BSC 1010C
General Biology I
Dr. Graeme Lindbeck
glindbeck@valenciacollege.edu

Cellular Respiration: Harvesting Chemical Energy

Outline

1. Cellular respiration and fermentation are catabolic (energy-yielding) pathways
2. Cells must recycle the ATP they spend for work
3. Redox reactions release energy when electrons move closer to electronegative atoms
4. Electrons "fall" from organic molecules to oxygen during cellular respiration
5. The "fall" of electrons during respiration is stepwise, via NAD+ and an electron transport chain
6. Respiration is a cumulative function of glycolysis, the Krebs cycle, and electron transport
7. Glycolysis harvests chemical energy by oxidizing glucose to pyruvate
8. The Krebs cycle completes the energy-yielding oxidation of organic molecules
9. The inner mitochondrial membrane couples electron transport to ATP synthesis
   1. The Pathway of Electron Transport
   2. Chemiosmosis: The Energy-Coupling Mechanism
   3. Biological Themes and Oxidative Phosphorylation
10. Cellular respiration generates many ATP molecules for each sugar molecule it oxidizes
11. Fermentation enables some cells to produce ATP without the help of oxygen
    1. Comparison of Fermentation and Respiration
    2. The Evolutionary Significance of Glycolysis
12. Glycolysis and the Krebs cycle connect to many other metabolic pathways
    1. Comparison of Fermentation and Respiration
    2. The Evolutionary Significance of Glycolysis
13. Feedback mechanisms control cellular respiration

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I. Cellular respiration and fermentation are catabolic (.energy-yielding) pathways

• Can be an anaerobic process.
• Results in a partial degradation of sugars.

• Most prevalent and efficient catabolic pathway.
• Is an exergonic process (DG = 2870 kJ/mol or - 686 kcal/mol).
• Can be summarized as:
  Organic compounds + Oxygen ® Carbon dioxide + Water + Energy (food)

  Carbohydrates, proteins and fats can all be metabolized as fuel, but cellular respiration is most often described as the oxidation of glucose:

  C₆H₁₂O₆ + 6O₂ ® 6CO₂ + 6H₂O + Energy (ATP + Heat)

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II. Cells must recycle the ATP they spend for work

ATP (adenosine triphosphate ) = Nucleotide with unstable phosphate bonds that the cell hydrolyzes for energy to drive endergonic reactions.

• The cell taps energy stored in ATP by enzymatically transferring terminal phosphate groups from ATP to other compounds.
• The compound receiving the phosphate group from ATP is said to be phosphorylated and becomes more reactive in the process.
• The phosphorylated compound loses its phosphate group as cellular work is performed; inorganic phosphate and ADP are formed in the process.
• Cells must replenish the ATP supply to continue cellular work. Respiration provides the energy to regenerate ATP from ADP and inorganic phosphate.

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III. Redox reactions release energy when electrons move closer to electronegative atoms

Oxidation-reduction reactions = Chemical reactions which involve a partial or complete transfer of electrons from one reactant to another; called redox reactions for short.

Oxidation = Partial or complete loss of electrons.

Reduction = Partial or complete gain of electrons.

Generalized Redox Reaction:

Electron transfer requires both a donor and acceptor, so when one reactant is oxidized the other is reduced

Not all redox reactions involve a complete transfer of electrons, but, instead, may just change the degree of sharing in covalent bonds. For example:

• Covalent electrons of methane are equally shared, because carbon and hydrogen have similar electronegativities.
• As methane reacts with oxygen to form carbon dioxide, electrons shift away from carbon and hydrogen to the more electronegative oxygen.
• Since electrons lose potential energy when they shift toward more electronegative atoms, redox reactions that move electrons closer to oxygen release energy.
• Oxygen is a powerful oxidizing agent because it is so electronegative.

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IV. Electrons "fall" from organic molecules to oxygen during cellular respiration

• Valence electrons of carbon and hydrogen lose potential energy as they shift toward electronegative oxygen.
• Released energy is used by cells to produce ATP.
• Carbohydrates and fats are excellent energy stores, because they are rich in C-H bonds.

