plant ch 7

In what ways do C4 plants have an advantage over C3 plants?

C4 plants outcompete C3 plants when grown at high temperatures because the optimum temperature range for photosynthesis is higher for C4 plants than C3 plants. In addition, C4 plants outcompete C3 plants when grown under dry conditions because C4 plants utilize CO2 more efficiently and thus can attain the same photosynthetic rate as C3 plants but with smaller stomatal openings. Also, C4 plants are able to utilize nitrogen more efficiently than C3 plants.

Chlorophyll absorbs light principally in the green wavelength.

False

Explain how the Hill reaction and the use of 18O provided evidence for van Niel's proposal that water, not carbon dioxide, is the source of the oxygen evolved in photosynthesis.

In 1937, Robin Hill showed that isolated chloroplasts, when exposed to light, were able to produce O2 in the absence of CO2. This light-driven release of O2 in the absence of CO2—called the Hill reaction—occurred only when the chloroplasts were illuminated and provided with an artificial electron acceptor. More convincing evidence that the O2 released in photosynthesis is derived from H2O came in 1941, when Samuel Ruben and Martin Kamen used a heavy isotope of oxygen (18O) to trace the oxygen from water to oxygen gas: CO2 + 2H2^18O -light-> (CH2O) + H2O + ^18O2 Thus, in the case of algae and green plants, in which water serves as the electron donor, a complete, balanced equation for photosynthesis can be written as follows: 3CO2 +6H2O -light-> C3h6O3 + 3O2 + 3H2O

Briefly explain the role of each of the following protein complexes in photosynthesis: Photosystem II, cytochrome b6/f, Photosystem I, and ATP synthase.

In the chloroplast, the pigment molecules (chlorophylls a and b and carotenoids) are embedded in the thylakoids in discrete units of organization called photosystems.These units consist of 250 to 400 pigment molecules and two closely linked components: an antenna complex and a reaction center. The antenna complex is a group of pigment molecules that gather light energy and "funnel" it to the reaction center. The reaction center is made up of a complex of proteins and chlorophyll molecules that enable light energy to be converted into chemical energy. Two different kinds of photosystems, Photosystem I and Photosystem II, are linked together by an electron transport chain. In Photosystem I, a pair of chlorophyll a molecules in the reaction center is known as P700. The "P" stands for pigment and the subscript "700" designates the optimal absorption peak in nanometers. The reaction center of Photosystem II also contains a special pair of chlorophyll a molecules. Its optimal absorption peak is at 680 nanometers, and accordingly it is called P680. In general, Photosystem I and Photosystem II work together simultaneously and continuously but are spatially separated. Photosystem II is located primarily in the grana thylakoids and Photosystem I almost entirely in the stroma thylakoids and at the marginsof the grana thylakoids. In Photosystem II, light energy is absorbed by molecules of P680 in the reaction center by resonance energy transfer from one or more of the antenna molecules. When a P680 molecule is excited, its energized electron is transferred to a primary acceptor molecule, Pheophytin, a modified chlorophyll molecule in which the central magnesium atom has been replaced by two protons. Pheophytin then passes the electron to PQA, a plastoquinone, which is tightly bound to the reaction center. Next, PQA passes two electrons to PQB, another plastoquinone, which simultaneously picks up two protons from the stroma, thereby becoming reduced to plastoquinol, PQBH2. The plastoquinol then joins a pool of mobile plastoquinol molecules in the interior lipid portion of the thylakoid membrane. The plastoquinol can now pass two electrons and two protons (H +) to the cytochrome b6/f complex and is thus oxidized back to PQB. Photosystem II has the unique ability to extract electrons from water and to use these electrons to replace those lost by P680 (now P680+) to plastoquinone. This light-dependent oxidative splitting of water molecules is called water photolysis. The oxygen-evolving complex is located on the inside of the thylakoid membrane, and the protons are released into the lumen of the thylakoid. The pumping of protons across the thylakoid membrane and into the lumen via the cytochrome b6/f complex generates an electrochemical proton gradient that drives the synthesis of ATP. Thus, the photolysis of water molecules contributes to the generation of a proton gradient across the thylakoid membrane—the sole means by which ATP is generated during photosynthesis. ATP synthase complexes, embedded in the thylakoid membrane, provide a channel through which the protons can flow down the gradient, back into the stroma (see Figure 7-12). As they do so, the potential energy of the gradient drives the synthesis of ATP from ADP and Pi. In Photosystem I, light energy excites antenna molecules, which pass the energy to the P700 molecules at the reaction center. When a P700 molecule is excited in this way, its energized electron is passed to a primary acceptor molecule called A0, a special chlorophyll with a function similar to the pheophytin of Photosystem II. The electrons are then passed downhill through a chain of carriers, including phylloquinone (A1), and iron-sulfur proteins, including ferredoxin. Ferredoxin (Fd), a mobile iron-sulfur protein, is found in the chloroplast stroma. It is the final electron acceptor of Photosystem I. Electrons are transferred from ferredoxin to NADP+. This results in the reduction of the NADP+ to NADPH and oxidation of the P700 molecule. The electrons removed from the P700 molecule are replaced by the electrons that have moved down the electron transport chain from Photosystem II, and are carried to P700 by plastocyanin.

