Energy for Life
Chapter
6
Outline
	6.1	Overview of Photosynthesis
	•	Plants, algae, and cyanobacteria are photosynthetic organisms that produce most of the carbohydrates used for energy by the living world.85
	•	In flowering plants, photosynthesis takes place within chloroplasts, organelles that contain membranous thylakoids surrounded by a fluid called stroma.85
	•	Photosynthesis has two sets of reactions: Pigments in the thylakoids capture solar energy, and enzymes in the stroma reduce carbon dioxide.86
	6.2	Light Reactions
	•	Plants use solar energy in the visible light range when they carry on photosynthesis.87
	•	Solar energy energizes electrons and permits a buildup of ATP and NADPH molecules.87–89
	6.3	Calvin Cycle Reactions
	•	Carbon dioxide reduction requires ATP and NADPH from the light reactions.90
	6.4	Other Types of Photosynthesis
	•	Plants use C3, C4, or CAM photosynthesis, which are distinguishable by the manner in which CO2 is fixed.92–93
Most people are aware that plants produce oxygen gas. Further, they often think plants produce this oxygen as some sort of altruistic act to benefit humans and other animals! But the facts don’t bear this out. During the process of photosynthesis, plants are actually making food for themselves. To do this, they reduce the carbon dioxide you exhale to carbohydrates after converting energy from the sun into chemical energy. This conversion process is complex, and depends on the energizing of electrons removed from water. In the process, water splits and releases oxygen. This oxygen is the gas we and other animals depend upon. Thus, plants provide the oxygen you inhale and the glucose you use as a source of energy only because they are in the process of photosynthesizing to keep themselves alive. Plants are solar powered, and indirectly so are all other living things on Earth, including ourselves. 
Understanding the process of photosynthesis and its critical importance to nearly all living organisms will help you appreciate just how dependent we humans are on plants. In this chapter, you will learn about basic plant cell structure and how plants perform photosynthesis.
6.1		Overview of Photosynthesis
Photosynthesis transforms solar energy into the chemical energy of a carbohydrate. Photosynthetic organisms, including plants, algae, and cyanobacteria, produce an enormous amount of carbohydrate (Fig. 6.1). If the amount of carbohydrate were instantly converted to coal, and the coal loaded into standard railroad cars (each car holding about 50 tons), the photosynthesizers of the biosphere would fill more than 100 cars per second with coal.
It is no wonder, then, that photosynthetic organisms are able to -sustain themselves and, with a few exceptions,1 all of the other living things on Earth. To appreciate this, consider that most food chains lead back to plants. In other words, producers, which have the ability to synthesize carbohydrates, feed not only themselves but also consumers, which must take in preformed organic molecules. 
Our analogy about photosynthetic products becoming coal is apt because the bodies of plants formed the coal we burn today. This process occurred hundreds of thousands of years ago, and that is why coal is called a fossil fuel. The wood of trees is also commonly used as fuel. In addition, the fermentation of plant materials produces alcohol, which can be used to fuel automobiles directly or as a gasoline additive. 
Flowering Plants as Photosynthesizers
The green portions of plants, particularly the leaves, carry on photosynthesis. The raw materials for photosynthesis are water and carbon dioxide. The roots of a plant absorb water, which then moves in vascular tissue up the stem to a leaf. Water exits into a leaf by way of leaf veins. The leaf of a flowering plant contains mesophyll tissue in which cells are specialized to carry on photosynthesis. Carbon dioxide in the air enters a leaf through small openings called stomata (sing., stoma). Carbon dioxide and water diffuse into mesophyll cells and then into chloroplasts, the organelles that carry on photosynthesis (Fig. 6.2).
In a chloroplast, a double membrane surrounds a fluid called the stroma. A third membrane system within the stroma forms flattened sacs called thylakoids, which in some places are stacked to form grana (sing., granum), so called because early microscopists thought they looked like piles of seeds. The space within each thylakoid is believed to be connected to the space within every other thylakoid, thereby forming an inner compartment within chloroplasts called the thylakoid space.
Chlorophyll and other pigments reside within the membranes of the thylakoids. These pigments are capable of absorbing solar energy, the energy that drives photosynthesis. The stroma is an enzyme-rich solution in which carbon dioxide is first attached to an organic compound and then reduced to a carbohydrate. 