Without the activation barrier, glucose would combine spontaneously with oxygen

• Igniting glucose provides the activation energy for the reaction to proceed; a mole of glucose yields 686 kcal (2870 kJ) of heat when burned in air.
• Cellular respiration does not oxidize glucose in one explosive step, as the energy could not be efficiently harnessed in a form available to perform cellular work.
• Enzymes lower the activation energy in cells, so glucose can be slowly oxidized in a stepwise fashion during glycolysis and Krebs cycle.

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V. The "fall" of electrons during respiration is stepwise, via NAD⁺ and an electron transport chain

Nicotinamide adenine dinucleotide (NAD⁺) = A dinucleotide that functions as a coenzyme in the redox reactions of metabolism.

• Found in all cells.
• Assists enzymes in electron transfer durinq redox reactions of metabolism.

Coenzyme = Small nonprotein organic molecule that is required for certain enzymes to function.

Dinucleotide = A molecule consisting of two nucleotides.

During the oxidation of glucose, NAD⁺ functions as an oxidizing agent by trapping energy-rich electrons from glucose or food. These reactions are catalyzed by enzymes called dehydrogenases, which:

• Remove a pair of hydrogen atoms (2 electrons and 2 protons) from substrate.
• Deliver the two electrons and one proton to NAD⁺.
• Release the remaining proton into the surrounding solution.

Electron transport chains convert some of the chemical energy extracted from food to a fon-n that can be used to make ATP. These transport chains:

• Are composed of electron-carrier molecules built into the inner mitochondrial membrane.
  Structure of this membrane correlates with its functional role (form fits function).
• Accept energy-rich electrons from reduced coenzymes (NADH and FADH₂); and during a series of redox reactions, pass these electrons down the chain to oxygen, the final electron acceptor. The electronegative oxygen accepts these electrons, along with hydrogen nuclei, to form water.
• Release energy from energy-rich electrons in a controlled stepwise fashion; a form that can be harnessed by the cell to power ATP production. If the reaction between hydrogen and oxygen during respiration occurred in a single explosive step, much of the energy released would be lost as heat, a form unavailable to do cellular work.

• Since electrons lose potential energy when they shift toward a more electronegative atom, this series of redox reactions releases energy.
• Each successive carrier in the chain has a higher electronegativity than the carrier before it, so the electrons are pulled downhill towards oxygen, the final electron acceptor and the molecule with the highest electronegtativity.

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VI. Respiration is a cumulative function of glycolysis, the Krebs cycle and electron transport

1. Glycolysis
2. Krebs Cycle
3. Electron transport chain (ETC) and oxidative phosphorylation

• Occurs in the cytosol.
• Partially oxidizes glucose(6C) into two pyruvate(3C) molecules.

• Occurs in the mitochondrial matrix.
• Completes glucose oxidation by breaking down a pyruvate derivative (acetyl CoA) into carbon dioxide.

• A small amount of ATP by substrate-level phosphorylation.
• NADH by transferring electrons from substrate to NAD⁺. (Krebs Cycle also produces FADH₂ by transferring electrons to FAD.)

• Is located at the inner membrane of the mitochondrion.
• Accepts energized electrons from reduced coenzymes (NADH and FADH₂) that are harvested during glycolysis and Krebs cycle. Oxygen pulls these electrons down the electron transport chain to a lower energy state.
• Couples this exergonic slide of electrons to ATP synthesis or oxidative phosphorylation. This process produces most (90%) of the ATP.

A small amount of ATP is produced directly by the reactions of glycolvsis and Krebs Cycle. This mechanism of producing ATP is called substrate-level phosphorylation.

Substrate-level phosphorylation = ATP production by direct enzymatic transfer of phosphate from an intermediate substrate in catabolism to ADP.

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VII. Glycolysis harvests chemical energy by oxidizing glucose to pyruvate

• Each reaction is catalyzed by specific enzymes dissolved in the cytosol.
• No CO₂ is released as glucose is oxidized to pyruvate; all carbon in glucose can be accounted for in the two molecules of pyruvate.
• Occurs whether or not oxygen is present.

