Chapter 10 - Photosynthesis

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.
What is the life on earth powered by?
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.
How does photosynthesis go about nourishing the entire living world?
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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.

What 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).

What organisms are autotrophs?
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.
How do heterotrophs obtain their organic material?
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.

What are some forms of heterotrophic consumption?
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.
How did the fossil fuels that we use today in our cars and other things come about?
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.
What are the two stages of photosynthesis?
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.

What are some of the types of alternative fuels that are coming from plants and algae?
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.

Why should we be researching biofuels?
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.

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?
It is the conversion of light energy to chemical energy that is stored in sugars or other organic compounds.
What is photosynthesis?
It occurs in plants, algae, and certain prokaryotes.
Where does photosynthesis occur?
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.
What is a chloroplast?
It is the the tissue in the interior of a leaf.
What is mesophyll?
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?)
How are the veins utilized in leaves?
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.

How is ATP and NADPH formed?
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.

2) Which of the following are products of the light reactions of photosynthesis that are utilized in the Calvin cycle?

A) CO2 and glucose
B) H2O and O2
C) ADP, Pi, and NADP+
D) electrons and H+
E) ATP and NADPH

Chlorophyll, the green pigment that gives leaves their color, resides in the thylakoid membranes of the chloroplast.
What is chlorophyll and where can it be found?
The light energy that is absorbed by the chlorophyll drives the synthesis of organic molecules.
What drives the synthesis of organic molecules?
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.

3) What are the products of the light reactions that are subsequently used by the Calvin cycle?

A) oxygen and carbon dioxide
B) carbon dioxide and RuBP
C) water and carbon
D) electrons and photons
E) ATP and NADPH

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.

4) Where does the Calvin cycle take place?

A) stroma of the chloroplast
B) thylakoid membrane
C) cytoplasm surrounding the chloroplast
D) chlorophyll molecule
E) outer membrane of the chloroplast

Source Page 188 – They utilize the solar energy to make ATP and NADPH, which supply chemical and reducing power, respectively to the Calvin cycle.
What do the light reactions do with solar energy?
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).
What are the end products of the Calvin cycle?
D) autotrophs
5) In any ecosystem, terrestrial or aquatic, what group(s) is (are) always necessary?

A) autotrophs and heterotrophs
B) producers and primary consumers
C) photosynthesizers
D) autotrophs
E) green plants

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.

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
C) chemiosmosis
D) the electron transfer system of photosystem I
E) the electron transfer system of photosystem II

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.

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
C) green and yellow
D) blue, green, and red
E) green, blue, and yellow

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).

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.
C) there are 700 photosystem I components to each chloroplast.
D) it absorbs 700 photons per microsecond.
E) the plastoquinone reflects light with a wavelength of 700 nm.

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.

17) Which of the events listed below occur in the light reactions of photosynthesis?

A) NADP is produced.
B) NADPH is reduced to NADP+.
C) carbon dioxide is incorporated into PGA.
D) ATP is phosphorylated to yield ADP.
E) light is absorbed and funneled to reaction-center chlorophyll a.

D) The electron vacancies in P680 are filled by electrons derived from water.

Source – Figure 10.14 –

18) Which statement describes the functioning of photosystem II?

A) Light energy excites electrons in the electron transport chain in a photosynthetic unit.
B) The excitation is passed along to a molecule of P700 chlorophyll in the photosynthetic unit.
C) The P680 chlorophyll donates a pair of protons to NADPH, which is thus converted to NADP+.
D) The electron vacancies in P680 are filled by electrons derived from water.
E) The splitting of water yields molecular carbon dioxide as a by-product.

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.
What is photophosphorylation?
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.

21) What are the products of linear photophosphorylation?

A) heat and fluorescence
B) ATP and P700
C) ATP and NADPH
D) ADP and NADP
E) P700 and P680

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.

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
B) photosystem I
C) cyclic electron flow
D) linear electron flow
E) chlorophyll

D) the synthesis of ATP

Source Figure 10.14

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
B) the absorption of light energy by chlorophyll
C) the flow of electrons from photosystem II to photosystem I
D) the synthesis of ATP
E) the reduction of NADP+

It is an energy coupling mechanism that uses energy stored in the form of a hydrogen gradient
What is chemiosmosis?
A) establishment of a proton gradient
24) What does the chemiosmotic process in chloroplasts involve?