As you well know, human beings, and indeed nearly all organisms, release carbon dioxide into the air. This same carbon dioxide enters leaves and is converted to a carbohydrate. Carbohydrate in the form of glucose is the chief organic source of energy for most organisms. 
The Photosynthetic Process
The overall equation for photosynthesis is sometimes written like this: 
This equation tells us that photosynthesis begins with the end products of cellular respiration, CO2 and H2O, which are low-energy molecules. In contrast, a high-energy carbohydrate, symbolized here by CH2O, is an end product of photosynthesis. To form a carbohydrate, hydrogen atoms have been removed from water and added to CO2. To reduce CO2, energy is required, and this energy is provided by the sun. The oxygen released by photosynthesis, so necessary to cellular respiration, is simply a by-product of the oxidation of water. 
Notice that if you multiplied (CH2O) by six you would get C6H12O6, which is the formula for glucose. Some prefer to emphasize that glucose is an end product of photosynthesis.
Two Sets of Reactions
An overall equation for photosynthesis tells us the beginning reactant and the end products of the pathway. But much goes on in between. The word photosynthesis suggests that the process requires two sets of reactions. Photo, which means light, refers to the reactions that capture solar energy, and synthesis refers to the reactions that produce a carbohydrate. The two sets of reactions are called the light reactions and the Calvin cycle reactions (Fig. 6.3).
Light ReactionsThe following events occur in the thylakoid membrane during the light reactions: 
	•	Chlorophyll within the thylakoid membranes absorbs solar energy and energizes electrons.
	•	ATP is produced from ADP  sP  with the help of an electron transport chain.
	•	NADP1,1 an enzyme helper, accepts electrons and becomes NADPH.
Calvin Cycle ReactionsThe following events occur in the stroma during the Calvin cycle reactions: 
	•	CO2 is taken up by one of the substrates in the cycle. 
	•	ATP and NADPH from the light reactions reduce CO2 to a carbohydrate.
In the following sections, we discuss details of these two reactions.
6.2		Light Reactions
During the light reactions, the pigments within the thylakoid membranes absorb solar (radiant) energy. Solar energy can be described in terms of its wavelength and its energy content. Figure 6.4 lists the different types of radiant energy, from the shortest wavelength, gamma rays, to the longest, radio waves. Visible light is only a small portion of this spectrum.
Visible light contains various wavelengths of light, as can be proven by passing it through a prism; the different wavelengths appear to us as the colors of the rainbow, ranging from violet (the shortest wavelength) to blue, green, yellow, orange, and red (the longest wavelength). The energy content is highest for violet light and lowest for red light.
Only about 42% of the solar radiation that hits the Earth’s atmosphere ever reaches the surface, and most of this radiation is within the visible-light range. Higher-energy wavelengths are screened out by the ozone layer in the -atmosphere, and lower-energy wavelengths are screened out by water vapor and CO2 before they reach the Earth’s surface. Both the organic molecules within organisms and certain life processes, such as vision and photosynthesis, are adapted to the radiation that is most prevalent in the environment.
Photosynthetic Pigments
The pigments found within most types of photosynthesizing cells are chlorophylls and carotenoids. These pigments are capable of absorbing portions of visible light. Both chlorophyll a and chlorophyll b absorb violet, blue, and red wavelengths better than those of other colors. Because green light is reflected and only minimally absorbed, leaves appear green to us. Accessory pigments such as the carotenoids appear yellow or orange because they are able to absorb light in the violet-blue-green range, but not the yellow-orange range. These pigments and others become noticeable in the fall when chlorophyll breaks down and the other pigments are uncovered (Fig. 6.5).
The Electron Pathway of the Light Reactions
The light reactions consist of an electron pathway that is necessary to the production of ATP and NADPH (Fig. 6.6). This pathway uses two photo-systems, called photosystem I (PS I) and photosystem II (PS II). The photosystems are named for the order in which they were discovered, not for the order in which they participate in the photosynthetic process. Here is a summary of how the pathway works:
	•	The photosystemsPS II and PS I consist of a pigment complex (contains chlorophyll and carotenoid molecules) and an electron acceptor. The pigment complex serves as an “antenna” for gathering solar energy, which is then passed from one pigment to the other until it is concentrated in a particular pair of chlorophyll a molecules, called the -reaction center.
