11. Write About a Theme: Energy Transfer
See the general information on grading short-answer essays and the suggested rubric at the
beginning of this document.
Sample key points:
 Energy enters an animal cell in the form of the chemical potential energy of fuel molecules
such as glucose.
 Energy enters a plant cell in the form of light, which is converted to the chemical energy of
sugars.
 The transfer and transformation of energy increase entropy, and a cell returns heat and the
low-energy molecules of CO2 and H2O to its surroundings.
 In energy coupling, the free energy released by exergonic chemical reactions is used to drive
endergonic reactions.
 ATP represents a store of energy that links catabolic pathways to anabolic pathways.
 The hydrolysis of ATP also drives the mechanical and transport work of a cell.
 Enzymes are required for all the chemical reactions of a cell. By lowering the activation
energy, enzymes enable specific reactions to occur more quickly.
Sample top-scoring answer:
The work of a cell requires energy. Energy enters an animal cell as the chemical energy of fuel
molecules such as glucose. In a photosynthesizing cell, light energy drives the synthesis of
sugars, which in turn provide the energy for metabolism. The free energy released by the
exergonic reactions of catabolic pathways is stored in ATP. The energy released by the
hydrolysis of ATP to ADP and inorganic phosphate drives the endergonic reactions of anabolic
pathways, which synthesize the complex molecules of a cell. Also, during the hydrolysis of ATP
in mechanical and transport work, potential energy is transformed into kinetic energy. In these
transfers and transformations, some energy is lost as heat, and the cell returns the less ordered
molecules of CO2 and H2O to its surroundings. All the reactions of a cell require enzymes, which
ensure efficient cell function by binding substrates and lowering activation energy, thus speeding
up metabolic reactions.
Chapter 9 Cellular Respiration and Fermentation
9. Evolution Connection
The presence of ATP synthases in all of these places suggests the likelihood of common
ancestry. As described in Chapter 6, the endosymbiont theory says that mitochondria evolved
from a free-living, aerobically respiring, nonphotosynthetic prokaryote that was engulfed by an
ancestral eukaryotic cell. It further holds that chloroplasts evolved from a free-living,
photosynthetic prokaryote engulfed in the same way. One would expect to observe related amino
acid sequences for the ATP synthases of mitochondria, chloroplasts, and prokaryotes living
today that are thought to be related to the ancestral prokaryotes that were engulfed.
10. Scientific Inquiry
In a person treated with uncoupling agents like DNP, the proton gradient established during
respiratory electron transport is no longer tied to ATP synthesis. As a result, oxidation of glucose
during the citric acid cycle yields very little ATP, since ATP is normally produced as protons
flow back through the ATP synthase sites in the inner mitochondrial membrane, while still
generating heat. Without large amounts of ATP available, biosynthesis cannot take place and
new organic molecules cannot be synthesized. Low ATP levels would signal the body to
continue breaking down its own molecules and feeding them into the cellular respiration
pathway, leading to excessive weight loss and severe overheating and dehydration. One or a
combination of these factors can cause death.
11. Write About a Theme: Emergent Properties
See the general information on grading short-answer essays and the suggested rubric at the
beginning of this document.
Sample key points:
 An electron transport chain consists of complexes of membrane proteins that transfer
electrons from carrier to carrier in a series of redox reactions.
 Redox reactions release energy as electrons move to lower energy levels, while the protein
complexes shuttle protons across the membrane.
 Thousands of electron transport chains embedded in the inner mitochondrial membrane
establish a concentration gradient of protons.
 Through the energy-coupling process of chemiosmosis, the resulting proton-motive force
drives diffusion of H+ through ATP synthase.
 The organization of proteins in the electron transport chain and of ATP synthase in the inner
mitochondrial membrane are key to this process.
 The arrangement of proteins in the ATP synthase complex allow them to function.
