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What is the life on earth powered by? |
Life on earth is solar powered. The chloroplasts of plants capture light energy that has traveled 150 million kilometers from the sun and convert it to chemical energy that is stored in sugar and other organic molecules. This process is called photosynthesis. |
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How does photosynthesis go about nourishing the entire living world? |
An organism acquires the organic compounds it uses for energy and carbon skeletons by one of the two major modes: autotrophic nutrition or heterotrophic nutrition. |
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What are autotrophs? |
Autotrophs are "self-feeders" (auto-means "self," and trophos means "feeder"); they sustain themselves without anything derived from living beings. Autotrophs produce their organic molecules from CO2 and other inorganic raw materials obtained from the environment. They are the ultimate source of organic compounds for all nonautotrophic organisms, and for this reason, biologists refer to autotrophs as the "producers" of the biosphere. |
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What organisms are autotrophs? |
Almost all plants are autotrophs; the only nutrients they require are water and minerals from the soil and carbon dioxide from the air. Specifically, plants are photoautotrophs, organisms that use light as a source of energy to synthesize organic substances. Photosynthesis also occurs in algae, certain other protists, and some prokaryotes (i think this also makes them autotrophic). |
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How do heterotrophs obtain their organic material? |
Heterotrophs obtain their organic material by the second major mode of nutrition. Unable to make their own food, they live on compounds produced by other organisms (hetero- means "other"). Heterotrophs are the biosphere’s consumers. |
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What are some forms of heterotrophic consumption? |
The most obvious form of heterotrophic consumption ("other feeding") occurs when an animal eats plants or other animals. But heterotrophic nutrition may be more subtle. Some heterotrophs consume the remains of dead organisms by decomposing and feeding on organic litter such as carcasses, feces, and fallen leaves; they are known as decomposers. Most fungi and many types of prokaryotes get their nourishment this way. Almost all heterotrophs, including humans, are completely dependent, either directly or indirectly, on photoautrotrophs for food-and also for oxygen, a by-product of photosynthesis. |
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How did the fossil fuels that we use today in our cars and other things come about? |
The Earth’s supply of fossil fuels was formed from remains of organisms that died hundreds of millions of years ago. In a sense, then, fossil fuels represent stores of the sun’s energy from the distant past. Because these resources are being used at a much higher rate than they are replenished, researchers are exploring methods of capitalizing on the photosynthetic process to provide alternative fuels. |
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What are the two stages of photosynthesis? |
The light reactions, in which solar energy is captured and transformed into chemical energy; and the Calvin cycle, in which the chemical energy is used to make organic molecules of food. |
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What are some of the types of alternative fuels that are coming from plants and algae? |
Biofuels from crops such as corn, soybeans, and cassava have been proposed as a supplement or even replacement for fossil fuels. To produce "bioethanol," the starch made naturally by the plants is simply converted to glucose and then fermented to ethanol by microorganisms. Alternatively, a simple chemical process can yield "biodiesel" from plant oils. Either product can be mixed with gasoline or used alone to power vehicles. Some species of unicellular algae are especially prolific oil producers, and they can be easily cultured in containers such as the tubular plastic bags. |
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Why should we be researching biofuels? |
The rate of fossil fuel use by humans far outpaces its formation in the earth: Fossil fuels are a nonrenewable source of energy. Tapping the power of sunlight by using products of photosynthesis to generate energy is a sustainable alternative if cost effective techniques can be developed. It is generally agreed that using algae is preferable to growing crops for this purpose because this use of cropland diminishes the food supply and drives up food prices. |
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The main product of fossil fuel combustion is CO2, and this combustion is the source of the increase in atmospheric CO2 concentration. Scientists have proposed strategically situating containers of these algae near industrial plants, or near highly congested city streets. Why does this arrangement make sense? |
I believe this arrangement makes sense because the algae can utilize the CO2 in order to produce even more and more fuels, at the same time removing the CO2 away from the atmosphere, which will in turn lessen the green house effect. Situating containers of algae near sources of CO2 emissions makes sense because algae need CO2 to carry out photosynthesis. The higher their rate of photosynthesis, the more plant oil they will produce (this is a win win win situation!). At the same time, algae would be absorbing the CO2 emitted from industrial plants or from car engines, reducing the amount of CO2 entering the atmosphere. |
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What is photosynthesis? |
It is the conversion of light energy to chemical energy that is stored in sugars or other organic compounds. |
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Where does photosynthesis occur? |
It occurs in plants, algae, and certain prokaryotes. |
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What is a chloroplast? |
It is an organelle found in plants and photosynthetic protists that absorbs sunlight and uses it to drive the synthesis of organic compounds from carbon dioxide and water. |
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What is mesophyll? |
It is the the tissue in the interior of a leaf. |
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How are the veins utilized in leaves? |
The water absorbed by the roots is delivered to the leaves in veins. The leaves are also use veins to export sugar to roots and other nonphotosynthetic parts of the plant. (How do we know this?) |
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How is ATP and NADPH formed? |
Source: Page 188 – Light absorbed by chlorophyll drives a transfer of the electrons and hydrogen ions from water to an acceptor called NADP+, where they are temporarily stored.The light reactions use solar power to reduce NADP+ NADPH by adding a pair of electrons along with an proton (H+). The light reactions also generate ATP using chemiosmosis to power the addition of a phosphate group to ADP, a process called photophosphorylation. |
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2) Which of the following are products of the light reactions of photosynthesis that are utilized in the Calvin cycle? A) CO2 and glucose |
E) ATP and NADPH Source: Page 188 – Light absorbed by chlorophyll drives a transfer of the electrons and hydrogen ions from water to an acceptor called NADP+, where they are temporarily stored.The light reactions use solar power to reduce NADP+ NADPH by adding a pair of electrons along with an proton (H+). The light reactions also generate ATP using chemiosmosis to power the addition of a phosphate group to ADP, a process called photophosphorylation. |
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What is chlorophyll and where can it be found? |
Chlorophyll, the green pigment that gives leaves their color, resides in the thylakoid membranes of the chloroplast. |
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What drives the synthesis of organic molecules? |
The light energy that is absorbed by the chlorophyll drives the synthesis of organic molecules. |
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3) What are the products of the light reactions that are subsequently used by the Calvin cycle? A) oxygen and carbon dioxide |
E) ATP and NADPH Source: Page 188 – The light energy is converted to chemical energy in a form of two compounds: NADPH, a source of electrons as "reducing power" that can be passed along to an electron acceptor, reducing it, and ATP, the versatile energy currency of cells. Notice that the light reactions produce no sugar; that happens in the second stage of photosynthesis, the Calvin Cycle. |
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4) Where does the Calvin cycle take place? A) stroma of the chloroplast |
A) stroma of the chloroplast Source Page 188 – In the chloroplast, the thylakoid membranes are the sites of the light reactions, whereas the Calvin cycle occurs in the stroma. |
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What do the light reactions do with solar energy? |
Source Page 188 – They utilize the solar energy to make ATP and NADPH, which supply chemical and reducing power, respectively to the Calvin cycle. |