A) establishment of a proton gradient
B) diffusion of electrons through the thylakoid membrane
C) reduction of water to produce ATP energy
D) movement of water by osmosis into the thylakoid space from the stroma
E) formation of glucose, using carbon dioxide, NADPH, and ATP

A) The isolated chloroplasts will make ATP.
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.
B) The Calvin cycle will be activated.
C) Cyclic photophosphorylation will occur.
D) Only A and B will occur.
E) A, B, and C will occur.

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.

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.
B) the matrix to the stroma.
C) the stroma to the thylakoid space.
D) the intermembrane space to the matrix.
E) ATP synthase to NADP+ reductase.

B) Photosynthesis stores energy in complex organic molecules, while respiration releases it.
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.
C) Photosynthesis occurs only in plants and respiration occurs only in animals.
D) ATP molecules are produced in photosynthesis and used up in respiration.
E) Respiration is anabolic and photosynthesis is catabolic.

A) thylakoid membranes of chloroplasts
29) Where are the molecules of the electron transport chain found in plant cells?

A) thylakoid membranes of chloroplasts
B) stroma of chloroplasts
C) outer membrane of mitochondria
D) matrix of mitochondria
E) cytoplasm

C) both photosynthesis and respiration
30) Synthesis of ATP by the chemiosmotic mechanism occurs during

A) photosynthesis.
B) respiration.
C) both photosynthesis and respiration.
D) neither photosynthesis nor respiration.
E) photorespiration

B) respiration.
31) Reduction of oxygen which forms water occurs during

A) photosynthesis.
B) respiration.
C) both photosynthesis and respiration.
D) neither photosynthesis nor respiration.
E) photorespiration.

A) photosynthesis.
32) Reduction of NADP+ occurs during

A) photosynthesis.
B) respiration.
C) both photosynthesis and respiration.
D) neither photosynthesis nor respiration.
E) photorespiration.

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.

33) The splitting of carbon dioxide to form oxygen gas and carbon compounds occurs during

A) photosynthesis.
B) respiration.
C) both photosynthesis and respiration.
D) neither photosynthesis nor respiration.
E) photorespiration.

C) both photosynthesis and respiration.

Source: Page 197 – Figure 10.17

34) Generation of proton gradients across membranes occurs during

A) photosynthesis.
B) respiration.
C) both photosynthesis and respiration.
D) neither photosynthesis nor respiration.
E) photorespiration.

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.

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.
C) They are logarithmically related.
D) They are separate phenomena.
E) They are only related in certain parts of the spectrum.

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.

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.
B) It is the molecule that transfers electrons to plastoquinone (Pq) of the electron transfer system.
C) NADP reductase will then catalyze the shift of the electron from Fd to NADP+ to reduce it to NADPH.
D) This molecule results from the transfer of an electron to the primary electron acceptor of photosystem II and strongly attracts another electron.
E) This molecule is found far more frequently among bacteria as well as in plants and plantlike Protists.

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.

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.
C) They cover the sensitive chromosomes of the plant.
D) They reflect orange light.
E) They take up toxins from the water.

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.
How do carotenoids good for humans too?
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

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.
B) The light reactions provide ATP and NADPH to the carbon fixation step of the Calvin cycle, and the cycle provides water and electrons to the light reactions.
C) The light reactions supply the Calvin cycle with CO2 to produce sugars, and the Calvin cycle supplies the light reactions with sugars to produce ATP.
D) The light reactions provide the Calvin cycle with oxygen for electron flow, and the Calvin cycle provides the light reactions with water to split.
E) There is no relationship between the light reactions and the Calvin cycle.

A) stroma of the chloroplast

Source: Page 188 – Figure 10.6 – The Calvin cycle occurs in the stroma.

43) Where do the enzymatic reactions of the Calvin cycle take place?

A) stroma of the chloroplast
B) thylakoid membranes
C) outer membrane of the chloroplast
D) electron transport chain
E) thylakoid space

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.

44) What is the primary function of the Calvin cycle?

A) use ATP to release carbon dioxide
B) use NADPH to release carbon dioxide
C) split water and release oxygen
D) transport RuBP out of the chloroplast
E) synthesize simple sugars from carbon dioxide

A) light reactions alone

Source: Page 188 – Water is split, providing a source of electrons and protons (hydrogen ions, H+).

45) Produces molecular oxygen (O2)

A) light reactions alone
B) the Calvin cycle alone
C) both the light reactions and the Calvin cycle
D) neither the light reactions nor the Calvin cycle
E) occurs in the chloroplast but is not part of photosynthesis

B) the Calvin cycle alone

Source: Page 198 – Figure 10.19 – Displays that the Calvin cycle requires ATP meanwhile the light reactions require ADP.

