Cellular Respiration and Fermentation

Chapter 9

Dr. Wendy Sera Houston Community College Biology 1406

Chapter 9 Concepts

1. Catabolic pathways yield energy by oxidizing organic fuels. 2. Glycolysis harvests chemical energy by oxidizing glucose to pyruvate.

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

4. During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesis.

5. Fermentation and anaerobic respiration enable cells to produce ATP without the use of oxygen. 6. Glycolysis and the citric acid cycle connect to many other metabolic pathways.

1 Life Is Work

. Living cells require energy from outside sources . Some animals, such as the giraffe, obtain energy by eating plants (herbivores/primary consumers), and some animals feed on other organisms that eat plants (carnivories/secondary consumers)

Figure 9.1—How do these leaves power the work of life for this giraffe?

Energy Flow and Nutrient Cycling in Ecosystems . Energy flows into an ecosystem as sunlight and leaves as heat

. Photosynthesis generates O2 and organic molecules (e.g., glucose), which are used in cellular respiration

. Cells use chemical energy stored in organic molecules to regenerate ATP, which powers cellular work

Figure 9.2— Light Energy flow energy and chemical ECOSYSTEM recycling in ecosystems Photosynthesis in chloroplasts Organic CO + H O O 2 2 molecules+ 2 Cellular respiration in mitochondria

ATP powers ATP most cellular work

Heat energy

2 BioFlix: The Carbon Cycle

Concept 9.1: Catabolic pathways yield energy by oxidizing organic fuels . Catabolic pathways release stored energy by breaking down complex molecules . Electron transfers (i.e., redox reactions) play a major role in these pathways . These processes are central to cellular respiration

Catabolic Pathways and Production of ATP

. The breakdown of organic molecules is exergonic . Fermentation is a partial degradation of sugars

that occurs without O2 and yields very little ATP . Aerobic respiration consumes organic molecules

and O2 and yields ATP . Anaerobic respiration is similar to aerobic respiration, but consumes compounds other

than O2 and yields same amount of ATP as aerobic respiration

3 . 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)

4 becomes oxidized (loses electron)

becomes reduced (gains electron)

becomes oxidized

becomes 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 (or the burning of any fuel!)

5 Figure 9.3—Methane combustion as an energy-yielding redox reaction Reactants Products becomes oxidized

becomes reduced

Methane Oxygen Carbon dioxide Water (reducing (oxidizing agent) agent)

Oxidation of Organic Fuel Molecules During Cellular Respiration . During cellular respiration, the fuel (such as

glucose) is oxidized, and O2 is reduced

becomes oxidized

becomes 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

6 Figure 9.4—NAD+ as an electron shuttle

2 e− + 2 H+ 2 e− + H+ + NAD NADH H+ Dehydrogenase Reduction of NAD+ 2[H] H+ (from food) Oxidation of NADH Nicotinamide Nicotinamide (oxidized form) (reduced form)

How does NAD+ trap electrons from glucose and other organic molecules?

• Enzymes called dehydrogenases remove a pair of hydrogen atoms (2 electrons & 2 protons) from the substrate (e.g., glucose), thereby oxidizing it. • The enzyme delivers the 2 electrons along with 1 proton to its coenzyme, NAD+ • The other proton is released as a hydrogen ion (H+)

dehydrogenase

. 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

7 Figure 9.5—An introduction to electron transport chains

H2 + ½ O2 2 H + ½ O2

Controlled release of 2 H+ + 2 e− energy

ATP

G G ATP Explosive

release ATP Free energy, energy, Free Free energy, energy, Free 2 e− ½ O 2 H+ 2

H2O H2O

(a) Uncontrolled reaction (b) Cellular respiration

Mitochondria: Chemical Energy Conversion

. Mitochondria are in nearly all eukaryotic cells . They have a smooth outer membrane and an inner membrane folded into cristae . The inner membrane creates two compartments: intermembrane space and mitochondrial matrix . Some metabolic steps of cellular respiration are catalyzed in the mitochondrial matrix . Cristae present a large surface area for enzymes that synthesize ATP

Figure 6.17—The mitochondrion: site of cellular respiration

Intermembrane space Outer membrane

Free ribosomes in the mitochondrial matrix Inner membrane Cristae Matrix

0.1 µm

8 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)

Figure 9.6—An overview of cellular respiration (step 1)

Electrons via NADH

GLYCOLYSIS

Glucose Pyruvate

CYTOSOL MITOCHONDRION

ATP

Substrate-level

Figure 9.6—An overview of cellular respiration (step 2)

