Photosynthesis Questions - Ch 10

In the carbon fixation step of the Calvin cycle, ribulose-1,5-bisphosphate (a five-carbon compound) is carboxylated to produce:

2 three-carbon compounds

Melvin Calvin introduced radiolabeled carbon dioxide into plants. Which of the following molecules should he have expected to see radiolabeled?

3-phosphoglycerate

Photophosphorylation results when:

ADP is phosphorylated by energy released when protons move from the thylakoid lumen to the stroma.

What do the light-dependent reactions of photosynthesis produce?

ATP and NADPH

What event accompanies energy absorption by chlorophyll (or other pigment molecules of the antenna complex)?

An electron is excited.

What is the difference between NAD+ and NADP?

Both function as electron carriers, but NADP has a phosphate group and NAD+ does not.

Compare and contrast mitochondria and chloroplasts. In what ways are their structures similar and different? What molecules or systems function in both types of organelles? Which enzymes or processes are unique to each organelle?

Both mitochondria and chloroplasts have internal membrane-bound structures (inner membrane and matrix in mitochondria and thylakoids in chloroplasts). They both have their own DNA and the capacity to make some of their own proteins. They both make ATP by chemiosmosis; thus, both have ATP synthase and electron transport chains. They both have cytochromes and quinone molecules in their electron transport chains. In contrast, only chloroplasts have chlorophyll and other pigment molecules that absorb light. Chloroplasts have NADPH electron carriers, whereas mitochondria have NADH. The Calvin cycle and all the enzymes related to it are unique to the chloroplast.

Make a sketch showing how C4 photosynthesis and CAM separate CO2 acquisition from the Calvin cycle in space and time, respectively.

C4 plants separate carbon acquisition and fixation from the Calvin cycle spatially. In C4 plants, carbon dioxide is fixed into organic acids in mesophyll cells via PEP carboxylase. Those organic acids are pumped and accumulated in bundle-sheath cells that then metabolize those molecules to release carbon dioxide to be used in the Calvin cycle. On the other hand, CAM plants separate carbon acquisition and fixation from the Calvin cycle temporally by fixing carbon dioxide at night, when they can leave stomata open, and using that carbon dioxide in the Calvin cycle during the day, when light is available to fuel ATP and NADPH formation.

Explain how the carbon reduction step of photosynthesis occurs. How is carbon dioxide fixed? Why are both ATP and NADPH required to produce sugar?

Carbon dioxide is fixed when the enzyme rubisco binds carbon dioxide and ribulose bisphosphate and combines them to make 3-phosphoglycerate. ATP is used to phosphorylate this compound and 1,3-bisphosphoglycerate. NADPH then donates electrons to 1,3-BPG, reducing it to form G3P—the final product of the Calvin cycle. ATP and NADPH are needed to provide the energy needed for the synthesis of the sugar

In addition to providing their protective function, carotenoids absorb certain wavelengths of light and pass the energy on to the reaction centers of photosystem I and II. Based on their function, predict exactly where carotenoids are located in the chloroplast. Explain your rationale. How would you test your hypothesis?

Carotenoids are located in the thylakoid membrane near the chlorophyll molecules in the reaction center. This allows them to be close enough to pass the energy along, and close enough to neutralize free radicals that could damage chlorophyll. This could be tested by shining red light on isolated chloroplasts. Because carotenoids reflect red light, but chlorophyll absorbs it, only carotenoids should be visible under the red light.

Energy from sunlight can excite electrons, kicking them out of their orbitals and creating free radicals. Free radicals are highly reactive atoms or molecules that have unpaired electrons. They degrade and destroy other compounds in their vicinity. Carotenoids, one of the pigments present in most chloroplasts, can stabilize these free radicals. This suggests that:

Carotenoids probably have a protective function in the cell.

What is a difference between chlorophyll a and chlorophyll b?

Chlorophyll a and b absorb light energy at slightly different wavelengths.

