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How Cells Acquire Energy

This chapter explores how cells acquire energy and the process of photosynthesis. It discusses the different types of autotrophs and heterotrophs and their sources of carbon and energy. The chapter also examines T.E. Englemann's experiment, the stages of photosynthesis, and the role of pigments and photosystems.

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How Cells Acquire Energy

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  1. How Cells Acquire Energy Chapter 6

  2. Carbon and Energy Sources • Photoautotrophs • Carbon source is carbon dioxide • Energy source is sunlight • Heterotrophs • Get carbon and energy by eating autotrophs or one another

  3. Photoautotrophs • Capture sunlight energy and use it to carry out photosynthesis • Plants • Some bacteria • Many protistans

  4. T.E. Englemann’s Experiment Background • Certain bacterial cells will move toward places where oxygen concentration is high • Photosynthesis produces oxygen

  5. T.E. Englemann’s Experiment Hypothesis • Movement of bacteria can be used to determine optimal light wavelengths for photosynthesis

  6. T.E. Englemann’s Experiment Method • Algal strand placed on microscope slide and illuminated by light of varying wavelengths • Oxygen-requiring bacteria placed on same slide

  7. T.E. Englemann’s Experiment

  8. T.E. Englemann’s Experiment Results Bacteria congregated where red and violet wavelengths illuminated alga Conclusion Bacteria moved to where algal cells released more oxygen--areas illuminated by the most effective light for photosynthesis

  9. Photosynthesis Energy-storing pathway Releases oxygen Requires carbon dioxide Aerobic Respiration Energy-releasing pathway Requires oxygen Releases carbon dioxide Linked Processes

  10. Chloroplasts Organelles of photosynthesis

  11. Photosynthesis Equation LIGHT ENERGY 6O2 + C2H12O6 + 6H2O 12H2O + 6CO2 oxygen glucose water carbon dioxide water

  12. Two Stages of Photosynthesis sunlight water uptake carbon dioxide uptake ATP ADP + Pi LIGHT DEPENDENT-REACTIONS LIGHT INDEPENDENT-REACTIONS NADPH NADP+ glucose P oxygen release new water

  13. Sunlight Energy • Continual input of solar energy into Earth’s atmosphere • Almost 1/3 is reflected back into space • Of the energy that reaches Earth’s surface, about 1% is intercepted by photoautotrophs

  14. Electromagnetic Spectrum Shortest Gamma rays wavelength X-rays UV radiation Visible light Infrared radiation Microwaves Longest Radio waves wavelength

  15. Visible Light • Wavelengths humans perceive as different colors • Violet (380 nm) to red (750 nm) • Longer wavelengths, lower energy

  16. Photons • Packets of light energy • Each type of photon has fixed amount of energy • Photons having most energy travel as shortest wavelength (blue-green light)

  17. Pigments • Light-absorbing molecules • Absorb some wavelengths and transmit others • Color you see are the wavelengths NOT absorbed chlorophyll a chlorophyll b Wavelength (nanometers)

  18. Pigment Structure • Light-catching part of molecule often has alternating single and double bonds • These bonds contain electrons that are capable of being moved to higher energy levels by absorbing light

  19. Excitation of Electrons • Excitation occurs only when the quantity of energy in an incoming photon matches the amount of energy necessary to boost the electrons of that specific pigment • Amount of energy needed varies among pigment molecules

  20. Variety of Pigments Chlorophylls a and b Carotenoids Anthocyanins Phycobilins

  21. Chlorophylls Main pigments in most photoautotrophs chlorophyll a Wavelength absorption (%) chlorophyll b Wavelength (nanometers)

  22. Carotenoids • Found in all photoautotrophs • Absorb blue-violet and blue-green that chlorophylls miss • Reflect red, yellow, orange wavelengths • Two types • Carotenes - pure hydrocarbons • Xanthophylls - contain oxygen

  23. Anthocyanins & Phycobilins Red to purple pigments • Anthocyanins • Give many flowers their colors • Phycobilins • Found in red algae and cyanobacteria

  24. Pigments in Photosynthesis • Bacteria • Pigments in plasma membranes • Plants • Pigments embedded in thylakoid membrane system • Pigments and proteins organized into photosystems • Photosystems located next to electron transport systems

  25. Photosystems and Electron Transporters water-splitting complex thylakoid compartment H2O 2H + 1/2O2 P680 P700 acceptor acceptor PHOTOSYSTEM II pool of electron transporters stroma PHOTOSYSTEM I

  26. Light-Dependent Reactions • Pigments absorb light energy, give up e- which enter electron transport systems • Water molecules are split, ATP and NADH are formed, and oxygen is released • Pigments that gave up electrons get replacements

  27. Photosystem Function: Harvester Pigments • Most pigments in photosystem are harvester pigments • When excited by light energy, these pigments transfer energy to adjacent pigment molecules • Each transfer involves energy loss