	•	PS II splits waterDue to the absorption of solar energy, electrons (e2) in the reaction center of PS II become so energized that they escape and move to a nearby electron-acceptor molecule. Replacement electrons are removed from water, which splits, releasing oxygen to the atmosphere. The electron acceptor sends energized electrons, received from the reaction center, down an electron transport chain.
	•	The electron transport chainIn an electron transport chain, a series of carriers pass electrons from one to the other, releasing energy that is stored in the form of a hydrogen ion (H1) gradient. Later, ATP is produced (see page 89).
	•	PS I produces NADPHWhen the PS I pigment complex absorbs solar energy, energized electrons leave its reaction center and are captured by a different electron acceptor. (Low-energy electrons from the electron transport chain adjacent to PS II replace those lost by PS I.) The electron acceptor in PS I passes its electrons to a NADP1 molecule. NADP1 accepts  electrons and becomes NADPH. 
Organization of the Thylakoid Membrane
PS II, PS I, and the electron transport chain are located within molecular complexes in the thylakoid membrane (Fig. 6.7). Also present is an ATP synthase complex.
ATP Production
During photosynthesis, the thylakoid space acts as a reservoir for hydrogen ions (H1). First, each time water is oxidized, two H1 remain in the thylakoid space. Second, as the electrons move from carrier to carrier down the electron transport chain, the electrons give up energy. This energy is used to pump H1 from the stroma into the thylakoid space. Therefore, there are many more H1 ions in the thylakoid space than in the stroma, and an H1 gradient has been established.
The H1 ions now flow down their concentration gradient, across the thylakoid membrane at the ATP synthase complex. This causes the enzyme ATP synthase to change its shape and produce ATP from ADP 1 sP. 
NADPH Production
Enzymes often have nonprotein helpers called coenzymes. NADP is a coenzyme that accepts electrons from a substrate, becoming NADPH. When it gives up electrons to a substrate, the substrate is reduced. During the light reactions, NADP receives electrons at the end of the electron pathway in the thylakoid membrane, and then it picks up a hydrogen ion to become NADPH.
6.3		Calvin Cycle Reactions
The Calvin cycle is a series of reactions that produce carbohydrate before returning to the starting point (Fig. 6.8). The end product of the Calvin cycle is often considered to be glucose, as mentioned. The cycle is named for Melvin Calvin who, with colleagues, used the radioactive isotope 14C as a tracer to discover the reactions that make up the cycle.
This series of reactions utilizes carbon dioxide from the atmosphere to produce carbohydrate. The Calvin cycle includes (1) carbon dioxide fixation, (2) carbon dioxide reduction, and (3) regeneration of the first substrate, RuBP (ribulose 1,5-bisphosphate).
Fixation of Carbon Dioxide
Carbon dioxide fixation is the first step of the Calvin cycle. During this reaction, CO2 from the atmo-sphere is attached to RuBP, a 5-carbon molecule. The enzyme for this reaction is called RuBP carboxylase (rubisco), and the result is a 6-carbon molecule that splits into two 3-carbon -molecules.
Reduction of Carbon Dioxide
Reduction of CO2 is the sequence of reactions that uses NADPH from the light reactions, and also uses some ATP from the same source. Carbon dioxide is reduced to a carbohydrate as R–CO2 becomes R–CH2O. Electrons and energy are needed for this reduction reaction, and these are supplied by NADPH and ATP, respectively.
Regeneration of RuBP 
The product of the Calvin cycle is actually glyceraldehyde-3-phosphate (G3P), which is used to form glucose. Notice that the Calvin cycle reactions in Figure 6.8 are multiplied by three because it takes three turns of the Calvin cycle to allow one G3P to exit. Why? Because, for every three turns of the Calvin cycle, five molecules of G3P are used to re-form three molecules of RuBP, a C5 molecule. These reactions also utilize some of the ATP produced by the light reactions.
The Importance of the Calvin Cycle
Compared to animal cells, algae and plants have enormous biochemical capabilities. From a G3P molecule, they can make all the molecules they need (Fig. 6.9). A plant can utilize the hydrocarbon skeleton of G3P to form fatty acids and glycerol, which are combined in plant oils. We are all familiar with corn oil, sunflower oil, and olive oil used in cooking. Also, when nitrogen is added to the hydrocarbon skeleton derived from G3P, amino acids are formed.
