9. Cellular Respiration and Fermentation (Biology 1)
How does the mitochondrion couple the electron transport and energy release to ATP Synthesis?
Chemiosmosis
Movement of electrons through Complex 1 and NADH (ETC)
1. Electrons transferred from NADH to first molecule of ETC in Complex 1. - This mooleculue is fMN 2. Next redox reaction the flavoprotein returns to its oxidized form as it passes electrons to an iron-sulfide protein 3. Iron-sulfide protein then passes the electrons to a compound called Q, a small hydrophobic molecules (only part of ETC not a protein) - Q is individually mobile and not in a particular complex 4. Most remaining electron carriers between Q and Oxygen are proteins called cytochromes - prosthetic group is a heme group (has an iron atom that accepts/donates electrons) 5. The last cytochrome, Cyt a3 passes its electrons to Oxygen which is very electronegative - Each oxygen also picks up a pair of hydrogen ions from the aqueous solution forming water.
Stages of Cellular Respiration
1. Glycolysis - breaks down glucose into 2 pyruvate 2. Pyruvate Oxidation - makes acetyl CoA 4. Citric Acid Cycle - completes the breakdown of glucose 3. Oxidative Phosphorylation - ATP synthesis
What are reasons why the number of ATP produced by cellular respiration is not exactly known?
1. Phosphorylation and the redox reactions are not directly coupled to each another (ratio of NADH to ATP is into a whole number) - not exactly sure how much H+must reenter to make ATP 2. ATP yield varies slightly dpending on the type of shuttle used to transprt electrons from the cytosol into the mitochondrion. - NADH (3 ATP) vs. FADH2 (1.5 ATP) 3. The use of the proton-motive force generated to drive other kinds of work can reduce the ATP yield.
Fermentation
A catabolic process that makes a limited amount of ATP from glucose without an electron transport chain and that produces a characteristic end product, such as ethyl alcohol or lactic acid. Fermentation is a partial degradation of sugars or other organic fuel that occurs without the use of oxygen.
ATP Yield per molecule of Glugcose
ATP profit when celluar respiration oxidizes 1 glucose to 6 CO2 molecules. Substrate-Level Phosphorylation: 4 ATP - 2 Glycolysis - 2 Citric Acid Cycle Oxidative Phosphorylation: 26-28 ATP
Where is the Carbon accounted for in Glycolysis?
All of the carbon originally present in glucose is accounted for in the 2 molecules of pyruvate no carbon is released as CO2 during glycolysis.
Chemiosmosis
An energy-coupling mechanism that uses energy stored in the form of a hydrogen ion gradient across a membrane to drive cellular work, such as the synthesis of ATP.
Cytochromes
An iron-containing protein that is a component of ETC in mitochondria and chloroplasts. Most remaining electron carriers between Q and Oxygen are proteins called cytochromes prosthetic group is a heme group (has an iron atom that accepts/donates electrons) The ETC has several types of cytochromes each named "cyt" with a letter and number to distinguish it as a different protein w/ slightly different electron-carrying heme group. The last cytochrome, Cyt a3 passes its electrons to Oxygen which is very electronegative
Biosynthesis
Anabolic Pathways The body uses small molecules to build other substances. Not all the organic molecules of food are destined to be oxidized as fuel to make ATP These small molecules may come directly from food, from glycolysis, or from the citric acid cycle.
The Evolutionary Significance of Glycolysis
Ancient prokaryotes are thought to have used glycolysis long before there was oxygen in the atmosphere Very little O2 was available in the atmosphere until about 2.7 billion years ago, so early prokaryotes likely used only glycolysis to generate ATP Glycolysis is a very ancient process
Glucose Cellular Respiration Equation
C6H12O6 + 6O2 --> 6CO2 + 6H2O + ATP
Why don't energy yielding foods just react with O2 if that is the cellular respiration formula
Carbs and Fats are reservoirs of electrons associated with hydrogen, often in the form of C-H bonds. Only the barrier of activation energy holds back the flood of electrons to a lower energy state. - without this barrier, a food substance would combine instantaneously with O2 Enzymes in your cells will lower the barrier of activation energy, allowing sugar to be oxidized in a series of steps - burning it outside of the body also does this. (w/ fire)
Obligate Anaerobes
Carry out fermentation or anaerobic respiration and cannot survive in the presence of O2
The Versatility of Catabolism
Catabolic pathways funnel electrons from many kinds of organic molecules into cellular respiration 1. Glycolysis accepts a wide range of carbohydrates 2. Proteins must be digested to amino acids; amino groups can feed glycolysis or the citric acid cycle - their amino group must be removed, given out as waste ammonia (NH3) in urine 3. Fats are digested too glycerol (used in glycolysis) and fatty acids (used in generating acetyl coA) 4. Fatty acids are broken down by beta oxidation and yield acetyl CoA 5. An oxidized gram of fat produces more than twice as much ATP as an oxidized gram of carbs - Fats make excellent fuels, due to their chemical structure and high energy level of their electrons C-H bonds, compared to carbohydrates
How does the ETC pump hydrogen ions?