Electrons Electrons via NADH via NADH and FADH2

GLYCOLYSIS PYRUVATE OXIDATION CITRIC ACID Glucose Pyruvate Acetyl CoA CYCLE

CYTOSOL MITOCHONDRION

ATP ATP

Substrate-level Substrate-level

9 Figure 9.6—An overview of cellular respiration (step 3)

Electrons Electrons via NADH via NADH and FADH2

GLYCOLYSIS PYRUVATE OXIDATIVE OXIDATION CITRIC PHOSPHORYLATION ACID Glucose Pyruvate Acetyl CoA CYCLE (Electron transport and chemiosmosis)

CYTOSOL MITOCHONDRION

ATP ATP ATP

Substrate-level Substrate-level Oxidative

BioFlix: Cellular Respiration

Oxidative vs. Substrate-level Phosphorylation . 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 . In total, for each molecule of glucose degraded to

CO2 and water by respiration, the cell makes up to 32 molecules of ATP

10 Figure 9.7—Substrate-level phosphorylation

Enzyme Enzyme

ADP P Substrate ATP Product

Concept 9.2: Glycolysis harvests chemical energy by oxidizing glucose to pyruvate . Glycolysis (“sugar splitting”) 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

Figure 9.8—The energy Energy Investment Phase input and output of Glucose glycolysis

2 ATP used 2 ADP + 2 P

Energy Payoff Phase

4 ADP + 4 P 4 ATP formed

+ + 2 NAD + 4 e− + 4 H 2 NADH + 2 H+

2 Pyruvate + 2 H2O

Net Glucose 2 Pyruvate + 2 H2O 4 ATP formed − 2 ATP used 2 ATP 2 NAD+ + 4 e− + 4 H+ 2 NADH + 2 H+

11 Figure 9.9—A closer look at glycolysis

GLYCOLYSIS: Energy Investment Phase

ATP Glucose Fructose Glucose 6-phosphate 6-phosphate ADP

Hexokinase Phosphogluco- isomerase 1 2

Figure 9.9 (cont.)—A closer look at glycolysis

GLYCOLYSIS: Energy Investment Phase Glyceraldehyde 3-phosphate (G3P) Fructose ATP Fructose 6-phosphate 1,6-bisphosphate ADP Isomerase 5 Phospho- Aldolase fructokinase Dihydroxyacetone 4 phosphate (DHAP) 3

Figure 9.9 (cont.)—A closer look at glycolysis

GLYCOLYSIS: Energy Payoff Phase

Glyceraldehyde 2 NADH 2 ATP 3-phosphate (G3P) 2 NAD+ 2 H+ 2 ADP 2 2

Triose Phospho- phosphate 2 glycerokinase Isomerase dehydrogenase 7 Aldolase 5 6 1,3-Bisphospho- 3-Phospho- 4 Dihydroxyacetone glycerate glycerate phosphate (DHAP)

12 Figure 9.9 (cont.)—A closer look at glycolysis

GLYCOLYSIS: Energy Payoff Phase

2 H O 2 ATP 2 2 ADP 2 2 2 2

Phospho- Enolase Pyruvate glyceromutase kinase 9 8 10 3-Phospho- 2-Phospho- Phosphoenol- Pyruvate glycerate glycerate pyruvate (PEP)

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, however, pyruvate must first be converted to acetyl Coenzyme A (acetyl CoA), which links glycolysis to the citric acid cycle . This step is carried out by a multi-enzyme complex that catalyzes three reactions

13 OXIDATIVE PYRUVATE CITRIC GLYCOLYSIS PHOSPHORYL- OXIDATION ACID CYCLE ATION

Figure 9.10—Oxidation of pyruvate to acetyl CoA, the step before the citric acid cycle

MITOCHONDRION CYTOSOL Coenzyme A CO2 1 3

NAD+ NADH + H+ Acetyl CoA Pyruvate

Transport protein

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 (per pyruvate molecule) . (or double the numbers above if you are calculating for 1 glucose molecule!)

14 CITRIC OXIDATIVE PYRUVATE GLYCOLYSIS ACID PHOSPHORYL- OXIDATION CYCLE ATION

ATP

PYRUVATE OXIDATION Figure 9.11—An Pyruvate (from glycolysis, 2 molecules per glucose) overview of pyruvate oxidation and the

CO2 NAD+ citric acid cycle CoA NADH + H+ Acetyl CoA CoA

CoA

CITRIC ACID 2 CO CYCLE 2

+ FADH2 3 NAD

FAD CoA 3 NADH + 3 H+

P ADP + i ATP

The Citric Acid Cycle, cont.