Notice also that glucose phosphate is among the organic molecules that result from G3P metabolism. Glucose is the molecule that plants and animals most often metabolize to produce ATP molecules to meet their energy needs. Glucose phosphate can be combined with fructose (and the phosphate removed) to form sucrose, the molecule that plants use to transport carbohydrates from one part of the body to another. Glucose phosphate is also the starting point for the synthesis of starch and cellulose. Starch is the storage form of glucose. Some starch is stored in chloroplasts, but most starch is stored in amyloplasts in plant roots. Cellulose is a structural component of plant cell walls; it serves as fiber in our diet because we are unable to digest it.
6.4		Other Types of Photosynthesis
Plants are physically adapted to their environments. In the cold, windy climates of the north, evergreen trees have small, narrow leaves that look like needles. In the warm, wet climates of the south, some evergreen trees have large, flat leaves to catch the rays of the sun. 
In the same way, plants are metabolically adapted to their environments. Where temperature and rainfall tend to be moderate, plants carry on C3 photosynthesis, and are therefore called C3 plants. In a C3 plant, the first detectable molecule after CO2 fixation is a C3 molecule composed of three carbon atoms (Fig. 6.10). Look again at the Calvin cycle (see Fig. 6.8), and notice that the C6 molecule formed when RuBP combines with carbon dioxide immediately breaks down to two C3 molecules. 
If the weather is hot and dry, stomata close, preventing the loss of water. (Water loss might cause the plant to wilt and die.) Now CO2 decreases, and O2, a by-product of photosynthesis, increases in leaf spaces. In C3 plants, this O2 competes with CO2 for the active site of rubisco, the first enzyme of the Calvin cycle, and less C3 is produced. Such yield decreases are of concern because many food crops are C3 plants.
In hot, dry climates, successful plants tend to carry on C4 photosynthesis instead of C3 photosynthesis. In a C4 plant, the first detectable molecule following CO2 fixation is a C4 molecule composed of four carbon atoms. C4 plants are able to avoid the uptake of O2 by rubisco. Let’s explore how C4 plants do this.  
C4 Photosynthesis 
The anatomy of a C4 plant is different from that of a C3 plant. In a C3 leaf, mesophyll cells arranged in parallel rows contain well-formed chloroplasts. The Calvin cycle reactions occur in the chloroplasts of mesophyll cells (Fig. 6.11). In a C4 leaf, chloroplasts are located in the mesophyll cells, but they are also located in bundle sheath cells, which surround the leaf vein. Further, the mesophyll cells are arranged concentrically around the bundle sheath cells, shielding the bundle sheath cells from leaf spaces including O2 from photosynthesis:
In C4 plants, the Calvin cycle reactions occur in the bundle sheath cells and not in the mesophyll cells. Therefore, CO2 from the air is not fixed by the Calvin cycle. Instead, CO2 is fixed by a C3 molecule, and the C4 that results is modified and then pumped into the bundle sheath cells (Fig. 6.11). Now CO2 enters the Calvin cycle. This represents partitioning of the pathways in space.
It takes energy to pump molecules, and you would think that the C4 pathway would be disadvantageous. Yet in hot, dry climates, the net photosynthetic rate of C4 plants such as sugarcane, corn, and Bermuda grass is two to three times that of C3 plants such as wheat, rice, and oats. Why do C4 plants enjoy such an advantage? The answer is that when the weather is hot and dry and stomata close, rubisco is not exposed to oxygen, and yield is maintained. 
When the weather is moderate, C3 plants ordinarily have the advantage, but when the weather becomes hot and dry, C4 plants have their chance to take over, and we can expect them to predominate. In the early summer, C3 plants such as Kentucky bluegrass and creeping bent grass predominate in lawns in the cooler parts of the United States, but by midsummer, crabgrass, a C4 plant, begins to take over. 
CAM Photosynthesis
Another type of photosynthesis is called CAM, which stands for crassulacean-acid metabolism. It gets its name from the Crassulaceae, a family of flowering succulent (water-containing) plants that live in warm, arid regions of the world. CAM was first discovered in these plants, but now it is known to be prevalent among most succulent plants that grow in desert environments, including cactuses.
Whereas a C4 plant represents partitioning in space—that is, carbon dioxide fixation occurs in mesophyll cells, and the Calvin cycle reactions occur in bundle sheath cells—CAM is partitioning by the use of time. During the night, CAM plants use a C3 molecule to fix some CO2, forming C4 molecules. These molecules are stored in large vacuoles in mesophyll cells. During the day, the C4 molecules release CO2 to the Calvin cycle when NADPH and ATP are available from the light reactions (Fig. 6.12). 