1. Energy-investment phase. The cell uses ATP to phosphorylate the intermediates of glycolysis.
2. Energy-yielding phase. Two three-carbon intermediates are oxidized. For each glucose molecule entering glycolysis:
   1. A net gain of two ATPs is produced by substrate-level phosphorylation.
   2. Two molecules of NAD⁺ are reduced to NADH.

Energy-Investment Phase:

The energy investment phase includes five preparatory steps that split glucose in two. This process actually consumes ATP.

Step 1: Glucose enters the cell, and carbon six is phosphorylated. This ATP-coupled reaction:

• Is catalyzed by hexokinase. (Kinase is an enzyme involved in phosphate transfer.)
• Requires an initial investment of ATP.
• Makes glucose more chemically reactive.
• Produces glucose-6-phosphate. Since the plasma membrane is relatively impermeable to ions, addition of an electrically charged phosphate group traps the sugar in the cell.

Step 2: An isomerase catalyzes the rearrangement of glucose-6-phosphate to its isomer, fructose-6-phosphate.

Step 3: Carbon one of fructose-6-phosphate is phosphorylated. This reaction:

• Requires an investment of another ATP.
• Is catalyzed by phosphoftuctokinase, an allosteric enzyme that controls the rate of glycolysis.

Step 4: Aldolase cleaves the six-carbon sugar into two isomeric three-carbon sugars.

• This is the reaction for which glycolysis is named.
• For each glucose molecule that begins glycolysis, there are two product molecules for this and each succeeding step.

Step 5: An isomerase catalyzes the reversible conversion between the two three-carbon sugars. This reaction:

• Never reaches equilibrium because only one isomer, glycealdehyde phosphate, is used in the next step of glycolysis.
• Is thus pulled towards the direction of glyceraldehyde phosphate, which is removed as fast as it forms.
• Results in the net effect that for each glucose molecule, two molecules of glyceraldehyde phosphate process through glycolysis.

The energy-yielding phase occurs after glucose is split into two three-carbon sugars. During these reactions, sugar is oxidized, and ATP and NADH are produced.

Step 6: An enzyme catalyzes two sequential reactions:

1. Glyceraldehyde phosphate is oxidized and NAD⁺ is reduced to NADH + H⁺
   • This reaction is very exergonic and is coupled to the endergonic phosphorylation phase (DG = -10.3 kcal/mol).
   • For every glucose molecule, 2 NADH are produced.
2. Glyceraldehyde phosphate is phosphorylated on carbon one.
   • The phosphate source is inorganic phosphate, which is alwavs present in the cytosol.
   • The new phosphate bond is a high energy bond with even more potential to transfer a phosphate group than ATP.

Step 7: ATP is produced by substrate-level phosphorylation

• In a very exergonic reaction, the phosphate group with the high energy bond is transferred from 1,3-diphosphoglycerate to ADP.
• For each glucose molecule, two ATP molecules are produced. The ATP ledger now stands at zero as the initial debt of two ATP from steps one and three is repaid.

Step 8: In preparation for the next reaction, a phosphate group on carbon three is enzymatically transferred to carbon two.

Step 9: Enzymatic removal of a water

• Creates a bond between carbons one and two of the substrate.
• Rearranges the substrate's electrons, which transforms the remaining phosphate bond into an unstable bond.

Step 10: ATP is produced by substrate-level phosphorylation.

• In a highly exergonic reaction, a phosphate group is transferred from phosphoenolpyruvate (PEP) to ADP.
• For each glucose molecule, this step produces two ATP.

• Glucose has been oxidized into two pyruvate molecules.
• The process is exergonic (DG = -140 kcal/mol or -586 kJ/mol); most of the energy harnessed is conserved in the high-energy electrons of NADH and in the phosphate bonds of ATP.

VIII. The Krebs Cycle completes the energy-yielding oxidation of organic molecules

Most of the chemical energy originally stored in glucose still resides in the two pyruvate molecules produced by glycolysis. The fate of pyruvate depends upon the presence or absence of oxygen. If oxygen is present pyruvate enters the mitochondrion where it is completely oxidized by a series of enzyme-controlled reactions.

