn Recall - RESPIRATION • C6H12O6 + 6 O2 + 6 H2O  6 CO2 + 12 H2O + Energy OR • C6H12O6 + 6 O2  6 CO2 + 6 H2O + Energy • Glucose is oxidized to form CO2 • Oxygen is reduced, forming water • Reaction is EXERGONIC

  11. Photosynthesis Equation • Photosynthesis reverses aerobic respiration • Net process of photosynthesis is: 6CO2 + 6H2O + light energy -> C6H12O6 + 6O2 • Water split and e- transferred to CO2, reducing it to sugar • Byproduct: 6O2 • Reaction is ENDERGONIC

  12. Free Oxygen • Plants give off O2, split from H2O not CO2 • C.B. van Neil, studies with H2S in bacteria • Later scientists used radioactive tracer 18O to confirm van Neil’s H2O hypothesis

  13. Photosynthesis Equation • Where does the energy to power the reaction come from? • In reality, photosynthesis adds one CO2 at a time (carbon fixation) CO2 + H2O + light energy -> [CH2O]* + O2 *CH2O represents the general formula for a sugar.

  14. Photosynthesis: A closer look… Two major components • LIGHT REACTIONS (PHOTOPHOSPHORYLATION) - conversion of light energy to chemical energy • CALVIN CYCLE (DARK REACTIONS) – transforms atmospheric CO2 to organic molecule; uses energy from light rxn to reduce to sugar.

  15. Light Reactions: Overview • Light energy absorbed by chlorophyll in thylakoids • Drives the transfer of e- to NADP+ (nicotinamide adenine dinucleotide phosphate), forming NADPH • Generates ATP by photophosphorylation

  16. What is LIGHT? • Electromagnetic energy – travels in waves • Distance between waves is the wavelength • ↓ wavelength = ↑ energy • ↑ wavelength = ↓ energy • Measured via electromagnetic spectrum • Visible light = 380 – 750 nm

  17. Photons • Direct particles of energy • Intensity inversely related to wavelength • Purple/blue light carries much more energy than orange/red range of spectrum

  18. When light meets matter, it is either reflected, transmitted or absorbed • Different pigments absorb photons of different wavelengths • WHY ARE LEAVES GREEN?

  19. Spectrophotometer • Measures pigment’s ability to absorb wavelengths • Uses transmittance • Absorption spectrum

  20. Thylakoids - 3 major pigments • Chlorophyll a – dominant pigment. Red & blue absorption • Chlorophyll b and carotenoids • Slightly different absorption • Funnel energy to chloro a • PHOTOPROTECTION (carotenoids)

  21. Action Spectrum • All the pigments together determine “action spectrum” for photosynthesis

  22. Action spectrum ≠ absorption spectrum of ONE pigment • Engelmann 1883 – aerobic bacteria indic. O2 & absorption

  23. Capturing Light Energy • Molecule absorbs photon • Causes e- to elevate to orbital with more potential energy • “Ground” state to “excited” state • Molecules absorb photons that match the energy difference between ground and excited state of e- • Corresponds to specific wavelengths, absorption spectrum

  24. Photons are absorbed by clusters of pigment molecules in thylakoid membranes • Energy of photon converted to potential energy of e- raised from ground state to excited state • In chlorophyll a and b, an electron from Mg in the porphyrin ring is excited

  25. Chlorophyll “head”

  26. Excited e- unstable • Drop to ground state in billionth of a second, releasing heat energy • Chlorophyll & other pigments release photon of light (fluorescence) without an e- acceptor

  27. Photosystems • In thylakoid membrane, chlorophyll organized photosystems • Acts like a light-gathering “antenna” • Hundreds of chloro a,b, and carotenoids • Some proteins, other small organic molec.

  28. Photon absorbed by any antenna molecule • Transmitted from molecule to molecule until reaches reaction center • At reaction center is a primary electron acceptor • Removes an excited e- from chloro a in reaction center • This starts the light reactions

  29. Photosystem I & II • Photosystem I has an absorption peak at 700nm - its rxn center is called the P700 center • Photosystem II - rxn center at 680nm. • Differences between reaction centers due to the associated proteins • Photosystems work together to generate ATP and NADPH.

  30. Cyclic & Noncyclic Electron Flow • During light rxn, e- can flow 1 of 2 ways: • Noncyclic electron flow, the predominant route, produces both ATP and NADPH • Under certain conditions, photoexcited electrons from photosystem I, but not photosystem II, can take an alternative pathway, cyclic electron flow

  31. Noncyclic Pathway Similar to oxidative phosphorylation

  32. Photosystem II absorbs light, captures an excited electron (rxn ctr oxidized) • Enzyme extracts e- from H2O and donates to oxidized reaction center • P680 is the strongest oxidizing agent known – it must be filled with e- • Photoexcited e- pass along ETC from PSII to PSI

  33. Electron carriers: • Pq (plastoquinone), a cytochrome complex • Pc (plastocyanin), a protein • Exergonic fall of e- provides energy for ATP synthesis Meanwhile - in Photosystem I…

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