Biology Photosynthesis
The process of photosynthesis probably originated _____.
in prokaryotes.
The light reactions supply the Calvin cycle with
ATP and NADPH
Which of the following questions is most relevant to understanding the Calvin cycle?
How is ATP used in the formation of 3-carbon carbohydrates?
How is photosynthesis similar in C4 plants and CAM plants?
In both cases, rubisco is not used to fix carbon initially.
In phase two, the reduction stage, what molecule will donate electrons, and therefore is the source of the reducing power?
NADPH
What most likely causes the trends in oxygen concentration shown in the graph above?
Photosynthesis produces more oxygen than is consumed by respiration during the day.
What is the source of O2 in the atmosphere?
Splitting of water
Where do the light reactions take place?
Thylakoid membrane
A photosystem is composed of a protein complex called a ________ - ________ complex surrounded by several __________ - _____________ complexes.
reaction - center, light - harvesting
How many turns of the Calvin cycle are required for 1 G3P to be synthesized.
3
What is the output for the light reactions?
ATP and NADPH
Which of the following best describes the role of ATP and NADPH in the process of photosynthesis?
ATP and NADPH are carriers of chemical energy that is used in the production of carbohydrates.
If the input water is labeled with a radioactive isotope of oxygen, 18 O. then the oxygen gas released as the reaction proceeds is also labeled with 18 O. Which of the following is the most likely explanation?
During the light reactions of photosynthesis, water is split, removing electrons and protons, and oxygen gas is released.
Study Inquiry Figure 10.9 on p. 194 which describes the classic experiment of Theodor W. Englemann. Describe how he determined an action spectrum long before the invention of a spectrophotometer.
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 O 2 and thus photosynthesizing most. Bacteria congregated in greatest numbers around the parts of the alga illuminated with violet-blue or red light.
Cyclic electron flow can be visualized in Figure 10.15 in your text. Cyclic electron flow is thought to be similar to the first forms of photosynthesis to evolve. In cyclic electron flow no water is split, there is no production of ____________ , and there is no release of _____________. (The Big Picture: Why did oxygen not accumulate in the atmosphere until cyclic electron flow evolved?)
NADPH, Oxygen, If cyclic electron flow is thought to be like the first forms of photosynthesis, this series of reactions are not associated with a release of oxygen, therefore, an accumulation of oxygen would not occur. It is only through noncyclic electron flow that oxygen would be released and would be responsible for accumulating oxygen in the atmosphere.
Plants photosynthesize _____.
Only in the light but respire in light and dark.
Explain how photorespiration can be a problem in agriculture.
Rice, wheat, and soybeans are C3 plants that are important in agriculture. When their stomata partially close on hot, dry days, photorespiration drains away as much as 50% of the carbon fixed by the Calvin cycle. C 3 plants produce less sugar because the declining level of CO2 in the leaf starves the Calvin cycle, limiting growth.
Photosynthesis is not a single process, but two processes, each with multiple steps. a. Explain what occurs in the light reactions stage of photosynthesis. Be sure to use NADP + and photophosphorylation in your discussion.
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 (hydrogen ions, H+) 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 + (nicotinamide adenine dinucleotide phosphate), where they are temporarily stored. The light reactions use solar power to reduce NADP+ to NADPH by adding a pair of electrons along with an H+. The light reactions also generate ATP, using chemiosmosis to power the addition of a phosphate group to ADP, a process called photophosphorylation.
Which of the following is an important difference between light-dependent and light-independent reactions of photosynthesis?
The light-dependent reactions produce ATP and NADPH; the light-independent reactions use energy stored in ATP and NADPH.
As electrons fall from photosystem II to photosystem I, the cytochrome complex uses the energy to pump ____________ ions. This builds a proton gradient that is used in chemiosmosis to produce what molecule? _______
hydrogen, ATP.
Some of the types of energy in the electromagnetic spectrum will be familiar, such as X-rays, microwaves, and radio waves. The most important part of the spectrum in photosynthesis is visible light. In order, what are the colors of the visible spectrum?
Violet, indigo, blue, green, yellow, orange, red.
