CHAPTER
6
How Cells Harvest Chemical Energy
Lecture Outline
I. Introduction
A.Oxygen is a reactant in cellular respiration, the process that breaks down sugar and other food molecules and generates ATP, the energy currency in cells, and heat.
B. Brown fat has a “short circuit” in its cellular respiration, which generates only heat, not ATP.
C.Brown fat is important for heat production in small mammals, including humans.
II. Cellular Respiration: Aerobic Harvesting of Energy
A.6.1 Photosynthesis and cellular respiration provide energy for life
1. Life requires energy.
2. In almost all ecosystems, energy ultimately comes from the sun.
3. In photosynthesis,
a. some of the energy in sunlight is captured by chloroplasts,
b.atoms of carbon dioxide and water are rearranged, and
c. sugar and oxygen are produced.
4. In cellular respiration,
a. sugar is broken down to carbon dioxide and water and
b.the cell captures some of the released energy to make ATP.
5. Cellular respiration takes place in the mitochondria of eukaryotic cells.
a. In those energy conversions, some energy is lost as heat.
B. 6.2 Breathing supplies O2 for use in cellular respiration and removes CO2
1. Respiration, as it relates to breathing, and cellular respiration are not the same.
a. Respiration, in the breathing sense, refers to an exchange of gases. Usually an organism brings in oxygen from the environment and releases waste CO2.
b.Cellular respiration is the aerobic (oxygen-requiring) harvesting of energy from food molecules by cells.
C.6.3 Cellular respiration banks energy in ATP molecules
1. Cellular respiration is an exergonic (energy-releasing) process that transfers energy from the bonds in glucose to form ATP.
2. Cellular respiration
a. can produce up to 32 ATP molecules for each glucose molecule,
b.uses about 34% of the energy originally stored in glucose, and
c. releases the other 66% as heat.
3. This energy conversion efficiency is better than most energy conversion systems.
a. Only about 25% of the energy in gasoline produces the kinetic energy of movement.
D.6.4 CONNECTION: The human body uses energy from ATP for all its activities
2. A kilocalorie (kcal) is
a. the quantity of heat required to raise the temperature of 1 kilogram (kg) of water by 1oC,
b.the same as a food calorie, and
c. used to measure the nutritional values indicated on food labels.
3. The average adult human needs about 2,200 kcal of energy per day.
a. About 75% of these calories is used to maintain a healthy body.
b.The remaining 25% is used to power physical activities.
4. A balance of energy intake and expenditure is required to maintain a healthy weight.
E. 6.5 Cells capture energy from electrons “falling” from organic fuels to oxygen
1. How do your cells extract energy from glucose?
2. The answer involves the transfer of electrons during chemical reactions.
3. During cellular respiration,
a. electrons are transferred from glucose to oxygen and
b.energy is released.
4. Oxygen attracts electrons very strongly.
5. An electron loses potential energy when it is transferred to oxygen.
6. Energy can be released from glucose by simply burning it.
7. This electron “fall” happens very rapidly.
8. This energy is dissipated as heat and light and is not available to living organisms.
9. Cellular respiration is a more controlled descent of electrons, like rolling down an energy hill.
10. Energy is released in small amounts and can be stored in the chemical bonds of ATP.
11. The movement of electrons from one molecule to another is an oxidation-reduction reaction, or redox reaction. In a redox reaction,
a. the loss of electrons from one substance is called oxidation,
b.the addition of electrons to another substance is called reduction,
c. a molecule is oxidized when it loses one or more electrons, and
d.a molecule is reduced when it gains one or more electrons.
12. A cellular respiration equation is helpful to show the changes in hydrogen atom distribution.
13. Glucose loses its hydrogen atoms and becomes oxidized to CO2.
14. Oxygen gains hydrogen atoms and becomes reduced to H2O.
15. An important player in the process of oxidizing glucose is a coenzyme called NAD+,
which
a. accepts electrons, and
b.becomes reduced to NADH.
16. NADH delivers electrons to a string of electron carrier molecules, which moves electrons down a hill.
17. These carrier molecules constitute an electron transport chain.
18. At the bottom of the hill is oxygen (½ O2), which
a. accepts two electrons,
b.picks up two H+, and
III. Stages of Cellular Respiration
A.6.6 Overview: Cellular respiration occurs in three main stages
1. Cellular respiration consists of a sequence of steps that can be divided into three stages.
a. Stage 1: Glycolysis
b.Stage 2: Pyruvate oxidation and citric acid cycle
c. Stage 3: Oxidative phosphorylation
2. Stage 1: Glycolysis a. occurs in the cytosol,
b.begins cellular respiration, and
c. breaks down glucose into two molecules of a three-carbon compound called pyruvate.