Certain members of the ETC accept and release protons (H+) along with electrons - The aqueous solutions inside and surrounding the cell are a ready source of H+
The Citric Acid Cycle
Completes the breakdown of glucose by oxidizing Acetyl CoA of pyruvate to carbon dioxide. Cycle has 8 steps, each catalyzed by a specific enzyme The acetyl group of acetyl CoA joins the cycle by combining with oxaloacetate, forming citrate The next seven steps decompose the citrate back to oxaloacetate, making the process a cycle
Anaerobic Respiration
Does not use oxygen. Anaerobic respiration uses an ETC with a final electron acceptor other than O2 (for example S)
Cellular Respiration Equation: What is Oxidized? What is Reduced?
During cellular respiration, fuel (such as glucose) is oxidized and O2 is reduced Oxidized: Glucose == CO2 Reduced: O2 == H20
Where is the Electron Transport Chain?
ETC is a collection of molecules embedded in the inner membrane of the mitochondrion in eukaryotic cells - In prokaryotes these reside in the plasma membrane The folding of the inner membrane to form cristae increases its surface area, providing space for thousands of copies of each component of the ETC in a mitochondrion.
Chemiosmosis and ATP Synthase
Electron transfer in the ETC causes proteins to pump H+ from the mitochondrial matrix to the inter membrane space H+ then moves back across the membrane passing through the protein, ATP synthase ATP synthase uses the exergonic flow of H+ to drive phosphorylation of ATP The energy stored in a H+ gradient across a membrane couples the redox reactions of the electron transport chain to ATP Synthesis The H+ gradient is referred too as a proton motive force (emphasizing capacity to do work)
Do electrons lose potential energy when they are transferred to NAD+?
Electrons lose very little of their potential energy when they are transferred from glucose to NAD+. Each NADH molecule formed during respiration represents stored energy
Each NADH that transfers a pair of electrons from glucose to the ETC contributes... ?
Enough proton-motive force t generate a max of about 3 ATP
How does NAD+ trap electrons from glucose and other organic molecules?
Enzymes called dehydrogenases remove a pair oof hydrogen atoms (2 electrons, 2 protons) from the substrate, oxidizing it. The enzyme delivers the 2 electrons alone with 1 proton to its coenzyme NAD+ forming NADH The other proton is released as a hydrogen ion (H+) into the surrounding solution
How does the mitochondrial membrane generate and maintain the H+ gradient that drives ATP synthesis?
Establishing the H+ gradient is a major function of the ETC The chain is an energy converter that uses the exergonic flow of electrons from NADH and FADH2 t pump H+ across the membrane - from mitochondrial matrix to intermembrane space
The breakdown of molecules is...
Exergonic Compounds that can participate in exergonic reactions can act as fuels.
FADH2 and ETC
FADH2 adds its electrons within complex 2, at a lower energy level than NADH does Although NADH and FADH2 each donate an equivalent number of electrons (2) for oxygen reduction, the ETC chain provides about 1/3 less energy for ATP when the donor os FADH2.
