Cellular Respiration: Harvesting Chemical Energy
I. Enzymes catalyze the systematic degradation of organic molecules that are rich in energy to simpler waste products with less energy. Some of the released energy is used to do work; the rest is dissipated as heat.
A. Carbohydrates, fats, and proteins can all be used as the fuel, but it is most useful to consider glucose.
C6H12O6 + 6O2 à 6CO2 + 6H2O + Energy (ATP + heat)
B. The catabolism of glucose is exergonic with a D G of -686 kcal per mole of glucose. Some of this energy is used to produce ATP, which can perform cellular work.
II. Catabolic pathways transfer the electrons stored in food molecules closer to more electronegative atoms, releasing energy that is used to synthesize ATP.
A. Reactions that result in the transfer of one or more electrons from one reactant to another are oxidation-reduction reactions, or redox reactions.
1. The loss of electrons is called oxidation.
2. The addition of electrons is called reduction.
3. In general: Xe- + Y à X + Ye-
a. X, the electron donor, is the reducing agent and reduces Y.
b. Y, the electron recipient, is the oxidizing agent and oxidizes X.
c. Redox reactions require both a donor and acceptor.
B. Redox reactions also occur when the transfer of electrons is not complete but involves a change in the degree of electron sharing in covalent bonds.
C. Oxygen is very electronegative, and is one of the most potent of all oxidizing agents.
D. Energy must be added to pull an electron away from an atom. The more electronegative the atom, the more energy is required to take an electron away from it.
E. An electron loses potential energy when it shifts from a less electronegative atom toward a more electronegative one. A redox reaction that relocates electrons closer to oxygen releases chemical energy that can do work.
III. 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. Rather, glucose and other fuels are broken down in a series of steps, each catalyzed by a specific enzyme. At key steps, electrons are stripped from the glucose.
B. In many oxidation reactions, the electron is transferred with a proton, as a hydrogen atom.
C. The hydrogen atoms are not transferred directly to oxygen but are passed first to a coenzyme called NAD+ (nicotinamide adenine dinucleotide).
D. Dehydrogenase enzymes strip two hydrogen atoms from the fuel (e.g., glucose), oxidizing it. The enzyme passes two electrons and one proton to NAD+. The other proton is released as H+ to the surrounding solution.
E. By receiving two electrons and only one proton, NAD+ has its charge neutralized when it is reduced to NADH. NAD+ functions as the oxidizing agent in many of the redox steps during the catabolism of glucose.
F. Each NADH molecule formed during respiration represents stored energy. This energy is tapped to synthesize ATP as electrons “fall” from NADH to oxygen.
G. Cellular respiration uses an electron transport chain to break the fall of electrons to O2 into several steps.
1. The electron transport chain consists of several molecules (primarily proteins) built into the inner membrane of a mitochondrion.
2. Electrons released from food are shuttled by NADH to the “top” higher-energy end of the chain.
3. At the “bottom” lower-energy end, oxygen captures the electrons along with H+ to form water.
4. Electrons are passed to increasingly electronegative molecules in the chain until they reduce oxygen, the most electronegative receptor.
IV. Cellular respiration occurs in three metabolic stages: glycolysis, the citric acid cycle, and the electron transport chain and oxidative phosphorylation.
A. Glycolysis occurs in the cytoplasm and begins catabolism by breaking glucose into two molecules of pyruvate.
B. The citric acid cycle occurs in the mitochondrial matrix. It completes the breakdown of glucose by oxidizing a derivative of pyruvate to carbon dioxide.
C. Several steps in glycolysis and the citric acid cycle are redox reactions in which dehydrogenase enzymes transfer electrons from substrates to NAD+, forming NADH. NADH passes these electrons to the electron transport chain.
D. In the electron transport chain, the electrons move from molecule to molecule until they combine with molecular oxygen and hydrogen ions to form water. As they are passed along the chain, the energy carried by these electrons is transformed in the mitochondrion into a form that can be used to synthesize ATP via oxidative phosphorylation.
1. The inner membrane of the mitochondrion is the site of electron transport and chemiosmosis, processes that together constitute oxidative phosphorylation.
