Three key pathways of Cellular Respiration
Glycolysis, the citric acid cycle, and oxidative phosphorylation.
Overview of Life and Work
To perform their many tasks, living cells require energy from outside sources.
Energy enters most ecosystems as sunlight and leaves as heat.
Photosynthesis generates oxygen and organic molecules that the mitochondria of eukaryotes use as fuel for cellular respiration.
Cells harvest the chemical energy stored in organic molecules and use it to regenerate ATP, the molecule that drives most cellular work.
A relatively inefficient catabolic process that leads to the partial degradation of sugars in the absence of oxygen. Gycolysis plus reactions that regenerate NAD+ by transferring electrons from NADH to pyruvate or derivatives of pyruvate. The NAD+ can then be reused to oxidize sugar by glycolysis, which nets two molecules of ATP by substrate-level phosphorylation. There are many types of fermentation, differing in the end products formed from pyruvate. Two common types are alcohol fermentation and lactic acid fermentation.
A more efficient and widespread catabolic process, in which oxygen is consumed as a reactant to complete the breakdown of a variety of organic molecules. Similar in broad principle to the combustion of gasoline in an automobile engine after oxygen is mixed with hydrocarbon fuel. Food is the fuel for respiration. The exhaust is carbon dioxide and water. Includes both aerobic and anaerobic processes. However, it originated as a synonym for aerobic respiration because of the relationship of that process to organismal respiration, in which an animal breathes in oxygen.
In eukaryotic cells, they are the site of most of the processes of cellular respiration. The folding of the cristae increases its surface area, providing space for thousands of copies of the ETC in each mitochondrion.
Overall process of Cellular Respirtation
organic compounds + O2 --> CO2 + H2O + energy (ATP + heat).
Which substances are fuels in respiration?
Carbohydrates, fats, and proteins can all be used as the fuel, but it is most useful to consider glucose. Catabolic pathways transfer the electrons stored in food molecules, releasing energy that is used to synthesize ATP.
C6H12O6 + 6O2 --> 6CO2 + 6H2O + Energy (ATP + heat) Exergonic with a delta G of -686 kcal per mole of glucose. Some of this energy is used to produce ATP, which can perform cellular work.
For each molecule of glucose degraded to carbon dioxide and water by respiration, the cell makes up to 38 ATP (30 or 32 ATP), each with 7.3 kcal/mol of free energy. Respiration uses the small steps in the respiratory pathway to break the large denomination of energy contained in glucose into the small change of ATP. The quantity of energy in ATP is more appropriate for the level of work required in the cell.
Reactions that result in the transfer of one or more electrons from one reactant to another are aka oxidation-reduction reactions. These reactions release energy when electrons move closer to electronegative atoms. Redox reactions require both a donor and acceptor.
The loss of electrons. 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.
The addition of electrons. An electron loses potential energy when it shifts from a less electronegative atom toward a more electronegative one.
Na + Cl -->
Na+ + Cl-
The formation of table salt from sodium and chloride is a redox reaction.
Here sodium is oxidized and chlorine is reduced (its charge drops from 0 to -1)
General Redox Equation
Xe- + Y ---+ X + Ye-
Xe is oxidized and Y is reduced. Xe is reducing agent, and Y is oxidizing agent.
Can redox reactions only occur with a transfer of electrons?
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. In the combustion of methane to form water and carbon dioxide, the nonpolar covalent bonds of methane (C—H) and oxygen (O=O) are converted to polar covalent bonds (C=O and O—H). Electrons move father away from the carbon atom and closer to their new covalent partners, the oxygen atoms, which are very electronegative. Thus, methane has been oxidized. The two atoms of the oxygen molecule share their electrons equally. When oxygen reacts with the hydrogen from methane to form water, the electrons of the covalent bonds are drawn closer to the oxygen. Each oxygen atom has partially "gained" electrons, and so the oxygen molecule has been reduced.
A substance that is very electronegative, and is one of the most potent of all oxidizing agents. Cyt a3 (3 is a subscript) passes its electron to oxygen which also recieves two hydrogen ions from the aqueous solution, to form H2O.