Distinguish between noncyclic and cyclic electron flow and photophosphorylation. What products are produced by each? Why is cyclic photophosphorylation essential to the Calvin cycle?

In the light, electrons flow continuously from water through Photosystems II and I to NADP+, resulting in the oxidation of water (H2O) to oxygen (O2) and the reduction of NADP+ to NADPH. This unidirectional flow of electrons from water to NADP+ is called noncyclic electron flow, and the ATP production that occurs is called noncyclic photophosphorylation. In the process, called cyclic electron flow, energized electrons are transferred from P700 to A0, as before. Instead of being passed downhill to NADP+, however, the electrons are shunted to an acceptor in the electron transport chain between Photosystems I and II. The electrons then pass downhill through that chain back into the reaction center of Photosystem I, driving the transport of protons across the thylakoid membrane and hence powering the generation of ATP. Because this process involves a cyclic flow of electrons, it is called cyclic photophosphorylation. The total energy harvest from noncyclic electron flow (based on the passage of 6 pairs of electrons from H2O to NADP+) is 6 ATP and 6 NADPH. Yet, the carbon-fixation reactions require more ATP than NADPH—at a ratio of about 3:2. Cyclic photophosphorylation, providing extra ATP, is therefore an ongoing necessity to meet the needs of the Calvin cycle, as well as to drive a host of other energy-requiring processes within the chloroplast.

Suaeda aralocaspica is different from many other C4 plants because it: a. has only one type of chloroplast. b. has Kranz anatomy. c. lacks Kranz anatomy. d. has bundle-sheath cells. e. lacks bundle-sheath cells.

c. lacks Kranz anatomy.

Whereas the C4 pathway and the Calvin cycle (C3 pathway) are spatially separated in C4 plants, in CAM plants these two pathways are temporally separated. Explain.

Oxaloacetate is formed when CO2 is fixed to phosphoenolpyruvate (PEP) in a reaction catalyzed by the enzyme PEP carboxylase, which is found in the cytosol of mesophyll cells of C4 plants. The oxaloacetate is then reduced to malate or converted, with the addition of an amino group, to the amino acid aspartate in the chloroplast of the same cell. The malate or aspartate (depending on the species) moves from the mesophyll cells to bundle-sheath cells surrounding the vascular bundles of the leaf, where the malate or aspartate is decarboxylated to yield CO2 and pyruvate. The CO2 then enters the Calvin cycle by reacting with RuBP to form 3-phosphoglycerate. Meanwhile, the pyruvate returns to the mesophyll cells, where it reacts with ATP to regenerate PEP. Hence, the anatomy of the leaves of C4 plants establishes a spatial separation between the C4 pathway and the Calvin cycle, which occur in two different types of cells. CAM plants, like C4 plants, use both the C4 pathway and the Calvin cycle. In CAM plants, however, there is a temporal separation—a separation in time—rather than a spatial separation of the two pathways. Because CAM involves the formation of malic acid at night and its disappearance during daytime, CAM plants are known as plants that taste sour at night and sweet during the day. (a) CO2 is first fixed at night when the stomata are open. At night, starch from the chloroplast is broken down as far as phosphoenolpyruvate (PEP). Carbon dioxide, hydrated to form HCO3- (bicarbonate ion), reacts with PEP to form oxaloacetate, which then is reduced to malate. Most of the malate is pumped into the vacuole and stored there as malic acid. (b) During the daytime, the malic acid is recovered from the vacuole and decarboxylated, producing CO2 and pyruvate. The CO2 enters the Calvin cycle, where it is refixed by Rubisco. Much of the pyruvate may be converted to sugars and starch by reverse glycolysis. Stomatal closure during the daytime prevents loss of water and of the CO2 released by decarboxylation of malate.

