Photosynthesis - Review the Knowledge You Need to Score High - 5 Steps to a 5: AP Biology 2017 (2016)

5 Steps to a 5: AP Biology 2017 (2016)

STEP 4

Review the Knowledge You Need to Score High

CHAPTER 8

Photosynthesis

IN THIS CHAPTER

Summary: This chapter discusses the basics behind the energy-creation process known as photosynthesis. It also teaches you how plants generate their energy from light. You will learn to differentiate between the two stages—the light-dependent and the light-independent reactions.

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Key Ideas

Image Overall photosynthesis reaction: H2 O + CO2 + light → O2 + glucose + H2 O.

Image Light-dependent reactions: inputs are water and light; products are ATP, NADPH, and O2.

Image The oxygen produced in photosynthesis comes from the water.

Image The carbon in the glucose produced in photosynthesis comes from the CO2.

Image Light-independent reactions (dark reactions): inputs are NADPH, ATP and CO2 ; products are ADP, NADP+ , and sugar.


Introduction

In Chapter 7 , we discussed how human and animal cells generate the energy needed to survive and perform on a day-to-day basis. Now we are going to look at how plants generate their energy from light—the process of photosynthesis. We stress again in this chapter what we said about respiration—do not get caught up in the memorization of every fact. Make sure that you understand the basic, overall concepts and the major ideas. Remember that most of plant photosynthesis occurs in the plant’s leaves. The majority of the chloroplasts of a plant are found in mesophyll cells. Remember that there are two stages to photosynthesis: the light-dependent reactions and the light-independent reactions, commonly called the “dark reactions.” The simplified equation of photosynthesis is

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H2 O + CO2 + light → O2 + glucose + H2 O

The Players in Photosynthesis

The host organelle for photosynthesis is the chloroplast, which is divided into an inner and outer portion. The inner fluid portion is called the stroma, which is surrounded by two outer membranes. In Figure 8.1 , you can see that winding through the stroma is an inner membrane called the thylakoid membrane system. This is where the first stage of photosynthesis occurs. This membrane consists of flattened channels and disks arranged in stacks called grana. We always remember the thylakoid system as resembling stacks of poker chips, where each chip is a single thylakoid. It is within these poker chips that the light-dependent reactions of photosynthesis occur.

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Figure 8.1 An overall view of photosynthesis. (From Biology , 8th ed., by Sylvia S. Mader, © 1985, 1987, 1990, 1993, 1996, 1998, 2001, 2004 by the McGraw Hill Companies, Inc. Reproduced with permission of The McGraw-Hill Companies .)

Before we examine the process of photosynthesis, here are some definitions that will make things a bit easier as you read this chapter.

Autotroph: an organism that is self-nourishing. It obtains carbon and energy without ingesting other organisms. Plants and algae are good examples of autotrophic organisms—they obtain their energy from carbon dioxide, water, and light. They are the producers of the world.

Bundle sheath cells: cells that are tightly wrapped around the veins of a leaf. They are the site for the Calvin cycle in C4 plants.

C4 plant: plant that has adapted its photosynthetic process to more efficiently handle hot and dry conditions.

Heterotroph: organisms that must consume other organisms to obtain nourishment. They are the consumers of the world.

Mesophyll: interior tissue of a leaf.

Mesophyll cells: cells that contain many chloroplasts and host the majority of photosynthesis.

Photolysis: process by which water is broken up by an enzyme into hydrogen ions and oxygen atoms; occurs during the light-dependent reactions of photosynthesis.

Photophosphorylation: process by which ATP is produced during the light-dependent reactions of photosynthesis. It is the chloroplast equivalent of oxidative phosphorylation.

Photorespiration: process by which oxygen competes with carbon dioxide and attaches to RuBP. Plants that experience photorespiration have a lowered capacity for growth.

Photosystem: a cluster of light-trapping pigments involved in the process of photosynthesis. Photosystems vary tremendously in their organization and can possess hundreds of pigments. The two most important are photosystems I and II of the light reactions.

Pigment: a molecule that absorbs light of a particular wavelength. Pigments are vital to the process of photosynthesis and include chlorophyll, carotenoids, and phycobilins.