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What are the end products of the Calvin cycle? |
Source Page 188 – The Calvin cycle incorporates CO2 into organic molecules, which are converted to sugar. (Recall that most simple sugars have formulas that are some multiple of CH2O). |
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5) In any ecosystem, terrestrial or aquatic, what group(s) is (are) always necessary? A) autotrophs and heterotrophs |
D) autotrophs |
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7) When oxygen is released as a result of photosynthesis, it is a by-product of which of the following? A) reducing NADP+ |
B) splitting the water molecules Source Page 188 – Water is split, providing a source of electrons and protons and giving off O2 as a by-product. |
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8) A plant has a unique photosynthetic pigment. The leaves of this plant appear to be reddish yellow. What wavelengths of visible light are being absorbed by this pigment? A) red and yellow |
B) blue and violet Source Page 189 – It cannot be A, because there is red and yellow, It cannot be C, because there is yellow. It cannot be D because there is red. It cannot be E because there is yellow. |
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16) The reaction-center chlorophyll of photosystem I is known as P700 because A) there are 700 chlorophyll molecules in the center. |
B) this pigment is best at absorbing light with a wavelength of 700 nm. Source Page 193 – The chlorophyll (a) at the reaction-center complex of photosystem 1 is called P700 because it most effectively absorbs light of wavelength 700 nm (in the far-red part of the spectrum). |
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17) Which of the events listed below occur in the light reactions of photosynthesis? A) NADP is produced. |
E) light is absorbed and funneled to reaction-center chlorophyll a. Source Page 193 – The solar powered transfer of an electron from the reaction center chlorophyll (a) pair to the primary electron acceptor is the first step of the light reactions. |
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18) Which statement describes the functioning of photosystem II? A) Light energy excites electrons in the electron transport chain in a photosynthetic unit. |
D) The electron vacancies in P680 are filled by electrons derived from water. Source – Figure 10.14 – |
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What is photophosphorylation? |
It is the process of generating ATP from ADP and phosphate by means of chemiosmosis, using a proton motive force generated across the thylakoid membrane of the chloroplast or the membrane of certain prokaryotes during the light reactions of photosynthesis. |
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21) What are the products of linear photophosphorylation? A) heat and fluorescence |
C) ATP and NADPH Source Page 193 – The two photosystems work together in using light energy to generate ATP and NADPH , the two main products of the light reactions. Source Figure 10.14 – How a linear electron flow during the light reactions generates ATP and NADPH. |
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22) As a research scientist, you measure the amount of ATP and NADPH consumed by the Calvin cycle in 1 hour. You find 30,000 molecules of ATP consumed, but only 20,000 molecules of NADPH. Where did the extra ATP molecules come from? A) photosystem II |
C) cyclic electron flow Source Page 195 – In certain cases, photoexcited electrons can take an alternative path called cyclic electron flow, which uses photosystem 1 but not photosystem 2. There is no production of NADPH and no release of oxygen. Cyclic flow does, however, generate ATP. |
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23) Assume a thylakoid is somehow punctured so that the interior of the thylakoid is no longer separated from the stroma. This damage will have the most direct effect on which of the following processes? A) the splitting of water |
D) the synthesis of ATP Source Figure 10.14 |
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What is chemiosmosis? |
It is an energy coupling mechanism that uses energy stored in the form of a hydrogen gradient |
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24) What does the chemiosmotic process in chloroplasts involve? A) establishment of a proton gradient |
A) establishment of a proton gradient |
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25) Suppose the interior of the thylakoids of isolated chloroplasts were made acidic and then transferred in the dark to a pH-8 solution. What would be likely to happen? A) The isolated chloroplasts will make ATP. |
A) The isolated chloroplasts will make ATP. |
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27) In mitochondria, chemiosmosis translocates protons from the matrix into the intermembrane space, whereas in chloroplasts, chemiosmosis translocates protons from A) the stroma to the photosystem II. |
C) the stroma to the thylakoid space. Source Page 196 – In the mitochondrion, protons diffuse their concentration gradient from the intermembrane space through ATP synthase to the matrix, driving ATP synthesis. In the chloroplast, ATP is synthesized as the hydrogen ions diffuse from the thylakoid space back to the stroma through ATP synthase complexes, whose catalytic knobs are on the stroma side of the membrane. |
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28) Which of the following statements best describes the relationship between photosynthesis and respiration? A) Respiration is the reversal of the biochemical pathways of photosynthesis. |
B) Photosynthesis stores energy in complex organic molecules, while respiration releases it. |
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29) Where are the molecules of the electron transport chain found in plant cells? A) thylakoid membranes of chloroplasts |
A) thylakoid membranes of chloroplasts |
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30) Synthesis of ATP by the chemiosmotic mechanism occurs during A) photosynthesis. |
C) both photosynthesis and respiration |
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31) Reduction of oxygen which forms water occurs during A) photosynthesis. |
B) respiration. |
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32) Reduction of NADP+ occurs during A) photosynthesis. |
A) photosynthesis. |
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33) The splitting of carbon dioxide to form oxygen gas and carbon compounds occurs during A) photosynthesis. |
D) neither photosynthesis nor respiration. Source Page 187 – One of the first clues to the mechanism of photosynthesis came from the discovery that the O2 given off by plants is derived from water and not from carbon dioxide. The chloroplast splits water into hydrogen and oxygen. Before this discovery, the prevailing hypothesis was that photosynthesis split carbon dioxide and then added water to the carbon. |
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34) Generation of proton gradients across membranes occurs during A) photosynthesis. |
C) both photosynthesis and respiration. Source: Page 197 – Figure 10.17 |
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35) What is the relationship between wavelength of light and the quantity of energy per photon? A) They have a direct, linear relationship. |
B) They are inversely related. Source: Page 189 – The amount of energy is inversely related to the wavelength of light: The shorter the wavelength, the greater the energy of each photon of that light. |
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37) P680+ is said to be the strongest biological oxidizing agent. Why? A) It is the receptor for the most excited electron in either photosystem. |
D) This molecule results from the transfer of an electron to the primary electron acceptor of photosystem II and strongly attracts another electron. Source: Page 194 – P680 is one of the strongest biological oxidizing agents known; its electron "hole" must be filled. This greatly facilitates the transfer of electrons from the split water molecule. |
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40) Carotenoids are often found in foods that are considered to have antioxidant properties in human nutrition. What related function do they have in plants? A) They serve as accessory pigments. |
B) They dissipate excessive light energy. Source: Page 191 – Some carotenoids seem to be function as photoprotectors. These compounds absorb and dissipate excessive light energy that would otherwise damage chlorophyll or interact with oxygen, forming reactive oxidative molecules that are dangerous to the cell. |
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How do carotenoids good for humans too? |
Carotenoids similar to the photoprotective ones in chloroplasts have a photoprotective role in the human eye. These and related molecules, often found in health food products, are valued as "phytochemicals, compounds with antioxidant properties. |
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42) Which of the following statements best represents the relationships between the light reactions and the Calvin cycle? A) The light reactions provide ATP and NADPH to the Calvin cycle, and the cycle returns ADP, Pi, and NADP+ to the light reactions. |