What is photophosphorylation, and what is the relationship between this process and the thylakoid membrane?

Photophosphorylation refers to the formation of ATP as a consequence of a proton gradient that results from the light-driven flow of electrons. The electron carriers of the photosynthetic electron transport chain are embedded in the thylakoid membrane. The flow of electrons along the electron transport chain causes protons to be pumped from the stroma across the thylakoid membrane into the thylakoid lumen, thus establishing an electrochemical gradient. Protons then flow down this gradient back across the thylakoid membrane into the stroma through the ATP synthase complex, resulting in the synthesis of ATP.

What is the relationship between the absorption spectrum of a pigment and the action spectrum of a process that depends on that same pigment?

The absorption spectrum of a pigment demonstrates which wavelengths of light the pigment absorbs to the greatest extent, and the action spectrum demonstrates the relative effectiveness of different wavelengths of light for a specific light-requiring process.

Under extremely dry conditions, CAM plants outcompete C3 and C4 plants.

true

CAM plants are said to taste sweet during the day and sour at night. Explain why.

The sour taste at night is due to the accumulation of malic acid that results from the nighttime fixation of CO2. During the day, the sour malic acid is removed by decarboxylation, and the plant tastes relatively sweet.

By means of a labeled diagram, explain the term "Kranz anatomy."

Typically, the leaves of C4 plants are characterized by an orderly arrangement of the mesophyll cells around a layer of large bundle-sheath cells, so that together the two form concentric layers around the vascular bundle. This wreathlike arrangement has been termed Kranz anatomy (Kranz is the German word for "wreath").

As excited electrons return to ground level, the energy released has three possible fates. What are those fates, and which two are useful energy-releasing events in photosynthesis?

When chlorophyll molecules (or other pigment molecules) absorb light, electrons are temporarily boosted to a higher energy level, called the excited state. As the electrons return to their lower energy level, or ground level, the energy released has three possible fates. The first possibility is that the energy is converted to heat, or partly to heat, but is mostly released as another, less energetic photon, a phenomenon known as fluorescence. The wavelength of the emitted light is slightly longer (and of lower energy) than the absorbed light, because a portion of the excitation energy is converted to heat before the less energetic, or fluorescent, photon is emitted. A second possibility is that the energy—but not the electron—may be transferred from the excited chlorophyll molecule to a neighboring chlorophyll molecule, exciting the second molecule and allowing the first one to return to its ground (unexcited) state. This process is known as resonance energy transfer, and it may be repeated to a third, a fourth, or further chlorophyll molecules. The third possibility is that the high-energy electron itself may be transferred to a neighboring molecule (an electron acceptor) that is part of an electron transport chain, leaving an "electron hole" in the excited chlorophyll molecule. This possibility results in the oxidation of the chlorophyll molecule and the reduction of an electron acceptor.

F. F. Blackman showed that: a. air "restored" by vegetation could support the breathing of animals. b. air is "restored" only in the presence of light and only by the green parts of the plant. c. photosynthesis has a light-dependent stage and a light-independent stage. d. isolated chloroplasts are able to produce O2 in the absence of light. e. all the substance of a plant is provided by water and not the soil.

c. photosynthesis has a light-dependent stage and a light-independent stage.

In the antenna complex, light energy is transferred from one pigment molecule to another by: a. pigment activation. b. fluorescence. c. resonance energy transfer. d. reduction. e. oxidation.

c. resonance energy transfer.