2. Oxidative phosphorylation produces almost 90% of the ATP generated by respiration. Some ATP is also formed directly during glycolysis and the citric acid cycle by substrate-level phosphorylation.
a. Here an enzyme transfers a phosphate group from an organic substrate to ADP, forming ATP.
E. For each molecule of glucose degraded to carbon dioxide and water by respiration, the cell makes up to 38 ATP, each with 7.3 kcal/mol of free energy.
F. Glycolysis harvests chemical energy by oxidizing glucose to pyruvate.
1. During glycolysis, glucose, a six carbon-sugar, is split into two three-carbon sugars.
2. These smaller sugars are oxidized and rearranged to form two molecules of pyruvate, the ionized form of pyruvic acid.
G. The citric acid cycle completes the energy-yielding oxidation of organic molecules.
1. More than three-quarters of the original energy in glucose is still present in the two molecules of pyruvate.
2. If oxygen is present, pyruvate enters the mitochondrion where enzymes of the citric acid cycle complete the oxidation of the organic fuel to carbon dioxide.
3. Most of the chemical energy is transferred to NAD+ and FAD by redox reactions. The reduced coenzymes NADH and FADH2 then transfer high-energy electrons to the electron transport chain.
H. The inner mitochondrial membrane couples electron transport to ATP synthesis.
1. Only 4 of 38 ATP ultimately produced by respiration of glucose are produced by substrate-level phosphorylation. Two are produced during glycolysis, and 2 are produced during the citric acid cycle.
2. NADH and FADH2 account for the vast majority of the energy extracted from the food. These reduced coenzymes link glycolysis and the citric acid cycle to oxidative phosphorylation, which uses energy released by the electron transport chain to power ATP synthesis.
3. The electron transport chain is a collection of molecules embedded in the cristae, the folded inner membrane of the mitochondrion. The folding of the cristae increases its surface area, providing space for thousands of copies of the chain in each mitochondrion.
4. Electrons drop in free energy as they pass down the electron transport chain.
I. During electron transport along the chain, electron carriers alternate between reduced and oxidized states as they accept and donate electrons.
1. Each component of the chain becomes reduced when it accepts electrons from its “uphill” neighbor, which is less electronegative. It then returns to its oxidized form as it passes electrons to its more electronegative “downhill” neighbor.
2. The electrons continue along the chain that includes several cytochrome proteins and one lipid carrier. The last cytochrome of the chain passes its electrons to oxygen, which is very electronegative. Each oxygen atom also picks up a pair of hydrogen ions from the aqueous solution to form water.
3. The electron transport chain generates no ATP directly. Its function is to break the large free energy drop from food to oxygen into a series of smaller steps that release energy in manageable amounts.
J. The mitochondrion couples electron transport and energy release to ATP synthesis using a mechanism called chemiosmosis.
1. A protein complex, ATP synthase, in the cristae actually makes ATP from ADP and Pi.
2. ATP uses the energy of an existing proton gradient to power ATP synthesis.
3. The chain is an energy converter that uses the exergonic flow of electrons to pump H+ from the matrix into the intermembrane space.
a. Certain members of the electron transport chain accept and release H+ along with electrons.
b. At certain steps along the chain, electron transfers cause H+ to be taken up and released into the surrounding solution.
c. The electron carriers are spatially arranged in the membrane in such a way that protons are accepted from the mitochondrial matrix and deposited in the intermembrane space.
d. The H+ gradient that results is the proton-motive force.
4. The protons pass back to the matrix through a channel in ATP synthase, using the exergonic flow of H+ to drive the phosphorylation of ADP.
K. Prokaryotes generate H+ gradients across their plasma membrane. They can use this proton-motive force not only to generate ATP, but also to pump nutrients and waste products across the membrane and to rotate their flagella.
L. There are three reasons that we cannot state an exact number of ATP molecules generated by one molecule of glucose.
1. Phosphorylation and the redox reactions are not directly coupled to each other, so the ratio of number of NADH to number of ATP is not a whole number.
2. One NADH results in 10 H+ being transported across the inner mitochondrial membrane. Between 3 and 4 H+ must reenter the mitochondrial matrix via ATP synthase to generate 1 ATP. Therefore, 1 NADH generates enough proton-motive force for synthesis of 2.5 to 3.3 ATP.