Rubisco: an enzyme that catalyzes the first step of the Calvin cycle in C3 plants.

Stomata: structure through which CO2 enters a plant and water vapor and O2 leave.

Transpiration: natural process by which plants lose H2 O via evaporation through their leaves.

BIG IDEA 4.C.1

Molecular variation in pigment molecules allows plants to absorb a greater range of wavelengths .

The Reactions of Photosynthesis

BIG IDEA 2.A.1

All living things require input of energy .

The process of photosynthesis can be neatly divided into two sets of reactions: the light-dependent reactions and the light-independent reactions. The light-dependent reactions occur first and require an input of water and light. They produce three things: the oxygen we breathe, NADPH, and ATP. These last two products of the light reactions are then consumed during the second stage of photosynthesis: the dark reactions. These reactions, which need CO2 , NADPH, and ATP as inputs, produce sugar and recycle the NADP+ and ADP to be used by the next set of light-dependent reactions. Now, we would be too kind if we left the discussion there. Let’s look at the reactions in more detail. Stop groaning . . . you know we have to go there.

BIG IDEA 2.A.2

Autotrophs capture free energy present in sunlight through photosynthesis .

Light-Dependent Reactions

Light-dependent reactions occur in the thylakoid membrane system. The thylakoid system is composed of the various stacks of poker chip look-alikes located within the stroma of the chloroplast. Within the thylakoid membrane is a photosynthetic participant termed chlorophyll. There are two main types of chlorophyll that you should remember: chlorophyll a and chlorophyll b . Chlorophyll a is the major pigment of photosynthesis, while chlorophyll b is considered to be an accessory pigment. The pigments are very similar structurally, but the minor differences are what account for the variance in their absorption of light. Chlorophyll absorbs light of a particular wavelength, and when it does, one of its electrons is elevated to a higher energy level (it is “excited”). Almost immediately, the excited electron drops back down to the ground state, giving off heat in the process. This energy is passed along until it finds chlorophyll a , which, when excited, passes its electron to the primary electron acceptor; then, the light-dependent reactions are under way.

The pigments of the thylakoid space organize themselves into groups called photosystems . These photosystems consist of varying combinations of chlorophylls a , b , and others; pigments called phycobilins; and another type of pigment called carotenoids. The accessory pigments help pick up light when chlorophyll a cannot do it as effectively. An example is red algae on the ocean bottom. When light is picked up by the accessory pigments, it is fluoresced and altered so that chlorophyll a can use it.

Imagine that the plant represented in Figure 8.2 is struck by light from the sun. This light excites the photosystem of the thylakoid space, which absorbs the photon and transmits the energy from one pigment molecule to another. As this energy is passed along, it loses a bit of energy with each step and eventually reaches chlorophyll a , which proceeds to kick off the process of photosynthesis. It initiates the first step of photosynthesis by passing the electron to the primary electron acceptor.

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Figure 8.2 Light-dependent reactions. (From Biology, 8th ed., by Sylvia S. Mader, © 1985, 1987, 1990, 1993, 1996, 1998, 2001, 2004 by the McGraw Hill Companies, Inc. Reproduced with permission of The McGraw-Hill Companies.)

Before we continue, there are two major photosystems we want to tell you about—you might want to get out a pen or pencil here to jot this down, because the names for these photosystems may seem confusing. They are photosystem I and photosystem II. The only difference between these two reaction centers is that the main chlorophyll of photosystem I absorbs light with a wavelength of 700 nm, while the main chlorophyll of photosystem II absorbs light with a wavelength of 680 nm. By interacting with different thylakoid membrane proteins, they are able to absorb light of slightly different wavelengths.

Now let’s get back to the reactions. Let’s go through the rest of Figure 8.2 and talk about the light-dependent reactions. For the sole purpose of confusing you, plants start photosynthesis by using photosystem II before photosystem I. As light strikes photosystem II, the energy is absorbed and passed along until it reaches the P680 chlorophyll. When this chlorophyll is excited, it passes its electrons to the primary electron acceptor. This is where the water molecule comes into play. Photolysis in the thylakoid space takes electrons from H2 O and passes them to P680 to replace the electrons given to the primary acceptor. With this reaction, a lone oxygen atom and a pair of hydrogen ions are formed from the water. The oxygen atom quickly finds another oxygen atom buddy, pairs up with it, and generates the O2 that the plants so graciously put out for us every day. This is the first product of the light reactions.