A) The light reactions provide ATP and NADPH to the Calvin cycle, and the Calvin cycle returns ADP, Pi, and NADP+ to the light reactions. Source: Page 198 – Figure 10.19 |
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43) Where do the enzymatic reactions of the Calvin cycle take place? A) stroma of the chloroplast |
A) stroma of the chloroplast Source: Page 188 – Figure 10.6 – The Calvin cycle occurs in the stroma. |
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44) What is the primary function of the Calvin cycle? A) use ATP to release carbon dioxide |
E) synthesize simple sugars from carbon dioxide Source: Page 198 – Carbon enters the Calvin cycle in the form of carbon dioxide and leaves in the form of sugar. |
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45) Produces molecular oxygen (O2) A) light reactions alone |
A) light reactions alone Source: Page 188 – Water is split, providing a source of electrons and protons (hydrogen ions, H+). |
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46) Requires ATP A) light reactions alone |
B) the Calvin cycle alone Source: Page 198 – Figure 10.19 – Displays that the Calvin cycle requires ATP meanwhile the light reactions require ADP. |
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48) Produces NADPH A) light reactions alone |
A) light reactions alone Source: Page 198 – Figure 10.19 |
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49) Produces three-carbon sugars A) light reactions alone |
B) the Calvin cycle alone Source: Page 198 – The carbohydrate produced directly from the Calvin cycle is actually not glucose, but a three carbon sugar called (G3P). |
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50) Requires CO2 A) light reactions alone |
B) the Calvin cycle alone Source: Page 199 – The Calvin cycle incorporates each carbon dioxide molecule, one at a time. |
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51) Requires glucose A) light reactions alone |
D) neither the light reactions nor the Calvin cycle |
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52) The sugar that results from three "turns" of the Calvin cycle is glyceraldehyde-3-phosphate (G3P). Which of the following is a consequence of this? A) Formation of a molecule of glucose would require 9 "turns." |
D) The formation of starch in plants involves assembling many G3P molecules, with or without further rearrangements. |
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53) In the process of carbon fixation, RuBP attaches a CO2 to produce a 6 carbon molecule, which is then split in two. After phosphorylation and reduction, what more needs to happen in the Calvin cycle? A) addition of a pair of electrons from NADPH |
D) regeneration of RuBP Source Page 199 – The 5 carbon skeletons of 5 molecules of G3P are rearranged by the last steps of the Calvin cycle into three molecules of RuBP. To accomplish this, the cycle spends three more molecules of ATP. The RuBP is now prepared to receive carbon dioxide again. |
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58) The pH of the inner thylakoid space has been measured, as have the pH of the stroma and of the cytosol of a particular plant cell. Which, if any, relationship would you expect to find? A) The pH within the thylakoid is less than that of the stroma. |
A) The pH within the thylakoid is less than that of the stroma. |
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59) Which of the following statements is true concerning Figure 10.3? A) It represents cell processes involved in C4 photosynthesis. |
A) It represents cell processes involved in C4 photosynthesis. Source: Page 201 – Figure 10.20 |
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60) Referring to Figure 10.3, oxygen would inhibit the CO2 fixation reactions in A) cell I only. |
B) cell II only. |
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What is photorespiration? |
It is a metabolic pathway that consumes oxygen and ATP, releases carbon dioxide, and decreases photosynthetic output. Photorespiration generally occurs on hot, dry, bright days, when the stomata close and there is favoring the binding of oxygen rather than carbon dioxide. Source – Glossary |
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61) In which cell would you expect photorespiration? A) Cell I |
B) Cell II |
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62) In an experiment studying photosynthesis performed during the day, you provide a plant with radioactive carbon (14C) dioxide as a metabolic tracer. The 14C is incorporated first into oxaloacetate. The plant is best characterized as a A) C4 plant. |
A) C4 plant. Source: Page 201 – Figure 10.20 – The first step is carried out by an enzyme present only in mesophyll cells called PEP carboxylase. This enzyme adds CO2 to PEP, forming the four carbon product oxaloacetate. |
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63) Why are C4 plants able to photosynthesize with no apparent photorespiration? A) They do not participate in the Calvin cycle. |
B) They use PEP carboxylase to initially fix CO2. |
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64) CAM plants keep stomata closed in daytime, thus reducing loss of water. They can do this because they A) fix CO2 into organic acids during the night. |
A) fix CO2 into organic acids during the night. Source: Page 202 – Figure 10.21 – The carbon dioxide, incoroprated into four-carbon organic acids (carbon fixation). |
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65) Photorespiration lowers the efficiency of photosynthesis by preventing the formation of A) carbon dioxide molecules. |
B) 3-phosphoglycerate molecules |
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66) The alternative pathways of photosynthesis using the C4 or CAM systems are said to be compromises. Why? A) Each one minimizes both water loss and rate of photosynthesis. |
C) Each one both minimizes photorespiration and optimizes the Calvin cycle. Source: Page 200 – In some plant species, alternate modes of carbon fixation have evolved that minimize photorespiration and optimize the Calvin cycle – even in hot, arid climates. |
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67) If plant gene alterations cause the plants to be deficient in photorespiration, what would most probably occur? A) Cells would carry on more photosynthesis. |
C) Less ATP would be generated. Source: Page 200 – Unlike normal cellular respiration, photorespiration generates no ATP; in fact, photorespiration consumes ATP. And unlike photosynthesis, photorespiration produces no sugar. In fact, photorespiration decreases photosynthetic output by siphoning organic material from the Calvin cycle and releasing CO2 that would otherwise be fixed. |
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1) The light reactions of photosynthesis supply the Calvin cycle with A) light energy. |
D) ATP and NADPH. |
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2) Which of the following sequences correctly represents the flow of electrons during photosynthesis? A) NADPH → O2 → CO2 |
B) H2O → NADPH → Calvin cycle |
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3) In mechanism, photophosphorylation is most similar to A) substrate-level phosphorylation in glycolysis. |
B) oxidative phosphorylation in cellular respiration. Source: G26 – Photophosphorylation is the process of generating ATP from ADP and phosphate by means of chemiosmosis, using a proton-motive force generated across the thylakoid membrane of the chloroplast or the membrane of certain prokaryotes during the light reactions of photosynthesis. |
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4) How is photosynthesis similar in C4 and CAM plants? A) In both cases, only photosystem I is used. |
C) In both cases, rubisco is not used to fix carbon initially. Source: Page 202 – Notice in Figure 10.21 that the CAM pathway is similar to the C4 pathway in that carbon dioxide is first incorporated into organic intermediates before it enters the Calvin cycle. The difference is that in C4 plants, the initial steps of carbon fixation are separated structurally from the Calvin cycle, whereas in CAM plants, the two steps occur at separate times within the same cell. |
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5) Which process is most directly driven by light energy? A) creation of a pH gradient by pumping protons across the thylakoid membrane |
D) removal of electrons from chlorophyll molecules |
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6) Which of the following statements is a correct distinction between autotrophs and heterotrophs? A) Only heterotrophs require chemical compounds from the environment. |
D) Autotrophs, but not heterotrophs, can nourish themselves beginning with CO2 and other nutrients that are inorganic. Source: Beginning |
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7) Which of the following does not occur during the Calvin cycle? A) carbon fixation |
C) release of oxygen |
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How is the Calvin cycle similar to the citric acid cycle in animals? |
The Calvin cycle is similar to the citric acid cycle in that a starting material is regenerated after molecules enter and leave the cycle. |
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How is the citric cycle different from the Calvin cycle? |