The enzyme responsible for carbon fixation in the Calvin cycle, and possibly the most abundant protein on Earth, is ____________.
rubisco
Chlorophyll b
An accessory photosynthetic pigment that transfers energy to chlorophyll a.
Reaction center complex
A complex of proteins associated with a special pair of chlorophyll a molecules and a primary electron acceptor. Located centrally in a photosystem, this complex triggers the light reactions of photosynthesis. Excited by light energy, the pair of chlorophylls donates an electron to the primary electron acceptor, which passes an electron to an electron transport chain.
action spectrum
A graph that profiles the relative effectiveness of different wavelengths of radiation in driving a particular process.
Chlorophyll
A green pigment located in membranes within the chloroplasts of plants and algae and in the membranes of certain prokaryotes. Chlorophyll a participates directly in the light reactions, which convert solar energy to chemical energy.
Photosystem I
A light-capturing unit in a chloroplast's thylakoid membrane or in the membrane of some prokaryotes; it has two molecules of P700 chlorophyll a at its reaction center.
crassulacean acid metabolism (CAM)
An adaptation for photosynthesis in arid conditions, first discovered in the family Crassulaceae. In this process, a plant takes up CO2 and incorporates it into a variety of organic acids at night; during the day, CO2 is released from organic acids for use in the Calvin cycle.
Which of the following is a reactant in photosynthesis?
Carbon Dioxide
A scientist claims that Elysia chlorotica, a species of sea slug, is capable of photosynthesis. Which of the following observations provides the best evidence to support the claim?
Elysia chlorotica grows when exposed to light in the absence of other food sources.
The major - light absorbing pigment in plant photosynthesis is a parotenoid.
False
In this reduction stage, the low-energy acid 1,3-bisphosphoglycerate is reduced by electrons from NADPH to form the three-carbon sugar _________ .
G3P
What is the output for the Calvin cycle?
G3P
The carbohydrate produced directly from the Calvin cycle is not glucose, but the three- carbon compound _________________. Each turn of the Calvin cycle fixes one molecule of CO 2 ; therefore, it will take __ turns of the Calvin cycle to net one G3P.
G3P, 3
Which of the following sequences correctly represents the flow of electrons during photosynthesis
H20 -> NADPH -> Calvin cycle
primary electron acceptor
In the thylakoid membrane of a chloroplast or in the membrane of some prokaryotes, a specialized molecule that shares the reaction-center complex with a pair of chlorophyll a molecules and that accepts an electron from them.
The rate of oxygen production equaled the rate of oxygen consumption during which of the following time periods?
J to K
Notice the colors and their corresponding wavelengths. Explain the relationship between wavelength and energy.
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. The distance between the crests of electromagnetic waves is called the wavelength. The amount of energy is inversely related to the wavelength of light: the shorter the wavelength, the greater the energy of each photon of light. Alternatively, the longer the wavelength of light, the lower the amount of energy of each photon.
Photosystem II
One of two light-capturing units in a chloroplast's thylakoid membrane or in the membrane of some prokaryotes; it has two molecules of P680 chlorophyll a at its reaction center.
Rubisco
Ribulose bisphosphate (RuBP) carboxylase-oxygenase, the enzyme that normally catalyzes the first step of the Calvin cycle (the addition of CO2 to RuBP). When excess O2 is present or CO2 levels are low, it can bind oxygen, resulting in photorespiration.
Calvin Cycle
The second of two major stages in photosynthesis (following the light reactions), involving fixation of atmospheric CO2 and reduction of the fixed carbon into carbohydrate.
Wavelength
The distance between crests of waves, such as those of the electromagnetic spectrum.
light reactions
The first of two major stages in photosynthesis (preceding the Calvin cycle). These reactions, which occur on the thylakoid membranes of the chloroplast or on membranes of certain prokaryotes, convert solar energy to the chemical energy of ATP and NADPH, releasing oxygen in the process.
carbon fixation
The initial incorporation of carbon from CO2 into an organic compound by an autotrophic organism (a plant, another photosynthetic organism, or a chemoautotrophic prokaryote).