The light reactions do not stop here, however. We need to consider what happens to the electron that has been passed to the primary electron acceptor. The electron is passed to photosystem I, P700, in a manner reminiscent of the electron transport chain. As the electrons are passed from P680 to P700, the lost energy is used to produce ATP (remember chemiosmosis). This ATP is the second product of the light reactions and is produced in a manner mechanistically similar to the way ATP is produced during oxidative phosphorylation of respiration. In plants, this process of ATP formation is called photophosphorylation.

After the photosystem I electrons are excited, photosystem I passes the energy to its own primary electron acceptor. These electrons are sent down another chain to ferredoxin, which then donates the electrons to NADP+ to produce NADPH, the third and final product of the light reactions. (Notice how in photosynthesis, there is NADPH instead of NADH. The symbol P can help you remember that it relates to photosynthesis. Images )

Remember the following about the light reactions:

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1. The light reactions occur in the thylakoid membrane.

2. The inputs to the light reactions are water and light.

3. The light reactions produce three products: ATP, NADPH, and O2 .

4. The oxygen produced in the light reactions comes from H2 O, not CO2 .

Two separate light-dependent pathways occur in plants. What we have just discussed is the noncyclic light reaction pathway. Considering the name of the first one, it is not shocking to discover that there is also a cyclic light reaction pathway (Figure 8.3 ). One key difference between the two is that in the noncyclic pathway, the electrons taken from chlorophyll a are not recycled back down to the ground state. This means that the electrons do not make their way back to the chlorophyll molecule when the reaction is complete. The electrons end up on NADPH. Another key difference between the two is that the cyclic pathway uses only photosystem I; photosystem II is not involved. In the cyclic pathway, sunlight hits P700, thus exciting the electrons and passing them from P700 to its primary electron acceptor. It is called the cyclic pathway because these electrons pass down the electron chain and eventually back to P700 to complete the cycle. The energy given off during the passage down the chain is harnessed to produce ATP—the only product of this pathway. Neither oxygen nor NADPH is produced from these reactions.

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Figure 8.3 Cyclic phosphorylation . (From Biology, 8th ed., by Sylvia S. Mader, © 1985, 1987, 1990, 1993, 1996, 1998, 2001, 2004 by the McGraw Hill Companies, Inc. Reproduced with permission of The McGraw-Hill Companies .)

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A question that might be forming as you read this is: “Why does this pathway continue to exist?” or perhaps you are wondering “Why do they insist on torturing me by writing about all of this photosynthesis stuff?” We will answer the first question and ignore the second one. The cyclic pathway exists because the Calvin cycle, which we discuss next, uses more ATP than it does NADPH. This eventually causes a problem because the light reactions produce equal amounts of ATP and NADPH. The plant compensates for this disparity by dropping into the cyclic phase when needed to produce the ATP necessary to keep the light-independent reactions from grinding to a halt.

Before moving on to the Calvin cycle, it is important to understand how ATP is formed. We know, we know. . . you thought we were finished . . . but we want you to be an expert in the field of photosynthesis. You never know when these facts might come in handy. For example, just the other day one of us was offered $10,000 by a random person on the street to recount the similarities between photosynthesis and respiration. So, this stuff is useful in everyday life. As the electrons are passing from the primary electron acceptor to the next photosystem, hydrogen ions are picked up from outside the membrane and brought back into the thylakoid compartment, creating an H+gradient similar to what we saw in oxidative phosphorylation. During the light-dependent reactions, when hydrogen ions are taken from water during photolysis, the proton gradient grows larger, causing some protons to leave, leading to the formation of ATP.

You’ll notice that this process in plants is a bit different from oxidative phosphorylation of the mitochondria, where the proton gradient is created by pumping protons from the matrix out to the intermembrane space. In the mitochondria, the ATP is produced when the protons move back in . But in plants, photophosphorylation creates the gradient by pumping protons in from the stroma to the thylakoid compartment, and the ATP is produced as the protons move back out . The opposing reactions produce the same happy result—more ATP for the cells.