Formation of Acetyl CoA: The junction between glycolysis and the Krebs Cycle is the oxidation of pyruvate to acetyl CoA:

• Pyruvate molecules are translocated from the cytosol into the mitochondrion by a carrier protein in the mitochoiidrial membrane.
• This step is catalyzed by a multienzyme complex which:
  1. Removes from the group of pyruvate, changing it from three-carbon to a two-carbon compound. This is the first step where CO₂ is released.
  2. Oxidizes the two-carbon frayment to acetate, while reducing NAD⁺ to NADH. Since glycolysis produces two pyruvate molecules per glucose, there are two NADH molecules produced.
  3. Attaches coenzyme A to the acetyl group, forming acetyl CoA. This bond is unstable, making the acetyl group very reactive.

Krebs Cycle: The Krebs Cycle reactions oxidize the remaining acetyl fragments of acetyl CoA to CO₂. Energy released from this exerconic process is used to reduce coenzyme (NAD⁺ and FAD) and to phosphorylate ATP (substrate-level phosphorylation).

• A German-British scientist, Hans Krebs, elucidated this catabolic pathway in the 1930's.
• The Krebs cycle, which is also known as the citric acid cycle or TCA cycle, has eight enzyme-controlled steps that occur in the mitochondrial matrix.

For every turn of Krebs Cycle:

• Two carbons enter in the acetyl fragment of acetyl CoA.
• Two different carbons are oxidized and leave as CO₂.
• Coenzymes are reduced; three NADH and one FADH₂ are produced.
• One ATP molecule is produced b substrate-level phosphorylation.
• Oxaloacetate is regenerated.

For every glucose molecule split during glycolysis:

• Two acetyl fragments are produced.
• It takes two turns of Krebs Cycle to complete the oxidation of glucose.

Step 1: The unstable bond of acetyl CoA breaks, and the two-carbon acetyl group bonds to the four-carbon oxaloacetate to form six-carbon citrate.

Step 2: Citrate is isomerized to isocitrate.

Step 3: Two major events occur during this step:

• Isocitrate loses CO₂ leaving a five-carbon molecule.
• The five-carbon compound is oxidized and NAD⁺ is reduced

Step 4: A multienzyme complex catalyzes:

• Removal of CO₂
• Oxidation of the remaining four-carbon compound and reduction of NAD⁺.
• Attachment of CoA with a high energy succinyl CoA

Step 5: Substrate-level phosphorylation occurs in a series of enzyme catalyzed reactions:

• The high energy bond of succinyl-CoA breaks, and some energy is conserved as CoA is displaced by a phosphate group.
• The phosphate group is transferred to GDP to form GTP and succinate.
• GTP donates a phosphate group to ADP to form ATP.

Step 6: Succinate is oxidized to fumarate and FAD is reduced.

• Two hydrogens are transferred to FAD to form FADH₂.
• The dehydrogenase that catalyzes this reaction is bound to the inner mitochondrial membrane.

Step 7: Water is added to fumarate which rearranges its chemical bonds to form malate.

Step 8: Malate is oxidized and NAD⁺ is reduced.

• A molecule of NADH is produced.
• Oxaloacetate is regenerated to begin the cycle again.

Two turns of the Krebs Cycle produces two ATPs by substrate-level phosphorylation. However, most ATP output of respiration results from oxidative phosphorylation.

• Reduced coenzymes produced by the Krebs Cycle (6 NADH and 2 FADH₂ per glucose) carry high energy electrons to the electron transport chain.
• The ETC couples electron flow down the chain to ATP synthesis.

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IX. Mitochondrial membrane couples electron transport to ATP synthesis

Only a few molecules of ATP are produced by substrate-level phosphorylation:

• 2 net ATPs per glucose from glycolysis.
• 2 ATPs per glucose from the Krebs Cycle.

Most molecules of ATP are produced by oxidative phosphorylation.

• At the end of the Krebs Cycle, most of the energy extracted from glucose is in molecules of NADH and ADH2.
• These reduced coenzymes link glycolysis and the Krebs Cycle to oxidative phosphorylation by passing their electrons down the electron transport chain to oxygen. (Though the Krebs Cycle occurs only under aerobic conditions, it does not use oxygen directly. The ETC and oxidative phosphorylation require oxygen as the final electron acceptor.)
• This exergonic transfer of electrons down the ETC to oxygen is coupled to ATP synthesis.

