BIOL 2107 Chapter 7

Fermentation, Aerobic, and Anaerobic Respiration

• The breakdown of organic molecules is exergonic

• Fermentation - partial degradation of sugars that occurs without oxygen

• Aerobic Respiration - consumes organic molecules and oxygen and yields ATP

• Anaerobic Respiration - consumes compounds other than oxygen

Cellular Respiration

• Cellular Respiration - includes both aerobic and anaerobic processes but is often used to refer to aerobic respiration

  • It is helpful to trace cellular respiration (CR) with the sugar glucose

Transfer of Electrons

• The transfer of electrons during chemical reactions releases energy stored in organic molecules

  • This released energy is ultimately used to synthesize ATP

Redox Reactions

• Redox Reactions - chemical reactions that transfer electrons between reactants

• Oxidation - a substance loses electrons, or is oxidized

• Reduction - a substance gains electrons, or is reduced

• Reducing Agent - electron donor

• Oxidizing Agent - electron acceptor

Oxidation of Organic Fuel Molecules During Cellular Respiration

• During CR, fuel (such as glucose) is oxidized, and oxygen is reduced

• Organic molecules with an abundance of hydrogen, like carbs and fats, are excellent fuels

• As hydrogen (with its electron) is transferred to oxygen, energy is released that can be used in ATP synthesis

Stepwise Energy Harvest via NAD+ and the ETC

• In CR, glucose and other organic molecules are broken down in a series of steps

• Electrons from organic compounds are usually first transferred to NAD+, a coenzyme

• As an electron acceptor, NAD+ functions as an oxidizing agent during CR

• Each NADH represents stored energy that is tapped to synthesize ATP

• Dehydrogenases - enzymes that facilitate the transfer of two electrons and one hydrogen ion to NAD+, forming NADH

  • One hydrogen ion is released in this process

• NADH passes the electrons to the ETC

• Electrons are passes to increasingly electronegative carrier molecules down the chain through a series of redox reactions

• Electron transfer to oxygen occurs in a series of energy-releasing steps instead of one explosive reaction

• The energy yielded is used to regenerate ATP

The Stages of Cellular Respiration

• Harvesting energy from glucose has three stages

  • Glycolysis breaks down glucose into two molecules of pyruvate in the cytosol

  • Pyruvate oxidation and the citric acid cycle completes the breakdown of glucose in the mitochondrial matrix

    • A small amount of ATP is formed in glycolysis and the citric acid cycle by substrate-level phosphorylation

  • Oxidative phosphorylation accounts for almost of the ATP synthesized and occurs in the inner membrane of the mitochondria

• For each molecule of glucose degraded to carbon dioxide and water by respiration, the cell makes up to about 32 molecules of ATP

Glycolysis Harvests Chemical Energy by Oxidizing Glucose to Pyruvate

• Glycolysis (“sugar splitting”) breaks down glucose into two molecules of pyruvate

• Two water molecules are produced as a by-product

• Glycolysis occurs in the cytoplasm and has two major phases

  • Energy investment phase

  • Energy payoff phase

• The net energy yield is 2 ATP plus 2 NADH per glucose molecule

• Glycolysis occurs whether or not oxygen is present

  • If oxygen is present, the energy stored in pyruvate and NADH can be extracted by pyruvate oxidation (PO), the citric acid cycle (CAC), and oxidative phosphorylation (OP)

After Pyruvate is Oxidized, the Citric Acid Cycle Completes the Energy-Yielding Oxidation of Organic Molecules

• In eukaryotic cells, if oxygen is present, pyruvate enters the mitochondrion to complete glucose oxidation

• Carbon dioxide is released and pyruvate is converted to acetyl coenyzme A (acetyl CoA) before entering the CAC

• This process yields 1 NADH per pyruvate (2 NADH per glucose molecule)

• Citric Acid Cycle (Krebs Cycle) - completes the breakdown of pyruvate to carbon dioxide (eight steps)