Light-Independent Reactions (Calvin Cycle)

After the light reactions have produced the necessary ATP and NADPH, the synthesis phase of photosynthesis is ready to proceed. The inputs into the Calvin cycle are NADPH (which provides hydrogen and electrons), ATP (which provides energy), and CO2 . From here on, just so we don’t drive you insane switching from term to term, we are going to call the dark reactions of photosynthesis the Calvin cycle (Figure 8.4 ). The Calvin cycle occurs in the stroma of the chloroplast, which is the fluid surrounding the thylakoid “poker chips.” (For further distinctions among the cyclic pathway, the noncyclic pathway, and the Calvin cycle, see Figure 8.5 .)

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Figure 8.4 The Calvin cycle. (From Biology, 8th ed., by Sylvia S. Mader, © 1985, 1987, 1990, 1993, 1996, 1998, 2001, 2004 by the McGraw Hill Companies, Inc. Reproduced with permission of The McGraw-Hill Companies .)

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Figure 8.5 Summary of photosynthesis.

The Calvin cycle begins with a step called carbon fixation. This is a tricky and complex term that makes it sound more confusing than it really is. Basically, carbon fixation is the binding of the carbon from CO2 to a molecule that is able to enter the Calvin cycle. Usually this molecule is ribulose bis-phosphate, a 5-carbon molecule known to its closer friends as RuBP. This reaction is assisted by the enzyme with one of the cooler names in the business: rubisco. The result of this reaction is a 6-carbon molecule that breaks into two 3-carbon molecules named 3-phosphoglycerate (3PG). ATP and NADPH step up at this point and donate a phosphate group and hydrogen electrons, respectively, to (3PG) to form glyceraldehyde 3-phosphate (G3P). Most of the G3P produced is converted back to RuBP so as to fix more carbon. The remaining G3P is converted into a 6-carbon sugar molecule, which is used to build carbohydrates for the plant. This process uses more ATP than it does NADPH. This is the disparity that makes cyclic photophosphorylation necessary in the light-dependent reactions.

We know that for some of you, the preceding discussion contains many difficult scientific names, strangely spelled words, and esoteric acronyms. So, here’s the bottom line—you should remember the following about the Calvin cycle:

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1. The Calvin cycle occurs in the stroma of the chloroplast.

2. The inputs into the Calvin cycle are NADPH, ATP, and CO2 .

3. The products of the Calvin cycle are NADP+ , ADP, and a sugar.

4. More ATP is used than NADPH, creating the need for cyclic photophosphorylation to create enough ATP for the reactions.

5. The carbon of the sugar produced in photosynthesis comes from the CO2 of the Calvin cycle.

Types of Photosynthesis

Plants do not always live under ideal photosynthetic conditions. Some plants must make changes to the system in order to successfully use light and produce energy. Plants contain a structure called a stomata, which consists of pores through which oxygen exits and carbon dioxide enters the leaf to be used in photosynthesis. Transpiration is the natural process by which plants lose water by evaporation from their leaves. When the temperature is very high, plants have to worry about excess transpiration. This is a potential problem for plants because they need the water to continue the process of photosynthesis. To combat this evaporation problem, plants must close their stomata to conserve water. But this solution leads to two different problems: (1) how will they bring in the CO2 required for photosynthesis? and (2) what will the plants do with the excess O2 that builds up when the stomata are closed?

When plants close their stomata to protect against water loss, they experience a shortage of CO2 , and the oxygen produced from the light reactions is unable to leave the plant. This excess oxygen competes with the carbon dioxide and attaches to RuBP in a reaction called photorespiration. This results in the formation of one molecule of PGA and one molecule of phosphoglycolate. This is not an ideal reaction because the sugar formed in photosynthesis comes from the PGA, not phosphoglycolate. As a result, plants that experience photorespiration have a lowered capacity for growth. Photorespiration tends to occur on hot, dry days when the stomata of the plant are closed.