The Pathway of Electron Transport

The electron transport chain is made of electron carrier molecules embedded in the inner mitochondrial membrane.

• Each successive carrier in the chain has a higher electronegativity than the carrier before it, so the electrons are pulled downhill towards oxygen, the final electron acceptor and the molecule with the highest electronegativity.
• Except for ubiquinone (Q), most of the carrier molecules are proteins and are tightly bound to prosthetic groups (nonprotein cofactors).
• Prosthetic groups alternate between reduced and oxidized states as they accept and donate electrons.

Protein Electron Carriers	Prosthetic Group
flavoproteins	flavin mononucleotide (FMN)
iron-sulfur proteins	iron and sulfur
cytochromes	heme group

Heme group = Prosthetic group composed of four organic rings surrounding a single iron atom.

Cytochrome = Type of protein molecule that contains a heme prosthetic group and that functions as an electron carrier in the electron transport chains of mitochondria and chloroplasts.

• There are several cytochromes, each a slightly different protein with a heme group.
• It is the iron of cytochromes that transfers electrons.

Sequence of Electron Transfers Along the Electron Transport Chain:

NADH is oxidized and flavoprotein is reduced as high energy electrons from NADH are transferred to FMN.

¯

Flavoprotein is oxidized as it passes electrons to an iron-sulfurprotein, Fe-S.

¯

Iron-sulfur protein is oxidized as it passes electrons to ubiquinone (Q).

¯

Ubiquinone passes electrons on to a succession of electron carriers, most of which are cytochromes.

¯

Cyt a3, the last cytochrome passes electrons to molecular oxygen, O2.

As molecular oxygen is reduced it also picks up wo protons from the medium to form water. For every two NADH's, one O₂ is reduced to two H₂O molecules.

• FADH₂ also donates electrons to the electron transport chain, but those electrons are added at a lower energy level than NADH.
• The electron transport chain does not make ATP directly. It generates a proton gradient across the inner mitochondrial membrane, which stores potential energy that can be used to phophorylate ADP.

Chemiosmosis: The Energy-Coupling Mechanism

The mechanism for coupling exergonic electron flow from the oxidation of food to the endergonic process of oxidative pliosphorylation is chemiosmosis.

Chemiosmosis = The coupling of exergonic electron flow down an electron transport chain to endergonic ATP production by the creation of a proton gradient across a membrane. The proton gradient drives ATP synthesis as protons diffuse back across the membrane.

• Proposed by British biochemist, Peter Mitchell (1961).
• The term chemiosmosis emphasizes a coupling between (1) chemical reactions (phosphorylation) and (2) transport processes (proton transport).
• Process involved in oxidative phosphorylation and photophosphorylation.

The site of oxidative phosphorylation is the inner mitochondrial membrane, which has many copies of a protein complex, ATP synthase. This complex:

• Is an enzyme that makes ATP.
• Uses an existing proton gradient across the inner mitochondrial membrane to power ATP synthesis.

Cristae or infoldings of the inner mitochondrial membrane increase the surface area available for chemiosmosis to occur.

Membrane structure correlates with the prominent functional role membranes play in chemiosmosis:

• Using energy from exergonic electron flow, the electron transport chain creates the proton gradient by pumping H⁺s from the mitochondrial matrix, across the inner membrane to the intermembrane space.
• This proton gradient is maintained, because the membrane's phospholipid bilayer is impermeable to H⁺s and prevents them from leaking back across the membrane by diffusion.
• ATP syntheses use the potential energy stored in a proton gradient to make ATP by allowing H⁺ to diffuse down the gradient, back across the membrane. Protons diffuse through the ATP synthase complex, which causes the phosphorylation of ADP.

How does the electron transport chain pump hydrogen ions from the matrix to the intermembrane space? The process is based on spatial organization of the electron transport chain in the membrane. Note that:

• Some electron carriers of the transport chain transport only electrons.
• Some electron carriers accept and release protons along with electrons. These carriers are spatially arranged so that protons are picked up from the matrix and are released into the intermembrane space.

Most of the electron carriers are organized into three complexes: 1) NADH dehydrogenase complex; 2) cytochrome b-c₁ complex; and 3) cytochrome oxidase complex.