46) Requires ATP

A) light reactions alone
B) the Calvin cycle alone
C) both the light reactions and the Calvin cycle
D) neither the light reactions nor the Calvin cycle
E) occurs in the chloroplast but is not part of photosynthesis

A) light reactions alone

Source: Page 198 – Figure 10.19

48) Produces NADPH

A) light reactions alone
B) the Calvin cycle alone
C) both the light reactions and the Calvin cycle
D) neither the light reactions nor the Calvin cycle
E) occurs in the chloroplast but is not part of photosynthesis

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).

49) Produces three-carbon sugars

A) light reactions alone
B) the Calvin cycle alone
C) both the light reactions and the Calvin cycle
D) neither the light reactions nor the Calvin cycle
E) occurs in the chloroplast but is not part of photosynthesis

B) the Calvin cycle alone

Source: Page 199 – The Calvin cycle incorporates each carbon dioxide molecule, one at a time.

50) Requires CO2

A) light reactions alone
B) the Calvin cycle alone
C) both the light reactions and the Calvin cycle
D) neither the light reactions nor the Calvin cycle
E) occurs in the chloroplast but is not part of photosynthesis

D) neither the light reactions nor the Calvin cycle
51) Requires glucose

A) light reactions alone
B) the Calvin cycle alone
C) both the light reactions and the Calvin cycle
D) neither the light reactions nor the Calvin cycle
E) occurs in the chloroplast but is not part of photosynthesis

D) The formation of starch in plants involves assembling many G3P molecules, with or without further rearrangements.
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.”
B) G3P more readily forms sucrose and other disaccharides than it does monosaccharides.
C) Some plants would not taste sweet to us.
D) The formation of starch in plants involves assembling many G3P molecules, with or without further rearrangements.
E) G3P is easier for a plant to store.

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.

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
B) inactivation of RuBP carboxylase enzyme
C) regeneration of ATP from ADP
D) regeneration of RuBP
E) a gain of NADPH

A) The pH within the thylakoid is less than that of the stroma.
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.
B) The pH of the stroma is higher than that of the other two measurements.
C) The pH of the stroma is higher than that of the thylakoid space but lower than that of the cytosol.
D) The pH of the thylakoid space is higher than that anywhere else in the cell.
E) There is no consistent relationship.

A) It represents cell processes involved in C4 photosynthesis.

Source: Page 201 – Figure 10.20

59) Which of the following statements is true concerning Figure 10.3?

A) It represents cell processes involved in C4 photosynthesis.
B) It represents the type of cell structures found in CAM plants.
C) It represents an adaptation that maximizes photorespiration.
D) It represents a C3 photosynthetic system.
E) It represents a relationship between plant cells that photosynthesize and those that cannot.

B) cell II only.
60) Referring to Figure 10.3, oxygen would inhibit the CO2 fixation reactions in

A) cell I only.
B) cell II only.
C) neither cell I nor cell II.
D) both cell I and cell II.
E) cell I during the night and cell II during the day.

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

What is photorespiration?
B) Cell II
61) In which cell would you expect photorespiration?

A) Cell I
B) Cell II
C) Cell I at night
D) Cell II at night
E) neither Cell I nor Cell II

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.

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.
B) C3 plant.
C) CAM plant.
D) heterotroph.
E) chemoautotroph.

B) They use PEP carboxylase to initially fix CO2.
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.
C) They are adapted to cold, wet climates.
D) They conserve water more efficiently.
E) They exclude oxygen from their tissues.

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).

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.
B) fix CO2 into sugars in the bundle-sheath cells.
C) fix CO2 into pyruvate in the mesophyll cells.
D) use the enzyme phosphofructokinase, which outcompetes rubisco for CO2.
E) use photosystems I and II at night.

B) 3-phosphoglycerate molecules
65) Photorespiration lowers the efficiency of photosynthesis by preventing the formation of

A) carbon dioxide molecules.
B) 3-phosphoglycerate molecules
C) ATP molecules.
D) ribulose bisphosphate molecules.
E) RuBP carboxylase molecules

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.

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.
B) C4 compromises on water loss and CAM compromises on photorespiration.
C) Each one both minimizes photorespiration and optimizes the Calvin cycle.
D) CAM plants allow more water loss, while C4 plants allow less CO2 into the plant.
E) C4 plants allow less water loss but Cam plants but allow more water loss.

