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How Cells Acquire Energy

How Cells Acquire Energy. Chapter 7. Photosynthesis: The Big Picture. Source of BOTH matter and energy for most living organisms Captures light energy from the sun and converts it into chemical energy Synthesized organic molecules from inorganic molecule BOTTOM LINE: Makes FOOD.

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How Cells Acquire Energy

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  1. How Cells Acquire Energy Chapter 7

  2. Photosynthesis: The Big Picture • Source of BOTH matter and energy for most living organisms • Captures light energy from the sun and converts it into chemical energy • Synthesized organic molecules from inorganic molecule BOTTOM LINE: Makes FOOD

  3. Definitions Autotroph: • Organisms that make their own food (energy-rich organic molecules) from simple, inorganic molecules Photoautotroph: • Organisms that make their own food through photosynthesis; obtain energy from the sun • Type of autotroph Heterotroph: • Get carbon and energy by eating autotrophs or one another

  4. Algea (Kelp) Plants Algae (spirogyra) Cyanobacteria Photoautotrophs • Capture sunlight energy and use it to carry out photosynthesis • Plants • Some bacteria • cyanobacteria • Many protistans • algae

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

  6. Photosynthesis Equation LIGHT ENERGY 12H2O + 6CO2 6O2 + C6H12O6 + 6H2O Water Carbon Dioxide Oxygen Glucose Water In-text figurePage 115

  7. Where Atoms End Up In-text figurePage 116

  8. Chloroplast Structure two outer membranes stroma inner membrane system (thylakoids connected by channels) Figure 7.3d, Page 116

  9. Two Stages of Photosynthesis Light dependent reactions • Converts light energy into chemical energy (ADP ATP) • Gathers e- and H+ from water (NADP+ NADPH) • Occurs in thylakoid membranes Light independent reactions (Calvin-Benson Cycle) • Reduces CO2 to synthesize glucose using energy and hydrogens (i.e. ATP and NADPH) generated in the light dependent reaction • Occurs in Stroma Notice that these reactions do not create NADH, but rather NADPH

  10. 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 In-text figurePage 117

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

  12. Visible Light • Electromagnetic energy with a wavelength of 308-750nm • Light energy is organized into packets called photons • The shorter the wavelength the greater the energy carried by the photons

  13. Properties of Light • White light (from the sun) contains all of the wavelengths of light • When light hits matter, it can be reflected (transmitted) or absorbed • White substances reflect all light • Black substances absorb all light

  14. Pigments • A substance that absorbs light • We see the color that is transmitted by pigment • The absorbed color disappears into pigment

  15. Plant pigments • Plant use a variety of pigments during photosynthesis: • Chlorophylls a and b • Carotenoids • Anthocyanins • Phycobilins • The main photosynthetic pigment is Chlorophyll a

  16. Chlorophylls • Chlorophyll aabsorbs red and blue light, and reflects green light (what we see) • Note: The colors that are absorbed are used for photosynthesis chlorophyll a Wavelength absorption (%) chlorophyll b Figure 7.7Page 120 Wavelength (nanometers) Figure 7.6a Page 119

  17. e- e- Effect of Light on Pigments What happens when light hits pigments? • The color disappears, but the energy does not • Absorbing photons of light excites electrons (e-), thus adding potential energy • Ground state: normal pigment • Excited state: pigment absorbing light (e- excited) Photon of light: Atom in pigment: Ground state Atom in pigment: Excited state

  18. Photosystems • In thylakoid membrane, pigments are organized in clusters called photosystems • These clusters contain several hundred pigment molecules • Two types of photosystems • Photosystem I = P700 (absorbs light at 700nm) • Photosystem II = P680 (absorbs light at 680nm)

  19. Reaction Center Chlorophyll • One of the pigments in each photosystem is known as the reaction center chlorophyll (RCC) • if any pigment within the photosystem gets hit by a photon, the energy is transferred to the RCC • The RCC will then transfer its excited e- into an electron transport chain

  20. Pigments in a Photosystem reaction center Figure 7.11Page 122

  21. Light Dependent Reactions • Location: the thylakoid membranes • Function: to generate ATP (energy!) and NADPH (reducing power!) that will be used in the light independent reaction Two processes: • Non-cyclic electron flow • Generates ATP and NADPH • Cyclic electron flow • Generates only ATP

  22. 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

  23. Machinery of Noncyclic Electron Flow H2O second electron transfer chain photolysis e– e– ATP SYNTHASE first electron transfer chain NADPH NADP+ ATP ADP + Pi PHOTOSYSTEM II PHOTOSYSTEM I Figure 7.13aPage 123

  24. Steps of Non-cyclic electron flow • Photosystem II gets hit by a photon; electron of RCC gets excited • The excited (high energy) e- gets picked up by an electron carrier and taken into an electron transfer chain (ETC) • The excited e- provides energy to pump protons (H+) into the thylakoid (tiny space) • Through chemiosmosis, ATP is generated