What is a redox reaction's significance towards energy?
It can relocate electrons closer to oxygen, such as the burning of methane, which releases chemical energy that can do work.
Stepwise nature of cellular respiration
The "fall" of electrons during respiration is stepwise, via NAD+ and an electron transport chain. 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. In many oxidation reactions, the electron is transferred with a proton, as a hydrogen atom.
This functions as the oxidizing agent in many of the redox steps during the catabolism of glucose. The hydrogen atoms are not transferred directly to oxygen but are passed first this coenzyme.
They 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.
Reduced form of NAD+. The electrons carried by this substance have lost very little of their potential energy in this process. The substance represents stored energy. Coenzyme that represents most of the energy stored in food.
Cellular Respiration Overview
Glycolysis occurs in the cytoplasm. It begins catabolism by breaking glucose into two molecules of pyruvate.The citric acid cycle occurs in the mitochondrial matrix. It completes the breakdown of glucose by oxidizing a derivative of pyruvate to carbon dioxide. 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. 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.
The inner membrane of the mitochondrion is the site of electron transport and chemiosmosis, processes that together constitute oxidative phosphorylation. 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. Here an enzyme transfers a phosphate group from an organic substrate to ADP, forming ATP.
First step of cellular respiration. A six carbon-sugar, is split into two three-carbon sugars. These smaller sugars are oxidized and rearranged to form two molecules of pyruvate, the ionized form of pyruvic acid. The 10 steps can be divided into two phases: an energy investment phase and an energy payoff phase. The net yield from glycolysis is 2 ATP and 2 NADH per glucose. No CO2 is produced during glycolysis. Can occur whether O2 is present or not.
Energy investment phase
The cell invests ATP to provide activation energy by phosphorylating glucose. This requires 2 ATP per glucose.
Energy payoff phase
ATP is produced by substrate-level phosphorylation and NAD+ is reduced to NADH by electrons released by the oxidation of glucose.
Step 1 of Glycolysis: 1st of the energy investment phase. Glucose enters the cell and is phosphorylated. A phosphate group from ATP is transferred to the sugar. The charge of the phosphate group traps the sugar in the cell because the plasma membrane is impermeable to large ions.
Step 2 of Glycolysis: Glucose-6-phosphate is converted to its isomer, fructose-6-phosphate.
Step 3 of Glycolysis (the committed step): This enzyme transfers a phosphate group from ATP to the sugar, investing another molecule of ATP in glycolysis, thus making fructose- 1,6-bisphosphate. So far, 2 ATP have been used. With phosphate groups on its opposite ends, the sugar is now ready to be split in half. This is a key step for regulation of glyolysis; This enzyme is allosterically regulated by ATP and its products.
Step 4 of Glycolysis:This is the reaction from which glycolysis gets its name. The enzyme cleaves the sugar molecule into two different threecarbon sugars: dihydroxyacetone phosphate and glyceraldehyde3- phosphate. These two sugars are isomers of each other.
Step 5 of Glycolysis: Isomerase catalyzes the reversible conversion between the two three carbon sugars. This reaction never reaches equilibrium in the cell because the next enzyme in glycolysis uses only glyceraldehyde-3-phosphate as its substrate (and not dihydroxyacetone phosphate). This pulls the equilibrium in the direction of glyceraldehyde-3-phosphate, which is removed as fast as it forms. Thus, the net result of steps 4 and 5 is cleavage of a six-carbon sugar into two molecules of glyceraldehyde-3-phosphate; each will progress through the remaining steps of glycolysis. This is the last step of energy investment.
Triose phosphate dehydrogenase
Step 6 of Glycolysis, 1st of the energy payoff phase.This enzyme catalyzes two sequential reactions while it holds glyceraldehyde 3-phosphate in its active site. First. the sugar is
oxidized by the transfer of electrons and W to NAD+, forming NADH (a redox reaction). This reaction is very exergonic, and the enzyme
uses the released energy to attach a phosphate group to the oxidized substrate, making a product of very high potential energy (1,3-Bisphosphoglycerate). The source of the phosphates is the pool of inorganic phosphate ions that are always present in the cytosol. Note that the coefficient 2 precedes all molecules in the energy payoff phase.
