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Chapter 9 Cellular Respiration
9.1 Catabolic pathways yield energy by oxidizing organic fuels
Cellular respiration includes both aerobic and anaerobic respiration but is often used to
refer to aerobic respiration
Although carbohydrates, fats, and proteins are all consumed as fuel, it is helpful to trace
cellular respiration with the sugar glucose
The breakdown of organic molecules is exergonic
_________ ____________consumes organic molecules and O and yields ATP
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_________ ___________ is similar to aerobic respiration but consumes compounds other
than O
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Chemical reactions that transfer electrons between reactants are called oxidation-reduction
reactions, or redox reactions
In __________, a substance loses electrons, or is oxidized
In __________, a substance gains electrons, or is reduced (the amount of positive charge is
reduced)
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Harvesting of energy from glucose has three stages
Glycolysis (breaks down glucose into two molecules of pyruvate)
The citric acid cycle (completes the breakdown of glucose)
Oxidative phosphorylation (accounts for most of the ATP synthesis)
Oxidative phosphorylation accounts for almost 90% of the ATP generated by cellular
respiration
A smaller amount of ATP is formed in glycolysis and the citric acid cycle by substrate-level
phosphorylation
For each molecule of glucose degraded to CO2 and water by respiration, the cell makes up
to 32 molecules of ATP
9.2 Glycolysis harvest chemical energy to oxidize glucose to pyruvate
Glycolysis (“splitting of sugar”) breaks down glucose into two molecules of pyruvate
Glycolysis occurs in the cytoplasm and has two major phases
Energy investment phase
Energy payoff phase
Glycolysis occurs whether or not O is present
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Before the citric acid cycle can begin, pyruvate must be converted to acetyl Coenzyme A
(acetyl CoA), which links glycolysis to the citric acid cycle
This step is carried out by a multienzyme complex that catalyses three reactions
9.3 After pyruvate is oxidized, the citric acid cycle completes the energy
yielding oxidation of organic molecules
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The citric acid cycle, also called the Krebs cycle, completes the breakdown of pyruvate to
CO
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The cycle oxidizes organic fuel derived from pyruvate, generating 1 ______, 3 ______, and 1
______ per turn
The citric acid cycle has eight steps, each catalyzed by a specific enzyme
The acetyl group of acetyl CoA joins the cycle by combining with oxaloacetate, forming
citrate
Concept 9.4: During oxidative phosphorylation, chemiosmosis couples electron
transport to ATP synthesis
Following glycolysis and the citric acid cycle, NADH and FADH2 account for most of the
energy extracted from food
These two electron carriers donate electrons to the electron transport chain, which powers
ATP synthesis via oxidative phosphorylation
In cellular respiration, glucose and other organic molecules are broken down in a series of
steps
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Electrons from organic compounds are usually first transferred to NAD , a coenzyme
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As an electron acceptor, NAD functions as an oxidizing agent during cellular respiration
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Each NADH (the reduced form of NAD ) represents stored energy that is tapped to
synthesize ATP
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9.5 Fermentation and anaerobic respiration enable cells to produce ATP
without the use of oxygen
+
Fermentation consists of glycolysis plus reactions that regenerate NAD , which can be
reused by glycolysis
Two common types are alcohol fermentation and lactic acid fermentation
Ancient prokaryotes are thought to have used glycolysis long before there was oxygen
in the atmosphere
Glycolysis is a very ancient process
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9.6 Regulation of Cellular Respiration via Feedback Mechanisms
Feedback inhibition is the most common mechanism for control
If ATP concentration begins to drop, respiration speeds up; when there is plenty of ATP,
respiration slows down
Control of catabolism is based mainly on regulating the activity of enzymes at strategic
points in the catabolic pathway
Chapter 10 Photosynthesis
Concept 10.1: Photosynthesis converts light energy to the chemical energy of food
Photosynthesis is the process that converts solar energy into chemical energy
Chloroplasts are structurally similar to and likely evolved from photosynthetic bacteria
The structural organization of these cells allows for the chemical reactions of
photosynthesis