  28. Photosystem Function: Reaction Center • Energy is reduced to level that can be captured by molecule of chlorophyll a • This molecule (P700 or P680) is the reaction center of a photosystem • Reaction center accepts energy and donates electron to acceptor molecule

  29. Pigments in a Photosystem reaction center (a specialized chlorophyll a molecule)

  30. Electron Transport System • Adjacent to photosystem • Acceptor molecule donates electrons from reaction center • As electrons flow through system, energy they release is used to produce ATP and, in some cases, NADPH

  31. Cyclic Electron Flow • Electrons • are donated by P700 in photosystem I to acceptor molecule • flow through electron transport system and back to P700 • Electron flow drives ATP formation • No NADPH is formed

  32. Cyclic Electron Flow e– electron acceptor electron transport system e– e– ATP e–

  33. Noncyclic Electron Flow • Two-step pathway for light absorption and electron excitation • Uses two photosystems: type I and type II • Produces ATP and NADPH • Involves photolysis - splitting of water

  34. Machinery of Noncyclic Electron Flow H2O photolysis e– e– ATP SYNTHASE NADPH NADP+ ATP ADP + Pi PHOTOSYSTEM II PHOTOSYSTEM I

  35. Energy Changes second transport system e– NADPH e– first transport system e– Potential to transfer energy (voids) e– (PHOTOSYSTEM I) (PHOTOSYSTEM II) 1/2 O2 + 2H+ H2O

  36. Chemiosmotic Model of ATP Formation • When water is split during photolysis, hydrogen ions are released into thylakoid compartment • More hydrogen ions are pumped into the thylakoid compartment when the electron transport system operates

  37. Chemiosmotic Model of ATP Formation • Electrical and H+concentration gradient exists between thylakoid compartment and stroma • H+ flows down gradients into stroma through ATP synthesis • Flow of ions drives formation of ATP

  38. Light-Independent Reactions • Synthesis part of photosynthesis • Can proceed in the dark • Take place in the stroma • Calvin-Benson cycle

  39. Overall reactants Carbon dioxide ATP NADPH Overall products Glucose ADP NADP+ Calvin-Benson Cycle Reaction pathway is cyclic and RuBP (ribulose bisphosphate) is regenerated

  40. 6 CO2 (from the air) Calvin- Benson Cycle CARBON FIXATION 6 6 RuBP unstable intermediate 12 PGA 6 ADP 12 ATP 6 ATP 12 NADPH 4 Pi 12 ADP 12 Pi 12NADP+ 10 PGAL 12 PGAL 2 PGAL Pi P glucose

  41. Building Glucose • PGA accepts • phosphate from ATP • hydrogen and electrons from NADPH • PGAL (phosphoglyceraldehyde) forms • When 12 PGAL have formed • 10 are used to regenerate RuBP • 2 combine to form phosphorylated glucose

  42. Using the Products of Photosynthesis • Phosphorylated glucose is the building block for: • sucrose • The most easily transported plant carbohydrate • starch • The most common storage form

  43. The C3 Pathway • In Calvin-Benson cycle, the first stable intermediate is a three-carbon PGA • Because the first intermediate has three carbons, the pathway is called the C3 pathway

  44. Photorespiration in C3 Plants • On hot, dry days stomata close • Inside leaf • Oxygen levels rise • Carbon dioxide levels drop • Rubisco attaches RuBP to oxygen instead of carbon dioxide • Only one PGAL forms instead of two

  45. C4 Plants • Carbon dioxide is fixed twice • In mesophyll cells, carbon dioxide is fixed to form four-carbon oxaloacetate • Oxaloacetate is transferred to bundle-sheath cells • Carbon dioxide is released and fixed again in Calvin-Benson cycle

  46. CAM Plants • Carbon is fixed twice (in same cells) • Night • Carbon dioxide is fixed to form organic acids • Day • Carbon dioxide is released and fixed in Calvin-Benson cycle

  47. Hydrothermal Vents • Fissures in sea-floor where seawater mixes with molten rock • Complex ecosystem is based on energy from these vents • Bacteria are producers in this system

  48. Light and Life at the Vents • Vents release faint radiation at low end of visible spectrum • These photons could be used to carry out photosynthesis • Nisbet and Van Dover hypothesize that the first cells may have arisen at hydrothermal vent systems

  49. Supporting Evidence • Absorption spectra for ancient photosynthetic bacteria correspond to wavelengths measured at the vents • Photosynthetic machinery contains iron, sulfur, manganese, and other minerals that are abundant at the vents

  50. Summary of Photosynthesis light LIGHT-DEPENDENT REACTIONS 6O2 12H2O ATP NADP+ NADPH ADP + Pi PGA CALVIN-BENSON CYCLE PGAL 6CO2 RuBP P C6H12O6 (phosphorylated glucose) end product (e.g. sucrose, starch, cellulose)

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