  25. Chemiosmotic Model of ATP Formation • Electrical and H+ concentration gradients are created between thylakoid compartment and stroma • H+ flow down gradients into stroma through ATP synthase • The energy driven by the flow of H+ powers the formation of ATP from ADP and Pi

  26. Chemiosmotic Model for ATP Formation H+ is shunted across membrane by some components of the first electron transfer chain Gradients propel H+ through ATP synthases; ATP forms by phosphate-group transfer Photolysis in the thylakoid compartment splits water H2O e– acceptor ATP SYNTHASE ATP ADP + Pi PHOTOSYSTEM II Figure 7.15Page 124

  27. Non-cyclic electron flow: Photolysis • While Photosystem II gets hit by light, etc., water is split: H2O  ½ O2 + 2H+ + 2e- • This process is called photolysis • The H+ are pumped into the thylakoid to create the proton gradient • The e- replace the excited e- that was taken away from the RCC

  28. Non-Cyclic Electron Flow:The saga continues • Photosystem I gets excited at the same time as photosystem II • Its excited e- gets taken into a second electron transfer chain that attaches the excited e- and the leftover H+ to NADP+ to make NADPH: NADP+ + H+ + e- NADPH

  29. Non-cyclic electron flow • The “electron hole” in photosystem I is then filled with the used up, low energy e- from photosystem II • Now everything is back to normal, and we can start all over again

  30. Energy Changes in Non-cyclic electron flow second transfer chain e– NADPH e– first transfer chain Potential to transfer energy (volts) e– e– (Photosystem I) (Photosystem II) 1/2O2 + 2H+ H2O Figure 7.13bPage 123

  31. Non-cyclic electron flow: Summary • After two excited photosystems, two ETCs and the splitting of water, both ATP and NADPH are generated!!!

  32. Cyclic electron flow • The light independent reactions require more ATP than NADPH • Cyclic electron flow is like a short cut to making extra ATP • Involves only Photosystem I

  33. Cyclic electron flow • Photosystem I gets excited • Excited e- is carried into the first ETC; energy goes to pump H+ into thylakoid compartment • Chemiosmosis powers formation of ATP • The same e- (now low energy) replaces itself in the “electron hole” in Photosystem I

  34. Cyclic electron flow H2O second electron transfer chain photolysis e– e– ATP SYNTHASE first electron transfer chain NADPH NADP+ ATP ADP + Pi PHOTOSYSTEM II PHOTOSYSTEM I Figure 7.13aPage 123

  35. Light dependent reactions:Summary • Non-cyclic electron flow • Generates ATP

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

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

  38. Calvin-Benson Cycle Three Phases: • Carbon Fixation • Reduction • Regeneration of RUBP

  39. Calvin-Benson Cycle: Carbon Fixation • Capturing atmospheric (gaseous) CO2 by attaching it to RuBP, a 5-carbon organic molecule • This process forms two 3-carbon molecules • The enzyme that catalyzes this process is called Rubisco

  40. Calvin Benson Cycle:Reduction • The captured CO2 has very little energy and no hydrogens • In order to make sugar, energy and hydrogens need to be added to the molecules formed by Carbon fixation • ATP and NADPH (made in the light dependent reactions) break down to form ADP and NADP+ and, in the process, transfer energy and hydrogens to the 3-carbon compounds formed by carbon fixation, resulting in sugar formation

  41. Calvin-Benson Cycle:Regeneration • Some of the sugar created by reduction leaves the Calvin cycle, and is used to build up glucose and other organic molecules • The rest of the sugar is used to remake (regenerate) RuBP • This process requires ATP (which was made in the light dependent reactions)

  42. Calvin-Benson Cycle:Summary • The cycle proceeds 6 times to form each molecule of glucose • In the process, ATP and NADPH is used up • 6CO2 are converted into C6H12O6 - glucose

  43. The C3 Pathway • In Calvin-Benson cycle, as described, 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 (holes in the leaf) close to prevent evaporation of water • As a result, within the leaf oxygen levels rise, and Carbon dioxide levels drop • Rubisco attaches RuBP to oxygen instead of carbon dioxide • Results in a VERY wasteful process known as Photorespiration – uses up ATP without generating sugar

  45. C4 and CAM Plants • To avoid photorespiration, plants that live in hot, dry climates evolved mechanisms to separate carbon fixation from the Calvin Cycle • The CO2 that enters the Calvin cycle is derived from the breakdown of previously synthesized organic acids • In this way, the enzyme that catalyzes the reaction that attaches CO2 to RuBP is not exposed to atmospheric oxygen

  46. C4 and CAM Plants • C4 plants do carbon fixation in a different location (cell type) than the Clavin cycle • CAM plants do carbon fixation at a different time (night) that the Calvin cycle (day)

  47. light LIGHT-DEPENDENT REACTIONS 6O2 12H2O ATP NADP+ NADPH ADP + Pi PGA CALVIN-BENSON CYCLE PGAL 6H2O 6CO2 RuBP P C6H12O6 (phosphorylated glucose) end product (e.g., sucrose, starch, cellulose) Summary of Photosynthesis LIGHT-INDEPENDENT REACTIONS Figure 7.21Page 129

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