The role of Rubisco is to catalyze the conversion of: a. CO2 to an unstable six-carbon compound. b. CO2 to glyceraldehyde 3-phosphate. c. 3-phosphoglycerate to glyceraldehyde 3-phosphate. d. glyceraldehyde 3-phosphate to sucrose. e. glyceraldehyde 3-phosphate to starch.

a. CO2 to an unstable six-carbon compound.

Which pigment occurs in all photosynthetic eukaryotes? a. Chlorophyll a b. Chlorophyll b c. Chlorophyll c d. Bacteriochlorophyll e. Chlorobium chlorophyll

a. Chlorophyll a

Which of the following statements about photorespiration is FALSE? a. It yields ATP but not NADPH. b. Phosphoglycolate is an intermediate. c. It consumes oxygen and releases CO2. d. It is a wasteful process. e. Three cellular organelles participate in the process.

a. It yields ATP but not NADPH.

The energy-transduction reactions of photosynthesis are also called the ______ reactions. a. light b. dark c. light-independent d. carbon-fixation e. biosynthetic

a. light

Following photolysis, the resulting protons are released into the ______, contributing to the proton gradient across the ______ membrane. a. lumen of the thylakoid; thylakoid b. chloroplast stroma; outer chloroplast c. chloroplast stroma; thylakoid d. chloroplast matrix; inner chloroplast e. cytosol; inner mitochondrial

a. lumen of the thylakoid; thylakoid

How many molecules of CO2 are fixed during each turn of the Calvin cycle? a. one b. two c. three d. four e. five

a. one

In the light reactions, the cytochrome b6/f complex receives electrons directly from: a. plastoquinol. b. ferredoxin. c. pheophytin d. manganese. e. plastocyanin.

a. plastoquinol.

The malate or aspartate produced in the C4 pathway moves next into: a. the bundle-sheath cells. b. the mesophyll cells. c. the stomata. d. Photosystem I. e. Photosystem II.

a. the bundle-sheath cells.

In CAM plants, malate formed as the end product of CO2 fixation in the dark is stored as malic acid in the: a. vacuole. b. chloroplast stroma. c. thylakoid lumen. d. cytosol. e. nucleus.

a. vacuole.

Which of the following does NOT occur in the Calvin cycle? a. ATP is hydrolyzed. b. ADP is phosphorylated to ATP. c. NADPH is oxidized. d. Ribulose 1,5-bisphosphate is regenerated. e. CO2 is fixed

b. ADP is phosphorylated to ATP.

Which of the following statements about CAM plants is FALSE? a. Not all CAM plants are succulent. b. All CAM plants are flowering plants. c. They use both C3 and C4 pathways. d. They are dependent on nighttime accumulation of CO2 for photosynthesis. e. Their water-use efficiency is higher than that of C3 and C4 plants

b. All CAM plants are flowering plants.

Which of the following statements concerning ferredoxin is FALSE? a. It is the final electron acceptor of Photosystem I. b. It is found in the chloroplast grana. c. It transfers electrons to NADP+. d. It is an iron-sulfur protein. e. It is a mobile protein.

b. It is found in the chloroplast grana.

Rubisco can use ______ or CO2 as a substrate. a. 3-phosphoglycerate b. O2 c. glyceraldehyde 3-phosphate d. serine e. oxaloacetate

b. O2

Carbon dioxide is "fixed" by bonding to: a. glyceraldehyde 3-phosphate. b. 3-phosphoglycerate. c. ribulose 1,5-bisphosphate. d. NADP+. e. ADP

c. ribulose 1,5-bisphosphate.

During cyclic electron flow, electrons are transferred directly from P700 to Ao to: a. P700. b. P680. c. the photosynthetic electron transport chain. d. the photosynthetic ATP synthase. e. the lumen of the thylakoid

c. the photosynthetic electron transport chain.

Which of the following events is NOT associated with Photosystem I? a. absorption of light by antenna molecules b. excitation of an electron from P700 c. transfer of electrons from cytochromes to iron-sulfur proteins d. reduction of NADP+ e. reduction of Ao

c. transfer of electrons from cytochromes to iron-sulfur proteins

The primary function of ______ is as an anti-oxidant. a. chlorophyll a b. carotenoids c. phycobilins d. bacteriochlorophyll e. chlorobium chlorophyll

b. carotenoids

In photophosphorylation, the role of the ATP synthase complex is to provide a channel for protons to flow back into the: a. lumen of the thylakoid. b. chloroplast stroma. c. intermembrane space of the mitochondrion. d. intermembrane space of the chloroplast. e. cytosol

b. chloroplast stroma.