• Each complex is an asymmetric particle that has a specific orientation in the membrane.
• As complexes transport electrons, they also harness energy from this exergonic process to pump protons across the inner mitochondrial membrane.

Mobile carriers transfer electrons between complexes. These mobile carriers are:

1. Ubiquinone (Q). Near the matrix, Q accepts electrons from the NADH dehydrogenase complex, diffuses across the lipid bilayer, and passes electrons to the cytochrome b-c_l complex.
2. Cytochrome c (Cyt c). Cyt c accepts electrons from the cytochrome b-c_l complex and conveys them to the cytochrome oxidase complex.

When the transport chain is operating:

• The pH in the intermembrane space is one or two pH units lower than in the matrix.
• The pH in the intermembrane space is the same as the pH of the cytosol because the outer mitochondrial membrane is permeable to protons.

The H⁺ gradient that results is called a proton-motive force to emphasize that the gradient represents potential energy.

Proton motive force = Potential energy stored in the proton gradient created across biological membranes that are involved in chemiosmosis.

This force is an electrochemical gradient with two components:

1. Concentration gradient of protons (chemical gradient).
2. Voltage across the membrane because of a higher concentration of positively charged protons on one side (electrical gradient).

It tends to drive protons across the membrane back into the matrix.

Chemiosmosis couples exergonic chemical reactions to endergonic H⁺ transport, which creates the proton-motive force used to drive cellular work, such as:

• ATP synthesis in mitochondria (oxidative phosphorylation). The energy to create the proton gradient comes from the oxidation of glucose and the ETC.
• ATP synthesis in chloroplasts (photophosphorylation). The energy to create the proton gradient comes from light trapped during the energy-capturing reactions of photosynthesis.
• ATP synthesis, transport processes and rotation of flagella in bacteria. The proton gradient is created across the plasma membrane. Peter Mitchell first postulated chemiosmosis as an energy-coupling mechanism based on experiments with bacteria.

Biological Themes and Oxidative Phosphorylation

The working model of how mitochondria harvest the energy of food, illustrates many of the text's integrative themes in the study of life:

• Energy conversion and utilization.
• Emergent properties. Oxidative phosphorylation is an emergent property of the intact mitochondrion that uses a precise interaction of molecules.
• Correlation of structure and function. The chemiosmotic model is based upon the spatial arrangement of membrane proteins.
• Evolution. In an effort to reconstruct the origin of oxidative phosphorylation and the evolution of cells, biologists compare similarities in the chemiosmotic machinery of mitochondria to that of chloroplasts and bacteria.

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X. Cellular respiration generates many ATP molecules for each sugar molecule it oxidizes

Glucose Þ NADH Þ electron transport chain Þ proton motive force Þ ATP

The net ATP yield from the oxidation of one glucose molecule to six carbon dioxide molecules can be estimated by adding:

1. ATP produced directly by substrate-level phosphorylation during glycoloysis and the Krebs cycle.
   • A net of two ATPs is produced during glycolysis. The debit of two ATPs used during the investment phase is subtracted from the four ATPs produced during the energy-yielding phase. Two ATPs are produced during the Krebs cycle.

2. ATP produced when chemiosmosis couples electron transport to oxidative phosphorylation.
   • The electron transport chain creates enough proton-motive force to produce a maximum of three ATPs for each electron pair that travels from NADH to oxygen. The average yield is actually between two and three ATPs per NADH (2.7).
   • FADH2 produced during the Krebs Cycle is worth a maximum of only two ATPs, since it donates electrons at a lower energy level to the electron transport chain.
   • In most eukaryotic cells, the ATP yield is lower from an NADH produced during glycolysis. The mitochondrial membrane is impermeable to NADH, so its electrons must be carried across the membrane in another molecule. These electrons are received inside the mitochondrion by FAD, a process which downgrades the energy level of those electrons.

Maximum ATP yield for each glucose oxidized during cellular respiration:

• This tally only estimates the ATP yield from respiration. Some variables that affect ATP yield include:
  Þ The proton-motive force may be used to drive other kinds of cellular work such as active transport.
  Þ The total ATP yield is inflated (-10%) by rounding off the number of ATPs produced per NADH to three.