While the citric acid cycle is catabolic (breaking down food to get energy), oxidizing acetyl CoA and using the energy to synthesize ATP, the Calvin cycle is anabolic, building carbohydrates from smaller molecules and consuming energy. Carbon enters the Calvin cycle in the form of CO2 and leaves in the form of sugar. The cycle spends ATP as an energy source and consumes NADPH as reducing power for adding high-energy electrons to make the sugar. |
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What is the carbohydrate produced directly from the Calvin cycle called? |
It is not glucose, but a three carbon sugar; the name of this sugar is glyceraldehyde 3-phosphate (G3P). For the net synthesis of one molecule of G3P, the cycle must take place three times, fixing three molecules of CO2. As we trace the steps of the Calvin cycle, keep in mind that we are following three molecules of CO2 through the reactions. |
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What is carbon fixation? |
It is the initial incorporation of CO2 into organic material. |
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What is phase 1 of the Calvin cycle? |
Phase 1 is carbon fixation. The Calvin cycle incorporates each CO2 molecule, one at a time, by attaching it to a five carbon sugar named ribulose biphosphate (RuBP). The enzyme that catalyzes this first step is RuBP carboxylase, or rubisco. (This is the most abundant protein in chloroplasts and is also thought to be the most abundant protein on Earth). The product of the reaction is a six-carbon intermediate so unstable that it immediately splits in half, forming two molecules of 3-phosphoglycerate (for each CO2 fixed). |
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What is phase 2 of the Calvin cycle? |
Reduction. Each molecule of 3-phosphoglycerate receives an additional phosphate group from ATP, becoming 1,3-biphosphoglycerate. Next, a pair of electrons donated from NADPh reduces 1,3-biphosphoglycerate, which also loses a phosphate group, becoming G3P. Specifically, the electrons from NADPH reduce a carboxyl group on 1,3-biphosphoglycerate to the aldehyde group of G3P, which stores potential energy. G3P is a sugar-the same three-carbon sugar formed in glycolysis by the splitting of glucose. For every three molecules of CO2 that enter the cycle, there are six molecules of G3P formed. But only one molecule of the three-carbon sugar can be counted as a net gain of carbohydrate. The cycle began with 15 carbons worth of carbohydrate in the form of three molecules of the five-carbon sugar RuBP. Now there are 18 carbons worth of carbohydrate in the form of six molecules of G3P. One molecule exits the cycle to be used by the plant cell, but the other five molecules must be recycled to regenerate the three molecules of RuBP. |
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What is phase 3 of the Calvin cycle? |
It is regeneration of the CO2 acceptor (RuBP). In a complex series of reactions, the carbon skeletons of five molecules of G3P are rearranged by the last steps of the Calvin cycle into three molecules of RuBP. |
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What are chloroplasts? |
They are chemical factories powered by the sun (SUNNN!). Their thylakoids transform light energy into the chemical energy of ATP and NADPH. To understand this conversion better, we need to know about some important properties of light. |
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What is light? |
Light is a form of energy known as electromagnetic energy, also called electromagnetic radiation. Electromagnetic energy travels in rhythmic waves analogous to those created by dropping a pebble into a pond. Electromagnetic waves, however, are disturbances of electric and magnetic fields rather than disturbances of a material medium such as water. |
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Define wavelength |
It is the distance between the crests of electromagnetic waves. Wavelengths range from less than a nanometer (for gamma rays) to more than a kilometer (for radio waves). This entire range of radiation is known as the electromagnetic spectrum. |
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What spectrum of EM is most important to life? |
It is the narrow band from about 380 nm to 750 nm in wavelength. This radiation is known as visible light because it can be detected as various colors by the human eye. |
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How is light related to photons? |
The model of light as waves explains many of light’s properties, but in certain respects light behaves as though its consists of discrete particles, called photons. |
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What are photons? |
Photons are not tangible objects, but they act like objects in that each of them has a fixed quantity of energy. The amount of energy is inversely related to the wavelength of the light: The shorter the wavelength, the greater the energy of each photon of that light. Thus, a photon of violet light packs nearly twice as much energy as a photon of red light. |
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How does the atmosphere effect light? |
Although the sun radiates the full spectrum of electromagnetic energy, the atmosphere acts like a selective window, allowing visible light to pass through while screening out a substantial fraction of radiation. The part of the spectrum we can see – visible light – is also the radiation that drives photosynthesis. |
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What is white light? |
It is a mixture of all wavelengths of visible light, a prism can sort white light into its component colors by bending light of different wavelengths at different angles. (Droplets of water in the atmosphere can act as prisms, forming a rainbow;) Visible light drives photosynthesis. |
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What happens when light meets matter? |
It may be reflected, transmitted, or absorbed. |
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What are pigments? |
Pigments are substances that absorb visible light. Different pigments absorb light of different wavelengths, and the wavelengths that are absorbed disappear. If a pigment is illuminated with white light, the color we see is the color most reflected or transmitted by the pigment. (If a pigment absorbs all wavelengths, it appears black). |
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Why do we see green when we look at a leaf? |
We see green when we look at a leaf because chlorophyll absorbs violet-blue and red light transmitting and reflecting green light. The ability of a pigment to absorb various wavelengths of light can be measured with an instrument called a spectrophotometer? |
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How does the spectrophotometer work? |
This machine directs beams of light of different wavelengths through a solution of the pigment and measures the fraction of the light transmitted at each wavelength. A graph plotting a pigment’s light absorption versus wavelength is called an absorption spectrum. |
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Why are leaves green? |
It is due to the interaction of light with chloroplasts. The chlorophyll molecules of chloroplasts absorb violet-blue and red light (the colors most effective in driving photosynthesis) and reflect or transmit green light. This is why leaves appear green. |
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Why should scientists learn about absorption spectra? |
Application: An absorption spectrum is a visual representation of how well a particular pigment absorbs different wavelengths of visible light. Absorption spectra of various chloroplast pigments help scientists decipher each pigment’s role in a plant. |
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What is the technique in determining absorption spectrum? |
A spectrophotometer measures the relative amounts of light different wavelengths absorbed and transmitted by a pigment solution. 1. White light is separated into colors (wavelengths) by a prism. 2. One by one, the different colors of light are passed through the sample (chlorophyll in this example). 3. The transmitted light strikes a photoelectric tube, which converts the light energy to electricity. 4. The electric current is measured by a galvanometer. The meter indicates the fraction of light transmitted through the sample, from which we can determine the amount of light absorbed. |
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What does the high transmittance indicate? How about low transmittance? |
The high transmittance (low absorption) reading indicates that chlorophyll absorbs very little green light. The low transmittance (high absorption) reading indicates that chlorophyll absorbs most blue light. |
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What can the absorption spectra of chloroplast pigments tell us? |
The absorption spectra of chloroplast pigments provide clues to the relative effectiveness of different wavelengths for driving photosynthesis, since light can perform work in chloroplasts only if it is absorbed. |
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What is the absorption spectra of three types of pigments in chloroplasts? |
Chlorophyll a, which participates directly in the light reactions; the accessory pigment chlorophyll b; and a group of accessory pigments called carotenoids. |
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What does the spectrum of chlorophyll a tell us? |