The Calvin cycle takes place in the: a. lumen of the thylakoid. b. chloroplast stroma. c. thylakoid membrane. d. cytoplasm. e. mitochondrial matrix.

b. chloroplast stroma.

The light-harvesting complex is different from a photosystem in that the light-harvesting complex: a. lacks chlorophyll a. b. lacks a reaction center. c. lacks carotenoids. d. collects light energy. e. contains pigment-binding proteins.

b. lacks a reaction center.

Kranz anatomy is characterized by a layer of ____ around a layer of ____. a. xylem cells; mesophyll cells b. mesophyll cells; bundle-sheath cells c. mesophyll cells; phloem cells d. bundle-sheath cells; phloem cells e. bundle-sheath cells; mesophyll cells

b. mesophyll cells; bundle-sheath cells

One of the benefits of photorespiration is removing toxic: a. 3-phosphoglyceraldehyde. b. phosphoglycolate. c. ribulose-bisphosphate. d. 3-phosphoglycerate. e. oxaloacetate

b. phosphoglycolate.

The O2 evolved in photosynthesis comes from: a. carbon dioxide. b. water. c. glucose. d. (CH2O). e. (C3H3O3).

b. water.

Which of the following is the very next event that occurs when a chlorophyll molecule absorbs light? a. The energy is released as heat. b. Fluorescence occurs. c. The electron is boosted to an excited state. d. Resonance energy transfer occurs. e. The electron is transferred to an electron transport chain.

c. The electron is boosted to an excited state.

Xanthophylls and carotenes: a. are the principal photosynthetic pigments in green plants. b. are the principal sources of vitamin C for humans. c. are carotenoids. d. are normally present in the cytosol rather than in plastids. e. can substitute for chlorophylls in photosynthesis.

c. are carotenoids.

Chlorophyll absorbs light principally in the ______ wavelengths. a. blue and green b. green and violet c. blue and violet d. violet and green e. green and red

c. blue and violet

Which of the following is most likely to occur in a leaf cell of a CAM plant during the day? a. entry of CO2 through stomata b. exit of water through stomata c. decarboxylation of malic acid d. fixation of CO2 by PEP carboxylase e. conversion of oxaloacetate to malate

c. decarboxylation of malic acid

In contrast to Photosystem I, Photosystem II is located primarily: a. in the chloroplast. b. in the cytosol. c. in grana thylakoids. d. in stroma thylokoids e. in the plasma membrane.

c. in grana thylakoids.

Which of the following is produced during noncyclic AND cyclic electron flow? a. water b. NADPH c. sugar d. ATP e. O2

d. ATP

Which of the following conditions favors photorespiration? a. a ratio of CO2 to O2 that favors CO2 b. conditions that cause the stomata to open c. plants growing far apart d. a hot, dry environment e. darkness

d. a hot, dry environment

Which of the following is a C4 plant? a. rice b. oats c. Kentucky bluegrass d. crabgrass e. wheat

d. crabgrass

Most of the glyceraldehyde 3-phosphate not exported to the cytosol is converted to ______ and stored in the chloroplasts. a. 3-phosphoglycerate b. sucrose c. glucose d. starch e. ribulose 1,5-bisphosphate

d. starch

Compared with C3 plants, C4 plants: a. are well adapted to low light intensities. b. are well adapted to low temperatures. c. are well adapted to moist areas. d. use nitrogen more efficiently. e. fix CO2 less efficiently.

d. use nitrogen more efficiently.