NADP
The oxidized form of nicotinamide adenine dinucleotide phosphate, an electron carrier that can accept electrons, becoming NADPH. NADPH temporarily stores energized electrons produced during the light reactions.
Photophosphorylation
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.
absorption spectrum
The range of a pigment's ability to absorb various wavelengths of light; also a graph of such a range.
bundle-sheath cells
In C4 plants, a type of photosynthetic cell arranged into tightly packed sheaths around the veins of a leaf.
Which of the following directly provides the energy needed for cell functions?
A phosphate group is removed from ATP.
glyceraldehyde 3-phosphate (G3P)
A three-carbon carbohydrate that is the direct product of the Calvin cycle; it is also an intermediate in glycolysis.
In chloroplasts, ATP is synthesized from ADP plus inorganic phosphate (Pi)(Pi) in a reaction catalyzed by ATP synthase molecules that are embedded in the thylakoid membrane. Which of the following statements provides evidence to support the claim that no ATP will be synthesized in the absence of a proton gradient across the thylakoid membrane?
No ATP is synthesized when channel proteins that allow the free passage of protons are inserted into the thylakoid membrane.
What is the name of the chlorophyll a at the reaction center of PS I?
P700
All organisms need energy to carry out life processes.
True
What is the input for the light reactions?
light energy and H2O
Chlorophyll a
A photosynthetic pigment that participates directly in the light reactions, which convert solar energy to chemical energy.
C4 plants
A plant in which the Calvin cycle is preceded by reactions that incorporate CO2 into a four-carbon compound, the end product of which supplies CO2 for the Calvin cycle.
Which of the following describes the pathway of electrons in the light reactions? (O2 = oxygen, PS = photosystem)
O2 -> PS II -> PS I -> NADPH
Explain the important events that occur in the carbon fixation stage of the Calvin cycle.
The Calvin cycle incorporates each CO 2 molecule, one at a time, by attaching it to a five-carbon sugar named ribulose bisphosphate. The enzyme that catalyzes this first step is RuBP carboxylase, or rubisco. The product of the reaction is a six-carbon intermediate so unstable that it immediately splits in half, forming two molecules of 3-phosphoglycerate.
Explain the role of PEP carboxylase in C4 plants, including key differences between it and rubisco.
This enzyme adds CO2 to phosphoenolpyruvate (PEP), forming the four-carbon product oxaloacetate. PEP carboxylase has a much higher affinity for CO2 than does rubisco and no affinity for O2 . Therefore, PEP carboxylase can fix carbon efficiently when rubisco cannot—that is, when it is hot and dry and stomata are partially closed, causing CO2 concentration in the leaf to fall and O2 concentration to rise.
Photosynthesis is a process that takes place only in photoautotrophs.
True
Which process is most directly driven by light energy?
transfer of energy from pigment molecule to pigment molecule
How many ATP's are required for the Calvin Cycle to make a G3P molecule?
9
Photosynthesis is not a single process, but two processes, each with multiple steps. b. Explain the Calvin cycle, using the term carbon fixation in your discussion.
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 as known as carbon fixation. The Calvin cycle then reduces the fixed carbon to carbohydrate by the 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.
Use both chemical symbols and words to write out the formula for photosynthesis. (Use the one that indicates only the net consumption of water.) Notice that the formula is essentially the opposite of cellular respiration.
6 CO2 (carbon dioxide) + 6 H2O (water) + Light energy → C6H12O6 (glucose) + 6O2 (oxygen)
CAM plants
A plant that uses crassulacean acid metabolism, an adaptation for photosynthesis in arid conditions. In this process, CO2 entering open stomata during the night is converted to organic acids, which release CO2 for the Calvin cycle during the day, when stomata are closed.
C3 plants
A plant that uses the Calvin cycle for the initial steps that incorporate CO2 into organic material, forming a three-carbon compound as the first stable intermediate.
linear flow of electrons
A route of electron flow during the light reactions of photosynthesis that involves both photosystems (I and II) and produces ATP, NADPH, and O2. The net electron flow is from H2O to NADP+.
cyclic electron flow
A route of electron flow during the light reactions of photosynthesis that involves only one photosystem and that produces ATP but not NADPH or O2.
