A. The Principles of Energy Harvest (Fig. 9.1)
1. Cellular respiration and fermentation are catabolic, energy-yielding pathways.
a. Organic molecules store energy in their arrangement of atoms.
b. Enzymes catalyze the systematic degradation of organic molecules that are rich in energy to simpler waste products with less energy.
c. Some of the released energy is used to do work and the rest is dissipated as heat.
d. Metabolic pathways that release the energy stored in complex organic molecules are catabolic.
e. One type of catabolic process, fermentation, leads to the partial degradation of sugars in the absence of oxygen.
f. A more efficient and widespread catabolic process, aerobic cellular respiration, uses oxygen as a reactant to complete the breakdown of a variety of organic molecules.
1. Most of the processes in aerobic cellular respiration occur in mitochondria.
g. Cellular respiration is similar to burning wood.
1. The overall process is: Organic compounds + O2 � CO2 + H2O + Energy
h. Carbohydrates, fats, and proteins can all be used as the fuel, but it is traditional to start learning with glucose.
1. The overall reaction for the aerobic cellular respiration of 1 molecule of glucose is:
C6H12O6 + 6O2 � 6CO2 + 6H2O + Energy (ATP + heat)
2. Cells recycle the ATP they use for work.
a. ATP, adenosine triphosphate, is the pivotal molecule in cellular energetics.
1. The close packing of three negatively charged phosphate groups is an unstable, energy-storing arrangement.
2. Loss of the end phosphate group releases energy.
b. Cellular work results in the conversion of ATP to ADP and inorganic phosphate (Pi).
c. An animal cell regenerates ATP from ADP and Pi by the catabolism of organic molecules.
d. The transfer of the terminal phosphate group from ATP to another molecule is phosphorylation. (Fig. 9.2)
1. This changes the receiving molecule, performing work (transport, mechanical, or chemical).
3. Reactions release energy when electrons move closer to oxygen atoms.
a. Catabolic pathways yield energy by relocating the electrons stored in food molecules. Released energy is used to synthesize ATP.
b. Relocation of electrons in these reactions involves a change in the degree of electron sharing in covalent bonds.
c. Example: In the combustion of methane to form water and carbon dioxide (on a gas stove), the nonpolar covalent bonds of methane (C-H) are converted to polar covalent bonds (O-H). (Fig. 9.3)
1. When these bonds shift from nonpolar to polar, the electrons move from positions equidistant between the two atoms to a closer position to oxygen.
d. An electron loses energy as it shifts to oxygen because the oxygen atom holds the electrons closer and that requires less energy.
e. A reaction that relocates electrons closer to oxygen releases chemical energy that can do work.
4. Electrons �fall� from organic molecules to oxygen during cellular respiration.
a. In the summary equation of cellular respiration:
C6H12O6 + 6O2 � 6CO2 + 6H2O + energy
The hydrogen atoms in glucose are transferred to oxygen and the electrons lose energy.
b. Molecules that have an abundance of hydrogen are excellent fuels because their bonds are a source of electrons that �fall� closer to oxygen.
c. The cell has a reservoir of electrons associated with hydrogen, especially in carbohydrates and fats.
d. However, these fuels do not spontaneously combine with O2 because they lack the activation energy.
e. Enzymes lower the barrier of activation energy, allowing the reactions to proceed.
5. The �fall� of electrons during respiration is stepwise, via NAD+ and an electron transport chain.
a. Cellular respiration does not oxidize glucose in a single step that transfers all the hydrogen in the fuel to oxygen at one time.
b. Rather, glucose and other fuels are broken down gradually in a series of steps, each catalyzed by a specific enzyme.
c. At key steps, hydrogen atoms are stripped from glucose and passed first to a coenzyme, like NAD+ (nicotinamide adenine dinucleotide).
d. Dehydrogenase enzymes strip two hydrogen atoms from the fuel (e.g., glucose), pass two electrons and one H+ to NAD+ and release one H+.
1. This changes NAD+ to NADH. (Fig. 9.4)
e. Electrons do not go directly from NADH to oxygen. Cellular respiration uses an electron transport chain to break the fall of electrons into several steps.
1. The electron transport chain, consisting of several molecules (primarily proteins), is built into the inner membrane of a mitochondrion.
2. NADH shuttles electrons from food to the �top� of the chain.
3. At the �bottom,� oxygen captures the electrons and H+ to form water.
B. The Process of Cellular Respiration
1. Respiration involves 4 stages: an overview
a. Respiration occurs in 4 metabolic stages: glycolysis, pyruvate oxidation, the Krebs cycle, and the electron transport chain/chemiosmosis. (Fig. 9.6)
b. Stage 1: Glycolysis occurs in the cytoplasm.