3. The ATP yield varies slightly depending on the type of shuttle used to transport electrons from the cytosol into the mitochondrion.
a. The mitochondrial inner membrane is impermeable to NADH, so the two electrons of the NADH produced in glycolysis must be conveyed into the mitochondrion by one of several electron shuttle systems.
b. In some shuttle systems, the electrons are passed to NAD+, which generates 3 ATP. In others, the electrons are passed to FAD, which generates only 2 ATP.
4. The proton-motive force generated by the redox reactions of respiration may drive other kinds of work, such as mitochondrial uptake of pyruvate from the cytosol.
M. How efficient is respiration in generating ATP?
1. Complete oxidation of glucose releases 686 kcal/mol.
2. Phosphorylation of ADP to form ATP requires at least 7.3 kcal/mol.
3. Efficiency of respiration is 7.3 kcal/mol times 38 ATP/glucose divided by 686 kcal/mol glucose, which equals 0.4 or 40%.
4. Approximately 60% of the energy from glucose is lost as heat.
V. Fermentation enables some cells to produce ATP without the help of oxygen.
A. Without electronegative oxygen to pull electrons down the transport chain, oxidative phosphorylation ceases. However, fermentation provides a mechanism by which some cells can oxidize organic fuel and generate ATP without the use of oxygen.
B. In glycolysis, glucose is oxidized to two pyruvate molecules with NAD+ as the oxidizing agent. Glycolysis is exergonic and produces 2 ATP (net).
C. Fermentation can generate ATP from glucose by substrate-level phosphorylation as long as there is a supply of NAD+ to accept electrons. If the NAD+ pool is exhausted, glycolysis shuts down.
D. Under aerobic conditions, NADH transfers its electrons to the electron transfer chain, recycling NAD+.
E. Under anaerobic conditions, various fermentation pathways generate ATP by glycolysis and recycle NAD+ by transferring electrons from NADH to pyruvate or derivatives of pyruvate.
VI. Glycolysis can accept a wide range of carbohydrates for catabolism.
A. Polysaccharides like starch or glycogen can be hydrolyzed to glucose monomers that enter glycolysis.
B. Other hexose sugars, such as galactose and fructose, can also be modified to undergo glycolysis.
C. The other two major fuels, proteins and fats, can also enter the respiratory pathways used by carbohydrates.
1. Proteins must first be digested to individual amino acids.
2. Amino acids that will be catabolized must have their amino groups removed via deamination.
3. The nitrogenous waste is excreted as ammonia, urea, or another waste product.
4. The carbon skeletons are modified by enzymes and enter as intermediaries into glycolysis or the citric acid cycle, depending on their structure.
D. 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 via beta oxidation. These molecules enter the citric acid cycle as acetyl CoA.
E. Intermediates in glycolysis and the citric acid cycle can be diverted to anabolic pathways.
1. For example, a human cell can synthesize about half the 20 different amino acids by modifying compounds from the citric acid cycle.
2. Glucose can be synthesized from pyruvate; fatty acids can be synthesized from acetyl CoA.
3. Excess carbohydrates and proteins can be converted to fats through intermediaries of glycolysis and the citric acid cycle.
VII. The rate of catabolism is also regulated, typically by the level of ATP in the cell.
A. If ATP levels drop, catabolism speeds up to produce more ATP.
B. The third step of glycolysis is catalyzed by phosphofructokinase. Allosteric regulation of phosphofructokinase sets the pace of respiration.
1. It is inhibited by ATP and stimulated by AMP (derived from ADP).
2. When ATP levels are high, inhibition of this enzyme slows glycolysis.
3. As ATP levels drop and ADP and AMP levels rise, the enzyme becomes active again and glycolysis speeds up.
4. Citrate, the first product of the citric acid cycle, is also an inhibitor of phosphofructokinase.
5. This synchronizes the rate of glycolysis and the citric acid cycle.
6. If intermediaries from the citric acid cycle are diverted to other uses (e.g., amino acid synthesis), glycolysis speeds up to replace these molecules.