The primary advantage of this partitioning again relates to the conservation of water. CAM plants open their stomata only at night, and therefore only at that time is atmospheric CO2 available. During the day, the sto-mata close. This conserves water, but CO2 cannot enter the plant.
Photosynthesis in a CAM plant is minimal because of the limited amount of CO2 fixed at night, but it does allow CAM plants to live under stressful conditions. 
Evolutionary Trends
C4 plants most likely evolved in, and are adapted to, areas of high light intensities, high temperatures, and limited rainfall. However, C4 plants are more sensitive to cold, and C3 plants probably do better than C4 plants below 258C. 
CAM plants, on the other hand, compete well with either type of plant when the environment is extremely arid. Surprisingly, CAM is quite widespread and has evolved in 30 families of flowering plants, including cactuses, stonecrops, orchids, and bromeliads. It is also found among nonflowering plants, such as some ferns and cone-bearing trees.  
The Chapter in Review
Summary
6.1		Overview of Photosynthesis
Cyanobacteria, algae, and plants carry on photosynthesis, a process in which water is oxidized and carbon dioxide is reduced using solar energy. The end products of photosynthesis include carbohydrate and oxygen:
In plants, photosynthesis takes place in chloroplasts. A chloroplast is bounded by a double membrane and contains two main components: the semiliquid stroma and the membranous grana made up of thylakoids. During photosynthesis, the light reactions take place in the thylakoid membrane, and the Calvin cycle reactions take place in the stroma:
6.2		Light Reactions
The light reactions use solar energy in the visible-light range. Pigment complexes in two photosystems (PS II and PS I) absorb various wavelengths of light, and the energy is concentrated in the reaction centers of the photosystems. The light reactions involve the following electron pathway: 
	•	Solar energy enters PS II, and energized electrons are picked up by an electron acceptor. The oxidation (splitting) of water replaces these electrons in the reaction center. O2 is released to the atmosphere, and hydrogen ions (H1) remain in the thylakoid space. 
	•	As electrons pass down an electron transport chain (ETC), the release of energy allows the carriers to pump H1 into the thylakoid space. The buildup of H1 establishes an electrochemical gradient. 
	•	When solar energy is absorbed by PS I, energized electrons leave and are ultimately received by NADP1, which also combines with H1 from the stroma to become NADPH. Electrons from PS II replace those lost by PS I. 
	•	When H1 flows down its concentration gradient through the channel present in ATP synthase complexes, ATP is synthesized from ADP and sP by ATP synthase. 
6.3		Calvin Cycle Reactions
The Calvin cycle consists of the following stages:
CO2 fixationThe enzyme rubisco fixes CO2 to RuBP, producing a 6-carbon molecule that immediately breaks down to two C3 molecules. 
CO2 reductionCO2 (incorporated into an organic molecule) is reduced to carbohydrate (CH2O). This step requires the NADPH and some of the ATP from the light reactions. 
Regeneration of RuBPFor every three turns of the Calvin cycle, the net gain is one G3P molecule; the other five G3P molecules are used to re-form three molecules of RuBP. This step also requires the energy of ATP. G3P is then converted to all the organic molecules a plant needs. It takes two G3P molecules to make one glucose molecule. 
6.4 		Other Types of Photosynthesis
C4 Photosynthesis
When the weather is hot and dry, C3 plants are at a disadvantage because stomata close and O2 from photosynthesis competes with CO2 for the active site of rubisco. C4 plants avoid this drawback. In C4 plants, CO2 fixation occurs in mesophyll cells, and the Calvin cycle reactions occur in bundle sheath cells. In mesophyll cells, a C3 molecule fixes CO2, and the result is a C4 molecule, for which this type of photosynthesis is named. A modified form of this molecule is pumped into bundle sheath cells, where CO2 is released to the Calvin cycle. This represents partitioning in space.
CAM Photosynthesis
CAM plants live in hot, dry environments. At night when stomata remain open, a C3 molecule fixes CO2 to produce a C4 molecule. The next day, CO2 is released and enters the Calvin cycle within the same cells. This represents a partitioning of pathways in time: Carbon dioxide fixation occurs at night, and the Calvin cycle occurs during the day.