1. It begins catabolism by breaking glucose into two molecules of pyruvate.
c. Stage 2: Pyruvate oxidation occurs in the mitochondrial matrix.
1. It forms acetyl CoA from pyruvate.
d. Stage 3: The Krebs cycle occurs in the mitochondrial matrix.
1. It degrades pyruvate to carbon dioxide.
e. Stage 4: Electron transport chain. Several steps in stages 1-3 transfer electrons from food molecules to NAD+, forming NADH.
1. NADH passes these electrons to the electron transport chain.
2. As they are passed along the chain, the energy carried by these electrons is stored in the mitochondrion in a form that can be used to synthesize ATP by chemiosmosis.
f. Some ATP is also generated in glycolysis and the Krebs cycle by substrate-level phosphorylation.
g. Ultimately 38 ATP are produced per molecule of glucose that is degraded by respiration.
2. Stage 1: Glycolysis
a. During glycolysis, glucose, a six carbon-sugar, is split into two three-carbon sugars.
b. These smaller sugars are oxidized and rearranged to form two molecules of pyruvate.
c. Each of the ten steps in glycolysis is catalyzed by a specific enzyme. (Fig. 9.9)
d. The net yield from glycolysis is 2 ATP and 2 NADH per glucose.
1. No CO2 is produced during glycolysis.
e. Glycolysis occurs whether O2 is present or not.
1. If O2 is present, pyruvate moves to the Krebs cycle and the energy stored in NADH can be converted to ATP by the electron transport system. (Textbook Activity 9C)
3. Stage 2: Pyruvate oxidation
a. More than three-quarters of the original energy in glucose is still present in two molecules of pyruvate.
b. If oxygen is present, pyruvate can be completely broken down into carbon dioxide.
c. As pyruvate enters the mitochondrion, a multienzyme complex modifies pyruvate to acetyl CoA. (Fig. 9.10)
1. A carboxyl group is removed as CO2.
2. A pair of electrons is transferred from the remaining two-carbon fragment to NAD+ to form NADH.
3. The oxidized fragment, acetate, combines with coenzyme A to form acetyl CoA.
4. Stage 3: The Krebs cycle.
a. This cycle begins when acetate from (2C) acetyl CoA combines with (4C) oxaloacetate to form (6C) citrate.
b. Ultimately, the oxaloacetate is recycled and the acetate is broken down to 2CO2.
c. Each cycle produces one ATP, three NADH, and one FADH2 (another electron carrier) per acetyl CoA.
1. The original glucose molecule yielded 2 acetyl CoAs so the total yield per glucose molecule is 2 ATP, 6 NADH, and 2 FADH2
d. The Krebs cycle consists of eight steps. (Fig. 9.11) (Textbook Activity 9D)
5. Stage 4: Electron transport chain/chemiosmosis
a. Only 4 of 38 ATP ultimately produced by respiration of glucose are derived directly from glycolysis and the Krebs cycle.
b. The vast majority of the ATP comes from the energy in the electrons carried by NADH (and FADH2).
c. The energy in these electrons is used in the electron transport system to power ATP synthesis.
1. Thousands of copies of the electron transport chain are found in the extensive surface of the cristae, the inner membrane of the mitochondrion. (Textbook Activity 9E)
d. Steps in the electron transport chain (Fig. 9.15)
1. Electrons carried by NADH are transferred to the first molecule in the electron transport chain (flavoprotein) and H+ is released.
2. The electrons carried by FADH2 are added at a later point in the chain.
3. The electrons continue along the chain which includes multiprotein complexes and electron carriers.
4. The electrons ultimately pass to oxygen.
5. The electrons give some of their energy to each of the proteins in the chain and the proteins use the energy to pump some of the released H+ into the intermembrane space.
6. Four electrons and 2H+ are used to combine with one oxygen to form water-this is where the O2 that we breathe gets used-as an acceptor for low energy electrons.
7. The electron transport chain generates no ATP directly-its function is to break the large energy drop from food to oxygen into a series of smaller steps that release energy in manageable amounts.
e. Chemiosmosis
1. A protein complex, ATP synthase, in the cristae actually makes ATP from ADP and Pi.
2. ATP synthase uses the energy of the existing H+ gradient to make ATP by chemiosmosis
a. This H+ gradient was developed between the intermembrane space and the matrix by the electron transport chain.
b. The H+ gradient is a form of stored energy.
c. As hydrogen ions flow down their gradient through ATP synthase, activating catalytic sites, ADP and inorganic phosphate combine to make ATP.