• Estimated efficiency in eukaryotic cells is about 38%.
• Energy lost in the process is released as heat.

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XI. Fermentation enables some cells to produce ATP without the help of oxygen

Aerobic = (Aer = air; bios = life) Existing in the presence of oxygen.

Anaerobic = (An = without; aer = air) Existing in the absence of free oxygen.

Fermentation = The anaerobic catabolism of organic nutrients.

Glycolysis oxidizes glucose to two pyruvate molecules, and the oxidizing agent for this process is NAD⁺, not oxygen.

• Some energy released from the exergonic process of glycolysis drives the production of 2 net ATPs by substrate-level phosphorylation.
• Glycolysis produces a net of 2 ATPs whether conditions are aerobic or anaerobic.

Aerobic conditions: Pyruvate is oxidized further, and more ATP is made as NADH passes electrons removed from glucose to the electron transport chain. NAD⁺ is regenerated in the process.

Anaerobic conditions: Pyruvate is reduced, and NAD⁺ is regenerated. This prevents the cell from depleting the pool of NAD⁺, which is the oxidizing agent necessary for glycolysis to continue. No additional ATP is produced.

Fermentation recycles NAD⁺ from NADH. This process consists of anaerobic glycolysis plus subsequent reactions that regenerate NAD+ by reducing pyruvate. Two of the most common types of fermentation are (1) alcohol fermentation and (2) lactic acid fermentation.

1. Alcohol Fermentation

Pyruvate is converted to ethanol in two steps:

1. a. Pyruvate loses carbon dioxide and is converted to the two-carbon compound acetaldehyde.
2. b. NADH is oxidized to NAD⁺ and acetaldehyde is reduced to ethanol.

Many bacteria and yeast carry out alcohol fermentation under anaerobic conditions.

2. Lactic Acid Fermentation

• Commercially important products of lactic acid fermentation include cheese and yogurt.
• When oxygen is scarce, human muscle cells switch from aerobic respiration to lactic acid fermentation. Lactate accumulates, but it is gradually carried to the liver where it is converted back to pyruvate when oxygen becomes available. Accumulation of lactic acid causes the muscles spasms we call cramps.

Comparison of Fermentation and Respiration

The anaerobic process of fermentation and aerobic process of cellular respiration are similar in that both metabolic pathways:
• use glycolysis to oxidize glucose and other substrates to pyruvate, producing a net of 2 ATPs by substrate phosphorylation.
• use NAD⁺ as the oxidizing agent that accepts electrons from food during glycolysis.

Fermentation and cellular respiration differ in:

• how NADH is oxidized back to NAD⁺.
  ÞRecall that the oxidized form, NAD⁺ is necessary for glycolysis to continue.
  Þ During fermentation, NADH passes electrons to pyruvate or some derivative. As pyruvate is reduced, NADH is oxidized to NAD⁺. Electrons transferred from NADH to pyruvate or other substrates are not used to power ATP production.
  Þ During cellular respiration, the stepwise electron transport from NADH to oxygen not only drives oxidative phosphorylation, but regenerates NAD⁺ in the process.

• the final electron acceptor.
  
  Þ In fermentation, the final electron acceptor is pyruvate (lactic acid fermentation), acetaldehyde (alcohol fermentation), or some other organic molecule.
  Þ In cellular respiration, the final electron acceptor is oxygen.

• the amount of energy harvested.
  
  Þ During fermentation, energy stored in pyruvate is unavailable to the cell.
  Þ Cellular respiration yields 18 times more ATP per glucose molecule than does fermentation. The higher energy yield is a consequence of the Krebs Cycle which completes the oxidation of glucose and thus taps the che mical bond energy still stored in pyruvate at the end of glycolysis.

• their requirement for oxygen.
  
  Þ Fermentation does not require oxygen.
  Þ Cellular respiration occurs only in the presence of oxygen.

Organisms can be classified based upon the effect oxygen has on growth and metabolism.

Strict (obligate) aerobes = Organisms that require oxygen for growth and as the final electron acceptor for aerobic respiration.

Strict (obligate) anaerobes = Microorganisms that only grow in the absence of oxygen and are, in fact, poisoned by it.