A group of plants called C4 plants combat photorespiration by altering the first step of their Calvin cycle. Normally, carbon fixation produces two 3-carbon molecules. In C4 plants, the carbon fixation step produces a 4-carbon molecule called oxaloacetate. This molecule is converted into malate and sent from the mesophyll cells to the bundle sheath cells, where the CO2 is used to build sugar. The mesophyll is the tissue of the interior of the leaf, and mesophyll cells are cells that contain bunches of chloroplasts. Bundle sheath cells are cells that are tightly wrapped around the veins of a leaf. They are the site for the Calvin cycle in C4 plants.

What is the difference between C3 plants and C4 plants? One difference is that C4 plants have two different types of photosynthetic cells: (1) tightly packed bundle sheath cells, which surround the vein of the leaf, and (2) mesophyll cells. Another difference involves the first product of carbon fixation. For C3 plants, it is PGA, for C4 plants, it is oxaloacetate. C4 plants are able to successfully perform photosynthesis in these hot areas because of the presence of an enzyme called PEP (phosphoenolpyruvate ) carboxylase . This enzyme really wants to bind to CO2 and is not tricked by the devious oxygen into using it instead of the necessary CO2 . PEP carboxylase prefers to pair up with CO2 rather than O2 , and this cuts down on photorespiration for C4 plants. The conversion of PEP to oxaloacetate occurs in the mesophyll cells; then, after being converted into malate, PEP is shipped to the bundle sheath cells. These cells contain the enzymes of photosynthesis, including our good pal rubisco. The malate releases the CO2 , which is then used by rubisco to perform the reactions of photosynthesis. This process counters the problem of photorespiration because the shuttling of CO2 from the mesophyll cells to the bundle sheath cells keeps the CO2 concentration high enough so that it is not beat out by oxygen for rubisco’s love and attention.

One last variation of photosynthesis that we should look at is the function performed by CAM (Crassulacean acid metabolizing) plants—water-storing plants, such as cacti, that close their stomata by day and open them by night to avoid transpiration during the hot days, without depleting the plant’s CO2 reserves. The CO2 taken in during the night is stored as organic acids in the vacuoles of mesophyll cells until daybreak when the stomata close. The Calvin cycle is able to proceed during the day because the stored CO2 is released, as needed, from the organic acids to be incorporated into the sugar product of the Calvin cycle.

To sum up these two variations of photosynthesis:

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C4 photosynthesis: photosynthetic process that first converts CO2 into a 4-carbon molecule in the mesophyll cells, converts that product to malate, and then shuttles the malate into the bundle sheath cells. There, malate releases CO2 , which reacts with rubisco to produce the carbohydrate product of photosynthesis.

CAM photosynthesis: plants close their stomata during the day, collect CO2 at night, and store the CO2 in the form of acids until it is needed during the day for photosynthesis.

Image Review Questions

Questions 1–4 refer to the following answer choices—use each answer only once.

A. Transpiration

B. Calvin cycle

C. CAM photosynthesis

D. Cyclic photophosphorylation

E. Noncyclic photophosphorylation

1 . Plants use this process so that they can open their stomata at night and close their stomata during the day to avoid water loss during the hot days, without depleting the plant’s CO2 reserves.

2 . Uses NADPH, ATP, and CO2 as the inputs to its reactions.

3 . Photosynthetic process that has ATP as its sole product. There is no oxygen and no NADPH produced from these reactions.

4 . The process by which plants lose water via evaporation through their leaves.

5 . The photosynthetic process performed by some plants in an effort to survive the hot and dry conditions of climates such as the desert is called

A. carbon fixation.

B. C3 photosynthesis.

C. C4 photosynthesis.

D. cyclic photophosphorylation.

E. noncyclic photophosphorylation.

6 . Which of the following is the photosynthetic stage that produces oxygen?

A. The light-dependent reactions

B. Chemiosmosis

C. The Calvin cycle

D. Carbon fixation

E. Photorespiration

7 . Which of the following reactions occur in both cellular respiration and photosynthesis?

A. Carbon fixation

B. Fermentation

C. Reduction of NADP+

D. Chemiosmosis

E. Formation of NADH

8 . Which of the following is not a product of the light-dependent reactions of photosynthesis?

A. O2

B. ATP

C. NADPH

D. Sugar

9 . Which of the following is an advantage held by a C4 plant?

