Capturing and Releasing Energy

5.1 A Burning Concern 82

5.2 Capturing Rainbows 83

5.3 Storing Energy in Sugar 85

5.4 The Light-Dependent Reactions 86

5.5 The Light-Independent Reactions 87

5.6 Photosynthesis and Aerobic Respiration: A Global Connection 89

5.7 Fermentation 92

5.8 Alternative Energy Sources in Food 93

5.9 A Burning Concern (revisited) 95

5

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96   Unit One How Cells Work

Summary Section 5.6 Photosynthesis changed the composition of Earth’s early atmosphere, with profound effects on life’s evolution. Organ-isms that could not tolerate the increased

oxygen content persisted only in anaerobic habitats. Oxygen-detoxifying pathways evolved,

allowing organisms to thrive in aerobic conditions. Most modern organisms convert the chemical energy of carbohy-

drates to the chemical energy of ATP by oxygen-requiring aerobic respiration. In eukaryotes, this pathway starts with glycolysis in cytoplasm, and ends in mitochondria. Coenzymes pick up electrons in aerobic respiration’s first two stages, glycolysis and the Krebs cycle. The energy of those electrons drives ATP synthesis in the third stage, electron transfer phosphorylation. At the end of the electron transfer chains, oxygen accepts electrons and hydrogen ions, so water forms.

Section 5.7 Anaerobic fermentation pathways begin with glycolysis and finish in the cytoplasm. A molecule other than oxygen accepts electrons at the end of these reactions. The end product of alcoholic fermentation is ethyl alcohol, or ethanol. The end product of lactate fermentation is lactate. The final steps of fer-mentation serve to regenerate NAD+, which is required for glycolysis to continue, but they produce no ATP. Thus, the breakdown of one glucose molecule in either alcoholic or lactate fermentation yields only the two ATP that form in glycolysis.

Section 5.8 In humans and other organisms, the simple sugars from carbohydrate breakdown, the glycerol and fatty acids from fat breakdown, and the carbon back-bones of amino acids from protein breakdown may

enter aerobic respiration at various reaction steps.

Self-Quiz AnswersinAppendixI

1. In a land plant, most of the carbon dioxide used in photosynthe-sis comes from .

a. glucose c. waterb. the atmosphere d. soil

2. is/are the main energy source that drives photosynthesis.a. Sunlight c. Oxygenb. Hydrogen ions d. Carbon dioxide

3. In the light-dependent reactions, .a. carbon dioxide is fixed c. CO2 accepts electronsb. ATP forms d. sugars form

4. When a photosystem absorbs light, .a. sugar phosphates are producedb. electrons are transferred to ATPc. RuBP accepts electronsd. it ejects electrons

5. The atoms in the oxygen molecules released during photosynthe-sis come from .

a. glucose c. waterb. carbon dioxide d. hydrogen ions

6. Is the following statement true or false? Plants make all of their ATP by photosynthesis.

Section 5.1 By the metabolic pathway of photosyn-thesis, the energy of light is used to build glucose from water and carbon dioxide. Plants and other photosyn-thetic producers remove CO2 from the atmosphere; aerobic respiration by producers and consumers puts it back. Humans disrupt the balance of these two

processes by burning fossil fuels, which adds extra CO2 to the atmo-sphere. The resulting imbalance is contributing to global warming.

Section 5.2 Photosynthetic pigments absorb visible light of particular wavelengths for photo-synthesis. Light that is not absorbed is reflected as a pigment’s characteristic color. The main photo-synthetic pigment, chlorophyll a, absorbs violet and red light, so it appears green. Accessory pigments absorb additional wavelengths.

Section 5.3 In chloroplasts, the light-dependent reactions of pho-tosynthesis occur at a much-folded thylakoid membrane. The membrane forms a continuous compartment in the chloroplast’s stroma, where the light-independent reactions occur.

The following diagram summarizes the two stages of photosyn-thesis:

Section 5.4 Photosynthetic pigments in the thy lakoid membrane absorb light energy and pass it to photo-systems, which then release electrons. The electrons

flow through electron transfer chains in the thylakoid membrane, and end up in NADPH. Molecules of the electron transfer chain use energy released by the electrons to set up a hydrogen ion gradient across the thylakoid membrane. The ions flow back across the mem-brane through ATP synthases, causing these proteins to produce ATP (a process called electron transfer phosphorylation). Photosynthe-sis releases oxygen because a photosystem replaces lost electrons by pulling them from water molecules, which break apart as a result.