• Each turn of the cycle oxidizes molecules derived from one pyruvate molecule

• The cycle turns twice, generating 2 ATP, 6 NADH, and 2 FADH per glucose molecule

• 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

• The NADH and FADH2 produced by the cycle relay electrons extracted from food to the ETC

During Oxidative Phosphorylation, Chemiosmosis Couples Electron Transport to ATP Synthesis

• Glycolysis and the CAC produce four ATP molecules by substrate-level phosphorylation

• NADH and FADH2 account for most of the energy extracted from glucose

• These electrons carriers donate electrons to the ETC, which powers ATP synthesis via OP

The Pathway of Electron Transport

• The ETC is located in the inner membrane of the mitochondrion in eukaryote cells (plasma membrane in prokaryotes)

• Most of the chain’s components are complexes of proteins with electron carriers numbered I to IV

• Most of the proteins are cytochromes with attached heme groups that function as electron carriers

• Electron carriers alternate between reduced and oxidized states as they accept and donate electrons

• Electrons drop in free energy with each transfer between carriers down the chain to oxygen

• Breaking the free energy drop into small, stepwise reactions releases energy in manageable amounts

• Water is formed as a by-product when oxygen is reduced

• ATP is not produced directly by the function of the ETC

Chemiosmosis: The Energy-Coupling Mechanism

• The energy released by electron transfer through the ETC is used to pump hydrogen ions into the intermembrane space

• This establishes a hydrogen ion gradient across the inner mitochondrial membrane

  • Hydrogen ions diffuse down their concentration gradient into the mitochondrial matrix

• Hydrogen ions can only cross the inner membrane through protein complexes called ATP synthase

• ATP synthase - uses the exergonic flow of hydrogen ions to drive phosphorylation of ATP

• Chemiosmosis - the use of energy in a hydrogen ion gradient to drive cellular work

• The energy stored in a hydrogen ion gradient across a membrane couples the redox reactions of the ETC to ATP synthesis

  • The H2 gradient is referred to a proton-motive force, emphasizing its capacity to do work.

An Accounting of ATP Production by Cellular Respiration

• During CR, most energy flows in the following sequence:

  • glucose → NADH → ETC → proton-motive force → ATP

• About 34% of the energy in a glucose molecule is transferred to ATP making about 32 ATP; the rest is lost as heat

• The exact number of ATP molecules is uncertain because

  • Phosphorylation is not directly coupled to the redox reactions

  • ATP yield varies depending on whether electrons are carried by NAD+ or FAD

  • The proton-motive force generated by the ETC is also used for other kinds of work

Fermentation and Anaerobic Respiration Enable Cells to Produce ATP Without the Use of Oxygen

• Most of the ATP produced during CR is due to OP

• Without oxygen, the ETC will stop operating and OP will cease

• In the absence of oxygen, cells generate ATP using either anaerobic respiration or fermentation; both, of which, begin with glycolysis

• An ETC is used in anaerobic respiration but not in fermentation

• In anaerobic respiration, another electronegative molecule, like sulfate, is used as the final electron acceptor in the place of oxygen

• Fermentation allows continuous production of ATP by the substrate-level phosphorylation of glycolysis

Comparing Fermentation with Anaerobic and Aerobic Respiration

• All three processes use glycolysis (net ATP = 2) to oxidize glucose and other organic fuels to pyruvate

• In all three, NAD+ is the oxidizing agent that accepts electrons from food during glycolysis

• All three produce some ATP by substrate-level phosphorylation

• The mechanism of NADH oxidation differs

  • In fermentation, the final electron acceptor is an organic molecule such as pyruvate or acetaldehyde

  • CR transfers electrons from NADH to a carrier molecule in the ETC

Types of Fermentation

• Fermentation consists of glycolysis plus reactions that regenerate NAD+ for use in glycolysis

• NAD+ is regenerated by electron transfer from NADH to pyruvate or its derivatives

• Two common types are alcohol fermentation and lactic acid fermentation

Obligate and Facultative Anaerobes

• Obligate Anaerobes - use only fermentation or anaerobic respiration and cannot survive in