Facultative anaerobes = Organisms capable of growth in either aerobic or anaerobic environments.

• Yeasts, many bacteria and mammalian muscle cells are facultative anaerobes.
• Can make ATP by fermentation in the absence of oxygen or by respiration in the presence of oxygen.
• Glycolysis is common to both fermentation and respiration, so pyruvate is a key point in catabolism.

The Evolutionary Significance of Glycolsysis

The first prokaryotes probably produced ATP by glycolysis. Evidence includes the following:

• Glycolysis does not require oxygen, and the oldest known bacterial fossils date back to three-and-a-half billion years ago when oxygen was not present in the atmosphere.
• Glycolysis is the most widespread metabolic pathway, so it probably evolved early.
• Glycolysis occurs in the cytosol and does not require membrane-bound organelles. Eukaryotic cells with organelles probably evolved about two billion years after prokaryotic cells.

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XII. Glycolysis and the Krebs cycle connect to many other metabolic pathways

The Versatility of Catabolism

Respiration can oxidize organic molecules other than glucose to make ATP. Organisms obtain most calories from fats, proteins, disaccharides and polysaccharides. These complex molecules must be enzymatically hydrolyzed into simpler molecules or monomers that can enter an intermediate reaction of glycolysis or the Krebs cycle.

Glycolysis can accept a wide range of carbohydrates for catabolism.

• Starch is hydrolyzed to glucose in the digestive tract of animals.
• In between meals, the liver hydrolyzes glycogen to glucose.
• Enzymes in the small intestine break down disaccharides to glucose or other monosaccharides.

Proteins are hydrolyzed to amino acids.

• Organisms synthesize new proteins from some of these amino acids.
• Excess amino acids are enzymatically converted to intermediates of glycolysis and the Krebs cycle. Common intermediates are pyruvate, acetyl CoA and a-ketoglutarate.
• This conversion process deaminates amino acids, and the resulting nitrogenous wastes are excreted.

Fats are excellent fuels because they are rich in hydrogens with high energy electrons. Oxidation of one grain of fat produces twice as much ATP as a gram of carbohydrate.

• Fat sources may be from the diet or from storage cells in the body.
• Fats are digested into glycerol and fifty acids.
• Glycerol can be converted to glyceraldehyde phosphate, an intermediate of glycolysis.
• Most energy in fats is in fatty acids, which are converted into acetyl CoA by beta oxidation. The resulting two-carbon fragments can enter the Krebs cycle.

Biosynthesis (Anabolic Pathways)

Some organic molecules of food provide the carbon skeletons or raw materials for the synthesis of new macromolecules.

• Some organic monomers from digestion can be used directly in anabolic pathwavs.
• Some precursors for biosvntliesis do not come directly from digested food. but instead come from glycolysis or Krebs cycle intermediates which are diverted into anabolic pathways.
• These anabolic pathways consume ATP produced by catabolic pathways of glycolysis and respiration.
• Glycolysis and the Krebs cycle are metabolic interchanges that can convert one type of macromolecule to another in response to the cell's metabolic demands.

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XIII. Feedback mechanisms control cellular respiration

• Anabolic pathways are switched off when their products are in ample supply. The most common mechanism of control is feedback inhibition.
• Catabolic pathways, such as glycolysis and Krebs cycle, are controlled by regulating enzyme activity at strategic points.

A key control point of catabolism is the third step of glycolysis, which is catalyzed by an allosteric enzyme, phosphoftuctokinase.

• The ratio of ATP to ADP and AMP reflects the energy status of the cell, and phosphofructokinase is sensitive to changes in this ratio.
• Citrate (produced in Krebs cycle) and ATP are allosteric inhibitors of phosphofructokinase, so when their concentrations rise, the enzyme slows glycolysis. As the rate of glycolysis slows, Krebs cycle also slows since the supply of acetyl CoA is reduced. This synchronizes the rates of glycolysis and Krebs cycle.
• ADP and AMP are allosteric activators for phosphofructokinase, so when their concentrations relative to ATP rise, the enzyme speeds up glycolysis which speeds up the Krebs cycle. There are other allosteric enzymes that also control the rates of glycolysis and the Krebs cycle.

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