3. Stage 2: Pyruvate oxidation and the citric acid cycle a. take place in mitochondria,
b.oxidize pyruvate to a two-carbon compound, and
c. supply the third stage with electrons.
4. The cell makes a small amount of ATP during glycolysis and the citric acid cycle.
5. Stage 3: Oxidative phosphorylation
a. NADH and a related electron carrier, FADH2, shuttle electrons to an electron
trans-port chain embedded in the inner mitochondrial membrane.
b.Most ATP produced by cellular respiration is generated by oxidative phosphoryla-tion, which uses the energy released by the downhill fall of electrons from NADH and FADH2 to oxygen to phosphorylate ADP.
c. As the electron transport chain passes electrons down the energy hill, it also pumps hydrogen ions (H+) across the inner mitochondrial membrane, into the narrow
intermembrane space, and produces a concentration gradient of H+ across the
membrane.
d.In chemiosmosis, the potential energy of this concentration gradient is used to make ATP.
B. 6.7 Glycolysis harvests chemical energy by oxidizing glucose to pyruvate
1. In glycolysis,
a. a single molecule of glucose is enzymatically cut in half through a series of steps,
b.two molecules of pyruvate are produced,
c. two molecules of NAD+ are reduced to two molecules of NADH, and
d.there is a net of two molecules of ATP.
2. ATP is formed in glycolysis by substrate-level phosphorylation during which
a. an enzyme transfers a phosphate group from a substrate molecule to ADP and
b.ATP is formed.
3. The compounds that form between the initial reactant, glucose, and the final product, pyruvate, are known as intermediates.
4. The steps of glycolysis have two main phases.
a. In steps 1–4, the energy investment phase,
i. energy is consumed as two ATP molecules are used to energize a glucose molecule,
ii. which is then split into two small sugars.
5. There is a net gain of two ATP molecules for each glucose molecule that enters glycolysis.
C.6.8 Pyruvate is oxidized in preparation for the citric acid cycle
1. The pyruvate formed in glycolysis is transported from the cytosol into a mitochondrion where the citric acid cycle and oxidative phosphorylation will occur.
2. Two molecules of pyruvate are produced for each molecule of glucose that enters glycolysis.
3. Pyruvate does not enter the citric acid cycle but undergoes some chemical grooming in which
a. a carboxyl group is removed and given off as CO2,
b.the two-carbon compound remaining is oxidized while a molecule of NAD+ is
reduced to NADH, and
c. coenzyme A joins with the two-carbon group to form acetyl coenzyme A, abbreviated as acetyl CoA.
4. Then two molecules ofacetyl CoA enter the citric acid cycle.
D.6.9 The citric acid cycle completes the oxidation of organic molecules, generating many NADH and FADH2 molecules
1. The citric acid cycle
a. is also called the Krebs cycle (after the German-British researcher Hans Krebs, who worked out much of this pathway in the 1930s),
b.completes the oxidation of organic molecules, and
c. generates many NADH and FADH2 molecules.
2. During the citric acid cycle,
a. the two-carbon group of acetyl CoA is added to a four-carbon compound, forming citrate,
b.citrate is degraded back to the four-carbon compound,
c. two CO2 are released, and
d.one ATP, three NADH, and one FADH2 are produced.
3. Remember that the citric acid cycle processes two molecules of acetyl CoA for each initial glucose.
4. Thus, after two turns of the citric acid cycle, the overall yield per glucose molecule is
a. 2 ATP,
b.6 NADH, and
c. 2 FADH2.
5. Thus, after glycolysis and the citric acid cycle, the cell has gained
a. 4 ATM,
b.10 NADH, and
c. 2 FADH2.
6. To harvest the energy banked in NADH and FADH2, these molecules must shuttle
their high-energy electrons to an electron transport chain.
E. 6.10 Most ATP production occurs by oxidative phosphorylation
1. The final stage of cellular respiration is oxidative phosphorylation, which
a. involves electron transport and chemiosmosis and
b.requires an adequate supply of oxygen.
