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7 HOW CELLS HARVEST
ENERGY FROM FOOD
CHAPTER OUTLINE
An Overview of Cellular Respiration (p. 138)
7.1
Where Is the Energy in Food? (p. 138; Figs. 7.1, 7.2, 7.3)
A. Plants store energy from sunlight in the form of organic compounds, but all other organisms,
plants included, must oxidize the organic compounds, using a process known as cellular
respiration, to supply the energy needed to drive cellular activities.
B. Aerobic Respiration
1. The first stage of cellular respiration, called glycolysis, occurs in the cytoplasm of the
cell, needs no oxygen, and is an ancient process.
2. The second stage, oxidation, occurs in mitochondria, which only eukaryotes possess.
3. The second stage is more powerful at recovering energy from food molecules than
glycolysis is.
Respiration Without Oxygen: Glycolysis (p. 140)
7.2 Using Coupled Reactions to Make ATP (p. 140; Figs. 7.4)
A.
Glycolysis
1. Glycolysis is a biochemical pathway that involves a sequential series of ten enzymecatalyzed reactions that cleave the six-carbon molecule glucose into two three-carbon
molecules called private.
2. During glycolysis, two coupled reactions also occur, leading to the production of ATP
via substrate-level phosphorylation.
3. In addition, two electrons are carried by NADH to the electron transport chain.
4. Glycolysis can be carried out by every living creature.
Respiration With Oxygen: The Krebs Cycle (p. 142)
7.3
Harvesting Electrons from Chemical Bonds (p. 142; Figs. 7.5, 7.6)
A. When oxygen is available, the cell harvests the energy of pyruvate, the end product of
glycolysis, by first oxidizing pyruvate to form acetyl coenzyme A (acetyl-CoA), and then by
oxidizing acetyl-CoA in the Krebs cycle.
B. Step One: Producing Acetyl-CoA
1. Pyruvate is oxidized in a single decarboxylation reaction that cleaves one of pyruvate’s
three carbons.
2. This reaction produces a 2-carbon acetyl group and a pair of electrons that reduce NAD +
to NADH.
3. The acetyl group combines with coenzyme A to become acetyl-CoA.
4. If the cell has plenty of energy, acetyl-CoA is transferred to a fat-depositing pathway;
otherwise it enters the Krebs cycle.
C. Step Two: The Krebs Cycle
1. The Krebs cycle takes place in the mitochondria, and its nine reactions can be grouped
into three stages.
2. In the first stage, acetyl-CoA joins the cycle, binding to a four-carbon molecule to
produce a six-carbon molecule.
3. Second, two carbons are removed as CO2, their electrons donated to NAD+, and a fourcarbon molecule is left.
4. Third, more electrons are extracted and taken away as NADH or FADH 2, and the fourcarbon starting material is regenerated.
5. In the process of cellular respiration, glucose is entirely consumed. All that is left over
are six molecules of CO2, energy in the form of four ATP molecules, and the electrons
carried by 10 NADH and 2 FADH2 carriers.
7.4
Using Electrons to Make ATP (p. 146)
A. Mitochondria use chemiosmosis to make ATP.
B. Moving Electrons Through the Electron Transport Chain
1. The NADH and FADH2 molecules carry their electrons to the inner mitochondrial
membrane where they transfer the electrons to a series of membrane-associated proteins
known as the electron transport chain.
2. Several protein complexes in the electron transport chain function as proton pumps,
using the energy of the electron to pump protons into the intermembrane space of the
mitochondria.
3. At the last of these protein complexes, the electrons are donated to oxygen to form
water. It is the supply of oxygen at this step that makes oxidative respiration possible.
C. Producing ATP: Chemiosmosis
1. As the energy from the electrons drive the proton pumps in the electron transport chain,
the proton concentration in the intermembrane space rises above the matrix.
2. Due to this concentration gradient, protons are induced to reenter the matrix by diffusion
through special ATP synthase channels.
3. As the protons pass through these channels, ATP is produced from ADP plus P I in the
matrix.
4. This process is called chemiosmosis and is the same chemiosmotic process encountered
in photosynthesis.
Harvesting Electrons Without Oxygen: Fermentation (p.148)
7.5
Cells Can Metabolize Food Without Oxygen (p. 148; Fig.7.7)
A.
Fermentation
1. In the absence of oxygen, fermentation, rather than oxidation, is the pathway that occurs
after glycolysis.
2. In fermentation, an organic molecule accepts hydrogen atoms from the NADH generated
by glycolysis, thus recycling NAD+ and allowing glycolysis to continue.
3. Bacteria carryout more than a dozen kinds of fermentation.
4. Eukaryotic cells are capable of only a few types of fermentation. In ethanol fermentation,
which occurs in yeasts, NAD+ and ethanol are produced; this is the source of ethanol for
wine and beer. In lactic acid fermentation, carried out in the muscle cells of animals,
NAD+ and lactate (lactic acid) are produced.
Other Sources of Energy (p.150)
7.6
Glucose Is Not the Only Food Molecule (p. 150; Fig. 7.8)
A. Other molecules besides glucose are consumed in food.
B. Cellular Respiration of Proteins
1. Proteins are first broken down into their individual amino acids.
2. Deamination reactions convert the amino acids into molecules that can take part in the
Krebs cycle.
C. Cellular Respiration of Fat
1. Fats are first broken down into fatty acids.
2. A process called -oxidation then converts the fatty acid tails into acetyl groups that can
be combined with coenzyme A to form acetyl-CoA, which feeds into the Krebs cycle.
KEY TERMS
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cellular respiration (p. 138)
glycolysis (p. 140) Anaerobic phase of cellular respiration.
oxidation (p. 138) This represents a second definition of this term and refers to the aerobic steps of
cellular respiration.
reduction (p. 138) The process in which an atom or molecule gains an electron.
Krebs cycle (p. 144)
electron transport chain (p. 146)
fermentation (p. 148)
LECTURE SUGGESTIONS AND ENRICHMENT TIPS
1.
Lactic Acid and Athletic Performance. Discuss with your students the relationship between athletic
conditioning and lactic acid buildup. When oxygen in muscle tissue has been used up during strenuous
exercise, pyruvate enters fermentation pathways instead of the Krebs cycle, and lactic acid
accumulates. Over a period of time, the blood carries away the lactic acid to the liver, where it is
converted back to glucose. In fact, the oxygen debt is the amount of oxygen needed to convert the
lactic acid back to glucose plus the amount needed to restore lost ATP in muscle tissue. Interestingly
enough, athletes or individuals who engage in regular aerobic exercise do not accumulate lactic acid as
rapidly. There appear to be several reasons for this. First of all, their muscles are conditioned and
stronger and have a better blood supply. Regular exercise promotes the development of more
capillaries within muscle, so the delivery of oxygen to muscle tissue is better than in non-athletes. The
hearts of athletes are stronger and can deliver more blood per pumping action. And last, physical
exercise induces the production of more mitochondria in muscle cells, so they can carry on the Krebs
cycle at an increased rate, supplying ATP longer.
CRITICAL THINKING QUESTIONS
1.
2.
How could a person engaging in vigorous exercise ensure that fermentation does not begin in their
muscle cells?
Explain how the process of cellular respiration can be used as evidence for evolution.