A. More efficient light absorption

B. More efficient photolysis

C. More efficient carbon fixation

D. More efficient uptake of carbon dioxide into the stomata

E. More efficient ATP synthesis during chemiosmosis

10 . Carbon dioxide enters the plant through the

A. Stomata

B. Stroma

C. Thylakoid membrane

D. Bundle sheath cell

11 . Which of the following is the source of the oxygen released during photosynthesis?

A. CO2

B. H2 O

C. Rubisco

D. PEP carboxylase

E. Pyruvate

12 . Which of the following is an incorrect statement about the Calvin cycle?

A. The main inputs to the reactions are NADPH, ATP, and CO2 .

B. The main outputs of the reactions are NADP+ , ADP, and sugar.

C. More NADPH is used than ATP during the Calvin cycle.

D. Carbon fixation is the first step of the process.

E. The reactions occur in the stroma of the chloroplast.

13 . Which of the following is the source of the carbon in sugar produced during photosynthesis?

A. CO2

B. H2 O

C. Rubisco

D. PEP carboxylase

E. Pyruvate

14 . The light-dependent reactions of photosynthesis occur in the

A. stroma.

B. mitochondrial matrix.

C. thylakoid membrane.

D. cytoplasm.

E. nucleus.

Image Answers and Explanations

1 . C —CAM plants open their stomata at night and close their stomata during the day to avoid water loss due to heat. The carbon dioxide taken in during the night is incorporated into organic acids and stored in vacuoles until the next day, when the stomata close and CO2 is needed for the Calvin cycle.

2 . B —The Calvin cycle uses ATP, NADPH, and CO2 to produce the desired sugar output of photosynthesis.

3 . D —Cyclic photophosphorylation occurs because the Calvin cycle uses more ATP than it does NADPH. This is a problem because the light reactions produce an equal amount of ATP and NADPH. The plant compensates for this disparity by dropping into the cyclic phase when needed to produce the ATP necessary to keep the light-independent reactions from grinding to a halt.

4 . A —Transpiration is the process by which plants lose water through their leaves. Not much else to be said about that. Images

5 . C —One of the major problems encountered by plants in hot and dry conditions is of photo-respiration. In hot conditions, plants close their stomata to avoid losing water to transpiration. The problem with this is that the plants run low on CO2 and fill with O2 . The oxygen competes with the carbon dioxide and attaches to RuBP, leaving the plant with a lowered capacity for growth. C4 plants cycle CO2 from mesophyll cells to bundle sheath cells, creating a higher concentration of CO2 in that region, thus allowing rubisco to carry out the Calvin cycle without being distracted by the O2 competitor.

6 . A —The light-dependent reactions are the source of the oxygen given off by plants.

7 . D —Chemiosmosis occurs in both photosynthesis and cellular respiration. This is the process by which the formation of ATP is driven by electrochemical gradients in the cell. Hydrogen ions accumulate on one side of a membrane, creating a proton gradient that causes them to move through channels to the other side of that membrane, thus leading, with the assistance of ATP synthase, to the production of ATP.

8 . D —Sugar is a product not of the light-dependent reactions of photosynthesis but of the Calvin cycle (the dark reactions). The outputs of the light-dependent reactions are ATP, NADPH, and O2 .

9 . C —C4 plants fix carbon more efficiently than do C3 plants. Please see the explanation for question 5 for a more detailed explanation of this answer.

10 . A —The stomata is the structure through which the CO2 enters a plant and the oxygen produced in the light-dependent reactions leaves the plant.

11 . B —The source of the oxygen produced during photosynthesis is the water that is split by the process of photolysis during the light-dependent reactions of photosynthesis. In this reaction, two hydrogen ions and an oxygen atom are formed from the water. The oxygen atom immediately finds and pairs up with another oxygen atom to form the oxygen product of the light-dependent reactions.

12 . C —This is a trick question. We reversed the two compounds (NADPH and ATP) in this one. More ATP than NADPH is used in the Calvin cycle. It is for this reason that cyclic photophosphorylation exists—to produce ATP to make up for this disparity.

13 . A —The carbon of CO2 is used to produce the sugar created during the Calvin cycle.