2. The arrangement of electron carriers built into a membrane makes it possible to
a. create an H+ concentration gradient across the membrane and then
3. Electrons from NADH and FADH2 travel down the electron transport chain to O2, the
final electron acceptor.
a. Oxygen picks up H+ to form water.
b.Energy released by these redox reactions is used to pump H+ from the mitochondrial
matrix into the intermembrane space.
4. In chemiosmosis, the H+ diffuses back across the inner membrane, through ATP
synthase complexes, driving the synthesis of ATP.
F. 6.11 SCIENTIFIC THINKING: Scientists have discovered heat-producing, calorie-burning brown fat in adults
1. Mitochondria in brown fat can burn fuel and produce heat without making ATP.
2. Ion channels spanning the inner mitochondrial membrane
a. allow H+ to flow freely across the membrane and
b.dissipate the H+ gradient that the electron transport chain produced, which does not
allow ATP synthase to make ATP.
3. Scientific studies of humans indicate that
a. brown fat may be present in most people and
b.when activated by cold environments, the brown fat of lean individuals is more active.
G.6.12 Review: Each molecule of glucose yields many molecules of ATP
1. Recall that the energy payoff of cellular respiration involves
a. glycolysis,
b.alteration of pyruvate,
c. the citric acid cycle, and
d.oxidative phosphorylation.
2. The total yield is about 32 ATP molecules per glucose molecule.
3. The number of ATP molecules cannot be stated exactly for several reasons.
a. The NADH produced in glycolysis passes its electrons across the mitochondrial membrane to either NAD+ or FAD. Because FADH
2 adds its electrons farther along
the electron transport chain, it contributes less to the H+ gradient and thus generates
less ATP.
b.Some of the energy of the H+ gradient may be used for work other than ATP
production, such as the active transport of pyruvate into the mitochondrion.
IV. Fermentation: Anaerobic Harvesting of Energy
A.6.13 Fermentation enables cells to produce ATP without oxygen
1. Fermentation is a way of harvesting chemical energy that does not require oxygen. Fermentation
a. uses glycolysis,
b.produces two ATP molecules per glucose, and
c. reduces NAD+ to NADH.
2. Fermentation also provides an anaerobic path for recycling NADH back to NAD+.
3. Your muscle cells and certain bacteria can regenerate NAD+ through lactic acid
fermentation, in which
a. NADH is oxidized back to NAD+, and
b.pyruvate is reduced to lactate.
5. The dairy industry uses lactic acid fermentation by bacteria to make cheese and yogurt.
6. Other types of microbial fermentation turn soybeans into soy sauce and cabbage into sauerkraut.
7. The baking and winemaking industries have used alcohol fermentation for thousands of years.
8. In this process, yeast (single-celled fungi)
a. oxidize NADH back to NAD+ and
b.convert pyruvate to CO2 and ethanol.
9. Obligate anaerobes
a. require anaerobic conditions, b. are poisoned by oxygen, and
c. live in stagnant ponds and deep soils.
10. Facultative anaerobes
a. can make ATP by fermentation or oxidative phosphorylation and
b.include yeasts and many bacteria.
B. 6.14 EVOLUTION CONNECTION: Glycolysis evolved early in the history of life on Earth
1. Glycolysis is the universal energy-harvesting process of life.
2. The role of glycolysis in fermentation and respiration dates back tolife long before oxygen was present,when only prokaryotes inhabited the Earth, about 3.5 billion years ago.
3. The ancient history of glycolysis is supported by its
a. occurrence in all the domains of life and
b.location within the cell, using pathways that do not involve any membrane-enclosed organelles of the eukaryotic cell.
V. Connections Between Metabolic Pathways
A.6.15 Cells use many kinds of organic molecules as fuel for cellular respiration
1. Although glucose is considered to be the primary source of sugar for respiration and fermentation, ATP is generated using
a. carbohydrates,
b.fats, and
c. proteins.
2. Fats make excellent cellular fuel because they
a. contain many hydrogen atoms and thus many energy-rich electrons and
b.yield more than twice as much ATP per gram as a gram of carbohydrate.
3. Proteins can also be used for fuel, although your body preferentially burns sugars and fats first.
B. 6.16 Organic molecules from food provide raw materials for biosynthesis
1. A cell must be able to make its own molecules to
a. build its structures and
b.perform its functions.
2. Food provides the raw materials your cells use for biosynthesis, the production of organic molecules, using energy-requiring metabolic pathways.