Step 7 of Glycolysis: ATP is produced by substrate-level phosphorylation. The phosphate group added in the previous step is transferred to ADP in an exergonic reaction. For each glucose molecule that began glycolysis, step 7 produces 2 ATp, since every product after the sugarsplitting
step (step 4) is doubled. Recall that 2 ATP were invested to get sugar ready for splitting; this ATP debt has now been repaid. Glucose has been converted to two molecules of 3-phosphoglycerate, which is not a sugar. The carbonyl group that characterizes a sugar has been oxidized to a carboxyl group (- COO-), the hallmark of an organic acid.
Step 8 of Glycolysis: This enzyme relocates the remaining phosphate group, preparing the substrate for the next reaction. 3-phosphoglycerate ---> 2-phosphoglycerate
Step 9 of Glycolysis: This enzyme causes a double bond to form in the substrate by extracting a water molecule, yielding phosphoenolpyruvate (PEP). The electrons of the substrate are rearranged in such a way that the
resulting phosphorylated compound has a very high potential energy, allowing step 10 to occur.
Step 10 of Glycolysis: Production of 2 more ATP by transferring the phosphate group from PEP to ADP (2nd instance of substratelevel
phosphorylation). Since this step occurs twice for each glucose molecule, 2 ATP are produced. Pyruvate is produced.
Epic Glycolysis Concluding Statement
Overall, glycolysis has used 2 ATP in the energy investment phase (steps 1 and 3) and produced 4 ATP in the energy payoff phase (steps 7 and 10), for a net gain of 2 ATP. Glycolysis has repaid the ATP investment with 100% interest. Additional energy was stored by step 6 in NADH, which can be used to make ATP by oxidative phosphorylation if oxygen is present. Glucose has been broken down and oxidized to two molecules of pyruvate, the end product of the glycolytic pathway. If oxygen is present, the chemical energy in pyruvate can be extracted by the
citric acid cycle. If oxygen is not present, fermentation may occur
The end product of glycolysis. If oxygen is present, it enters the mitochondrion where enzymes of the citric acid cycle complete the oxidation of the organic fuel to carbon dioxide. (In prokaryotic cells, this process occurs in the cytosoI.) Upon entering the mitochondrion via active transport, it is first converted to a compound called acetyl coenzyme A or acetyl CoA.
The junction between glycolysis and the citric acid cycle which is accomplished by a multi-enzyme complex that catalyzes three reactions.
1st Step of Transition Phase
Pyruvate's carboxyl group (-COO-) " which is already fully oxidized and thus has little chemical energy, is removed and given off as a molecule of CO2 (This is the first step in which CO2 is released during respiration.)
2nd Step of Transition Phase
The remaining two-carbon fragment is oxidized. forming a compound named acetate (the ionized form of acetic acid). An enzyme transfers the extracted electrons to NAO+, storing energy in the form of NADH.
3rd Step of Transition Phase
Finally, coenzyme A (CoA), a sulfur containing
compound derived from a B vitamin. is attached to the acetate by an unstable bond that makes the acetyl group (the attached acetate) very reactive.
Because of the chemical nature of the CoA group, the product of this chemical grooming, acetyl CoA, has a high potential energy; in other words. the reaction of acetyl CoA to yield lower-energy
products is highly exergonic. This molecule is now ready to feed its acetyl group into the citric acid cycle for further oxidation.
Citric acid cycle
(Szent-Györgyi-Krebs or Tricarboxylic acid cycle) who was largely responsible for elucidating its pathways in the 1930s. This cycle oxidizes organic fuel derived from pyruvate. The acetyl group of acetyl CoA joins the cycle by combining with the compound oxaloacetate, forming citrate. The next seven steps decompose the citrate back to oxaloacetate. The regeneration of oxaloacetate makes this a cycle. 3 CO2 molecules are released, including the one in transition phase. Two turns of the cycle makes 6 CO2.
