Mastering Biology: Section 9.4
Describe how H+ travels through an ATP synthase in detail. (Figure 9.14)
1. H+ ions flowing down their gradient enter a half channel in a stator, which is anchored in the membrane. 2. H+ ions enter binding sites within a rotor, changing the shape of each subunit so that the rotor spins the membrane. 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. 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. 5. Turning of the rod activates catalytic sites in the knob that produces ATP from ADP and phosphate.
How much of the potential chemical energy in glucose is transferred to ATP?
34% is transferred to ATP. The actual percentage is probably higher because delta G is lower under cellular conditions. Cellular respiration is remarkably efficient in its energy conversion. By comparison, the most efficient automobile converts only about 25% of the energy stored in gasoline to energy that moves the car (*** **** can we improve this efficiency) Maybe we can one day improve human efficiency too, 100% glucose to ATP conversion would be excellent.
How many ATP are produced by substrate-level phosphorylation during glycolysis?
4 ATP's are produced.
Under what conditions is it beneficial to reduce the efficiency of cellular respiration?
A remarkable adaptation is shown by hibernating mammals, which overwinter in a state of inactivity and lowered metabolism. Although their internal body temperature is lower than normal, it still must be kept significantly higher than the external air temperature. One type of tissue, called brown fat, is made up of cells packed full of mitochondria. The inner mitochondria membrane contains a channel protein called the uncoupling protein, which allows protons to flow back down their concentration gradient without generating ATP. Activation of these proteins in hibernating mammals results in ongoing oxidation of stored fuel stores (fats), generating heat without any ATP production. In the absence of such an adaptation, the ATP level would build up to a point that cellular respiration would be shut down due to regulatory mechanisms.
What is the third variable why numbers of ATP produced in cellular respiration is so inexact?
A third variable that reduces the yield of ATP is the use of the proton-motive force generated by the redox reactions of respiration to drive other kinds of work. For example, the proton-motive force powers the mitochondrion's uptake of pyruvate from the cytosol. However, if all the proton-motive force generated by the electron transport chain were used to drive ATP synthesis, one glucose molecule could generate a maximum of 28 ATP produced by oxidative phosphorylation plus 4 ATP from substrate level phosphorylation to give a total yield of about 32 ATP.
Explain in detail the chemiosmosis of oxidative phosphorylation.
During chemiosmosis, the protons flow back down their gradient via ATP synthase, which is built into the membrane nearby. The ATP synthase harnesses the proton-motive force to phosphorylate ADP, forming ATP. Together, electron transport and chemiosmosis make up oxidative phosphorylation.
How many ATP's can be produced from one glucose molecule?
Each NADH that transfers a pair of electrons from glucose to the electron transport chain contributes enough to the proton-motive force to generate a maximum of about 3 ATP.
How does the inner mitochondrial membrane or the prokaryotic plasma membrane generate and maintain the H+ gradient that drives ATP synthesis by the ATP synthase protein complex?
Establishing the H+ gradient is a major function of the electron transport chain. 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. And the ATP synthases are the only sites that provide a route through the membrane for H+. As described previously, the passage of H+ through ATP synthase uses the exergonic flow of H+ to drive the phosphorylation of ADP. Thus, the energy stored in an H+ gradient across a membrane couples the redox reactions of the electron transport chain to ATP synthesis, an example of chemiosmosis.
Why are the numbers of ATP so inexact? (Give me the first reason)
First, phosphorylation and the redox reactions are not directly coupled to each other, so the ratio of the number of NADH molecules to the number of ATP molecules is not a whole number. We know that 1 NADH results in 10 H+ being transported out across the inner mitochondrial membrane, but the exact number of H+ that must reenter the mitochondrial matrix via ATP synthase to generate 1 ATP has long been debated. Based on experimental data, however, most biochemists now agree that the most accurate number is 4 H+. Therefore, a single molecule of NADH generates enough proton-motive force for the synthesis of 2.5 ATP. The citric acid cycle also supplies electrons to the electron transport chain via FADH2, but since its electrons enter later in the chain, each molecule of this electron carrier is responsible for transport of only enough H+ for the synthesis of 1.5 ATP. These numbers take into account the slight energetic cost of moving the ATP formed in the mitochondrion out into the cytosol, where it will be used.