The spectrum of chlorophyll a suggests that violet-blue and red light work best for photosynthesis, since they are absorbed, while green is the least effective color. This is confirmed by an action spectrum for photosynthesis, which profiles the relative effectiveness of different wavelengths of radiation in driving the process. |
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How is an action spectrum prepared? |
An action spectrum is prepared by illuminating chloroplasts with lights of different colors and then plotting wavelength against some measure of photosynthetic rate, such as CO2 consumption or O2 release. |
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When was action spectrum for photosynthesis first demonstrated? |
The action spectrum for photosynthesis was first demonstrated by Theodor W. Engelmann, a German botanist, in 1883. Before equipment for measuring O2 levels had even been invented, Engelmann performed a clever experiment in which he used bacteria to measure rates of photosynthesis in filamentous algae. His results are a striking match to the modern action spectrum. |
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What was Engelmann’s experiment? |
In 1883, Theodor W. Engelmann illuminated a filamentous alga with light that had been passed through a prism, exposing different segments of the alga to different wavelengths. He used aerobic bacteria, which concentrate near an oxygen source, to determine which segments of the alga were releasing the most O2 and thus photosynthesizing most. Bacteria congregated in greatest numbers around the parts of the alga illuminated with violet-blue or red light. The conclusion of this experiment was that light in the violet-blue and red portions of the spectrum is most effective in driving photosynthesis. |
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If Engelmann had used a filter that allowed only red light to pass through, how would the results have differed? |
Red, but not violet blue, wavelengths would pass through the filter, so the bacteria would not congregate where the violet-blue light normally comes through. Therefore, the left "peak" of bacteria would not be present, but the right peak would be observed because the red wavelengths passing through the filter would be used for photosynthesis. |
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Why does the action spectrum for photosynthesis not exactly match the absorption spectrum of chlorophyll a? |
The absorption spectrum of chlorophyll a alone underestimates the effectiveness of certain wavelengths in driving photosynthesis. This is partly because accessory pigments with different absorption spectra are also photosynthetically important in chloroplasts and broaden the spectrum of colors that can be used for photosynthesis. Chlorophyll a and b differ only in one of the functional groups bonded to the porphyrin ring. A slight structural difference between them is enough to cause the two pigments to absorb at slightly different wavelengths in the red and blue parts of the spectrum. As a result, chlorophyll a is a blue green and chlorophyll b is olive green. |
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What is a porphyrin ring? |
It is the light-absorbing "head of molecule; note magnesium atom at center. |
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What is the purpose of the hydrocarbon tail? |
It interacts with hydrophobic regions of proteins inside thylakoid membranes of chloroplasts. |
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What are carotenoids? |
They are accessory pigments, hydrocarbons that are various shades of yellow and orange because they absorb violet and blue-green light. Carotenoids may broaden the spectrum of colors that can drive photosynthesis. However, a more important function of at least some carotenoids seems to be photoprotection: These compounds absorb and dissipate excessive light energy that would otherwise damage chlorophyll or interact with oxygen, forming reactive oxidative molecules that are dangerous to the cell. |
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How do carotenoids similar to the photoprotective ones in chloroplasts, protect the human eye? |
These and related molecules, often found in health food products, are valued as "phytochemicals" (from the Greek "phyton", plant) compounds with antioxidant properties. Plants can synthesize all the antioxidants they require, but humans and other animals must obtain some of them from their diets. |
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What exactly happens when chlorophyll and other pigments absorb light? |
The colors corresponding to the absorbed wavelengths disappear from the spectrum of the transmitted and reflected light, but energy cannot disappear. When a molecule absorbs a photon of light, one of the molecule’s electrons is elevated to an orbital where it has more potential energy. When the electron is in its normal orbital, the pigment molecule is said to be in its ground state. Absorption of a photon boosts an electron to an orbital of higher energy, and the pigment is then said to be in an excited state. The only photons absorbed are those whose energy is exactly equal to the energy difference between the ground state and an excited state, and this energy difference varies from one kind of molecule to another. Thus, a particular compound absorbs only photons corresponding to specific wavelengths, which is why each pigment has a unique absorption spectrum. |
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What happens when absorption of a photon raises an electron from the ground state to an excited state? |
Well the electron cannot remain in the excited state long. The excited state, like all high-energy states, is unstable. Generally, when isolated pigment molecules absorb light, their excited electrons drop back down to the ground state orbital in a billionth of a second (nanosecond!), releasing their excess energy as heat. This conversion of light energy to heat is what makes the top of an automobile so hot on a sunny day. (White cars are coolest because their paint reflects all wavelengths of visible light, although it may absorb ultraviolet and other invisible radiation). In isolation, some pigments, including chlorophyll, emit light as well as heat after absorbing photons. As excited electrons fall back to the ground state, photons are given off. This afterglow is called fluorescence. If a solution of chlorophyll isolated from chloroplasts; it will fluoresce in the red-orange part of the spectrum and also give off heat. |
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What happens in the excitation of isolated chlorophyll by light? |
Absorption of a photon causes a transition of the chlorophyll molecule from its ground state to its excited state. The photon boosts an electron to an orbital where it has more potential energy. If the illuminated molecule exists in isolation, its excited electron immediately drops back down to the ground-state orbital, and its excess energy is given off as heat and fluorescence (light). A chlorophyll solution excited with UV light fluoresces with a red-orange glow. |
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If a leaf containing a similar concentration of chlorophyll as the solution was exposed to the same UV light, no fluorescence would be seen. Explain the difference in fluorescence emission between the solution and the leaf. |
In the leaf most of the chlorophyll electrons excited by photon absorption are used to power the reactions of photosynthesis. |
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Why do chlorophyll molecules excited by the absorption of light energy produce very different results in an intact chloroplast than they do in isolation? |
In their native environment of the thylakoid membrane, chlorophyll molecules are organized along with other small organic molecules and proteins into complexes called photosystems. |
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What is a photosystem composed of? |
It is composed of a reaction-center complex surrounded by several light-harvesting complexes. The reaction-center complex is an organized association of proteins holding a special pair of chlorophyll a molecules. |
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What does each light-harvesting complex consist of? |
It consists of various pigment molecules (which may include chlorophyll a, chlorophyll b, and carotenoids) bound to proteins. The number and variety of pigment molecules enable a photosystem to harvest light over a larger surface area and a larger portion the spectrum than could any single pigment molecule alone. |
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How do the light-harvesting complexes work? |
Together, these light harvesting complexes act as an antenna for the reaction-center complex. When a pigment molecule absorbs a photon, the energy is transferred from pigment molecule to pigment molecule within a light-harvesting complex, somewhat like a human "wave" at a sports arena, until it is passed into the reaction-center complex. |
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What is the reaction-center complex contain? |