Thinking Scientifically
	1.	Greenhouse growers of horticultural plants have long known that carbon dioxide can be a limiting factor for photosynthesis. That is, if plants are supplied with higher than normal levels of carbon dioxide, they will grow faster. Based on the data graphed below, answer the following questions: 
			At 300 ppm (parts per million) CO2 (roughly the level in the atmosphere), did an increase in temperature result in an increase in photosynthetic rate? Did an increase in light intensity at 300 ppm CO2 result in an increase in photosynthetic rate? 
			At 1,300 ppm CO2 (a CO2-enriched atmosphere), did an increase in temperature result in an increase in photosynthetic rate? Did an increase in light intensity at 1,300 ppm CO2 result in an increase in photosynthetic rate? Is CO2 a major limiting factor for growth in this experiment?
	2.	Because of greenhouse studies similar to the one outlined in question 1, some scientists have suggested that global warming will increase rates of photosynthesis, causing plants to take up higher levels of CO2, and therefore decreasing atmospheric CO2 levels. However, studies at the ecosystem level indicate that higher atmospheric CO2 levels, combined with higher temperatures, do not result in higher levels of CO2 fixed by photosynthesis. How might plant responses in an ecosystem differ from those in a greenhouse?
	3.	In 1882, T. W. Engelmann carried out an ingenious experiment to demonstrate that chlorophyll absorbs light in the blue and red portions of the spectrum. He placed a single filament of a green alga in a drop of water on a microscope slide. Then he passed light through a prism and onto the string of algal cells. The slide also contained aerobic bacterial cells. After some time, he peered into the microscope and saw the bacteria clustered around the regions of the algal filament that were receiving blue light and red light, as shown in the following illustration. Why do you suppose the bacterial cells were clustered in this manner?
Testing Yourself
Choose the best answer for each question.
	1.	The raw materials for photosynthesis are
	a. oxygen and water.
	b. oxygen and carbon dioxide.
	c. carbon dioxide and water.
	d. carbohydrates and water.
	e. carbohydrates and carbon dioxide.
	2.	During photosynthesis, carbon dioxide is _________ and water is ________.
	a. reduced, oxidized	c. reduced, reduced
	b. oxidized, reduced	d. oxidized, oxidized
	3.	When electrons in the reaction center of PS I are passed to an energy acceptor molecule, they are replaced by electrons that have been given up by
	a. oxygen.	c. carbon dioxide.
	b. glucose.	d. water.
	4.	PS I, PS II, and the electron transport chain are located in the
	a. thylakoid membrane.
	b. stroma.
	c. outer chloroplast membrane.
	d. cell’s nucleus.
	5.	During the light reactions of photosynthesis, ATP is produced when hydrogen ions move
	a.	
down a concentration gradient from the thylakoid space to the stroma.
	b.	
against a concentration gradient from the thylakoid space to the stroma.
	c.	
down a concentration gradient from the stroma to the thylakoid space.
	d.	
against a concentration gradient from the stroma to the thylakoid space.
	6.	Oxygen is generated by the 
	a. light reactions.
	b. Calvin cycle.
	c. light reactions and the Calvin cycle reactions.
	d. None of these are correct.
	7.	The Calvin cycle requires _________ from the light reactions.
	a. carbon dioxide and water
	b. ATP and NADPH
	c. carbon dioxide and ATP
	d. ATP and water
	e. NADH and water
	8.	The product of the Calvin cycle and a reactant in many plant metabolic pathways is
	a. ribulose 1,5-bisphosphate, RuBP.
	b. adenosine diphosphate, ADP.
	c. 3-phosphoglycerate, 3PG.
	d. glyceraldehyde-3-phosphate, G3P.
	e. carbon dioxide, CO2.
	9.	Use these terms to label the following diagram of the light reactions.
	ATP
	thylakoid membrane
	electron transport chain
	stroma
	thylakoid space
	NADPH
	ATP synthase complex
	10.	In C4 plants, a 4-carbon molecule 
	a. is the product of the Calvin cycle.
	b. is the first electron carrier in the light reactions.
	c. is the storage form of carbohydrates.
	d. precedes the Calvin cycle.
	11.	A leaf of a C4 plant differs from that of a C3 plant because it
	a.	has more stomata.
	b.	has more layers of mesophyll.
	c.	
contains mesophyll cells in a concentric ring around the bundle sheath cells.
	d.	has needlelike leaves
For questions 12–15, match the items to those in the key. Answers can be used more than once, and each question can have more than one answer.