Carotenoids
An accessory pigment, either yellow or orange, in the chloroplasts of plants and in some prokaryotes. By absorbing wavelengths of light that chlorophyll cannot, they broaden the spectrum of colors that can drive photosynthesis.
spectrophotometer
An instrument that measures the proportions of light of different wavelengths absorbed and transmitted by a pigment solution.
Heterotroph
An organism that obtains organic food molecules by eating other organisms or substances derived from them.
Autotroph
An organism that obtains organic food molecules without eating other organisms or substances derived from other organisms. They use energy from the sun or from the oxidation of inorganic substances to make organic molecules from inorganic ones.
Using 18O as the basis of your discussion, explain how we know that the oxygen released in photosynthesis comes from water.
Scientists confirmed van Niel's hypothesis that oxygen released from photosynthesis comes from water with the use of 18O, a heavy isotope, as a tracer to follow the fate of oxygen atoms during photosynthesis. The experiments showed that the O2 from plants was labeled with 18O only if water was the source of the tracer. If the 18O was introduced to the plant in the form of CO2, the label did not turn up in the released O2.
Photosynthesis
The conversion of light energy to chemical energy that is stored in sugars or other organic compounds; occurs in plants, algae, and certain prokaryotes.
Stroma
The dense fluid within the chloroplast surrounding the thylakoid membrane and containing ribosomes and DNA; involved in the synthesis of organic molecules from carbon dioxide and water.
What compound is the source of electrons for linear electron flow?
Water
Which of the following best describes the role of water in photosynthesis?
Water molecules donate electrons to the electron transport chain.
The net production of one G3P requires ____ molecules of ATP and ____ molecules of NADPH.
9,6
Define autotrophs and heterotrophs. Which are the producers of the ecosystem? Which are the consumers? Give an example of each group.
Autotrophs are "self feeders; they sustain themselves without eating anything derived from other living beings. They produce their own organic molecules from CO2 and other inorganic raw materials obtained from the environment. Because they are the ultimate sources of organic compounds for all non autotrophic organisms, they are the producers of the biosphere. Some examples of autotrophs are the plants, multicellular alga, unicellular protists, cyanobacteria, and purple sulfur bacteria. Heterotrophs are unable to make their own food; they live on compounds produced by other organisms. They are the consumers of the biosphere. Examples of common heterotrophs are animals, decomposers, most fungi, and many prokaryotes.
Compare and contrast C 4 plants with CAM plants. In your explanation, give two key similarities and two key differences.
Both C4 and CAM plants thrive in hot, dry conditions. Both have evolved methods to reduce water loss and to "fix" carbon dioxide in an intermediate compound before it enters the Calvin cycle. C4 plants have Kranz anatomy, with thick-walled bundle-sheath cells. CO2 is added to PEP to make a four-carbon intermediate so that CO2 will not be lost through photorespiration. CAM plants prevent water loss by closing their stomata during the day, but in order to have CO2 available, it is fixed in crassulacean acid when the stomata are open at night. In C4 plants, the initial steps of carbon fixation followed by transfer of CO2 to the Calvin cycle are separated structurally by two different types of cells. In CAM plants, the two steps occur at different times (temporal separation of steps) but in the same cell.
Three turns of the Calvin cycle nets one G3P because the other five G3Ps must be recycled to three RuBPs. Explain how the regeneration of RuBP is accomplished.
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. To accomplish this, the cycle spends three more molecules of ATP. The RuBP is now prepared to receive CO2 again, and the cycle continues
In photosystem I, NADP + reductase catalyzes the transfer of the excited electron and H+ to NADP + to form ___________ .
NADPH
What is the input for the Calvin Cycle?
NADPH, CO2, H2O, G3P, ATP
:Which of the following is an important difference between light - dependent and light - independent reactions of photosynthesis?
The light - dependent reactions produce ATP and NADPH, the light - independent reactions use energy stored in ATP and NADPH.