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.

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.
B) Cells would carry on the Calvin cycle at a much slower rate.
C) Less ATP would be generated.
D) There would be more light-induced damage to the cells.
E) More sugars would be produced.

D) ATP and NADPH.
1) The light reactions of photosynthesis supply the Calvin cycle with

A) light energy.
B) CO2 and ATP.
C) H2O and NADPH.
D) ATP and NADPH.
E) sugar and O2.

B) H2O → NADPH → Calvin cycle
2) Which of the following sequences correctly represents the flow of electrons during photosynthesis?

A) NADPH → O2 → CO2
B) H2O → NADPH → Calvin cycle
C) NADPH → chlorophyll → Calvin cycle
D) H2O → photosystem I → photosystem II
E) NADPH → electron transport chain → O2

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.

3) In mechanism, photophosphorylation is most similar to

A) substrate-level phosphorylation in glycolysis.
B) oxidative phosphorylation in cellular respiration.
C) the Calvin cycle.
D) carbon fixation.
E) reduction of NADP+.

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.

4) How is photosynthesis similar in C4 and CAM plants?

A) In both cases, only photosystem I is used.
B) Both types of plants make sugar without the Calvin cycle.
C) In both cases, rubisco is not used to fix carbon initially.
D) Both types of plants make most of their sugar in the dark.
E) In both cases, thylakoids are not involved in photosynthesis.

D) removal of electrons from chlorophyll molecules
5) Which process is most directly driven by light energy?

A) creation of a pH gradient by pumping protons across the thylakoid membrane
B) carbon fixation in the stroma
C) reduction of NADP+ molecules
D) removal of electrons from chlorophyll molecules
E) ATP synthesis

D) Autotrophs, but not heterotrophs, can nourish themselves beginning with CO2 and other nutrients that are inorganic.

Source: Beginning

6) Which of the following statements is a correct distinction between autotrophs and heterotrophs?

A) Only heterotrophs require chemical compounds from the environment.
B) Cellular respiration is unique to heterotrophs.
C) Only heterotrophs have mitochondria.
D) Autotrophs, but not heterotrophs, can nourish themselves beginning with CO2 and other nutrients that are inorganic.
E) Only heterotrophs require oxygen.

C) release of oxygen
7) Which of the following does not occur during the Calvin cycle?

A) carbon fixation
B) oxidation of NADPH
C) release of oxygen
D) regeneration of the CO2 acceptor
E) consumption of ATP

The Calvin cycle is similar to the citric acid cycle in that a starting material is regenerated after molecules enter and leave the cycle.
How is the Calvin cycle similar to the citric acid cycle in animals?
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.

How is the citric cycle different from the Calvin cycle?
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.
What is the carbohydrate produced directly from the Calvin cycle called?
It is the initial incorporation of CO2 into organic material.
What is carbon fixation?
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).

What is phase 1 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.

What is phase 2 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.
What is phase 3 of the Calvin cycle?
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.
What are chloroplasts?
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.

What is light?
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.
Define wavelength
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.
What spectrum of EM is most important to life?
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.
How is light related to 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.
What are photons?
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.
How does the atmosphere effect 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.
What is white light?
It may be reflected, transmitted, or absorbed.
What happens when light meets matter?
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).
What are pigments?
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?

Why do we see green when we look at a leaf?
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.
How does the spectrophotometer work?
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.
Why are leaves green?
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.
Why should scientists learn about absorption spectra?
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.

What is the technique in determining absorption spectrum?
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.

What does the high transmittance indicate? How about low transmittance?
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.
What can the absorption spectra of chloroplast pigments tell us?
Chlorophyll a, which participates directly in the light reactions; the accessory pigment chlorophyll b; and a group of accessory pigments called carotenoids.
What is the absorption spectra of three types of pigments in chloroplasts?
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.
What does the spectrum of chlorophyll a tell us?
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.
How is an action spectrum prepared?
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.
When was action spectrum for photosynthesis first demonstrated?
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.

What was Engelmann’s experiment?
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.
If Engelmann had used a filter that allowed only red light to pass through, how would the results have differed?
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.

Why does the action spectrum for photosynthesis not exactly match the absorption spectrum of chlorophyll a?
It is the light-absorbing “head of molecule; note magnesium atom at center.
What is a porphyrin ring?
It interacts with hydrophobic regions of proteins inside thylakoid membranes of chloroplasts.
What is the purpose of the hydrocarbon tail?
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.