Scientists set out to determine how chlorophyll is arranged in the leaf chloroplast. They wanted to ascertain whether each chlorophyll molecule worked individually (without the aid of other light-absorbing pigments). They quantified the number of chlorophyll molecules in a sample of algae, and calculated how much oxygen these molecules could produce in photosynthesis if each of these chlorophyll molecules worked independently. When exposing the cells to light at an intensity that would provide maximal reaction, scientists found that oxygen production was approximately 1/3 of that expected if each chlorophyll molecule functioned independently. These results can be interpreted to come to which of the following conclusions?

Each antennae complex contains more than one chlorophyll molecule.

In an oxidation-reduction reaction, potential energy decreases as:

Electrons are passed from a reduced molecule to an oxidized molecule.

What happens when an excited electron is passed to an electron acceptor in a photosystem?

Energy in sunlight is transformed to chemical energy.

Photosystem II is active in noncyclic photophosphorylation (when electrons of photosystem II are replaced as in oxygenic photosynthesis); under conditions of cyclic photophosphorylation, both photosystem II and photosystem I are functioning. True or false?

False

Both C3 and C4 plants have the enzymes of the Calvin cycle. How are C4 plants more efficient than C3 plants at fixing carbon?

In C4 plants, carbon fixation takes place in mesophyll cells, whereas the Calvin cycle takes place in bundle-sheath cells.

Why is chlorophyll green?

It absorbs wavelengths in the blue and red parts of the visible spectrum and transmits wavelengths in the green part.

How does the sequestering of carbon dioxide in CAM plants help them to survive?

It allows carbon dioxide to be gathered and used at different times of the day.

What does it mean to say that CO2 becomes fixed?

It becomes bonded to an organic compound.

Why is it critical for plants to maintain a high concentration of carbon dioxide in the leaves?

It helps to prevent photorespiration.

As electrons are passed through the system of electron carriers associated with photosystem II, they lose energy. What happens to this energy?

It is used to establish and maintain a proton gradient.

The final electron acceptor(s) associated with photosystem I is/are:

NADP

The light-independent reactions of plants function to make organic molecules using carbon dioxide as a carbon source. What is the electron source that helps reduce carbon dioxide to sugars and other organic molecules?

NADPH

In an experiment conducted by Jagendorf, isolated thylakoids were incubated in an acidic solution at pH 4.0 until the pH was equilibrated across the thylakoid membrane. The thylakoids were then transferred to a buffer at pH 8.0 with ADP and inorganic phosphate. ATP was synthesized. Did this experiment require light to generate the ATP?

No, because ATP synthesis is dependent only on the presence of a hydrogen ion gradient and does not require light directly.

Consider plants that occupy the top, middle, or ground layer of a forest, and algae that live near the surface of the ocean or in deeper water. Would you expect the same photosynthetic pigments to be found in species that live in these different habitats? Why or why not? How would you test your hypothesis?

No, these plants and algae could not be expected to have the same complement of photosynthetic pigments. Plants living in the top layers of the forest would be expected have a wide variety of photosynthetic pigments, so that they can access the full spectrum of light from the Sun. They would be expected to have a high content of carotenoids to protect chlorophyll from UV damage. Plants that occupy spaces lower to the ground would receive less light than those plants at the top, but would likely still have various pigments to capture differing wavelengths of light. Plants and algae living at the ocean's surface could be expected to have pigment content similar to that of plants living on land, because most of the full spectrum of light reaches these plants. On the other hand, plants living in deeper water will probably have higher concentrations of pigments that are excited by the wavelengths of light that reach those water depths. This hypothesis could be tested by exposing samples of each plant to different wavelengths of light and measuring photosynthetic rates. Doing so generates an action spectrum that can help determine which pigments are present in a plant.

How would the experiments of van Niel with purple sulfur bacteria have been different if oxygen came from carbon dioxide during photosynthesis?

Oxygen would have been produced from carbon dioxide and hydrogen sulfide.