How does energy flow in respiration?
Glucose -> NADH -> electron transport chain -> proton-motive force -> ATP.
What is the overall function of cellular respiration?
Harvesting the energy of glucose for ATP synthesis.
What effect would an absence of O2 have on the process shown in figure 9.15
I believe if oxygen were missing water could not be formed... (dont know what else to add..) [9.4 - 1] Oxidative phosphorylation would eventually stop entirely, resulting in no ATP production by this process. Without oxygen to "pull" electrons down the electron transport chain, H+ would not be pumped into the mitochondrion's intermembrane and chemiosmosis.
How can one describe chemiosmosis in general terms?
In general terms, chemiosmosis is an energy coupling-mechanism that uses energy stored in the for of an H+ gradient across a membrane to drive cellular work.
Where does chemiosmosis occur?
In mitochondria, the energy gradient formation comes from exergonic redox reactions, and ATP synthesis is the work performed. But chemiosmosis also occurs elsewhere and in other variations. Chloroplasts use chemiosmosis to generate ATP during photosynthesis; in these organelles, light (rather than chemical energy) drives both electron flow down an electron transport chain and the resulting H+ gradient formation.
How do ion pumps work?
Ion pumps usually use ATP as an energy source to transport ions against their gradients.
What is the electron transport chain?
It is a collection of molecules embedded in the inner membrane of the mitochondrion in eukaryotic cells. (In prokaryotes, these molecules reside in the plasma membrane.) The folding of the inner membrane to form cristae increases the surface area, providing a space for thousands of copies of the chain in each mitochondrion. (Once again, we see that structure fits function.) Most components of the chain are proteins, which exist in multiprotein complexes numbered 1 through 1V. Tightly bound to these proteins are prosthetic groups, nonprotein components essential for the catalytic functions of certain enzymes.
What is the form and function of ATP synthase?
It is a multisubunit complex with four main parts, each made up of multiple polypeptides. Protons move one by one into binding sites on one of the parts (the rotor) 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. ATP synthase is the smallest molecular rotary motor known in nature. *** **** fking amazing, I hope one day I can utilize these parts to cure disease, to build biological machinery, and find human immortality, or perhaps understand where the fk consciousness come from amidst all the pile of meat, which we call humans.
Explain in detail the electron transport chain of oxidative phosphorylation.
NADH and FADH2 shuttle high-energy electrons extracted from food during glycolysis and the citric acid cycle into an electron transport chain built into the inner mitochondrial membrane. The gold arrows trace the transport of electrons, which finally pass to oxygen at the downhill end of the chain forming water. Two mobile carriers, ubiquinone (Q) and cytochrome (Cty, c), move rapidly, ferrying electrons between the large complexes. As complexes 1, 3, and IV accept and then donate electrons, they pump protons from the mitochondrial matrix into the intermembrane space. (In prokaryotes, protons are pumped outside the plasma membrane.) Note that FADH2 deposits its electrons via complex 2 and so results in fewer protons being pumped into the intermembrane space than occurs with NADH. Chemical energy originally harvested from food is transformed into a proton-motive force, a gradient of H+ across the membrane.
What is ATP synthase?
Populating the inner membrane of the mitochondrion or the prokaryotic plasma membrane are many copies of a protein complex called ATP synthase, the enzyme that actually makes ATP from ADP and inorganic phosphate. ATP synthase works like an ion pump running in reverse. Rather than hydrolyzing ATP to pump protons against their concentration gradient, under the conditions of cellular respiration ATP synthase uses the energy of an existing ion gradient to power ATP synthesis.
How does the electron transport chain pump hydrogen ions?
Researchers have found that certain members of the electron transport chain accept and release protons (H+) along with electrons. The aqueous solutions inside and surrounding the cell are a ready source of H+. At certain steps along the chain, electron transfers cause H+ to be taken up and released into the surrounding solution. In eukaryotic cells, the electron carriers are spatially arranged in the inner mitochondrial membrane in such a way that H+ is accepted from the mitochondrial matrix and deposited in the intermembrane space.