The energy required to run the Calvin cycle reactions of photosynthesis comes from which two substances produced during the light-dependent reactions?
ATP and NADPH
What is the input for the Calvin cycle?
NADPH, CO2, ATP
Use two key differences to explain how chemiosmosis is different in photosynthesis and cellular respiration.
1. There are noteworthy differences between oxidative phosphorylation in mitochondria and photophosphorylation in chloroplasts. In mitochondria, the high-energy electrons, extracted from organic molecules, are dropped down the transport chain (which are thus oxidized), whereas in chloroplasts, the source of high energy electrons that dropped down the transport chain come from water. 2. 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. The mitochondria use chemiosmosis to transfer chemical energy from food molecules to make ATP.
It is estimated that oxygen production first evolved in photosynthetic prokaryotes approximately 2.7 billion years ago. The first photosynthetic prokaryotes are presumed to be similar to today's cyanobacteria. Which of the following best supports the claim that photosynthetic prokaryotes were responsible for the oxygen in Earth's atmosphere?
The light reactions of photosynthesis split water into hydrogen ions and oxygen.
Within the photosystems, the critical conversion of solar energy to chemical energy occurs. This process is the essence of being a producer! Using Figure 10.12 from the main text as a guide, label the following diagram and then explain the role of the components of the photosystem listed below.
a. Reaction-center complex: This complex of proteins is associated with a special pair of chlorophyll a molecules and a primary electron acceptor. Located centrally in a photosystem, this complex triggers the light reactions of photosynthesis. Excited by light energy, the pair of chlorophylls donates an electron to the primary electron acceptor, which passes an electron to an electron transport chain. b. Light-harvesting complex: This complex of proteins associated with pigment molecules (including chlorophyll a, chlorophyll b, and carotenoids) captures light energy and transfers it to reaction-center pigments in a photosystem. c. Primary electron acceptor: In the thylakoid membrane of a chloroplast or in the membrane of some prokaryotes, the primary electron acceptor is a specialized molecule that shares the reaction center complex with a pair of chlorophyll a molecules and that accepts an electron from them.
Which of the following occurs during the calvin cycle?
carbon fixation
According to the results of this experiment, germination of tobacco seeds during the first week is
increased by exposure to light
How many NADPHs are required for the Calvin Cycle?
6
light-harvesting complex
A complex of proteins associated with pigment molecules (including chlorophyll a, chlorophyll b, and carotenoids) that captures light energy and transfers it to reaction-center pigments in a photosystem.
Photosystem
A light-capturing unit located in the thylakoid membrane of the chloroplast or in the membrane of some prokaryotes, consisting of a reaction-center complex surrounded by numerous light-harvesting complexes. There are two types of these, I and II; they absorb light best at different wavelengths.
Photorespiration
A metabolic pathway that consumes oxygen and ATP, releases carbon dioxide, and decreases photosynthetic output. It generally occurs on hot, dry, bright days, when the stomata close and the O2:CO2 ratio in the leaf increases, favoring the binding of O2 rather than CO2 by rubisco.
Which of the following statements is a correct distinction between autotrophs and heterotrophs?
Autotrophs, but not heterotrophs, can nourish themselves beginning with CO2 and other nutrients that are inorganic.
The absorption spectrum of chlorophyll a alone underestimates the effectiveness of certain wavelengths in driving photosynthesis. Explain why this is so.
This is partly because accessory pigments with different absorption spectra ̶ including chlorophyll b and carotenoids ̶ broaden the spectrum of colors that can be used for photosynthesis.
In mechanism, photophosphorylation is most similar to
oxidative phosphorylation in cellular respiration
When oxygen is released as a result of photosynthesis, it is a direct by product of:
photolysis of water
To test the hypothesis that a particular plant synthesizes storage lipids by using glyceraldehyde 3-phosphate (G3P) from photosynthesis, a researcher plans to use radiolabeled precursors to track the molecules through the biosynthetic pathway. Which of the following radiolabeled precursors is most appropriate for the researcher to use?
14C-labeled CO2, because atmospheric carbon is fixed to carbohydrates by photosynthesis
Explain what is meant by a C3 plant.