Key:
	a. C3 plant
	b. C4 plant
	c. CAM plant
	12.	Carbon fixation occurs in mesophyll cells. 
	13.	The Calvin cycle reactions occur in bundle sheath cells. 
	14.	Carbon fixation and the Calvin cycle reactions are separated by space. 
	15.	Carbon fixation and the Calvin cycle reactions are separated by time.
Go to www.mhhe.com/maderessentials for more quiz questions.
Bioethical Issue
In the United States, solar energy to grow food is greatly supplemented by fossil fuel energy. Even before crops are sowed, there is an input of fossil fuel energy for the production of seeds, tools, fertilizers, pesticides, and transportation of these to the farm. Fuel is then needed to plant seeds, apply fertilizers and pesticides, irrigate, and harvest crops. After harvesting, more fuel is used to process and package crops.
At this time, the fossil fuel energy to grow food is several hundred times its caloric content because of the limited amount of land devoted to agriculture, plus we use high-yielding plants that require additional care. It takes about 20 times the amount of energy to keep cattle in feedlots and feed them grain as it does to range-feed them. Because the combustion of fossil fuel energy contributes to environmental problems, such as global warming and air pollution, we should take steps to reduce the amount of fossil fuel energy used to grow food. What can be done? We could devote as much land as possible to farming and animal husbandry, and plant breeders could sacrifice some yield to develop plants that would require less fossil fuel energy. If cattle were range-fed, manure could substitute, in part, for chemical fertilizers. Biological control—the use of natural enemies to control pests—would cut down on pesticide use and possibly improve the health of farm families. The use of solar energy and wind power would also reduce the use of fossil fuel energy.
As consumers, we could eat blemished fruits and vegetables, thus eliminating the need for some pesticides. We could also reduce our consumption of processed foods, eat less meat, and avoid using electrically powered devices when preparing food at home. 
Understanding the Terms
ATP synthase89
C3 plant92
C4 plant92
Calvin cycle reaction86
CAM93
carbon dioxide (CO2) 
fixation90
carotenoid87
chlorophyll85, 87
chloroplast85
coenzyme89
electron transport chain88
grana (sing., granum)85
light reaction86
photosynthesis84
photosystem88
RuBP carboxylase (rubisco)90
stomata (sing., stoma)85
stroma85
thylakoid85
Match the terms to these definitions:
a. _______________ The fluid component of a chloroplast.
b. _______________ Carbon dioxide enters leaves through these openings.
c. _______________ This enzyme produces adenosine triphosphate.
d. _______________ Hydrogen ions accumulate here during the light reactions.
e. _______________ These nonprotein molecules help enzymes catalyze reactions.
f. _______________ The first step of the Calvin cycle.
g. _______________ A cluster of pigment molecules that absorbs energy during the light reactions.
h. _______________ The photosynthetic pigment that absorbs violet, blue, and green light.
i. _______________ These plants fix carbon at night and carry out Calvin cycle reactions during the day.
1 A few types of bacteria are chemosynthetic organisms, which obtain the necessary energy to produce their own organic nutrients by oxidizing inorganic compounds.
Plants produce oxygen by splitting water molecules.
Plants grow fastest when exposed to red and blue light.
The oceans teem with photosynthesizers.
Figure 6.1Photosynthetic organisms.
Photosynthetic organisms include plants, such as trees, flowers, and mosses, which typically live on land; photosynthetic protists, such as Euglena, diatoms, and kelp, which typically live in water; and cyanobacteria, a type of bacterium that lives everywhere.
Figure 6.2Leaves and photosynthesis.
The raw materials for photosynthesis are carbon dioxide and water. Water enters a leaf by way of leaf veins. Carbon dioxide enters a leaf by way of the stomata (sing., stoma). Chloroplasts have two major parts. The grana are made up of thylakoids, membranous disks that contain photosynthetic pigments such as chlorophylls. These pigments absorb solar energy. The stroma is a fluid-filled space where carbon dioxide is enzymatically reduced to a carbohydrate such as glucose.
Figure 6.3The photosynthetic process.
The process of photosynthesis consists of the light reactions and the Calvin cycle reactions. The light reactions, which produce ATP and NADPH, occur in the thylakoid membrane. These molecules are used in the Calvin cycle in the stroma to reduce carbon dioxide to a carbohydrate.
Figure 6.4Radiant energy.