Products of Citric Acid Cycle
A GTP molecule is formed by substrate-level phosphorylation. The GTP is then used to synthesize an ATP, the only ATP generated directly by the citric acid cycle. (one ATP for each cycle). Most of the chemical energy is transferred to NAD+ and FAD during the redox reaction. The reduced coenzymes NADH and FADH2 then transfer high-energy electrons to the electron transport chain. Each cycle produces one ATP by substrate-level phosphorylation, three NADH, and one FADH2 per acetyl CoA.
Step 1 of Kreb's Cycle
Acetyl CoA adds its two-carbon acetyl group to oxaloacetate. Producing citrate.
Step 2 of Kreb's Cycle
Citrate is converted to its isomer, isocitrate, by removal of one water molecule and addition of another.
Step 3 of Kreb's Cycle
lsocitrate is oxidized, reducing NAD to NADH. Then the resulting compound loses a CO2 molecule. Result is alpha-Ketoglutarate.
Step 4 of Kreb's Cycle
Another CO2 is lost, and the resulting compound is oxidized, reducing NAD+ to NADH. The remaining molecule is then attached to coenzyme A by an unstable bond. Result is Succinyl CoA.
Step 5 of Kreb's Cycle
CoA is displaced by a phosphate group. which is transferred to GDP forming GTP, a molecule with function similar to ATP that in some cases, is used to generate ATP. Succynyl CoA --> Succinate This GTP may also be used to directly power work in the cell. ln the cells of plants, bacteria, and some animal tissues, step 5 forms an ATP molecule directly by substrate-level phosphorylation. The output from step 5 represents the only ATP generated directly by the citric acid cycle.
Step 6 of Kreb's Cycle
Two hydrogens are transferred to FAD, forming FADH2 and oxidizing succinate into fumarate.
Step 7 of Kreb's Cycle
Addition of a water molecule rearranges bonds in the substrate. Fumarate --> malate
Step 8 of Kreb's Cycle
The substrate (malate) is oxidized, reducing NAD+ to NADH and regenerating oxaloacetate.
The production of ATP using energy derived from the redox reactions of an electron transport chain. During this process chemiosmosis couples electron transport to ATP synthesis
Electron Transport Chain (ETC)
A collection of molecules embedded in the cristae, the folded inner membrane of the mitochondrion. Most components of the chain are proteins bound to prosthetic groups, nonprotein components essential for catalysis. Electrons drop in free energy as they pass down the electron transport chain. This does not produce any ATP directly. It releases energy which is coupled by chemiosmosis for ATP synthesis.
Electron Carrier Mechanism
They alternate between reduced and oxidized states as they accept and donate electrons. 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.
Two or more proteins (usually enzymes) bound together in a way that allows each protein to play a sequential role in the same multistep process.
Complexes I to IV
The majority of the components that make up the ETC. Bound to these proteins are prosthetic groups, nonprotein components essential for the catalytic functions of certain enzymes.
Electrons removed from glucose by NAD+, during glycolysis and the citric acid cycle, are transferred from NADH to this molecule, the first of the electron transport chain. This molecule is a flavoprotein.
Has a prosthethic group called flavin mononucleotide (FMN). It becomes reduced when it recieves an electron from NADH. It becomes oxidized again when it pases the electron downhill to Fe-S in complex I. (the dash is really a dot)
The overall energy drop (delta G) for electrons traveling from NADH to oxygen. This "fall" is broken up into a series of smaller steps by the eledron transport chain.
One of a family of proteins with both
iron and sulfur tightly bound. There is an Iron-sulfur protein in Complex I, II and III.
A small hydrophobic molecule, the only member of the electron transport chain that is not a protein. It is is individually mobile within the membrane
rather than residing in a particular complex.