It contains a molecule capable of accepting electrons and becoming reduced; this is called the primary electron acceptor. The pair of chlorophyll a molecules in the reaction-center complex are special because their molecular environment – their location and the other molecules which they are associated – enables them to use the energy from light not only to boost one of their electrons to a higher energy level, but also to transfer it to a different molecule – the primary electron acceptor. |
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How does a photosystem harvest light? |
When a photon strikes a pigment molecule in a light-harvesting complex, the energy is passed from molecule to molecule until it reaches the reaction-center complex. Here, an excited electron from the special pair of chlorophyll, a molecules is transferred to the primary electron acceptor. |
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What is the first step of the light reactions? |
The solar-powered transfer of an electron from the reaction-center chlorophyll a pair to the primary electron acceptor is the first step of the light reactions. As soon as the chlorophyll electron is excited to a higher level, the primary electron acceptor captures it; this is a redox reaction. |
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Why does isolated chlorophyll fluoresce? |
This is because there is no electron acceptor, so electrons of photoexcited chlorophyll drop right back to the ground state. (Which also releases heat.). In the structured environment of a chloroplast, however, an electron acceptor is readily available, and the potential energy represented by the excited electron is not dissipated as light and heat. Thus, each photosystem – a reaction-center complex surrounded by light-harvesting complexes – functions in the chloroplast as a unit. It converts light energy to chemical energy, which will ultimately be used for the synthesis of sugar. |
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What are the two types of photosystems that populate the thylakoid membrane? |
The two photosystems that cooperate in the light reactions of photosynthesis in the thylakoid membrane are photosystem 2 (PS 2) and photosystem 1 (PS 1). (They were named in order of their discover, but photosystem 2 functions first in the light reactions. Each has a characteristic reaction-center complex – a particular kind of primary electron acceptor next to a special pair of chlorophyll a molecules associated with specific proteins. |
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What is is P680? |
The reaction-center chlorophyll a of PS 2 is known as P680 because this pigment is best at absorbing light having a wavelength of 680 nm (in the red part of the spectrum) |
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What is P700? |
The chlorophyll a at the reaction-center complex of PS 1 is called P700 because it most effectively absorbs light of wavelength 700 nm (in the far-red part of the spectrum). |
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What are the differences between P680 and P700? |
These two pigments, P680 and P700, are nearly identical chlorophyll a molecules. However, their association with different proteins in the thylakoid membrane affects the electron distribution in the two pigments and accounts for the slight differences in their light-absorbing properties. |
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How do the two photosystems work together in using light energy to generate ATP and NADPH, the two main products of the light reactions? |
Light drives the synthesis of ATP and NADPH by energizing the two photosystems embedded in the thylakoid membranes of chloroplasts. The key to this energy transformation is a flow of electrons through the photosystems and other molecular components built into the thylakoid membrane. This is called linear electron flow and it occurs during the light reactions of photosynthesis. |
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What is the first step in LEF? |
1. A photon of light strikes a pigment molecule in a light-harvesting complex of PS 2, boosting one of its electrons to a higher energy level. As this electron falls back to its ground state, an electron in a nearby pigment molecule is simultaneously raised to an excited state. This process continues, with the energy being relayed to other pigment molecules until it reaches the P680 of chlorophyll a molecules in the PS 2 reaction-center complex. It excites an electron in this pair of chlorophylls to a higher energy state. |
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What are steps 2 and 3 in the linear electron flow? |
2. The electron is transferred from the excited P680 to the primary electron acceptor. We can refer the resulting form of P680, missing an electron as P680+. 3. An enzyme catalyzes (speeds up) the splitting of a water molecule into two electrons, two hydrogen ions (H+), and an oxygen atom. The electrons are supplied one by one to the P680+ pair, each electron replacing one transferred to the primary electron acceptor. (P680+ is the strongest biological oxidizing [stripping away electrons, high EN?] agent known; its electron "hole" must be filled. This greatly facilitates the transfer of electrons from the split water molecule.) The H+(protons) are released into the thylakoid lumen. The oxygen atom immediately combines with an oxygen atom generated by the splitting of another water molecule, forming O2. |
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What is step 4 in the linear electron flow? |
4. Each photoexcited electron passes from the primary electron acceptor of PS 2 to PS 1 via an electron transport chain, the components of which are similar to those of the electron transport chain that functions in cellular respiration. The electron transport chain between PS 2 and PS 1 is made up of the electron carrier plastoquinone (Pq), a cytochrome complex, and a protein called plastocyanin (Pc). |
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What is step 5 in the linear electron flow? |
5. The exergonic "fall" of electrons to a lower energy level provides energy for the synthesis of ATP. As electrons pass through the cytochrome complex, H+ are pumped into the thylakoid lumen, contributing to the proton gradient that is subsequently used in chemiosmosis (Proton osmosis?!) |
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What is chemiosmosis? |
Hydrogen ions (protons) will diffuse from an area of high proton concentration to an area of lower proton concentration. |
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What is step 6 in the linear electron flow? |
6. Meanwhile, light energy has been transferred via light-harvesting complex pigments to the PS 1 reaction-center complex, exciting an electron of the P700 pair of chlorophyll a molecules located there. The photoexcited electron was then transferred to PS 1’s primary electron acceptor, creating an electron "hole" in the P700 – which we now can call P700+. In other words, P700+ can now act as an electron acceptor, accepting an electron that reaches the bottom of the electron transport chain from PS 2. |
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What is step 7 in the linear electron flow? |
Photoexcited electrons are passed in a series of redox reactions from the primary electron acceptor of PS 1 down a second electron transport chain through the protein ferredoxin (FD). (This chain does not create a proton gradient and thus does not produce ATP). |
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What is step 8 in the linear electron flow? |
8. The enzyme NADP+ reductase catalyzes the transfer of electrons from FD to NADP+. Two electrons are required for its reduction to NADPH. This molecule is at a higher energy level than water, and its electrons are more readily available for the reactions of the Calvin cycle than were those of water. This process also removes an H+ from the stroma. |
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What is the function of the linear electron flow? |
The light reactions use solar power to generate ATP and NADPH, which provide chemical energy and reducing power, respectively, to the carbohydrate-synthesizing reactions of the Calvin cycle. |
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What is a cyclic electron flow? |
In certain cases, photoexcited electrons can take an alternative pathway called cyclic electron flow, which uses PS 1, but not PS 2. The cyclic flow is a short circuit. The electrons cycle back from FD to the cytochrome complex and from there continue on to a P700 chlorophyll in the PS 1 reaction-center complex. There is no production of NADPH and no release of oxygen. Cyclic flow does, however, generate ATP. |
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Explain what is occurring cyclic electron flow. Figure 10.16 |
Photoexcited electrons from PS 1 are occasionally shunted back from FD to chlorophyll via the cytochrome complex and PC. This electron shunt supplements the supply of ATP (via chemiosmosis – movement of protons down a gradient) but produces no NADPH. The "shadow" of linear electron flow is included in the diagram for comparison with the cyclic route. The two Fd molecules shown in the diagram are actually one and the same – the final electron carrier in the electron transport chain of PS 1. |
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Why do some currently existing groups of photosynthetic bacteria have a PS 1 but not a PS2? |
