BI-157 Ch 9: Cellular Respiration and Fermentation

Examples of Life Needing Energy

• Living cells require energy from outside sources

• Some animals, such as the giraffe, obtain energy by eating plants (Herbivores), and some animals feed on other organisms that eat plants (Carnivores)

• Energy flows into an ecosystem as sunlight and leaves as heat (IR Radiation)

• Photosynthesis generates O2 and organic molecules, which are used in cellular respiration

• Cells use chemical energy (Respire) stored in organic molecules to generate ATP, which powers work

How catabolic pathways yield energy

• Catabolic pathways release stored energy by breaking down complex molecules 

• Electron transfer plays a major role in these pathways

  • These processes are central to cellular respiration

Forms of Catabolic Pathways and Production of ATP

• The breakdown of organic molecules is exergonic

• Fermentation is a partial degradation of sugars that occurs without O2

• Aerobic respiration consumes organic molecules and O2 and yields ATP

• Anaerobic respiration is similar to aerobic respiration but consumes compounds other
  than O2

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

• Although carbohydrates, fats, and proteins are all consumed as fuel, it is helpful to trace cellular respiration with the sugar glucose

   C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O + Energy (ATP + heat)

Redox Reactions: Oxidation and Reduction

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

• This released energy is ultimately used to synthesize ATP

Redox Reactions

• Chemical reactions that transfer electrons between reactants are called oxidation-reduction reactions, or redox reactions

• In oxidation, a substance LOSES electrons, or is oxidized

• In reduction, a substance GAINS electrons, or is reduced (the amount of positive charge is reduced)

  • The electron donor is called the reducing agent

  • The electron receptor is called the oxidizing agent

• Some redox reactions do not transfer electrons but change the electron sharing in covalent bonds

  • An example is the reaction between methane and O₂

Oxidation of Organic Fuel Molecules During Cellular Respiration

• During cellular respiration, the fuel (such as glucose) is oxidized, and O₂ is reduced

Stepwise Energy Harvest via NAD+ and the Electron Transport Chain

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

• Electrons from organic compounds are usually first transferred to NAD+, (Nicotinamide Adenine Dinucleotide) a coenzyme

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

• Each NADH (the reduced form of NAD+) represents stored energy that is tapped to synthesize ATP

Electron Transport Chain

• NADH passes the electrons to the Electron Transport Chain

• Unlike an uncontrolled reaction, the electron transport chain passes electrons in a series of steps instead of one explosive (less efficient) reaction

• O2 pulls electrons down the chain in an energy-yielding tumble

• The energy yielded is used to regenerate ATP from ADP or AMP

The Stages of Cellular Respiration: A Preview

• Harvesting of energy from glucose has three stages

  • Glycolysis breaks down glucose into two molecules of pyruvate

  • Pyruvate Oxidation and The Citric Acid (Krebs) Cycle completes the breakdown of glucose

  • Oxidative phosphorylation (Electron Transport and Chemiosmosis) accounts for most of the ATP synthesis

• The process that generates most of the ATP is called Oxidative Phosphorylation because it is powered by redox reactions

• Oxidative phosphorylation accounts for almost 90% of the ATP generated by cellular respiration

• A smaller amount of ATP is formed in glycolysis and the citric acid cycle by Substrate-Level Phosphorylation

• For each molecule of glucose degraded to CO2 and water by respiration, the cell makes up to 32 molecules of ATP

Glycolysis

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

• Glycolysis occurs in the cytoplasm and has two major phases

1. Energy investment phase

2. Energy payback/payoff phase

Glycolysis occurs whether or not O₂ is present

Pyruvate with O₂

After pyruvate is oxidized, the citric acid cycle completes the energy-yielding oxidation of organic molecules

• In the presence of O₂, pyruvate enters the mitochondrion (in eukaryotic cells) where the oxidation of glucose is completed

Oxidation of Pyruvate to Acetyl CoA

• Before the citric acid cycle can begin, pyruvate must be converted to acetyl Coenzyme A (Acetyl CoA), which links glycolysis to the citric acid cycle

• This step is carried out by a multienzyme complex that catalyses three reactions

The Citric Acid Cycle

• The Citric Acid Cycle, also called the Krebs Cycle, completes break down of pyruvate to CO₂

• The cycle oxidizes organic fuel derived from pyruvate, generating 1 ATP, 3 NADH, and 1 FADH₂ per turn (2 turns per initial glucose)

• The citric acid cycle has eight 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

• The NADH and FADH₂ produced by the cycle relay electrons extracted from food to the Electron Transport Chain by Oxidative Phosphorolation