A group that comprises most of the remaining electron carriers in between ubiquinone and oxygen. Their prosthethic group has a heme group, which has an iron atom that accepts and
donates electrons. ETC has several types of these carriers, each a different protein with a slightly different electron~carrying heme group.
Pathway of Two Electrons from NADH
Transition from I to III
Ubiquinone (Q in diagram)
Cyt b, Fe-S, Cyt c1(1 is a subscript)
Transition from III to IV
Cyt a, Cyt a3 (3 is a subscript)
Cyt a3 to Oxygen + Two H+ ions from solution to form H20.
Similar to NADH as in it is a coenzyme that represents a majority of the energy stored in the food, and like NADH it also links the citric acid cycle to oxidative phosphorylation. Donates its electrons to the ETC at complex II a lower energy level than NADH. NADH and FADH2 each donate an equivalent number of electrons (2) for oxygen
reduction, but the latter gives one third less energy for ATP production.
Pathway of Two Electrons from FADH2
FADH2 releases two electrons into Complex II, resulting to an FAD which is ejected as the electrons pass to the next carrier, Fe-S.
Transition between II and III
Complex III to IV
Same as pathway for electrons from NADH
An energy-coupling mechanism that uses energy stored in the form of an H+ gradient across a membrane to drive cellular work. In mitochondria,
the energy for gradient formation comes from exergonic redox reactions (ETC), and ATP synthesis is the work performed. This process also occurs elsewhere and in other variations (such as photosynthesis).
The enzyme that makes ATP from ADP. It is a proton pump. The power source for the ATP synthase is a difference in the concentration of H+ on opposite sides of the inner mitochondrial membrane. (We can also think of this gradient as a difference in pH, since pH is a measure ofH+ concentration.) It is multisubunit complex
with four main parts, each made up of multiple polypeptides.
How is the H+ gradient of chemiosmosis established and maintained in the plasma membrane of prokaryotes or the inner mitochondriol membrane (IMM) of euks?
The electron transport chain meets this demand. The chain is an energy converter that uses the exergonic flow of electrons from NADH and FADH2 to pump H+ across the membrane, from the mitochondrial matrix into the intermembrane space. The H+ has a tendency to move back across the membrane, diffusing down its gradient. However the IMM is impermeable to ions and they can only come back through ATP synthases. This flow of protons cause ATP synthesis (look at definition for rotor).
How does the electron transport chain pumps hydrogen ions?
Certain members of the electron transport chain
accept and release protons (H+) along with the electrons. (The aqueous solutions inside and surrounding the cell are a ready source of H+.)
The potential energy stored in the form of an electrochemical gradient, generated by the pumping of hydrogen ions across biological membranes during chemiosmosis This force drives H+ back across the membrane through the H+ channels provided by ATP synthases.
The fixed part of ATP synthase where ADP reacts with Pi to form ATP as a result of the extension of the internal rod from when the rotor spun because of a proton.
ATP synthesis Step 1
H+ ions flowing down their gradient enter a half channel in a stator, which is anchored in the membrane.
ATP synthesis Step 2
H+ ions enter binding sites within a rotor, changing the shape of each subunit so that the rotor spins within the membrane.
ATP synthesis Step 3
Each H+ ion makes one complete turn before
leaving the rotor and passing through a second
half channel in the stator into the mitochondrial matrix.
ATP synthesis Step 4
Spinning of the rotor causes an internal rod to spin as well. This rod extends like a stalk into the knob below it which is held stationary by part of the stator.
ATP synthesis Step 5
Turning of the rod activates catalytic sites in the knob that produce ATP from ADP and Pi. (Pi is inorganic phosphate and the i is a subscript)
A part of ATP synthase on which protons one by one bind one to and release from, causing it to spin in a way that catalyzes ATP production from ADP and inorganic phosphate. The flow of protons thus behaves somewhat like a rushing stream that turns a waterwheel
How do Prokaryotes carry out oxidative phosphorylation?
They generate H+ gradients across their
plasma membranes. They then tap the proton-motive force not only to make ATP inside the cell but also to rotate their flagella and to pump nutrients and waste products across the membrane.
