Photosynthesis

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1. What is a phototroph?

Back 1

an organism that converts solar energy to chemical energy in the form of ATP

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2. What two subgroups of phototrophs exist, and what is the definition of each?

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The two subgroups of phototrophs are: photoheterotrophs, these organisms acquire energy from sunlight, but depend on organic sources of reduced carbon; photoautotrophs are organisms that use solar energy to drive the biosynthesis of energy rich organic molecules from simple inorganic materials.

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3. What are the two major biochemical processes that occur during photosynthesis? How are they complementary?

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The two major biochemical processes that occur during photosynthesis are energy transduction reaction and carbon assimilation reaction. Energy transduction reaction uses chlorophyll molecules to capture light energy and convert it to chemical energy in the form of ATP and the reduced coenzyme NADPH. Carbon assimilation reaction, also known as the Calvin Cycle, fully oxidizes carbon atoms from carbon dioxide and fixes (reduces and covalently attaches) it to organic acceptor molecules and then rearranges it to form carbohydrates and other organic compounds that are required to build a living cell. They are complementary because the ATP and NADPH that is generated by the energy transduction reaction subsequently provide energy and reducing power for the carbon assimilation reaction.

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4. What are oxygenic phototrophs? How do they differ from anoxygenic phototrophs? Give an example of each.

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Oxygenic phototrophs (plants, algae and cynobacteria) require light energy absorption by chlorophyll and other pigment molecules; this drives the movement of electrons from water. Anoxygenic phototrophs (green and purple bacteria) contain compounds with less positive reduction potentials than that of water, such as sulfide (SH-), thiosulfate (S2O32-), or succinate, serve as electron donors.

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5. What is photoreduction?

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The light dependent generation of NADPH in oxygenic and anoxygenic phototrophs.

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6. Write a general reaction for the complete process of photosynthesis.

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Light + CO2 + 2H2A  [CH2O] + 2A + H2O

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7. Write a reaction for photosynthesis as it occurs in oxygenic phototrophs.

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Light + 3CO2 + 6H2O  C3H6O3 + 3H2O + 3H2O

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8. What are chloroplasts? Do all plant cells contain them?

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Chloroplasts are specialized organelles that have the job of photosynthetic energy transduction and carbon assimilation. No all plants do not have chloroplasts.

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9. Briefly define each of the following: proplastids, plastids, amyloplasts, chromoplasts, proteinoplasts, and elaioplasts.

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Proplastids are small, double – membrane – enclosed, plant cytoplasmic organelles that can develop into several kinds of plastids, including chloroplasts. Plastids are any of several types of plant cytoplasmic organelles derived from proplastids. Amyloplasts are sites for storing starch. Chromoplasts give flowers their color. Proteinoplasts store protein. Elaioplasts store lipids.

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10. Describe the basic anatomy of a chloroplast. How is it similar to and different from the anatomy of a mitochondrion?

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Chloroplasts are composed of a three membrane system; an outer membrane, which contains porins, (Transmembrane proteins that permit the passage of solutes with molecular weights up to about 5000 daltons), it is freely permeable to most small organic molecules and ions. An inner membrane which encloses the stroma (a gel – like matrix – teeming with enzymes for carbon, nitrogen, and sulfur assimilation). The third membrane, the thylakoid are flat, saclike structures suspended in the stroma and usually arranged in stacks called grana. The grana resemble stacks of coins and are interconnected by a network of longer thylakoids called stroma thylakoid. Essentially all of the photosynthetic pigments, the enzymes required for the photoreactions, the carriers involved in electron transport and the proteins that couple electron transport to proton pumping and ATP synthesis are localized in or in the thylakoid membranes. The grana and stroma thylakoids enclose a single continuous compartment called the thylakoid lu

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11. What is the significance of the thylakoid membranes of a chloroplast?

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The significance of the thylakoid membrane is the generation of an electrochemical proton gradient and the synthesis of ATP, because protons pumped into the lumen during light – driven electron transport drive ATP synthesis as they return to the stroma.

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12. How do photosynthetic bacteria alter their cellular structures in order to carry out photosynthesis?

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They alter their plasma membrane by folding it inward and forming photosynthetic membranes, these structures are similar to the thylakoids.

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13. What is photoexcitation?

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Photoexcitation is when a photon is absorbed by a pigment (light – absorbing molecule), such as chlorophyll, the energy of the photon is transferred to an electron, which is energized from its grounded state in a low – energy orbital to an excited state in a high – energy orbital.

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14. What is resonance energy transfer? Why is it important for the success of photosynthesis?

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It’s the transfer of most or all of the excitation energy from the photoexcited electron to an electron in an adjacent pigment molecule, this excites the second electron to a higher – energy orbital. This is important for the success of photosynthesis because it moves the captured energy from light - absorbing molecules to molecules that are capable of passing an excited electron to an organic acceptor molecule.

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15. What is photochemical reduction?

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The transfer of the photoexcited electron itself to a high – energy orbital in another molecule.

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16. Describe how the structure of a chlorophyll molecule is related to its function.

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The skeleton of each molecule consists of a central porphyrin ring and a strongly hydrophobic phytol side chain. The alternating double bonds in the porphyrin ring are responsible for absorbing visible light, while the phytol side chain interacts with lipids of the thylakoid or cynobacterial membranes, anchoring the light – absorbing molecules in these membranes.

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17. For each of the following types of chlorophyll, list the organisms that commonly possess them: chlorophyll a, chlorophyll b, chlorophyll c, chlorophyll d, phycobilin, and bacteriochlorophyll.