CO2 enters and O2 exits the leaf through microscopic pores called __________
The chlorophyll is in the membranes of __________ (connected sacs in the chloroplast);
thylakoids may be stacked in columns called grana
Chloroplasts also contain __________, a dense interior fluid
Tracking Atoms Through Photosynthesis: Scientific Inquiry Photosynthesis is a
complex series of reactions that can be summarized as the following equation:
6 CO2 + 12 H2O + Light energy  C6H12O6 + 6 O2 + 6 H2O
Chloroplasts split H2O into hydrogen and oxygen, incorporating the electrons of hydrogen
into sugar molecules and releasing oxygen as a by-product
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The Two Stages of Photosynthesis: A Preview
Photosynthesis consists of the light reactions (the photo part) and Calvin cycle (the
synthesis part)
The light reactions (in the thylakoids)
Split H O
2
Release O
2
+
Reduce NADP to NADPH
Generate ATP from ADP by photophosphorylation
The Calvin cycle (in the stroma) forms sugar from CO2, using ATP and NADPH
The Calvin cycle begins with carbon fixation, incorporating CO2 into organic
molecules
Concept 10.2: The light reactions convert solar energy to the chemical energy of ATP
and NADPH
Chloroplasts are solar-powered chemical factories
Their thylakoids transform light energy into the chemical energy of ATP and NADPH
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The Nature of Sunlight Light is a form of electromagnetic energy, also called
electromagnetic radiation
Wavelength determines the type of electromagnetic energy
Photosynthetic Pigments: The Light Receptors
Pigments are substances that absorb visible light
Different pigments absorb different wavelengths
An absorption spectrum is a graph plotting a pigment’s light absorption versus
wavelength.
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A Photosystem: A Reaction-Center Complex Associated with Light-Harvesting
Complexes
A photosystem consists of a reaction-center complex (a type of protein complex)
surrounded by light-harvesting complexes
A primary electron acceptor in the reaction center accepts excited electrons and is
reduced as a result
H2O is split by enzymes, and the electrons are transferred from the hydrogen atoms
to P680+, thus reducing it to P680
O2 is released as a by-product of this reaction
In PS I (like PS II), transferred light energy excites P700, which loses an electron to an
electron acceptor
P700+ (P700 that is missing an electron) accepts an electron passed down from PS II via
the electron transport chain
Cyclic electron flow uses only photosystem I and produces ATP, but not NADPH
Cyclic electron flow generates surplus ATP, satisfying the higher demand in the Calvin
cycle
In chloroplasts, protons are pumped into the thylakoid space and drive ATP synthesis
as they diffuse back into the stroma
ATP and NADPH are produced on the side facing the stroma, where the Calvin cycle
takes place
In summary, light reactions generate ATP and increase the potential energy of electrons
by moving them from ________ to ____________
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Concept 10.3: The Calvin cycle uses the chemical energy of ATP and NADPH to reduce
CO to sugar
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The Calvin cycle, like the citric acid cycle, regenerates its starting material after molecules
enter and leave the cycle
The cycle builds sugar from smaller molecules by using ATP and the reducing power of
electrons carried by NADPH
Carbon enters the cycle as CO2 and leaves as a sugar named glyceraldehyde 3-phospate
(G3P)
For net synthesis of 1 G3P, the cycle must take place three times, fixing 3 molecules of
CO2
The Calvin cycle has three phases
Carbon fixation (catalyzed by rubisco)
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Reduction
Regeneration of the CO2 acceptor (RuBP)
10.4 Photorespiration: An Evolutionary Relic?
In most plants (C3 plants), initial fixation of CO2, via rubisco, forms a three-carbon
compound (3-phosphoglycerate)
Photorespiration consumes O2 and organic fuel and releases CO2 without producing
ATP or sugar
C4 plants minimize the cost of photorespiration by incorporating CO2 into fourcarbon compounds in mesophyll cells
These four-carbon compounds are exported to bundle-sheath cells, where they
release CO2 that is then used in the Calvin cycle
Some plants, including succulents, use crassulacean acid metabolism (CAM) to fix
carbon
CAM plants open their stomata at night, incorporating CO2 into organic acids
The Importance of Photosynthesis: A Review
The energy entering chloroplasts as _________ gets stored as chemical energy in
____________ ____________
Sugar made in the chloroplasts supplies chemical energy and carbon skeletons to
synthesize the organic molecules of cells
Plants store excess sugar as starch in structures such as roots, tubers, seeds, and fruits
In addition to food production, photosynthesis produces the O2 in our atmosphere