Basic Energy Flow in Respiration
Glucose ---> NADH ---> electron transport chain ---> proton-motive force ---> ATP.
NADH to ATP conversion
Each NADH that transfers a pair of electrons
from glucose to the electron transport chain contributes enough energy to the proton-motive force to generate a maximum of about 2.5 ATP.
FADH2 to ATP conversion
Each FADH2 that transfers a pair of electrons
from glucose to the electron transport chain contributes enough energy to the proton-motive force to generate a maximum of about 1.5 ATP.
Why are the number of ATP produced inexact?
First, phosphorylation and the redox reactions are not directly coupled to each other, so the ratio of number of NADH molecules to number of ATP molecules is not a whole number. Second, the ATP yield varies slightly depending on the type
of shuttle used to transport electrons from the cytosol into the mitochondrion.The mitochondrial inner membrane is impermeable to NADH, so NADH in the cytosol is segregated from the machinery of oxidative phosphorylation. The two electrons of NADH captured in glycolysis must be conveyed into the mitochondrion by one of several electron shuttle systems. Depending on the type of shuttle in a particular cell type, the electrons are passed either to NAD+ or to FAD in the mitochondrial matrix. Athird variable that reduces the yield ofATP is the use of the proton-motive force generated by the redox reactions of respiration to drive other kinds of work. For example, the protonmotive force powers the mitochondrion's uptake of pyruvate from the cytosol.
Efficiency of Cellular Respiration and its Derivation
The complete oxidation of a mole of glucose releases 686 kcal of energy under standard conditions (.6.G = -686 kcal/mol). Phosphorylation of ADP to form ATP stores at least 7.3 kcal per mole of ATP. Therefore, the efficiency of respiration is 7.3 kcal per mole of ATP times 38 moles of ATP per mole of glucose divided by 686 kcal per mole ofglucose, which equals 0.4. Thus, about 40% of the potential chemical energy in glucose has been transferred to ATP; the actual percentage is probably higher because.1.G is lower under cellular conditions. The rest
of the stored energy is lost as heat. Cellular respiration is remarkably efficient in its energy conversion. By comparison, the most efficient automobile converts only about 25% ofthe energy stored in gasoline to energy that moves the car.
The function of heat from Cellular Respiration
Humans use some of this heat to maintain our relatively high body temperature (30 degrees C), and we dissipate the rest through sweating and other cooling mechanisms.
Three electron driven proton pumps embedded in inner membrane. They are:
NADH-Q oxidoreductase (Complex I)-
Q-cytochrome c oxidoreductase (complex III)
Cytochrome c oxidase (Complex IV)
The use of inorganic molecules other than oxygen to accept electrons at the "downhill" end of electron transport chains. Takes place in certain prokaryotic organisms. Some "sulfate-reducing~ marine bacteria, for instance, use the sulfate ion (SO4 2-) at the end of their respiratory chain. Operation of the chain builds up a proton motive
force used to produce ATP, but H2S (hydrogen sulfide) is produced as a by-product rather than water.
Transfers amino group from one amino acid to the other. Transferring amino acid becomes alpha-ketoacid commonly Glutamate <-> alpha-ketoglutarate 18 transaminases (not Thr, Lys)
Pyruvate is converted to ethanol (ethyl alcohol) in two steps. The first step releases carbon dioxide from the pyruvate, which is converted to the two-carbon compound acetaldehyde. In the second step, acetaldehyde is reduced by NADH to ethanol. This regenerates the supply of NAD+ needed for the continuation of glycolysis. Net production of 2 ATP by substrate-level phosphorylation.
A fungus that carries out alcohol fermentation. The CO2 bubbles generated by baker's yeast during alcohol fermentation allow bread to rise.
Lactic Acid Fermentation
Pyruvate is reduced directly by NADH to form lactate as an end product, with no release of CO2, (Lactate is the ionized form of lactic acid.) This process is used by certain fungi and bacteria is used in the dairy industry to make
cheese and yogurt. Net production of 2 ATP by substrate-level phosphorylation.