What is the second reason why the numbers of ATP produced in cellular respiration is so inexact?
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 2 electrons of NADH captured in glycolysis must be conveyed into the mitochondrion by one of several electron shuttle systems. Depending on the kind of shuttle in a particular cell type, the electrons are passed either to NAD+ or to FAD in the mitochondria matrix. If the electrons are passed to FAD, as in brain cells, only about 1.5 ATP can result from each NADH that was originally generated in the cytosol. If the electrons are passed to mitochondrial NAD+, as in liver cells and heart cells, the yield is about 2.5 ATP per NADH.
How does the mitochondrion (or the prokaryotic plasma membrane) couple this electron transport and energy release to ATP synthesis?
The answer is a mechanism called chemiosmosis (The energy coupling mechanism).
How does the electron transport chain generate ATP's?
The electron transport chain makes no ATP directly. Instead, it eases the fall of electrons from food to oxygen, breaking a large free-energy drop into a series of smaller steps that release energy in manageable amounts.
Where does the electrons travel to after leaving complex 1 in the electron transport chain?
The iron-sulfur protein then passes the electrons to a compound called ubiquinone (Q). This electron carrier is a small hydrophobic molecule, the only member of the electron transport chain that is not a protein. Ubiquinone is individually mobile within the membrane rather than residing in a particular complex. (Another name for ubiquinone is coenzyme Q, or CoQ; you may have seen it sold as a nutritional supplement - nope...not yet...)
What is the power source for ATP synthase?
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 of H+ concentration.)
Where is most of the energy that is not used from glucose to ATP lost to?
The rest of the energy stored in glucose is lost as heat. We humans use some of this heat to maintain our relatively high body temperature (37 C), and we dissipate the rest through sweating and other cooling mechanisms.
What are the steps of oxidative phosphorylation?
The steps of oxidative phosphorylation are 1. Electron transport chain: Electron transport and pumping of protons (H+), which creates an H+ gradient across the membrane. 2. Chemiosmosis: ATP synthesis powered by the flow of H+ back across the membrane.
What occurs in the first complex?
This molecule is a flavoprotein, so named because it has a prosthetic group called flavin mononucleotide (FMN). In the next redox reaction, the flavoprotein returns to its oxidized form as it passes electrons to an iron-sulfur protein (Fe*S), one of a family of proteins with both iron and sulfur tightly bound.
What is chemiosmosis.
This process, in which energy stored in the form of a hydrogen ion gradient across a membrane is used to drive cellular work such as the synthesis of ATP, is called chemiosmosis. We have previously used the word osmosis in discussing water transport, but here it refers to the flow of H+ across a membrane.
In the absence of O2, as in question 1, what do you think would happen if you decreased the pH of the intermembrane space of the mitochondrion? Explain your answer.
[9.4 - 2] Decreasing the pH means addition of H+. This would establish a proton gradient even without the function of the electron transport chain and we would expect ATP synthase to function and synthesize ATP. (In fact, it was experiments like this that provided support for chemiosmosis as an energy-coupling mechanism.)
In concept 71. you learned that membranes must be fluid to function properly. How does the operation of the electron transport chain support that assertion?
[9.4 - 3.] One of the components of the electron transport chain, ubiquinone (Q), must be able to diffuse within the membrane. It could not do so if the membrane were locked rigidly into place.
Explain exactly how the numbers 26 or 28 ATP were calculated.
[Figure 9.16] First there are 2 NADH from the oxidation of pyruvate plus 6 NADH from the citric acid cycle (CAC); 8 NADH * 2.5 ATP/NADH = 20 ATP. Second there are 2 FADH2 from the CAC; 2 FAD2 * 1.5 ATP/FADH2 = 3 ATP. Third, the 2 NADH from glycolysis enter the mitochondrion through one of two types of shuttle. They pass their electrons either to FAD, which become FADH2 and results in 3 ATP, or to 2 NAD+, which become NADH and result in 5 ATP. Thus, 20 + 3 + 3 = 26 ATP, or 20 + 3 + 5 = 28 ATP from all NADH and FADH2.