C. Related Metabolic Processes
1. Fermentation enables some cells to produce ATP without the help of oxygen.
a. Glycolysis generates 2 ATP whether oxygen is present (aerobic) or not (anaerobic).
b. Fermentation can generate ATP from glucose as long as there is a supply of NAD+ to accept electrons and hydrogen atoms.
1. If the NAD+ pool is exhausted, glycolysis shuts down.
c. Under anaerobic conditions, various fermentation pathways generate ATP by glycolysis, recycle NAD+ and remove pyruvate by converting it to other end products that can be removed from the cell. (Fig. 9.17)
1. Alcohol fermentation
a. First glycolysis occurs, generating pyruvate, 2 ATP and 2 NADH.
b. Pyruvate is then converted to a two-carbon compound, acetaldehyde, by the removal of CO2.
c. Acetaldehyde then receives 2 H atoms and electrons from NADH to form ethanol.
d. Alcohol fermentation by yeast is used in brewing and winemaking.
2. Lactic acid fermentation
a. Muscle cells switch from aerobic respiration to lactic acid fermentation to generate ATP when O2 is scarce (during heavy exercise).
b. The waste product, lactate, may cause muscle fatigue until it is carried away by the blood.
c. First glycolysis occurs, generating pyruvate, 2 ATP and 2 NADH.
d. Pyruvate receives 2 H atoms and electrons from NADH to form lactate (ionized form of lactic acid).
2. Comparison of aerobic respiration and fermentation
a. Aerobic respiration and fermentation are alternatives for producing ATP from sugars.
1. Both use glycolysis to convert sugars to pyruvate with a net production of 2 ATP.
2. Both use NAD+ as an electron and hydrogen acceptor.
b. In fermentation (no oxygen available), the electrons and hydrogen of NADH are passed to an organic molecule (acetaldehyde or pyruvate).
c. In aerobic respiration (oxygen available), the electrons and hydrogen of NADH are passed to O2, generating more ATP.
d. Without oxygen, the energy still stored in pyruvate is unavailable to the cell.
e. Under aerobic respiration, a molecule of glucose yields 38 ATP, but the same molecule of glucose yields only 2 ATP under fermentation.
3. Evolutionary significance of glycolysis
a. The oldest bacterial fossils are over 3.5 billion years old, appearing long before appreciable quantities of O2 accumulated in the atmosphere.
b. Therefore, the first prokaryotes may have generated ATP exclusively from glycolysis.
c. The fact that glycolysis is also the most widespread metabolic pathway and occurs in the cytosol without membrane-enclosed organelles, suggests that glycolysis evolved early in the history of life.
4. Glycolysis and the Krebs cycle connect to many other metabolic pathways. (Fig. 9.19)
a. Glycolysis can accept a wide range of carbohydrates.
1. Polysaccharides, like starch or glycogen, can be hydrolyzed to glucose monomers that enter glycolysis.
2. Other 6-carbon sugars, like galactose and fructose, can also be modified to undergo glycolysis.
b. Proteins can also enter the respiratory pathways, including glycolysis and the Krebs cycle.
1. Proteins must first be digested to individual amino acids.
2. Some amino acids are used to build proteins.
3. Excess amino acids can be catabolized after their amino groups are removed via deamination.
4. The carbon skeletons of the amino acids are modified by enzymes and enter as intermediates into glycolysis or the Krebs cycle depending on their structure.
c. Fats must be digested to glycerol and fatty acids.
1. Glycerol can be converted to glyceraldehyde phosphate, an intermediate of glycolysis.
2. The rich energy of fatty acids is accessed as fatty acids are split into two-carbon fragments.
3. These molecules enter the Krebs cycle as acetyl CoA.
5. Feedback mechanisms control cellular respiration.
a. The rate of catabolism is regulated, typically by the level of ATP in the cell.
1. If ATP levels drop, catabolism speeds up to produce more ATP.
2. When there is enough ATP respiration shuts down.
b. Control of catabolism is based mainly on regulating the activity of enzymes at strategic points in the catabolic pathway by feedback inhibition. (Fig. 9.20)
c. One example of feedback inhibition in the catabolic pathway occurs in the third step of glycolysis, catalyzed by phosphofructokinase.
1. This enzyme is inhibited by ATP at an allosteric site.
2. When ATP levels are high phosphofructokinase is inhibited and everything below that step is stopped.
3. When ATP levels drop, the enzyme is active again and glycolysis speeds up.
4. Citrate, the first product of the Krebs cycle, is also an inhibitor of phosphofructokinase.