For these species, which include the purple sulfur bacteria, cyclic electron flow is the sole means of generating ATP in photosynthesis. Evolutionary biologists hypothesize that these bacterial groups are descendants of the bacteria in which photosynthesis first evolved, in a form similar to cyclic electron flow. |
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Can cyclic electron flow occur in photosynthetic species that possess both photosystems? |
Yes, some prokaryotes such as cyanobacteria, as well as eukaryotic photosynthetic species that have been tested so far have cyclic electron flow. Although the process is probably in part an "evolutionary leftover," it clearly plays at least one beneficial role for these organisms. Mutant plants that are not able to carry out cyclic electron flow are capable of growing well in low light, but do not grow well where light is intense. This is evidence for the idea that cyclic electron flow may be photoprotective. Whether ATP synthesis is driven by linear or cyclic electron flow, the actual mechanism is the same. |
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How do chloroplasts and mitochondria generate ATP? |
They both generate ATP by the same basic mechanism: chemiosmosis. An electron transport chain assembled in a membrane pumps protons across the membrane as electrons are passed through a series of carriers that are progressively more electronegative. In this way, electron transport chains transform redox energy to a proton-motive force,potential energy stored in the form of an H+ gradient across a membrane. Built into the same membrane is an ATP synthase complex that couples the diffusion of hydrogen ions down their gradient to the phosphorylation of ADP. |
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How are chloroplasts and mitochondria alike? |
Some of the electron carriers, including the iron-containing proteins called cytochromes, are very similar in chloroplasts and mitochondria. The ATP synthase complexes of the two organelles are also very much alike. |
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What are some of the noteworthy differences between oxidative phosphorylation in mitochondria and photophosphorylation in chloroplasts? |
In mitochondria, the high-energy electrons dropped down the transport chain are extracted from organic molecules (which are thus oxidized), while in chloroplasts, the source of electrons is water. Chloroplasts do not need molecules from food to make ATP; their photosystems capture light energy and use it to drive the electrons from water to the top of the transport chain. In other words, mitochondria use chemiosmosis to transfer chemical energy from food molecules to ATP, whereas chloroplasts transform light energy into chemical energy in ATP. |
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What are the similarities in the spatial organization of chemiosmosis between chloroplasts and mitochondria? |
The inner membrane of the mitochondrion pumps protons from the mitochondrial matrix out to the intermembrane space, which then serves as a reservoir of hydrogen ions. The thylakoid membrane of the chloroplast pumps protons from the stroma into the thylakoid space (interior of the thylakoid), which functions as the H+ reservoir. If you can imagine the cristae of mitochondria pinching off from the inner membrane, this may help you see how the thylakoid space and the intermembrane space are comparable spaces in the two organelles, while the mitochondrial matrix is analogous to the stroma of the chloroplast. In the mitochondrion, protons diffuse down their concentration gradient from the intermembrane space through ATP synthase to the matrix, driving ATP synthesis. In chloroplast, ATP is synthesized as the hydrogen ions diffuse from the thylakoid space back to the stroma through ATP synthase complexes, whose catalytic knobs are on the stroma side of the membrane. Thus, ATP forms in the stroma, where it is used to help drive sugar synthesis during the Calvin cycle. |
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Figure 10.17 – Compare chemiosmosis in mitochondria and chloroplasts. |
In both kinds of organelles, electron transport chains pump protons (H+) across a membrane from a low region of H+ concentration to one of high H+ concentration. The protons then diffuse back across the membrane through ATP synthase, driving the synthesis of ATP. |
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What is the pH difference between the thylakoid space and the stroma? |
When chloroplasts are in an experimental setting are illuminated, the pH in the thylakoid space drops to about 5 (the H+ concentration increases), and the pH in the stroma increases to about 8 (the H+ concentration decreases). This gradient of three pH units corresponds to a thousandfold difference in H+ concentration (damn!). If in the laboratory lights are turned off, the pH gradient is abolished, but it can quickly be restored by turning the lights back on. Experiments such as this provided strong evidence in support of the chemiosmotic model. |
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Summarize the light reactions. |
Electron flow pushes electrons from the water, where they are at a low state of potential energy, ultimately to NADPH, where they are stored at a high state of potential energy. The light-driven electron current also generates ATP. Thus, the equipment of the thylakoid membrane converts light energy to chemical energy stored in ATP and NADPH. (Oxygen is a by-product.) |
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What color of light is least effective in driving photosynthesis? |
1. Green, because green light is mostly transmitted and reflected-not absorbed- by photosynthetic pigments. |
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Compared to a solution of isolated chlorophyll, why do intact chloroplasts release less heat and fluorescence when illuminated? |
2. In chloroplasts, light excited electrons are trapped by a primary electron acceptor, which prevents them from dropping back to the ground state. In isolated chlorophyll, there is no electron acceptor, so the photoexcited electrons immediately drops back down to the ground state, with the emission of light and heat. |
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In the light reactions, what is the initial electron donor? Where do the electrons finally end up? |
3. Water (H2O) is the initial electron donor; NADP+ accepts electrons at the end of the electron transport chain, becoming reduced to NADPH. |
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In an experiment, isolated chloroplasts placed in an illuminated solution with the appropriate chemicals can carry out ATP synthesis. Predict what would happen to the rate of synthesis if a compound is added to the solution that makes membranes freely permeable to hydrogen ions. |
4.In this experiment, the rate of ATP synthesis would slow and eventually stop. Because the added compound would not allow a proton gradient to build up across the membrane, ATP synthase could not catalyze ATP production. |
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What are the major sites of photosynthesis in plants? |
All green parts of a plant, including green stems and unripened fruit, have chloroplasts, but the leaves are the major sites of photosynthesis in most plants. |
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How many chloroplasts are in a chunk of leaf with a top surface area of 1 mm2? |
About half a million chloroplasts That is a lot of chloroplasts… I wonder if one day we can engineer plants to produce even more chloroplasts? Or have we already done that? |
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Where are chloroplasts usually found in plants? |
Chloroplasts are found mainly in the cells of mesophyll, the tissue in the interior of the leaf. Carbon dioxide enters the leaf, and oxygen exits by way of microscopic pores called stomata (singular, stoma; from the Greek, meaning "mouth"). |
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How do plants utilize water? |
Water is absorbed by the roots is delivered to the leaves in veins. Leaves also use veins to export sugar to roots and other nonphotosynthetic parts of the plant. |
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How many chloroplasts does a typical mesophyll cell have? |
A typical mesophyll cell has about 30-40 chloroplasts, each organelle measuring about 2-4 micrometers by 4-7 micrometers. A chloroplast has an envelope of two membranes surrounding a dense fluid called the stroma. |
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What is suspended within the stroma of a chloroplast? |
Suspended within the stroma is a third membrane system, made up of sacs called thylakoids, which segregates the stroma from the thylakoid space inside these sacs. In some places, thylakoid sacs are stacked in columns called grana (singular, granum). |
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What is chlorophyll? |
Chlorophyll, the green pigment that gives leaves their color, resides in the thylakoid membranes of the chloroplast. (The internal photosynthetic membranes of some prokaryotes are also called thylakoid membranes. It is the light energy by chlorophyll that drives the synthesis of organic molecules in chloroplast. Now that we have looked at the sites of photosynthesis in plants, we are ready to look more closely at the process of photosynthesis. |