Which of the following statements about an antenna complex is FALSE? a. It is part of a photosystem. b. It "funnels" energy to the reaction center. c. It contains chlorophyll molecules. d. It contains carotenoid pigments. e. It converts light energy into chemical energy

e. It converts light energy into chemical energy

Which of the following statements about the Calvin cycle is FALSE? a. It requires more ATP than NADPH. b. Each reaction is catalyzed by a specific enzyme. c. It regenerates ribulose 1,5-bisphosphate. d. It fixes CO2. e. It uses ATP from noncyclic, but not cyclic, photophosphorylation

e. It uses ATP from noncyclic, but not cyclic, photophosphorylation

Who provided the first experimental evidence that soil alone does not nourish the plant? a. Jan Ingenhousz b. Aristotle c. F. F. Blackman d. Joseph Priestley e. Jan Baptista van Helmont

e. Jan Baptista van Helmont

In Photosystem II, energized electrons are transferred from pheophytin directly to: a. chlorophyll a. b. NADP+. c. the oxygen-evolving complex. d. plastoquinol. e. PQA.

e. PQA.

Which of the following statements about the electromagnetic spectrum is FALSE? a. All radiations travel in waves. b. White light consists of a number of different colors. c. Different colors of light are refracted at different angles. d. A wavelength is the distance from one wave crest to the crest of another. e. The longer the wavelength of light, the more energy it has.

e. The longer the wavelength of light, the more energy it has.

In the C4 pathway, the enzyme PEP carboxylase: a. uses O2 as a substrate. b. uses CO2 as a substrate. c. operates inefficiently when the CO2 concentration is low. d. is active only in the chloroplasts of mesophyll cells. e. catalyzes the formation of oxaloacetate.

e. catalyzes the formation of oxaloacetate.

The ______ complex links photosystems I and II. a. ATP synthase b. light-harvesting c. oxygen-evolving d. water photolysis e. cytochrome b6/f

e. cytochrome b6/f

An action spectrum is different from an absorption spectrum in that an action spectrum: a. provides evidence that a particular pigment is responsible for a particular process. b. provides information about the extent of reflectance. c. is the light-transmitting pattern of a pigment. d. is the light-absorbing pattern of a pigment. e. is the relative effectiveness of different wavelengths for a specific process

e. is the relative effectiveness of different wavelengths for a specific process

Compared with a C3 plant, a C4 plant: a. carries out more photorespiration. b. has a lower photosynthetic efficiency. c. has more Rubisco. d. has a higher leaf nitrogen content. e. needs more ATP to fix CO2.

e. needs more ATP to fix CO2.

Light is composed of particles called: a. electrons. b. protons. c. neutrons. d. gamma rays. e. photons.

e. photons.

Which of the following is not considered a CAM plant? a. cactus b. pineapple c. Spanish "moss" d. Welwitschia e. wheat

e. wheat

Accessory pigments are directly involved in the energy-transduction processes of photosynthesis.

false

All photosynthetic eukaryotes contain both chlorophylls a and b.

false

At the reaction center of Photosystem II is P700.

false

C3-C4 intermediates provide evidence for the evolution of C3 plants from C4 ancestors.

false

In the process of cyclic photophosphorylation, both ATP and NADPH are produced.

false

Photorespiration is an energetically wasteful process that seldom occurs in nature.

false

Protons flow through the ATP synthesis from the stroma into the thylakoid lumen.

false

The concentration of CO2 in the Earth's atmosphere is approximately 3 percent.

false

The energy of a photon of light is directly proportional to its wavelength.

false

An action spectrum can be used to demonstrate the relative effectiveness of different wavelengths of light for photosynthesis.

true

C. B. van Neil's experiments provided evidence that water, not carbon dioxide, was the source of the oxygen in photosynthesis.

true

In Photosystem II, pheophytin passes an electron directly to plastoquinone.

true

One of the benefits of photorespiration is to remove phosphoglycolate from the plant.

true

Phycobilins are water-soluble accessory pigments found in cyanobacteria and red algae.

true

Rubisco can use either O2 or CO2 as a substrate.

true

Some CAM plants can keep their stomata closed night and day.

true

The ATP and NADPH generated by the light reactions are used in the Calvin cycle.

true

The cytochrome b6/f complex links Photosystems II and I.

true

The leaf anatomy of C4 plants provides a spatial separation between the C4 pathway and the Calvin cycle.

true

The light-harvesting complex does not contain a reaction center.

true

The photolysis of water occurs in conjunction with Photosystem II but not Photosystem I.

true