If pigments from a particular species of plant are extracted and subjected to paper chromatography, which of the following is the most believable result?

Paper chromatography would separate the pigments from a particular plant into several bands.Paper chromatography would separate the pigments from a particular plant into several bands

The Calvin cycle and rubisco are found in lineages of bacteria and archaea that evolved long before the origin of oxygenic photosynthesis. Based on this observation, biologists infer that rubisco evolved in an environment containing little, if any, oxygen. Some biologists propose that this inference explains why photorespiration occurs today. Do you agree with the hypothesis that photorespiration is an evolutionary holdover? Why or why not?

Photorespiration does seem to be an evolutionary holdover, because it actually burns energy and oxygen that the plant could use for metabolism and for photosynthesis. To date, there is no known "use" for photorespiration in today's plants. In fact, plants living in climates that make them susceptible to photorespiration have evolved mechanisms to avoid its occurrence.

Rubisco can bind either carbon dioxide or oxygen to its active site and catalyze the reaction of either molecule with ribulose-1,5-bisphosphate. Photorespiration is the reaction between oxygen and ribulose-1,5-bisphosphate. Which of the following statements about photorespiration is accurate?

Photorespiration is an energy-requiring process that reduces the efficiency of photosynthesis.

In what sense does photorespiration "undo" photosynthesis?

Photorespiration occurs when carbon dioxide levels are low (on hot, dry days when stomata are closed to conserve water), and rubisco binds to oxygen and combines it to ribulose bisphosphate instead of combining carbon dioxide with this organic compound. Photorespiration decreases photosynthetic output because it takes ribulose bisphosphate away from the photosynthetic process by combining it with oxygen. The product of photorespiration is ultimately broken down to CO2. Photorespiration effectively reverses photosynthesis because oxygen and energy are used, CO2 is released instead of fixed, and no sugars are produced.

Explain how photosystems I and II are similar and how they differ.

Photosystems I and II are similar in that they both use electrons excited by photons to reduce molecules at the beginning of electron transport chains. Both are located in the thylakoid membrane. On the other hand, photosystems I and II most easily use light of different wavelengths, and they use the potential energy of the excited electrons to produce different compounds (II makes ATP and I makes NADPH). They occur in different parts of the thylakoid membrane, and they replace electrons lost from their chlorophylls in different ways (II splits water, and I gets electrons from II).

Purple photosynthetic bacteria do not contain chloroplasts. They have an antenna system that includes two light-harvesting protein complexes, both of which are integral membrane proteins. The bacteriochlorophyll a molecules contain Mg2+ and absorb light at 800 nm. Which of the following statements comparing photosynthesis in purple bacteria and plant chloroplasts is true?

Plant chlorophyll a absorbs in the visible light range, whereas purple photosynthetic bacteriochlorophyll a absorbs at a longer wavelength.

Why do plants need both chloroplasts and mitochondria?

Plants need chloroplasts to convert light energy into the chemical energy that is trapped in the chemical bonds of the photosynthetic product, glucose. They need mitochondria to harvest the energy in the chemical bonds of glucose and use it to make ATP, which is the "energy currency" of the cell.

Data suggest that rubisco (ribulose-1,5-bisphosphate carboxylase) makes up 10% of the total protein found in spinach leaves. Research elucidating the structure of rubisco shows that it has four active sites. Why, with four active sites, might there be such a large concentration of rubisco in plant matter?

Rubisco is a very slow enzyme; what it lacks in speed, it makes up in numbers.

Why is rubisco considered an inefficient enzyme?

Rubisco is inefficient in that (1) it is slow, and (2) it catalyzes the toxic photorespiration reaction when cell CO2 concentration is low and O2concentration is high. It evolved when there was little O2 in Earth's atmosphere.

How does carbon fixation differ between C3 and C4 plants?

Rubisco is the primary enzyme that catalyzes carbon fixation in C3plants, and phosphoenolpyruvate carboxylase can catalyze carbon fixation in C4 plants.

