BIOL #07 Flashcards
Glucose Oxidation Summary
• Glucose oxidation produces ATP, NADH, FADH2, and CO2
.
• Glucose oxidation only produces 4 ATP (2 (net) from glycolysis, 2 from the Citric acid cycle)
• Most of glucose’s original energy is contained in the electrons transferred
to NADH and FADH2, which then carry them to oxygen, the final electron acceptor. In the process, the energy released powers ATP synthesis.
The Electron Transport Chain
• Fourth step in cellular respiration:
– High potential energy of the electrons carried by NADH and FADH2 is gradually decreased as they move through a series of redox reactions (these electron carriers become oxidized as they lose electrons).
• Proteins involved in these redox reactions make up what is called an electron transport chain (ETC).
– In eukaryotes, these molecules are embedded in the inner membrane of the mitochondrion (e.g. walls of the cristae)
• O2 is the final electron acceptor of the ETC – the transfer of electrons along with protons to oxygen forms water.
– O2 is reduced (GER), thus acts as an oxidizing agent.
Chemiosmosis
- The energy released as electrons move through the ETC is used to pump protons across the plasma membrane into the intermembrane space, forming a strong electrochemical gradient.
- The protons then move through the enzyme ATP synthase, driving the production of ATP from ADP and Pi.
Oxidative Phosphorylation
• Because this mode of ATP production links the phosphorylation of ADP with NADH and FADH2 oxidation, it is called oxidative phosphorylation.
– This step of cellular respiration is responsible for the production of the greatest ATP yield.
– Oxidative phosphorylation refers to the electron transport chain and chemiosmosis collectively.
ETC Molecules
• Most of the ETC molecules are proteins (typically cytochromes) containing chemical groups that facilitate redox reactions.
• All but one of these molecules are proteins and are embedded in the inner mitochondrial membrane.
– In contrast, the lipidsoluble ubiquinone (Q) can move throughout the membrane and is not a protein (aka CoQ)
During electron transport:
– NADH donates electrons to a flavin-containing protein at the top of the chain
– FADH2 donates electrons to an iron-sulfur protein that passes electrons directly to Q.
– Both NADH and FADH2 become oxidized during the process.
– The electrons are passed along the chain of molecules in a series of redox reactions until the most electronegative molecule (O2) accepts them and become reduced.
How Is the Electron Transport Chain Organized?
- ETC proteins are organized into four large multiprotein complexes (called complex I–IV) plus cofactors essential to the catalytic functions of certain enzymes.
- Q and the protein cytochrome c transfer electrons between complexes.
- The folding of the inner membrane to form cristae increases surface area, providing space for thousands of copies of the ETC chain in each mitochondrion.
What is the function of the ETC?
- The ETC does not make ATP directly.
- The ETC eases the fall of electrons from food to oxygen – breaking a large free energy drop by using a series of smaller steps to release energy in manageable amounts.
- The ETC is coupled with another process (chemiosmosis) which synthesizes ATP.
The ETC Serves as Proton Pumps
• The ETC pumps protons from the mitochondrial matrix to the intermembrane space.
– Proton pumping is done by complexes I, III, and IV.
– Q also carries protons across the membrane.
• The ETC acts as an energy converter that uses the exergonic movement of electrons to pump H+into the intermembrane space
The Chemiosmotic Hypothesis
• The difference in concentration of H+on opposite sides of the inner mitochondrial matrix provides the power source for chemiosmosis (called the proton-motive force)
– This electrochemical gradient can also be thought of as a pH gradient.
– The H+ has a tendency to move back across the membrane, diffusing down the gradient.
• Chemiosmosis is a process in which energy stored in the form of a H+ gradient across a membrane can be used to drive cellular work (in this case, the endergonic process of ATP synthesis).
ATP Synthase Structure
- ATP synthase is an enzyme complex used in chemiosmosis to make (synthesize) ATP.
- In eukaryotes, many copies of this protein complex are found on the inner mitochondrion membrane that forms the cristae.
• ATP synthase consisting of two components:
– A membrane-bound, protontransporting base (F0 unit)
– An ATPase “knob” (F1 unit)
- The units are connected by a rotor, which spins the F1 unit, and a stator, which interacts with the spinning F1 unit.
- ATP synthase structures provide the only route through the membrane for H+
- Protons flowing through the F0unit spin the rotor.
- As the F1 unit spins, its subunits change shape, and catalyze the phosphorylation of ADP to ATP.
ATP Yeild
The theoretical yield is 38 ATP. In reality, this maximum is never reached – some potential reasons:
– Losses (e.g. leaky membranes)
– Costs of moving pyruvate and ADP into the mitochondrial matrix
– Costs of moving ATP out into the cytosol for use by the cell
– Types of transport molecules used to move electrons across membranes for carrier molecules, such as NADH
• Thus, current estimates suggest ~ 29-32 ATP molecules produced per glucose molecule during cell respiration.
The efficiency of cellular respiration:
- Roughly 34% of the potential chemical energy in glucose can be transferred to ATP. This estimate will change slightly based on different cellular conditions.
- The mechanisms of cellular respiration carried out by the mitochondria are quite efficient in their energy transforming capacities. The most efficient automobile converts only ~25% of the energy stored in gasoline into mechanical energy that moves the car.
- The rest of the energy stored in glucose is lost as heat during cellular respiration.
Aerobic and Anaerobic Respiration
• Eukaryotes (and many prokaryotes)
– use oxygen as a final electron acceptor in the electron transport chain of aerobic respiration.
• Some prokaryotes (especially in oxygen-poor environments):
– use other electron acceptors in the process of anaerobic respiration.
• Sulfate-reducing marine bacteria use the sulfate ion as the final ETC acceptor.
– Certain pathways (such as fermentation) allows glycolysisto continue when the lack of an electron acceptor (i.e. oxygen) shuts down electron transport chains.
Oxygen as a Final Electron Acceptor
• Oxygen
– most effective electron acceptor due to high electronegativity.
• Large difference between the potential energy of NADH (and FADH2) and O2 electrons allows the generation of a large proton-motive force for ATP production.
– ‘Proton-motive’ force refers to the proton gradient driving chemiosmosis
– Movement of electrons from NADH (and FADH2) to O2 is a highly exergonic reaction (remember: an electron loses potential energy when it shifts to a more electronegative atom).
• Cells that don’t use oxygen as an electron acceptor cannot generate such a large potential energy difference and make less ATP than cells that use aerobic respiration.
