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Photosynthesis

Photosynthesis. Introduction. Autotrophs : any organism that can make their own food. Photoautotrophs : use light energy Chemoautotrophs change inorganic chemicals such as hydrogen sulfate into chemical energy. Important structures.

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Photosynthesis

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  1. Photosynthesis

  2. Introduction • Autotrophs: any organism that can make their own food. • Photoautotrophs : use light energy • Chemoautotrophs change inorganic chemicals such as hydrogen sulfate into chemical energy

  3. Important structures • Chloroplast: Site of photosynthesis in eukaryotic cells. • Thylakoids: Disk shaped membranes containing photosynthetic pigments. Site of light dependent reactions. • Grana: Stacks of thylakoids. • Stroma: Fluid filled space surrounding grana. Site of light independent reactions.

  4. Properties of Light • White light from the sun is composed of a range of wavelengths. • Chlorophyll is the main pigment that absorbs sunlight. • Chlorophyll absorbs blue light and to a lesser extent, red light. • It reflects green light hence its intense green color.

  5. What do chlorophyll and fireworks have in common? • Fireworks are made up of various metal salts that go through oxidation and reduction reactions, producing great heat that causes electrons to move from their usual level around the nucleus (ground state) up to a higher position further away from the nucleus (excited state). The energy from heat has been imparted to the electron(s), which now has more potential energy. These excited electrons move almost instantaneously back down to their ground state, releasing their stored potential energy in the form of light. The greater the electrons movement away from the nucleus to the excited state, the more potential energy it has and the more energy it will release. • It should be pointed out that chlorophyll absorbs light energy, specifically its electrons, and this energy is used to pump protons instead of producing light. This proton gradient in turn is used to produce ATP, which along with other energized electrons are transferred to the Calvin cycle.

  6. Suppose a large meteor hit the earth. How could smoke and soot in the atmosphere wipe out life far beyond the area of direct impact? • Because any particles in the atmosphere can block the light from the sun and reduce the excitation of electrons in chlorophyll molecules, photosynthesis depends on a relatively clean atmosphere. Any reduction in the available sunlight can have serious effects on plants. Scientists believe that if a large meteor hit the earth—as one did when the dinosaurs were wiped out 65 million years ago—smoke, soot, and dust in the atmosphere could block sunlight to such an extent that plants in the region, or even possibly all of the plants on earth, could not conduct photosynthesis at high enough levels to survive. And when plants die off, all of the animals and other species that rely on them for energy die as well. As dire as it sounds, all life on earth is completely dependent on the continued excitation of electrons by sunlight.

  7. Chloroplast two outer membranes thylakoid membrane system Organelle of photosynthesis in plants and algae stroma thylakoid compartment

  8. a A look inside the leaf b One of the photosynthetic cells inside leaf leaf’s upper epidermis photosynthetic cell in leaf leaf vein Leaf Structure leaf’s lower epidermis

  9. Photosynthesis Equation LIGHT ENERGY 6H2O + 6CO2 6O2 + C6H12O6 water carbon dioxide oxygen glucose

  10. Photosynthesis • Two stages: • light-dependent reactions require light to work • light-independent reactions do not require light

  11. Two Steps in Photosynthesis sunlight H2O O2 CO2 Where the two stages of photosynthesis occur inside the chloroplast NADPH, ATP light-dependent reactions light-independent reactions NADP+, ADP sugars

  12. Photosynthesis: Light-Dependent • Photosystems - pigments surrounding a central chlorophyll a molecule....the reaction center • Each pigment absorbs a different wavelength of light & transfers its energy to the reaction center which in turns energizes an electron. • Only the chlorophyll that is the reaction center can give the energized electrons to the electron acceptors! The energized electron is then used elsewhere to make ATP or NADPH

  13. Two Potential Fates of Excited Electrons Electron returns to resting, unexcited state. Excited electrons are passed to other atoms.

  14. Photosynthesis: Light-Dependent

  15. The Passing of Electrons in Their Excited State Chief way energy moves through cells Molecules that gain electrons always carry greater energy than before receiving them Can view this as passing of potential energy from molecule to molecule

  16. Photosynthesis: Light-Dependent

  17. Photosynthesis: Light-Dependent • As these pigments absorb photons from the sunlight that hits the leaves, electrons in the pigments become excited and then return to their resting state. • As the electrons return to their resting state, energy (but not the electrons) is transferred to neighboring pigment molecules. • This process continues until the transferred energy from many pigment molecules excites the electrons in a chlorophyll a molecule at the center of the photosystem.

  18. Photosynthesis: Light-Dependent • This is where the electron journey begins. • The special chlorophyll a continually loses its excited electrons to a nearby molecule, called the primary electron acceptor, which acts like an electron vacuum. • Why must plants get water for photosynthesis to occur? • As electrons keep getting taken away from the special chlorophyll a molecule, the electrons must be replaced. The replacement electrons come from water.