Regulation of Cellular Respiration via Feedback Mechanisms
Feedback inhibition is the most common mechanism for control If ATP concentration begins to drop, respiration speeds up; when there is plenty of ATP, respiration slows down Control of catabolism is based mainly on regulating the activity of enzymes at strategic points in the catabolic pathway
Fermentation
Fermentation is a way of harvesting chemical energy without using oxygen or ETC Fermentation consists of glycolysis plus reactions that regenerate NAD+, which can be reused by glycolysis. Uses substrate-level phosphorylation instead of ETC to generate ATP (glycolysis)
During cellular respiration, most energy flows in this sequence:
Glucose - NADH - ETC - Proton-motive force - ATP
Glycolysis
Glycolysis (splitting of sugar) breaks down glucose into two molecules of pyruvate Glycolysis occurs in the cytoplasm and has two major phases - Energy Investment Phase - Energy Payoff Phase Glycolysis occurs whether or not O2 is present - if O2 is present the chemical energy stored in pyruvate and NADH can be extracted by pyruvate oxidation, the citric acid cycle and oxidative phosphorylation
Glycolysis and the Citric acid cycle connect to many other metabolic pathways
Glycolysis and the citric acid cycle are major intersections to various catabolic (breakdown) and anabolic (biosynthetic) pathways
Alcohol Fermentation
Glycolysis followed by the reduction of pyruvate to ethyl alcohol, regenerating NAD+ and releasing carbon dioxide. Pyruvate is converted to ethanol in two steps. 1. releases co2 from pyruvate converting it to acetaldehyde 2. acetaldehyde is then reduced by NADH to ethanol, regenerating NAD+ Many bacteria carry out alcoholic fermentation under anaerobic conditions. - Yeast carries out bth types and used in brewing, winemaking and baking. Co2 generated by yeast makes bread rise
Pyruvate Oxidation
Glycolysis releases less than a . quarter of the chemical energy in glucose; most remains in the 2 molecules of pyruvate. When O2 is present, pyruvate enters the mitochondrion in Eukaryotic cells where the oxidation of glucose is completed This happens in the Cytosol Pyruvate enters mitochondria via active transport and is converted to a compound called: Acetyl coA
Comparing Fermentation with Anaerobic and Aerobic Respiration
In all 3: - use glycolysis (net ATP = 2) to oxidize glucose and harvest chemical energy of food - NAD+ is the oxidizing agent that accepts the electrons during glycolysis The processes have different final electron acceptors: - an organic molecule (pyruvate/acetaldhyde) in fermentation and O2 is cellular respiration Cellular respiration produces 32 ATP p/ glucose molecule; fermentation produces 2 ATP p/ glucose molecule
Stepwise Energy Harvest vis NAD+ and ETC
In cellular respiration, glucose and other organic molecules are broken down in a series of steps each one catalyzed by an enzyme - if energy is released all at once it can't be harnessed efficiently At . key steps electrons are stripped from the glucose - each electron travels with a proton (a hydrogen atom) - The electrons are not transferred directly to oxygen, but instead are usually passed first to an electron carrier (NAD+ and FAD)
Why are organic molecules excellent fuel?
In general, organic molecules that have an abundance oof hydrogen are excellent fuels because their bonds are a source of "hilltop" electrons, whose energy may be released as the electrons "fall" down an energy gradient during their transfer to oxygen. The energy state of the electron changes in the transfers the H from glucose to Oxygen. In respiration the oxidation of glucose transfers electrons to a lower energy state, liberating energy that becomes available. So in general, we see fuels with multiple C-H bonds oxidized into products with multiple C-O bonds
Lactic Acid Fermentation
In lactic acid fermentation, pyruvate is reduced to NADH, forming lactate as an end product with no release of CO2 Lactic acid fermentation by some fungi and bacteria is used to make cheese and yogurt Human muscle cells use lactic acid fermentation to generate ATP when O2 is scarce
The Electron Transport Chain
In the inner membrane of the mitochondrion Most of the chains components are proteins, which exist in multi-protein complexes Electrons are transferred from NADH or FADH2 to the electron transport chain Electrons drop in free energy as they go down the chain and are finally passed to O2 forming H2O The chain generates no ATP directly - It breaks the large free-energy drop from food to O2 into smaller steps that release energy in manageable amounts
ATP synthase
Large protein that uses energy from H+ ions to bind ADP and a phosphate group together to produce ATP ATP synthase is a multi-subunit 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
Life is Work
Living cells require energy from outside sources Energy flows into an ecosystem as sunlight and leaves as heat. Photosynthesis generates O2 and organic molecules, which are used in cellular respiration Cells use chemical energy stored in organic molecules to regenerate ATP which powers work.
Catabolic Pathway
Metabolic pathways that release stored energy by breaking down complex molecules Transfer oof electrons from fuel molecules (like glucose) t other molecules plays a major role in these pathways.
Proteins in the Electron Transport Chain
Most components of the chain are proteins, which exist in multi-protein complexes numbered 1 - 4. Tightly bound to these are prosthetic groups, nonprotein complexes such as cofactors and coenzymes essential for the catalytic functions of certain enzymes.