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How do we track atoms through photosynthesis? |
Scientists have tried for centuries to piece together the process by which plants make food. Although some of the steps are still not completely understood, the overall photosynthetic equation has been known since the 1800s. (Wow we still do not fully understand it yet.) In the presence of light, the green parts of plants produce organic compounds and oxygen from carbon dioxide and water. Using molecular formulas, we can summarize the complex series of chemical reactions in photosynthesis with this chemical equation: 6Carbons + 12 Waters + Light Energy -> 1 glucose + 6 Oxygens + 6 Waters |
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What was one of the first clues to the mechanism of photosynthesis? |
One of the first clues to the mechanism of photosynthesis came from the discovery that O2 given off by plants is derived from water and not from CO2. The chloroplast splits water into hydrogen and oxygen. Before this discovery, the prevailing hypothesis was that photosynthesis split carbon dioxide (CO2 -> C + O2) and then added water to the carbon (C + H2O -> CH2O). This hypothesis predicted that O2 release during photosynthesis came from CO2. |
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When was the idea that photosynthesis split carbon dioxide challenged and why? |
This idea was challenged in the 1930s by C.B. van Niel, of Stanford University. Van Niel was investigating photosynthesis in bacteria that make their carbohydrate from CO2 but do not release O2. He concluded that, at least in these bacteria, CO2 is not split into carbon and oxygen. One group of bacteria used hydrogen sulfide (H2S) rather than water for photosynthesis, forming yellow globules of sulfur as waste product. There is a different equation for photosynthesis in sulfur bacteria. Van Niel reasoned that the bacteria split H2S and used the hydrogen atoms to make sugar. He then generalized that idea, proposing that all photosynthetic organisms require a hydrogen source but that the sources varies, thus van Niel hypothesized that plants split H2O as a source of electrons from hydrogen atoms, releasing O2 as a by-product. |
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What happened 20 years later after Van Niels proposed his hypothesis of photosynthesis? |
Nearly 20 years later, scientists confirmed van Niels hypothesis by using oxygen-18, a heavy isotope, as a tracer to the follow the fate of oxygen atoms during photosynthesis. The experiments showed that the O2 from plants was labeled oxygen-18 only if water was the source of the tracer (experiment 1). If the oxygen-18 was introduced to the plant in the form of CO2, the label did not turn up in the released O2 (experiment 2). A significant result of the shuffling of atoms during photosynthesis is the extraction of hydrogen from water and its incorporation into sugar. The waste product of photosynthesis, O2, is released to the atmosphere. |
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How are photosynthesis and cellular respiration alike? |
Both processes involve redox reactions. During cellular respiration, energy is released from sugar when electrons associated with hydrogen are transported by carriers to oxygen, forming water as a by-product. The electrons lose potential energy as they "fall" down the electron transport chain toward electronegative oxygen, and the mitochondrion harnesses that energy to synthesize ATP. Photosynthesis reverses the direction of electron flow. Water is split, and electrons are transferred along with hydrogen ions from water to carbon dioxide, reducing it to sugar. Because the electrons increase potential energy as they move from water to sugar, this process requires energy – in words is endergonic. This energy boost is provided by light. |
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How does photosynthesis work in the chloroplast? |
In the chloroplast, the thylakoid membranes are the sites of the light reactions, whereas the Calvin cycle occurs in the stroma. Light reactions use solar energy to make ATP and NADPH, which supply chemical energy and reducing power, respectively to the Calvin cycle. The Calvin cycle incorporates CO2 into organic molecules, which are converted to sugar. (Recall that most simple sugars have formulas that are some multiple of CH2O.) |
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Is photosynthesis a single process? |
No it is not. Photosynthesis is two processes, each with multiple steps. These two steps of photosynthesis are known as the light reactions (the "photo" part of photosynthesis) and the Calvin cycle (the "synthesis" part). |
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Tell me a bit more about the "photo" part of photosynthesis, the light reactions. |
The light reactions are the steps of photosynthesis that convert solar energy to chemical energy. Water is split, providing a source of electrons and protons (H+, hydrogen ions) and giving off O2 as a by-product. Light absorbed by chlorophyll drives a transfer of the electrons and hydrogen ions from water to an acceptor called NADP+, where they are temporarily stored. The electron acceptor NADP+ is first cousin to NAD+, which functions as an electron carrier in cellular respiration. |
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How do NAD+ and NADP+ differ? |
The two molecules differ only by the presence of an extra phosphate group in the NADP+ molecule. The light reactions use solar power to reduce NADP+ to NADPH by adding a pair of electrons with an H+. |
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What is photophosphorylation? |
The light reactions also generate ATP, using chemiosmosis to power the addition of a phosphate group to ADP, a process called photophosphorylation. Thus light is initially converted to chemical energy in the form of two compounds: NADPH, a source of electrons as "reducing power" that can be passed along to an electron acceptor, reducing it, and ATP, the versatile energy currency of cells. Notice that the light reactions produce no sugar; that happens in the second stage of photosynthesis. |
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Who is the Calvin cycle named after? |
The Calvin cycle is named for Melvin Calvin, who, along with his colleagues, began to elucidate its steps in the late 1940s. The cycle begins by incorporating CO2 from the air into organic molecules already present in the chloroplast. This initial incorporation of carbon into organic compounds is known as carbon fixation. |
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What happens in the Calvin cycle after carbon fixation is completed? |
The Calvin cycle then reduces (adds electrons) the fixed carbon to carbohydrate by addition of electrons. The reducing power is provided by NADPH, which acquired its cargo of electrons in the light reactions. To convert CO2 to carbohydrate, the Calvin cycle also requires chemical energy in the form of ATP, which is also generated by the light reactions. Thus, it is the Calvin cycle that makes sugar, but it can do so only with the help of the NADPH and ATP produced by the light reactions. |
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Why are the metabolic steps of the Calvin cycle sometimes referred to as the dark reactions or light-independent reactions? |
This is because none of the steps requires light directly. Nevertheless, the Calvin cycle in most plants occurs during daylight, for only then can the light reactions provide the NADPH and ATP that the Calvin cycle requires. In essence, the chloroplast uses light energy to make sugar by coordinating the two stages of photosynthesis. |
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Where do the two processes of photosynthesis, the light reactions and the Calvin cycle occur? |
The thylakoids of the chloroplast are the sites of the light reactions, while the Calvin cycle occurs in the stroma. On the outside of the thylakoids, molecules of NADP+ and ADP pick up electrons and phosphate, respectively, and NADPH and ATP are then released to the stroma, where they play crucial roles in the Calvin cycle. The two stages of photosynthesis are treated in this figure as metabolic modules that take in ingredients and crank out products, in the form of sugars, ATP, and oxygen that we can breath. |
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How do the reactant molecules of photosynthesis reach the chloroplast in leaves? |
The CO2 enters leaves via stomata, and water enters via roots and is carried to leaves through veins. |
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How did the use of an oxygen isotope help elucidate the chemistry of photosynthesis? |
Using oxygen-18, a heavy isotope of oxygen, as a label, researchers were able to confirm van Niel’s hypothesis that oxygen produced during photosynthesis originates in water, not in carbon dioxide. |
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The Calvin cycle requires ATP and NADPH, products of the light reactions. If a classmate asserted that the light reactions don’t depend on the Calvin cycle and, with continual light, could just keep on producing ATP and NADPH, how would you respond? |
The light reactions could not keep producing NADPH and ATP without the NADP+, ATP, and Phosphate that the Calvin cycle generates. The two cycles are interdependent. |
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