Section 5.5 The ATP and NADPH that form in the light-dependent reactions power the light-independent reactions of the Calvin–Benson cycle, which builds glucose from CO2. Closing stomata allows a plant to conserve water, but it also limits gas exchange. Oxygen buildup in plant tissues reduces the efficiency of glucose production in C3 plants. Additional carbon fixation reactions in C4 plants and CAM plants make sugar production more efficient on hot, dry days.

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Chapter 5 Capturing and Releasing Energy     97

Digging into DataEnergy Efficiency of Biofuel Production From Corn, Soy, and Prairie Grasses

Most biomass currently used for biofuel production in the United States consists of food crops—mainly corn, soybeans, and sugarcane. In 2006, David Tilman and his colleagues published the results of a 10-year study comparing the net energy output of various biofuels. The researchers grew a mixture of native perennial grasses without irrigation, fertilizer, pesticides, or herbicides, in sandy soil that was so depleted by intensive agriculture that it had been abandoned. They measured the usable energy in biofuels made from the grasses, from corn, and from soy. They also measured the energy it took to grow and produce each kind of biofuel (Figure 5.16).

1. About how much energy did ethanol produced from one hectare of corn yield? How much energy did it take to grow and produce that ethanol?

2. Which biofuel tested had the highest ratio of energy output to energy input?

3. Which of the three crops would require the least amount of land to produce a given amount of biofuel energy?

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0

5

10

15

20

25outputinput

corn grainethanol

1.25

soybeanbiodiesel

1.93

grasssynfuel

8.09ratio of energyoutput to input:

Figure 5.16   Energy inputs and outputs of biofuels from corn and soy grown on fertile farmland, and grassland plants grown in infertile soil. One hectare is about 2.5 acres.© Cengage Learning.

7. After photosynthesis evolved, its by-product, , accumu-lated and changed the atmosphere.

8. Glycolysis starts and ends in the .a. nucleus c. plasma membraneb. mitochondrion d. cytoplasm

9. The Calvin–Benson cycle starts when .a. light is availableb. carbon dioxide is attached to RuBPc. electrons leave a photosystem

10. In eukaryotes, aerobic respiration is completed in the .a. nucleus c. plasma membraneb. mitochondrion d. cytoplasm

11. In eukaryotes, fermentation is completed in the .a. nucleus c. plasma membraneb. mitochondrion d. cytoplasm

12. In the third stage of aerobic respiration, is the final accep-tor of electrons.

a. water b. hydrogen c. oxygen d. NADH

13. Which of the following is not produced by an animal muscle cell operating under anaerobic conditions?

a. heat c. ATP e. pyruvateb. lactate d. NAD+ f. all are produced

14. Hydrogen ion flow drives ATP formation during .a. photosynthesis d. the Calvin–Benson cycle b. aerobic respiration e. both a and b c. fermentation f. all of the above

15. Your body cells can use as an alternative energy source when glucose is in short supply.

a. fatty acids c. amino acids b. glycerol d. all of the above

16. Match the term with the best description. pyruvate a. no oxygen required fermentation b. site of photosynthesis mitochondrion c. product of glycolysis pigment d. aerobic respiration ends here carbon dioxide e. carbon-fixing enzyme rubisco f. like an antenna chloroplast g. big in the atmosphere

Critical Thinking1. About 200 years ago, Jan Baptista van Helmont wanted to know where growing plants get the materials necessary for increases in size. He planted a tree seedling weighing 5 pounds in a barrel filled with 200 pounds of soil and then watered the tree regularly. After five years, the tree weighed 169 pounds, 3 ounces, and the soil weighed 199 pounds, 14 ounces. Because the tree had gained so much weight and the soil had lost so little, he concluded that the tree had gained all of its additional weight by absorbing the water he had added to the barrel, but of course he was incorrect. What really happened?

2. As you learned, membranes impermeable to hydrogen ions are required for electron transfer phosphorylation. Membranes in mitochondria serve this purpose in eukaryotes. Prokaryotic cells do not have this organelle, but they do make ATP by elec-tron transfer phosphorylation. How do you think they do it, given that they have no mitochondria?

3. While looking into an aquarium, you see bubbles coming from an aquatic plant (right). What are the bubbles? ©

E. R

. Deg

ging

er.

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