Pyruvate Oxidation Reactants / Products
Must have O2 2 Pyruvates: - 2 molecules CO2 - 4 NADH - 2 Acetyl CoA
NAD+
NAD+ is a coenzyme and acts as an electron carrier As an electron acceptor, NAD+ functions as an oxidizing agent and gets reduced during cellular respiration. It is well suited as an electron carrier because it can cycle easily between its oxidized form, NAD+ and its reduced form NADH. Each NADH represents stored energy that is tapped to synthesize ATP NADH passes the electrons to the electron transport chain
NADH and Electron Transport Chain
NADH passes the electrons to the electron transport chain Unlike an uncontrolled reaction, the electron transport chain passes electroons in a series of steps instead of one explosive reaction. O2 pulls electrons down the chain in an energy yielding tumble The energy yielded is used to regenerate ATP Each "downhill" protein carrieer is more electronegative than is neighbor thus capable of oxidizing the uphill neighbor. Therefore electrons transferred from glucose to NAD+, fall down an energy gradient in the ETC to a far more stable location in the electronegative oxygen atom (water)
Do all redox reactions transfer electrons?
No, some redox reactions do not transfer electrons but change the electron sharing in covalent bonds An example is most things with O which is very electronegative - because O is so electronegative, it is one of the most powerful of all oxidizing agents.
Does Cellular respiration transfer all of glucoses energy to ATP?
Only about 34% of the energy in a glucose molecule is transferred to ATP during cellular respiration, making 32 ATP
Facultative Anaerobes
Organisms that can survive using either fermentation or cellular respiration - Yeast and many bacteria In facultative anaerobes, pyruvate is a fork in the metabolic road that leads to two alternative catabolic routes
Catabolic pathways yield energy by...
Oxidizing organic Fuels Several processes are central to cellular respiration and related pathways.
In Glycolysis ATP is produced through...
Substrate Level phosphorylation
Cellular Respiration
The catabolic pathways of aerobic and anaerobic respiration, which break down organic molecules and use an electron transport chain for the production of ATP Food (carbs, fats, proteins) provides the fuel for respiration, and the exhaust is carbon dioxide and water Organic Compounds + Oxygen = CO2 + H2O + Energy
Substrate-Level Phosphorylation
The enzyme-catalyzed formation of ATP by direct transfer of a phosphate group to ADP from an intermediate substrate in catabolism.
Proton-Motive Force
The potential energy stored in the form of an electrochemical gradient, generated by the pumping of hydrogen ions across biological membranes during chemiosmosis.
Oxidative Phosphorylation
The production of ATP using energy derived from the redox reactions of an electron transport chain
Redox Reactions
The transfer of electrons during chemical reactions releases energy stored in organic molecules - This released energy is ultimately used to synthesize ATP
How do Catabolic pathways do work?
They do not directly do work. Catabolism is linked to work by a chemical shaft - ATP. To keep working, the cell must re-generate its supply oof ATP from ADP and P.
How is Acetyl CoA formed in Pyruvate Oxidation
This step, linking glycolysis and the citric acid cycle, is carried out by a multi-enzyme complex that catalyzes three reactions 1. Pyruvate's carboxyl group fully oxidized given off as molecule of CO2 2. Remaining 2 carbon fragment oxidized and electrons transferred to NAD+ 3.
Cells Catabolic Processes
Through activity oof enzymes, a cell systematically degrades complex organic molecules that are reich in potential energy to simpler waste products that have less energy. Some of the energy taken out of chemical storage can be used to do work; the rest is dissipated as heat.
Citric Acid Cycle (Reactants and Products)
Uses 2 Pyruvate (1 Pyruvate each turn) Generates p/ glucose: - 2 ATP - 6 NADH - 2 FADH2 - 4 CO2
Oxidative Phosphorylation (step)
Uses energy released by the Electron Transport chain to power ATP synthesis. 1. Electron Transport Chain 2. Chemiosmosis Following glycolysis and citric acid cycle, NADH and FADH2 account for most of the energy extracted from food.
brown fat
a dark-colored adipose tissue with many blood vessels, involved in the rapid production of heat in hibernating animals and human babies. Made up of cells packed full of mitochondria. The inner mitochondrial membrane contains a channel protein called the uncoupling protein that allows protons to flow back down their concentration gradient without generating ATP Results in ongoing oxidation of fuel generating heat without ATP
Anaerobic respiration
similar to aerobic respiration but consumes compounds other than O2 and does not end ETC with water but with another electronegative molecule.
Glycolysis Reactant / Product
glucose (or intermediate such as fructose/lipid) produces - 2 net ATP (uses 2 produces 4) - 2 NADH - 2 H+ - 2 pyruvate - 2 H2O
Aerobic Respiration
the most efficient catabolic pathway in which oxygen is consumed as a reactant along with the organic fuel Consumes organic molecules and O2 and yields ATP