A C3 plant is a plant that uses the Calvin cycle for the initial steps that incorporate CO2 into organic material, forming a three-carbon compound as the first stable intermediate.
Thylakoids
A flattened, membranous sac inside a chloroplast. They often exist in stacks called grana that are interconnected; their membranes contain molecular "machinery" used to convert light energy to chemical energy.
Stomata (Stoma singular)
A microscopic pore surrounded by guard cells in the epidermis of leaves and stems that allows gas exchange between the environment and the interior of the plant.
Which of the following statements about the rate of CO2 fixation in the two types of plants is supported by the data shown in the figures?
At 21% O2, plant type 2 has a lower rate of CO2 fixation than plant type 1 does in both types of soil.
A researcher claims that increased atmospheric carbon dioxide levels cause increased growth rates in plants. Which of the following statements best supports the researcher's claim?
Atmospheric carbon dioxide is the raw material for photosynthesis, which plants rely on for producing sugars and other organic compounds.
A researcher claims that increased atmospheric carbon dioxide levels cause increased growth rates in plants.Which of the following statements best supports the researcher's claim?
Atmospheric carbon dioxide is the raw material for photosynthesis, which plants rely on for producing sugars and other organic compounds.
Conceptually, it is important to know that the C4 pathway does not replace the Calvin cycle but works as a CO2 pump that prefaces the Calvin cycle. Explain how changes in leaf architecture (Figure 10.19) help isolate rubisco in regions of the leaf that are high in CO2 but low in O2 .
C4 plants use both mesophyll cells and bundle-sheath cells for photosynthesis. In mesophyll cells, CO2 is incorporated into organic compounds that then move into the bundle-sheath cells, where Calvin cycle takes place. Thus, keeping the CO2 concentration in the bundle-sheath cells high enough for rubisco to bind CO2 rather than O2.
Which of the following molecules found in the food we eat is most commonly broken down to make ATP?
Carbohydrates
Which of the following questions is most relevant to understanding the Calvin Cycle?
How is ATP used in the formation of 3-carbon carbohydrates?
Which of the following best justifies the use of tube 2 as a control treatment?
It was a positive control for measuring the effect of DCMU on the reaction.
Which of the following is NOT a part of photosynthesis?
Krebs Cycle
mesophyll
Leaf cells specialized for photosynthesis. In C3 and CAM plants, these cells are located between the upper and lower epidermis; in C4 plants, they are located between the bundle-sheath cells and the epidermis.
What is the source of energy that excites the electron in photosystem II?
Light
The process in which O2 is released as a by-product of oxidation - reduction reactions.
Light Dependent Reactions
Which of the following claims is best supported by the experimental results?
Light is required for the electron transport chain to transfer electrons.
Process in which O2 is released as a by-product of oxidation-reduction reactions
Light-dependent reactions of photosynthesis
What happens when a plant undergoes photorespiration?
Photorespiration is a metabolic process in which the plant consumes oxygen and ATP, releases carbon dioxide, and decreases photosynthetic output. For photorespiration to occur, C 3 plants will close their stomata on hot, dry days and produce less sugar because of declining levels of CO2 within the air spaces of the leaf. Rubisco is capable of easily binding O2 in place of CO2 . As CO2 becomes scarce within the air spaces of the leaf and O2 builds up, rubisco adds O2 to the Calvin cycle instead of CO2 . The product splits, and a two-carbon compound leaves the chloroplast. Peroxisomes and mitochondria split this compound, releasing CO2.
Explain this statement: "Only the green cells of a plant are the autotroph while the rest of the plant is a heterotroph."
Technically, green cells are the only autotrophic parts of the plant. The rest of the plant depends on organic molecules exported from leaves through veins. In most plants, carbohydrate is transported out of the leaves in the form of sucrose, a disaccharide. After arriving at nonphotosynthetic cells, the sucrose provides raw materials for cellular respiration and a multitude of anabolic pathways that synthesize proteins, lipids, and other products.
Explain what is meant by a C4 plant.