You are told in this chapter that purple sulfur bacteria were commonly used in research on photosystem I, and that heliobacteria were commonly used to research photosystem II. Why are these organisms better models for studying these processes than trees are?

Some advantages to working with bacteria are that they are simpler, faster-growing, and easy to keep lots of in the lab. Trees are more complex, take up a lot of space, and take a long time to grow and reproduce—making them, in many ways, more difficult to work with.

Why is it possible for the Calvin cycle to occur in the dark?

The Calvin cycle uses energy stored previously during the light-dependent reactions of photosynthesis.

Which photosystem's electron transport chain is more similar to that in the mitochondria, and why?

The ETC in photosystem II is more similar to that in the mitochondria than is the ETC in photosystem I, because photosystem II also creates a proton motive force that is used by ATP synthase to make ATP.

In autumn, the leaves of deciduous trees change colors. This is because the chlorophyll is degraded, and:

The carotenoids and other pigments are still present in the leaves.

What is the evidence for two photosystems?

The combination of light at 680 nm and 700 nm is much more effective in stimulating photosynthesis than is that at either wavelength alone.

Sketch the Z scheme. Explain how photosystem I and photosystem II interact by tracing the path of an electron through the Z scheme. What molecule connects the two photosystems?

The electrons from photosystem II are used to replace the electrons from P700 chlorophyll molecules that have been excited and donated electrons to ferredoxin in photosystem I, which are ultimately donated to NADP+ to form NADPH. Plastocyanin and chlorophyll P700 are the molecules that link the two photosystems.

Explain how the energy transformation step of photosynthesis occurs. How is light energy converted to chemical energy in the form of ATP and NADPH?

The energy transformation step of photosynthesis occurs when the energy from a photon of light is transferred to chlorophyll a in the reaction center (P680) and its excited electron is transferred to an oxidizing agent, pheophytin-I. During this step, the energy is transformed from electromagnetic energy to chemical energy. Electrons from the oxidation of water reduce P680+, so that it returns to its ground state, ready to absorb another photon. The electrons donated by P680+ are passed from pheophytin I through a series of exergonic redox reactions that are coupled to proton pumps, which in turn use the energy from redox reactions to create a proton-motive force. This allows for production of ATP by chemiosmosis and ATP synthase. In photosystem I, P700+ absorbs photons, sending electrons to an oxidizing agent called ferredoxin. P700 is now P700+ and is returned to its ground state by electrons coming out of the redox chain. Ferredoxin then reduces NADP+ reductase, which in turn reduces NADP+, making NADPH + H+ via NADP+ reductase (this also uses the two protons generated by oxidation of water). From these light reactions, the energy in light is transferred to chemical energy in ATP and NADPH that can be used in the Calvin cycle to reduce CO2 to make G3P.

The products of light-dependent reactions are used in light-independent reactions.

The products of light-dependent reactions are used in light-independent reactions.

What happens to the rate of ATP synthesis if the pH of the thylakoid lumen decreases?

The rate of ATP synthesis increases

Which of the following statements concerning chloroplasts is false?

They are unrelated to plastids.

What is/are the function(s) of accessory pigments in plants?

They extend the range of wavelengths a plant can use to drive photosynthesis, and they protect plants from the damaging effects of electromagnetic radiation.

Why are there several structurally different pigments in the reaction centers of photosystems?

This setup enables the plant to absorb energy from sunlight from a variety of wavelengths.

Six ATP are used for production of one three-carbon sugar (glyceraldehyde-3-phosphate) from RuBP and carbon dioxide in the Calvin cycle. Nevertheless, the Calvin cycle actually requires nine ATP to function. Why?

Three additional ATP are used to regenerate RuBP.

Under conditions of noncyclic electron flow (anoxygenic photosynthesis), both NADPH and ATP are products of light-dependent reactions; but under conditions of cyclic photophosphorylation, only ATP is produced. True or false?