Sample top-scoring answer:
Novel properties emerge from the arrangement and interaction of parts at each level of the
biological hierarchy. An electron transport chain consists of a sequence of electron carrier
molecules embedded in the inner mitochondrial membrane that transfer electrons through a
series of redox reactions, while shuttling protons across the membrane. The thousands of
electron transfer chains create a concentration gradient of H+. This proton-motive force drives
ATP formation by ATP synthase via chemiosmosis. It is the organization of the electron carrier
proteins in the electron transport chain and the location of ATP synthase that allows ATP
synthesis to occur. The function of the ATP synthase complex is also due to the specific, ordered
arrangement of the proteins that make up the complex. Thus, oxidative phosphorylation is an
emergent property resulting from the structural and functional organization of component parts,
none of which would be able to synthesize ATP on its own.
Chapter 10 Photosynthesis
8. Evolution Connection
Assuming that modern soybeans are the result of breeding programs meant to maximize the size
(mass) of the harvested soybeans, and assuming that soybean mass is due mostly to stored
carbohydrate (or matter ultimately derived from carbohydrate), it would seem logical that crop
scientists would have selected strains with reduced levels of photorespiration. This means that
the wild relatives of modern soybeans would be expected to have even higher levels of
photorespiration than 50%.
9. Scientific Inquiry
The ATP would end up outside the thylakoid. The chloroplasts were able to make ATP in the
dark because the researchers set up an artificial proton concentration gradient across the
thylakoid membrane; thus, the light reactions were not necessary to establish the H+ gradient
required for ATP synthesis by ATP synthase.
10. Science, Technology, and Society
Trees in tropical rain forests carry out photosynthesis, fixing CO2 as sugar and storing carbon as
they synthesize new biomass. However, trees also respire, using sugar as an energy source and
releasing CO2. While photosynthesis takes place in green leaves, respiration takes place
throughout the tree, in its leaves, stems, branches, and roots. Thus, living trees are not only sinks
for storing CO2, but also sources for the release of CO2. In addition, when a tree dies or its leaves
are eaten by an herbivore, microbes or animals digest its biomass and the sugars are broken
down by respiration to release CO2. Furthermore, mass cutting and burning of forests adds
significantly greater amounts of CO2 to the atmosphere, not offset in any way by the CO2
absorption that a living tree would be carrying out. (So the net result of cutting and burning
forests is to increase the atmospheric CO2.)
11. Write About a Theme: Energy Transfer
See the general information on grading short-answer essays and the suggested rubric at the
beginning of this document.
Sample key points:
 The light reactions take place in the thylakoids, where photosystems and electron transport
chains are embedded in membranes that enclose the thylakoid space.
 The energy of light is used to pump electrons from water through photosystem II, an electron
transport chain, and photosystem I to NADPH.
 In redox reactions in the electron transport chain, protons are pumped into the thylakoid
space. ATP synthase uses this proton-motive force to make ATP.
 The Calvin cycle, which occurs in the stroma, uses the energy of ATP and the reducing
power of NADPH produced by the light reactions to reduce CO2 to sugar.
 The three stages of the Calvin cycle are carbon fixation (using rubisco to attach CO2 to
RuBP), reduction (using ATP and electrons from NADPH), and regeneration of the CO2
accepter (using energy from ATP). The three-carbon sugar, G3P, exits the cycle and is
converted to glucose and other organic molecules.
Sample top-scoring answer:
The energy transformations of photosynthesis take place in chloroplasts. The light reactions
occur in the thylakoids, where photosystems containing light-harvesting complexes of pigment
molecules, a reaction-center chlorophyll a, and a primary electron acceptor are embedded. When
a pigment absorbs a photon, an electron is boosted to a higher-energy state. The energy of light is
used to pump electrons from water through photosystem II, an electron transport chain, and
photosystem I to NADPH. During energy-releasing redox reactions in the electron transport
chain, protons are pumped into the thylakoid space. ATP synthase uses this proton-motive force
to make ATP. The Calvin cycle occurs in the stroma. CO2 is fixed into organic compounds,
which are reduced using the energy of ATP and electrons from NADPH. The three-carbon sugar
G3P is produced and can be converted to sugars and other organic molecules, ultimately
providing the chemical energy and carbon source for all of life.