A C4 plant is plant in which the Calvin cycle is preceded by reactions that incorporate CO2 into a four-carbon compound, the end product of which supplies CO2 for the Calvin cycle.
To summarize, note that the light reactions store chemical energy in ___________ and _____________ , which shuttle the energy to the carbohydrate-producing __________ cycle.
ATP, NADPH, Calvin
Using Figure 10.19 in your text as a guide, explain the three key events—indicated by the arrows below—in the C4 pathway.
1. In mesophyll cells, the enzyme PEP carboxylase adds CO2 to PEP, forming a four-carbon compound. 2. The four-carbon compound moves into a bundle-sheath cell via plasmodesmata. 3. In bundle-sheath cells, an enzyme releases CO2 , which enters the Calvin cycle.
The last idea in this challenging concept is how chemiosmosis works in photosynthesis. Describe four ways that chemiosmosis is similar in photosynthesis and cellular respiration.
1. In photosynthesis and cellular respiration, an electron transport chain is assembled in a membrane that will pump protons across the membrane as electrons are passed through a series of carriers that have progressively more affinity for electrons. 2. In photosynthesis and cellular respiration, electron transport chains transform redox energy to a proton-motive force, potential energy stored in the form of an H+ gradient across a membrane. 3. In photosynthesis and cellular respiration, electron transport chains pump protons (H+) across a membrane from a region of low H+ concentration to one of high H+ concentration. 4. In photosynthesis and cellular respiration, an ATP synthase complex couples the diffusion of hydrogen ions down their concentration gradient to the phosphorylation of ADP, forming ATP.
List the three places in the light reactions where a proton-motive force is generated.
1. Water is split by photosystem II on the side of the membrane facing the thylakoid space contributing to the high [H+] in the thylakoid space. 2. As plastoquinone (Pq), a mobile carrier, transfers electrons to the cytochrome complex, four protons are pumped across the thylakoid membrane into the thylakoid space and increasing the [H+] in this area. 3. A hydrogen ion is removed from the stroma when it is taken up by NADP + adding to the decrease [H+] in the stroma.
To test the hypothesis that a particular plant synthesizes storage lipids by using glyceraldehyde 3-phosphate (G3P) from photosynthesis, a researcher plans to use radiolabeled precursors to track the molecules through the biosynthetic pathway. Which of the following radiolabeled precursors is most appropriate for the researcher to use?
14C-labeled CO2, because atmospheric carbon is fixed to carbohydrates by photosynthesis.
Examine Figure 10.18 in your text while we tally carbons. This figure is designed to show the production of one net G3P. That means the Calvin cycle must be turned three times. Each turn will require a starting molecule of ribulose bisphosphate (RuBP), a five-carbon compound. This means we start with ______ carbons distributed in three RuBPs. After fixing three molecules of CO2 using the enzyme _____________ , the Calvin cycle forms six G3Ps with a total of ________ carbons. At this point the net gain of carbons is __________ , or one net G3P molecule.
15, rubisco, 18, 3
Use this same figure to explain the correlation between an absorption spectrum and an action spectrum.
An absorption spectrum is the range of a pigment's ability to absorb various wavelengths of light; also a graph of such a range. An action spectrum is a graph that profiles the relative effectiveness of different wavelengths of radiation in driving photosynthesis. The broader peaks observed in the action spectrum seen in Figure 10.9 can be explained by the presence of pigments other than chlorophyll that capture light energy in the chloroplast, such as carotenoids and other photosynthetic pigments.
Photosystem II (PSII) has at its reaction center a special pair of chlorophyll a molecules called P680. What is the explanation for this name?
The reaction center chlorophyll a molecules of PS II are so named because these molecules are best at absorbing light energy at 680 nm. Nearly identical chlorophyll a molecules are found within the reaction center of PS I, however, these molecules of chlorophyll a absorb light energy best at 700 nm. Because of their association with different proteins in the thylakoid membrane that affect the electron distribution in the two pigments, this will account for the slight differences in their light-absorbing properties.
The carbon 'that makes up organic molecules in plants is derived directly from
carbon fixed in photosynthesis