What are carotenoids?
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.
How do carotenoids similar to the photoprotective ones in chloroplasts, protect the human eye?
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.

What exactly happens when chlorophyll and other pigments absorb light?
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.

What happens when absorption of a photon raises an electron from the ground state to an excited state?
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.
What happens in the excitation of isolated chlorophyll by light?
In the leaf most of the chlorophyll electrons excited by photon absorption are used to power the reactions of photosynthesis.
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 their native environment of the thylakoid membrane, chlorophyll molecules are organized along with other small organic molecules and proteins into complexes called photosystems.
Why do chlorophyll molecules excited by the absorption of light energy produce very different results in an intact chloroplast than they do in isolation?
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.
What is a photosystem composed 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.
What does each light-harvesting complex consist of?
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.
How do the light-harvesting complexes work?
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.
What is the reaction-center complex contain?
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.
How does a photosystem harvest light?
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.
What is the first step of the light reactions?
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.

Why does isolated chlorophyll fluoresce?
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.

What are the two types of photosystems that populate the thylakoid membrane?
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)
What is is P680?
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).
What is 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.
What are the differences between P680 and P700?
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.

How do the two photosystems work together in using light energy to generate ATP and NADPH, the two main products of the light reactions?
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.
What is the first step in LEF?
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.

What are steps 2 and 3 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).
What is step 4 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?!)
What is step 5 in the linear electron flow?
Hydrogen ions (protons) will diffuse from an area of high proton concentration to an area of lower proton concentration.
What is chemiosmosis?
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.
What is step 6 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).
What is step 7 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.
What is step 8 in 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.
What is the function of the linear 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.
What is a cyclic electron flow?
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.

Explain what is occurring cyclic electron flow. Figure 10.16
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.
Why do some currently existing groups of photosynthetic bacteria have a PS 1 but not a PS2?
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.

Can cyclic electron flow occur in photosynthetic species that possess both photosystems?
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.

How do chloroplasts and mitochondria generate ATP?
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.
How are chloroplasts and mitochondria alike?
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.

What are some of the noteworthy differences between oxidative phosphorylation in mitochondria and photophosphorylation in chloroplasts?
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.

What are the similarities in the spatial organization of chemiosmosis between chloroplasts and mitochondria?
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.
Figure 10.17 – Compare chemiosmosis in mitochondria and chloroplasts.
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.

What is the pH difference between the thylakoid space and the stroma?
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.)
Summarize the light reactions.
1. Green, because green light is mostly transmitted and reflected-not absorbed- by photosynthetic pigments.
What color of light is least effective in driving photosynthesis?
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.
Compared to a solution of isolated chlorophyll, why do intact chloroplasts release less heat and fluorescence when illuminated?
3. Water (H2O) is the initial electron donor; NADP+ accepts electrons at the end of the electron transport chain, becoming reduced to NADPH.
In the light reactions, what is the initial electron donor? Where do the electrons finally end up?
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.
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.
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.
What are the major sites of photosynthesis in plants?
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?

How many chloroplasts are in a chunk of leaf with a top surface area of 1 mm2?
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”).
Where are chloroplasts usually found in plants?
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.
How do plants utilize water?
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.
How many chloroplasts does a typical mesophyll cell have?
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).
What is suspended within the stroma of a chloroplast?
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.

What is chlorophyll?
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

How do we track atoms through 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.

What was one of the first clues to the mechanism of photosynthesis?
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.

When was the idea that photosynthesis split carbon dioxide challenged and why?
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.

What happened 20 years later after Van Niels proposed his hypothesis of photosynthesis?
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.

How are photosynthesis and cellular respiration alike?
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.)

How does photosynthesis work in the chloroplast?
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).
Is photosynthesis a single process?
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.

Tell me a bit more about the “photo” part of photosynthesis, the light reactions.
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+.
How do NAD+ and NADP+ differ?
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.

What is photophosphorylation?
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.
Who is the Calvin cycle named after?
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.

What happens in the Calvin cycle after carbon fixation is completed?
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.
Why are the metabolic steps of the Calvin cycle sometimes referred to as the dark reactions or light-independent reactions?
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.

Where do the two processes of photosynthesis, the light reactions and the Calvin cycle occur?
The CO2 enters leaves via stomata, and water enters via roots and is carried to leaves through veins.
How do the reactant molecules of photosynthesis reach the chloroplast in leaves?
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.
How did the use of an oxygen isotope help elucidate the chemistry of photosynthesis?
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.
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?
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