Role of NADH and FADH₂ In Metabolism

During Oxidative Phosphorylation, Chemiosmosis couples Electron Transport to ATP synthesis

• Following glycolysis and the citric acid cycle, NADH and FADH₂ account for/hold most of the energy extracted from food (glucose)

• These two electron carriers donate electrons to the electron transport chain, which powers ATP synthesis via Oxidative Phosphorylation

The Pathway of Electron Transport

• The electron transport chain is embedded in the inner membrane (cristae) of the mitochondrion

  • Most of the chain’s components are proteins, which exist in multiprotein complexes

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

• Electrons drop in free energy as they go down the chain, gradually releasing energy, and are finally passed to O₂, forming H₂O

• Electrons are transferred from NADH or FADH₂ to the Electron Transport Chain

  • Electrons are passed through a number of proteins including cytochromes (each with an iron atom) to O₂

  • The electron transport 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

Chemiosmosis

• Electron transfer in the electron transport chain causes proteins to pump H+ from the mitochondrial matrix to the intermembrane space

  • H+ then moves back across the membrane, passing through the protein complex, ATP Synthase 

  • ATP synthase uses the exergonic flow of H+ to drive phosphorylation of ATP

  • This is an example of Chemiosmosis, the use of energy in a H+ gradient to drive cellular work

• 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 to as a proton-motive force, emphasizing its capacity to do work

ATP Production by Cellular Respiration

• During cellular respiration, most energy flows in this sequence: 

glucose → NADH → electron transport chain → proton-motive force → ATP

• About 34% of the energy in a glucose molecule is transferred to ATP during cellular respiration, making about 32 ATP

• There are several reasons why the number of ATP is not known exactly, mostly having to do with sufficient oxygen availability

Production of ATP in Cellular Respiration

• Most cellular respiration requires O2 to produce ATP

• Without O₂, the electron transport chain will cease to operate

• In that case, glycolysis couples with anaerobic respiration or fermentation to produce ATP

  • Anaerobic respiration uses an electron transport chain with a final electron acceptor other than O₂, for example sulfate

  • Fermentation uses substrate-level phosphorylation instead of an electron transport chain to generate ATP

Types of Fermentation

• Fermentation consists of glycolysis plus reactions that regenerate NAD+, which can be reused by glycolysis

• Two common types are alcohol fermentation and lactic acid fermentation:

  • In Alcohol Fermentation, pyruvate is converted to ethanol in two steps

    • The first step releases CO₂ 

    • The second step produces ethanol

    • Alcohol fermentation by yeast is used in brewing, winemaking, and baking

  • In Lactic Acid Fermentation, pyruvate is reduced by 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

Fermentation vs. Anaerobic and Aerobic Respiration

• All use glycolysis (net ATP = 2) to oxidize glucose and harvest chemical energy of food

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

• The processes have different mechanisms for oxidizing NADH: 

  • In fermentation, an organic molecule (such as pyruvate or acetaldehyde) acts as a final electron acceptor

  • In cellular respiration electrons are transferred to the electron transport chain (Oxygen is final electron acceptor) 

• Cellular respiration produces 32 ATP per glucose molecule; fermentation produces 2 ATP per glucose molecule 

• Obligate Anaerobes carry out fermentation or anaerobic respiration and cannot survive in the presence of O2

• Yeast and many bacteria are Facultative Anaerobes, meaning that they can survive using either fermentation or cellular respiration

  • In a facultative anaerobe, pyruvate is a fork in the metabolic road that leads to two alternative catabolic routes

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 (when photosynthesis evolved), so early prokaryotes likely used only glycolysis to generate ATP

• Glycolysis is a very ancient process

The Versatility of Catabolism

• Catabolic pathways funnel electrons from many kinds of organic molecules into cellular respiration

• Glycolysis accepts a wide range of carbohydrates

• Proteins must be digested to amino acids; amino groups can feed glycolysis or the citric acid cycle

• Fats are digested to glycerol (used in glycolysis) and fatty acids (used in generating acetyl CoA) 

• Fatty acids are broken down by beta oxidation and yield acetyl CoA

• An oxidized gram of fat produces more than twice as much ATP as an oxidized gram of carbohydrate

Biosynthesis (Anabolic Pathways)

• The body uses small molecules to build other substances

• These small molecules may come directly from food, from glycolysis, or from the citric acid cycle

Regulation of Cellular Respiration via Feedback Mechanisms

• Feedback inhibition is the most common mechanism for metabolic 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