What happens to excess Lactate?
Excess lactate is gradually carried away by the blood to the liver. Lactate is converted
back to pyruvate by liver cells.
Random Fact # 1
A key difference is the contrasting mechanisms
for oxidizing NADH back to NAD+, which is required to sustain glycolysis. In fermentation, the final electron acceptor is an organic molecule such as pyruvate (lactic acid fermentation)
or acetaldehyde (alcohol fermentation).
Carry out only fermentation or anaerobic respiration and in fact cannot survive
in the presence ofoxygen.
can live with or without oxygen. On the cellular level, our muscle cells behave as these types of anaerobes.
Random Fact # 2
To make the same amount of ATP, a facultative anaerobe would have to consume sugar at a much faster rate when fermenting than when
The polysaccharide that humans and many other animals store in their liver and muscle cells, can be hydrolyzed to glucose between meals as
fuel for respiration.
How are amino acids used as fuel?
Before amino acids can feed into glycolysis or the citric acid cycle, their amino groups must be removed, a process called deamination. The nitrogenous refuse is excreted from the animal in the form of ammonia, urea, or other waste products.
A metabolic sequence that breaks fatty acids down to two-carbon fragments which enter the Krebs cycle as acetyl CoA. NADH and FADH1 are
also generated during this process; they can enter the electron transport chain, leading to further ATP production.
Biosynthesis (anabolic examples)
For example, humans can make
about half of the 20 amino acids in proteins by modifying compounds siphoned away from the citric acid cycle; the rest are "essential amino acids~ that must be obtained in the diet. Also, glucose can be made from pyruvate, and fatty acids can be synthesized from acetyl CoA. Of course, these anabolic, or biosynthetic, pathways do not generate ATP, but instead consume it.
Random Fact # 3
In addition, glycolysis and the citric acid cycle function as metabolic interchanges that enable our cells to convert some kinds of molecules to others as we need them. For example, an
intermediate compound generated during glycolysis, dihydroxyacetone phosphate can be converted to one of the major precursors of fats. If we eat more food than we need, we store fat even if our diet is fat-free. Metabolism
is remarkably versatile and adaptable.
The end product of the anabolic
pathway inhibits the enzyme that catalyzes an early step of the pathway. This prevents the needless diversion of key metabolic intermediates from uses that are
more urgent. This is the most common mechanism to control anabolic pathways.
Catabolism Various ****
The cell also controls its catabolism. If the cell is working hard and its ATP concentration begins to drop, respiration speeds up. When there is plenty of ATP to meet demand, respiration slows down, sparing valuable organic molecules for other functions. Again, control is based mainly on regulating the activity of enzymes at strategic points in the catabolic pathway. As shown in Figure 9.21, one important switch is phosphofructokinase, the enzyme that catalyzes step 3 of glycolysis. That is the first step that commits substrate irreversibly to the glycolytic pathway. By controlling the rate of this step, the cell can speed up or slow down the entire
catabolic process. Phosphofructokinase can thus be considered the pacemaker of respiration.
AMP ATP ****
Phosphofructokinase is also sensitive to citrate, the first product of the citric acid cycle. If
citrate accumulates in mitochondria, some of it passes into the cytosol and inhibits phosphofructokinase. This mechanism helps synchronize the rates of glycolysis and the citric acid cycle. As citrate accumulates, glycolysis
slows down, and the supply of acetyl groups to the citric acid cycle decreases. If citrate consumption increases, either because of a demand for more ATP or because anabolic
pathways are draining off intermediates of the citricacid cycle, glycolysis accelerates and meets the demand.
Enzyme that causes ATP molecules to release the energy stored in their terminal phosphate bonds
The tendency of a molecule to accept or donate electrons during a chemical reaction.
uncoupling of oxidative phosphorylation by DNP (2,4-dinitrophenol), is protonated in inner membrane space and then diffuses into matrix....diminishing H+ gradient