  19. Photosynthesis: Light-Dependent

  20. Photosynthesis: Light-Dependent • Think of a pump pushing water into an elevated tank, creating a store of potential energy that can run out of the tank with great force and kinetic energy, which can be harnessed to do work, such as moving a large paddle wheel. • Similarly, the protons eventually rush out of the thylakoid sacs with great force—and that force is harnessed to build energy-storing ATP molecules, one of the two products of the “photo” portion of photosynthesis.

  21. Photosynthesis: Light-Dependent • Product #1 of the “Photo” Portion of Photosynthesis: ATP • Product #2: NADPH • Product #3 (waste): O2

  22. Photosynthesis: Light-inDependent • Also known as the Calvin Cycle • They occur whether or not light is present. • Occur in the stroma of the chloroplast. • The purpose of the reactions is to take the energy from ATP and energized ions from NADPH and add them to carbon dioxide to make glucose or sugar. • The reactions reduce carbon dioxide by adding energize electrons and protons to it and removing one oxygen atom. This effectively converts the carbon dioxide into CH2O.

  23. Photosynthesis: Light-inDependent

  24. Photosynthesis: Light-inDependent

  25. Factors that Affect Rates • 3 factors can limit the speed of photosynthesis: light intensity, CO2 concentration, temperature • Without enough light, a plant cannot photosynthesize very quickly, even if there is plenty of water and CO2 .

  26. Factors that Affect Rates • Sometimes photosynthesis is limited by the concentration of carbon dioxide in the air. • Even if there is plenty of light, a plant cannot photosynthesize if there is insufficient carbon dioxide.

  27. Factors that Affect Rates • If it gets too cold, the rate of photosynthesis will decrease. Plants cannot photosynthesize if it gets too hot.

  28. Summary of Photosynthesis sunlight Light Dependent Reactions 6O2 12H2O ATP ADP + Pi NADP+ NADPH 6CO2 Calvin- Benson cycle Light Independent Reactions 6 RuBP 12 PGAL 6H2O phosphorylated glucose P end products (e.g., sucrose, starch, cellulose)

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

  30. Photosynthesis: The Movie • Animation

  31. The battle against world hunger can use plants adapted to water scarcity.

  32. Stomata Pores for gas exchange

  33. How to get CO2 when stomata are shut? • Primary sites for gas exchange in plants • CO2 for photosynthesis • O2 generated as a by-product in photosynthesis exits • water vapor evaporates • Closed stomata: • prevents water evaporation  • O2 cannot be released from the chloroplasts  • CO2 cannot enter them  • No carbon = no Calvin cycle • plant growth comes to a standstill; crops fail

  34. Evolutionary Adaptations • Some plants are able to thrive in hot, dry conditions. • Evolutionary adaptations – along with recent agricultural (technological) advances – help battle world hunger • plants close their stomatato combat water loss through evaporation

  35. C4 Photosynthesis C4 plants produce an enzyme that is better at attracting carbon (in addition to RUBISCO) called PEP Carboxylase allows CO2to be taken into the plant very quickly "delivers" the CO2 directly to RUBISCO Called C4 because the CO2is first incorporated into a 4-carbon compound Stomata are open during the day

  36. C4 Photosynthesis Adaptive Value: photosynthesizes faster than C3 plants under high light intensity and high temperatures better water use efficiency because PEP Carboxylase brings in CO2faster  doesn’t need to keep stomata open as much C4 plants include several thousand species in at least 19 plant families, including corn & sugarcane C4 photosynthesis also adds additional energy expense; outcompeted in mild climates by C3 plants.

  37. C4 Photosynthesis

  38. CAM Photosynthesis CAM = CrassulaceanAcid Metabolism after plant family in which it was first found (Crassulaceae) because CO2is stored as an acid before use in photosynthesis Stomata open at night (evaporation rates usually lower), closed during day CO2 converted to acid, stored @ night daytime: acid broken down CO2 released to RUBISCO for photosynthesis

  39. CAM Photosynthesis Adaptive Value: better water use efficiency than C3s under arid conditions open stomata @ night when transpiration rates are lower no sunlight, lower temperatures, lower wind speeds, etc. May CAM-idleunder extremely arid conditions leave stomata closed night and day O2 given off in photosynthesis is used for respiration CO2given off in respiration is used for photosynthesis kind of like a perpetual energy machine, but plant cannot CAM-idle forever allows plant to survive dry spells, recover very quickly when water is available again (unlike plants that drop their leaves and twigs and go dormant during dry spells)

  40. CAM Photosynthesis CACTUS AGAVE ORCHID BROMELIAD

  41. All Three Photosynthetic Pathways

  42. Evolutionary Adaptations & World Hunger C4 and CAM photosynthesis originally evolved as successful adaptations to hot/dry regions Researchers are now using these adaptations to fight world hunger. several genes that code for the C4 photosynthesis enzymes have been introduced from corn into rice once in rice, genes increase rice plant’s ability to photosynthesize  higher growth rates and food yields

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