Overview: Life Is Work

                  Living cells require energy from outside sources

                  Some animals, such as the chimpanzee, obtain energy by eating plants, and some animals feed on other organisms that eat plants

                  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

 

Concept 9.1: Catabolic pathways yield energy by oxidizing organic fuels

                  Several processes are central to cellular respiration and related pathways

 

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

The Principle of Redox

                  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+, 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

                  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 reaction

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

                  The energy yielded is used to regenerate ATP

 

The Stages of Cellular Respiration: A Preview

                  Harvesting of energy from glucose has three stages

               Glycolysis (breaks down glucose into two molecules of pyruvate)

               The citric acid cycle (completes the breakdown of glucose)

               Oxidative phosphorylation (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

 

Concept 9.2: Glycolysis harvests chemical energy by oxidizing glucose to pyruvate

                  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

 

Concept 9.3: 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 the break down of pyruvate to CO2

                  The cycle oxidizes organic fuel derived from pyruvate, generating 1 ATP, 3 NADH, and 1 FADH2 per turn

 

                  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

 

Concept 9.4: During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesis

                  Following glycolysis and the citric acid cycle, NADH and FADH2 account for most of the energy extracted from food

                  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 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 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: The Energy-Coupling Mechanism

                  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 proton, 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

 

An Accounting of 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

 

Concept 9.5: Fermentation and anaerobic
respiration enable cells to produce ATP without the use of oxygen

                  Most cellular respiration requires O2 to produce ATP

                  Without O2, the electron transport chain will cease to operate

                  In that case, glycolysis couples with fermentation or anaerobic respiration 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, with the first releasing CO2

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

                  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

 

Comparing Fermentation with 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 final electron acceptors: an organic molecule (such as pyruvate or acetaldehyde) in fermentation and O2 in cellular respiration

                  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, so early prokaryotes likely used only glycolysis to generate ATP

                  Glycolysis is a very ancient process

 

Concept 9.6: Glycolysis and the citric acid cycle connect to many other metabolic pathways

                  Gycolysis and the citric acid cycle are major intersections to various catabolic and anabolic pathways

 

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 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