14 . C —The light-dependent reactions occur in the thylakoid membrane of the chloroplast. Remember, the thylakoid system resembles the various stacks of poker chips located within the stroma of the chloroplast. The light-independent reactions occur in the stroma of the chloroplast.

Image Rapid Review

The following terms should be thoroughly familiar to you:

Photosynthesis: process by which plants use the energy from light to generate sugar.

• Occurs in chloroplasts

• Light reactions (thylakoid)

• Calvin cycle (stroma)

Autotroph: self-nourishing organism that is also known as a producer (plants).

Heterotroph: organisms that must consume other organisms to obtain energy—consumers (humans).

Transpiration: loss of water via evaporation through the stomata (natural process).

Photophosphorylation: process by which ATP is made during light reactions.

Photolysis: process by which water is split into hydrogen ions and oxygen atoms (light reactions).

Stomata: structure through which CO2 enters a plant, and water vapor and oxygen leave a plant.

Pigment: molecule that absorbs light of a particular wavelength (chlorophyll, carotenoid, phycobilins).

There are three types of photosynthesis reactions:

(Noncyclic ) light-dependent reactions

• Occur in thylakoid membrane of chloroplast.

• Inputs are light and water.

• Light strikes photosystem II (P680).

• Electrons pass along until they reach primary electron acceptor.

• Photolysis occurs—H2 O is split to H+ and O2 .

• Electrons pass down an ETC to P700 (photosystem I), forming ATP by chemiosmosis.

• Electrons of P700 pass down another ETC to produce NADPH.

• Three products of light reactions are NADPH, ATP, and O2 .

• Oxygen produced comes from H2 O.

(Cyclic ) light-dependent reactions

• Occur in thylakoid membrane.

• Only involves photosystem I; no photosystem II.

• ATP is the only product of these reactions.

• No NADPH or oxygen are produced.

• These reactions exist because the Calvin cycle uses more ATP than NADPH; this is how the difference is made up.

Light-independent reactions (Calvin cycle )

• Occurs in stroma of chloroplast.

• Inputs are NADPH, ATP, and CO2 .

• First step is carbon fixation, which is catalyzed by an enzyme named rubisco.

• A series of reactions lead to the production of NADP+ , ADP, and sugar.

• More ATP is used than NADPH, which creates the need for the cyclic light reactions.

• The carbon of the sugar product comes from CO2 .

Also:

C4 plants —plants that have adapted their photosynthetic process to more efficiently handle hot and dry conditions.

C4 photosynthesis —process that first converts CO2 into a 4-carbon molecule in the mesophyll cells, converts that product to malate, and then shuttles it to the bundle sheath cells, where the malate releases CO2 and rubisco picks it up as if all were normal.

CAM plants —plants that close their stomata during the day, collect CO2 at night, and store the CO2 in the form of acids until it is needed during the day for photosynthesis.

CHAPTER 8

Photosynthesis

1 . Which of the following do CAM and C4 plants have in common?

(A) They are known to survive well in excessively moist environments.

(B) They readily bind carbon dioxide.

(C) They produce sugar more efficiently than do C3 plants.

(D) They produce ATP less efficiently than do C3 plants.

2 . All of the following are directly involved in photosystems EXCEPT

(A) phycobilins.

(B) carotenoids.

(C) chlorophyll A.

(D) rubisco.

3 . Which of the following processes occurs in both cellular respiration and photosynthesis?

(A) Carbon fixation

(B) Fermentation

(C) Reduction of NADP+

(D) Chemiosmosis

4 . Which of the following processes represents an anaerobic pathway that produces ATP less efficiently than do oxygen-driven processes?

(A) Carbon fixation

(B) Fermentation

(C) Reduction of NADP+

(D) Chemiosmosis

images Answers and Explanations


1 . B —Both plants provide alternatives to carbon fixation and readily attach to carbon dioxide molecules.

2 . D —Rubisco is the only choice not directly involved in photosystems. Rubisco is an enzyme that catalyzes the first step of the Calvin cycle in C3 plants.

3 . D —Chemiosmosis is the process by which the formation of ATP is driven by electrochemical gradients in the cell. This process occurs in both respiration and photosynthesis.