Radiant energy exists in a range of wavelengths that extends from the very short wavelengths of gamma rays through the very long wavelengths of radio waves. Visible light, which drives photosynthesis, is expanded to show its component colors. The components differ according to wavelength and energy content. 
Figure 6.5Leaf colors.
During the summer, leaves appear green because the chlorophylls absorb other portions of the visible spectrum better than they absorb green. During the fall, chlorophyll breaks down, and the carotenoids that remain cause leaves to appear yellow to orange because they do not absorb these colors. 
Figure 6.6The electron pathway.
Energized electrons (replaced from water, which splits to release oxygen) leave PS II and pass down an electron transport chain, leading to the formation of ATP. Energized electrons (replaced from PS II) leave PS I and pass to NADP1, which then combines with H1, becoming NADPH.
Figure 6.7Organization of a thylakoid.
Molecular complexes of the electron transport chain within the thylakoid membrane pump hydrogen ions from the stroma into the thylakoid space. When hydrogen ions flow back out of the space into the stroma through the ATP synthase complex, ATP is produced from ADP 1 sP . NADP1 accepts two electrons and joins with H1 to become NADPH.
Figure 6.8The Calvin cycle reactions.
The Calvin cycle is divided into three portions: CO2 fixation, CO2 reduction, and regeneration of RuBP. Because five G3P are needed to re-form three RuBP, it takes three turns of the cycle to achieve a net gain of one G3P. Two G3P molecules are needed to form glucose.
Figure 6.9The fate of G3P.
G3P is the first reactant in a number of plant cell metabolic pathways. Two G3Ps are needed to form glucose phosphate; glucose is often considered the end product of photosynthesis. Sucrose is the transport sugar in plants; starch is the storage form of glucose; and cellulose is a major constituent of plant cell walls.
Figure 6.10Carbon dioxide fixation in C3 plants.
In C3 plants, such as columbines, CO2 is taken up by the Calvin cycle directly in mesophyll cells, and the first detectable molecule is a C3 molecule (red).
Figure 6.11Carbon dioxide fixation in C4 plants.
C4 plants, such as corn, form a C4 molecule (red) in mesophyll cells prior to releasing CO2 to the Calvin cycle in bundle sheath cells. 
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1NADP = Nicotinamide adenine dinucleotide phosphate.
Figure 6.12Carbon dioxide fixation in a CAM plant. 
CAM plants, such as pineapple, fix CO2 at night, forming a C4  molecule (red) that is released to the Calvin cycle during the day.
Check Your Progress
	1.	List three major groups of photosynthetic organisms.
	2.	What molecules are required in order for photosynthesis to begin, and what molecule is the most significant end product? 
Answers:1. Plants, algae, and cyanobacteria.2. During photosynthesis, carbon dioxide and water become a carbohydrate.
Check Your Progress
	1.	List the two sets of reactions that are carried out during photosynthesis.
	2.	 What two molecules are produced as a result of the electron pathway of the light reactions?
Answers:1. Light reactions and Calvin cycle reactions.2. ATP and NADPH.
Check Your Progress
	1.	What are three major steps of the Calvin cycle?
	2.	Where does the Calvin cycle get the NADPH and the ATP it uses to reduce carbon dioxide to a carbohydrate? 
	3.	List the products that a plant cell can make from G3P, the product of the Calvin cycle.
	4.	In what part of a chloroplast do the light reactions occur? In what part do the Calvin cycle reactions occur? 
Answers:1. Carbon dioxide fixation, carbon dioxide reduction, and regeneration of RuBP.2. From the light reactions.3. Glucose, sucrose, starch, cellulose, fatty acids, glycerol, and amino acids.4. The light reactions occur in the thylakoid membrane, and the Calvin cycle reactions occur in the stroma. 
Check Your Progress
	1.	Name some plants that use a method of photosynthesis other than C3 photosynthesis. 
	2.	 Explain why C4 photosynthesis is advantageous in hot, dry conditions.
Answers:1. C4 plants include many grasses, sugarcane, and corn; CAM plants include cactuses, stonecrops, orchids, and bromeliads.2. Stomata close under hot, dry conditions, increasing the concentration of oxygen relative to carbon dioxide in the leaf. The spatial separation of carbon fixation and the Calvin cycle reactions in C4 plants prevents oxygen from competing with carbon dioxide for an active site on the enzyme rubisco.