True

Why do the absorption spectrum for chlorophyll and the action spectrum for photosynthesis coincide?

Wavelengths of light that are absorbed by chlorophyll trigger the light-dependent reactions.

Photorespiration is:

a process involving the addition of oxygen to ribulose-1,5-bisphosphate and the formation of the products 3-phosphoglycerate and 2-phosphoglycolate

The proteins of the electron transport chain active in the light-dependent reactions:

are membrane proteins present in the thylakoid

Stoma, openings in the leaf, are important to photosynthesis for:

entry of carbon dioxide that is used in the Calvin cycle

Fill in the blanks: Plants have ____________ cells that open to form a pore on the leaf surface. This structure is called a ____________. In C4 plants, PEP carboxylase is found in ____________ cells located near the surface of the leaves. Other C4 cells that contain rubisco and surround vascular tissue in the interior of the leaf are called ____________ cells. All these leaf cells contain photosynthetic organelles called ____________. The internal membranes of these organelles form flat vesicle-like structures called ____________, which form stacks called ____________.

guard; stoma; mesophyll; bundle sheath; chloroplasts; thylakoids; grana.

The process of photosynthesis probably originated:

in prokaryotes

Melvin Calvin was surprised when radioactive carbon dioxide was incorporated into organic molecules:

in the absence of light

If atmospheric carbon dioxide levels are rising due to deforestation and burning of fossil fuels, you might expect the rate of photosynthesis to:

increase

Which of the following factors is not involved in determining whether light is absorbed by a molecule?

intensity of the light

Plastoquinone (PQ), an electron carrier of small molecular weight, is found in the electron transport chain associated with photosystem II. If PQ is not directly anchored to other membrane or cytoplasmic structures, it is probably:

mobile in the thylakoid membrane

CAM plants function using crassulacean acid metabolism. Like C4plants, CAM plants provide a preparatory step for the Calvin cycle. CAM plants are found in hot, dry environments; to prevent dessication, they keep their stomata closed during the day. They take in carbon dioxide at night while stomata are open. To increase the concentration of carbon dioxide available to the enzyme rubisco and minimize the degree of photorespiration, the CAM plants carboxylate:

organic acids

If green plant cells are incubated with 18O-labeled water, what molecule will become radioactive as the cells are exposed to light?

oxygen

Electrons excited by absorption of light in photosystem I are transferred to iron-sulfur electron acceptors, and therefore must be replaced. The replacements come directly from:

photosystem II

Besides proteins, the thylakoid must contain a large number of ____________ molecules in the reaction centers in order to harvest light energy.

pigment

Which of the following procedures would identify the enzyme that catalyzes the carboxylation of ribulose-1,5-bisphosphate?

purifying a variety of proteins from plant extracts and testing each protein individually to see if it can carboxylate ribulose-1,5-bisphosphate

Based on what you know about the structure and function of the antenna complex, irradiating a leaf with which of the following light types would be result in the release of the greatest quantities of oxygen?

red and blue light

The biochemical objective of photosystem I is to:

reduce NADPH+

In paper chromatography, molecules are separated based on their ____________ and ____________.

size and solubility

The Z scheme is the interaction between photosystem I and photosystem II. What does the "Z" represent?

the changes that occur in potential energy of electrons associated with the two photosystems

What is the stroma of a chloroplast?

the fluid inside the chloroplast but outside the thylakoids

The absorption spectrum of a plant shows what wavelengths of light the plant absorbs. The absorption spectrum depends on:

the pigments present in the plant

What capability of flavonoids, found in plants, protects their leaves from damage due to ultraviolet light?

their ability to absorb ultraviolet light

Chlorophyll absorbs light in the ____________ range?

visible

Early investigators thought the oxygen produced by photosynthetic plants came from carbon dioxide. In actual fact, it comes from:

water

The electrons of photosystem II are excited and transferred to electron carriers. From which molecule or structure do the photosystem II replacement electrons come?

water