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Chlorophyll a is found in brown, red, violet blue and blue green algae, chlorophyll b is found in higher plants and green algae. Chlorophyll c is found in brown algae and diatoms. Chlorophyll d is found in some red algae. Phycobilin is found in yellow and red algae . Bacteriochlorophyll is found in purple and green bacteria and purple bacterium.

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18. What are accessory pigments, and why are they important?

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Photosynthetic organisms which absorb photons that cannot be captured by chlorophyll. This feature enables organisms to collect energy from a much larger portion of the sunlight reaching the Earth’s surface.

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19. Why do different photosynthetic organisms have different amounts and types of accessory pigments?

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They have different accessory pigments.

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20. What are the most common types of carotenoids?

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B – carotene and lutein

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21. What is a photosystem?

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The localization of chlorophyll molecules, accessory pigments, and associated proteins to the thylakoid or photosynthetic membranes.

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22. What are antenna pigments?

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Pigments that serve as light gatherers, they absorb photons and pass the energy to a neighboring chlorophyll molecule or accessory pigment by resonance energy transfer.

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23. What is the reaction center of a photosystem?

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This is where two distinct chlorophyll a molecules known as the special pair reside.

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24. What is a light-harvesting complex, and what is its relationship to a photosystem?

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It collects light energy light a photosystem, but unlike a photosystem it does not have a reaction center; instead , it passes the collected energy to a nearby photosystem by resonance energy transfer.

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25. What are the characteristics of photosystem I and II?

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Photosystem I has an absorption maximum of 700 nm, it absorbs both short and long wavelengths of red light: photosystem II has an absorption maximum of 680 nm, it absorbs only short wavelengths or red light.

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26. Why is it important that photosystems I and II are mobile within the thylakoid membranes?

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This is important because it controls the mobility of luminal proteins.

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27. Why it is important that the reduction potential of a reaction center electron is drastically lowered by light energy in both photosystem I and II?

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Because if it is excited, it can not transfer its energy to another pigment using resonance energy transfer.

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28. What is the significance of the oxygen-evolving complex (OEC) in photosystem II? Briefly describe how it works.

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It delivers the four electrons from water to tyrosine side chain and then to photosystem
II. The oxygen-evolving complex is the site of water oxidation. It is a metallo-oxo cluster
comprising four manganese ions (in oxidation states ranging from +3 to +5) and one
divalent calcium ion. When it oxidizes water, producing dioxygen gas and protons, it
sequentially delivers the four electrons from water to a tyrosine (D1-Y161) sidechain
and then to P680 itself.

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29. Why is the use of water photolysis by cyanobacteria thought to have been a critical event in Earth’s history?

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Through this process, photosynthetic bacteria was able to evolve into the plants that we have today.

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30. What is noncyclic electron flow?

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It is the various components of the ETS that provide a continuous, unidirectional flow of electrons from water to NADP+.

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31. What is photophosphorylation?

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The process of creating ATP using a proton gradient created by the energy gathered from sunlight.

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32. Describe the ways in which ATP synthesis in the chloroplast is similar to that occurring in the mitochondrion.

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They both use ATP synthase to make ATP. The electron transport chain of both creates an electro-chemical gradient by pumping hydrogen ions to one side of the membrane, and the flow of those ions through the ATP synthase channel, "spins" this lolly pop-like enzyme and combines ADP with a phosphate group to make ATP.

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33. Under what conditions is cyclic electron flow important for the photosynthetic cell? Which photosystem is involved?

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When there is a need for ATP and it is made in Photosystem I.

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34. Why is there no release of oxygen during cyclic photophosphorylation?

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Because NADPH does not split H2O and there is no reducing power for CO2.

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35. What is the purpose of the ATP and NADPH that is produced during photosynthesis?

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The reduction of CO2 to carbohydrates in the Calvin Cycle.

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36. What is the purpose of the Calvin cycle?

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To use CO2 and energy to form ATP that are used to form sugar.

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37. How did Melvin Calvin demonstrate the fate of the carbon assimilated during photosynthesis? Where does this carbon come from?

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He used carbon 14 to trace the fate of the carbon assimilated during photosynthesis. The carbon comes from the atmosphere.

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38. Where does the Calvin cycle occur?

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It occurs in the stroma.

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39. What are the three stages of the Calvin cycle?

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The three stages of the Calvin Cycle are:
1.) Fixation of CO2 to form 3 – phosphoglycerate
2.) Reduction of 3 – phoshoglycerate to form hexose sugars
3.) The regeneration of ribulose 1,5 - bisphosphate

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40. What molecule is the beginning substrate of the Calvin cycle and is regenerated with each repetition of the cycle?

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RuBP

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41. What is rubisco? Why is it thought to be the most abundant protein on Earth?

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It is the shortened name for ribulose 1,5 – biphosphate carboxylase / oxygenase. It is responsible for the addition of carbon dioxide and oxygen to RuBP. The result of this being the wasteful cleavage of RuBP to form 3 – phosphoglycerate and phosphoglycolate. In other words, it is used in the Calvin Cycle to catalyze the first major step of carbon fixation. It is the most abundant protein on Earth because it is present in every plant that undergoes photosynthesis and molecular synthesis through the Calvin Cycle.

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42. For each repetition of the Calvin cycle, how many triose phosphate molecules are available for the biosynthesis of organic molecules?

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1 out of 6

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43. What are the major organic molecules that can be synthesized by photosynthetic cells as a result of the Calvin cycle?

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Sucrose, starch and cellulose