. 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 (i.e., citric acid)

. 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

15 Figure 9.12—A closer look at the citric acid Acetyl CoA CoA-SH cycle

NADH H O + H+ 1 2 NAD+ Oxaloacetate 8 2

Malate Citrate Isocitrate NAD+ CITRIC 3 NADH 7 ACID + H+ H2O CYCLE CO2

Fumarate CoA-SH -Ketoglutarate 4 6 CoA-SH

FADH2 5 CO2 NAD+ FAD NADH Succinate P i + H+ GTP GDP Succinyl CoA ADP

ATP

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 multi-protein 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

16 CITRIC OXIDATIVE PYRUVATE GLYCOLYSIS ACID PHOSPHORYL- OXIDATION CYCLE ATION

ATP

NADH Figure 9.13— 50 e− Free-energy

2 NAD+ ) FADH2 change during − mol 2 e Multiprotein I FAD electron 40 FMN complexes II

Fe•S Fe•S transport (kcal/

2 Q III Cyt b 30 Fe•S Cyt c1 Cyt c IV

Cyt a ) relative relative ) O to Cyt a3 G 20

10 2 e−

(originally from Free Free energy ( NADH or FADH2)

2 H+ + ½ O 0 2

The Pathway of Electron Transport, cont.

. 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

17 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 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

INTERMEMBRANE Figure 9.14— SPACE H+ Stator ATP synthase, Rotor a molecular mill

Internal rod Catalytic knob

Video: ATP Synthase 3-D Structure, Top View

18 Video: ATP Synthase 3-D Structure, Side View

Figure 9.15—Chemiosmosis couples the electron transport chain to ATP synthesis

H+ ATP H+ Protein H+ synthase complex H+ of electron Cyt c carriers IV Q I III

II + 2 H + ½ O2 H2O FADH2 FAD

+ NADH NAD ADP + P i ATP (carrying + electrons H from 1 Electron transport chain 2 Chemiosmosis food) Oxidative phosphorylation

Chemiosmosis = A Proto-Motive Force

. 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

19 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

Figure 9.16—ATP yield per molecule of glucose at each stage of cellular respiration Electron shuttles span membrane CYTOSOL MITOCHONDRION 2 NADH or

2 FADH2

2 NADH 2 NADH 6 NADH 2 FADH2

GLYCOLYSIS PYRUVATE OXIDATIVE CITRIC OXIDATION PHOSPHORYLATION ACID Glucose 2 2 Acetyl CoA (Electron transport CYCLE Pyruvate and chemiosmosis)

+ 2 ATP + 2 ATP + about 26 or 28 ATP

Maximum per glucose: About 30 or 32 ATP

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 anaerobic respiration or fermentation to produce ATP

20 . 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 . Lactic acid fermentation

Alcohol Fermentation

. In alcohol fermentation, pyruvate is converted to ethanol in two steps:

1. The first step releases CO2 2. The second step produces ethanol . Alcohol fermentation by yeast is used in brewing, winemaking, and baking

21 Figure 9.17—Fermentation

2 ADP + 2 P i 2 ATP

Glucose GLYCOLYSIS

2 Pyruvate

2 NAD+ 2 NADH 2 CO2 + 2 H+

2 Ethanol 2 Acetaldehyde (a) Alcohol fermentation

Lactic Acid Fermentation

. 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

Figure 9.17—Fermentation

2 ADP + 2 P i 2 ATP

Glucose GLYCOLYSIS

2 NAD+ 2 NADH + 2 H+ 2 Pyruvate

2 Lactate (b) Lactic acid fermentation

22 Animation: Fermentation Overview

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 . However, the processes have different final electron acceptors: an organic molecule (such as pyruvate or

acetaldehyde) in fermentation, O2 in aerobic respiration, or something else in anaerobic respiration . Cellular respiration produces 30-32 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

23 Glucose

Glycolysis CYTOSOL

Pyruvate

No O2 present: O2 present: Fermentation Aerobic cellular respiration

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

24 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

Proteins Carbohydrates Fats

Amino Sugars Glycerol Fatty acids acids

GLYCOLYSIS Glucose

Glyceraldehyde 3- P

NH3 Pyruvate

Acetyl CoA

Biosynthesis (Anabolic Pathways)

. The body uses small molecules to build other substances . These small molecules may come: 1. directly from food, 2. from glycolysis, 3. or from the citric acid cycle.

25 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

Glucose AMP GLYCOLYSIS Fructose 6-phosphate Stimulates

Phosphofructokinase

Fructose 1,6-bisphosphate Inhibits Inhibits

Pyruvate

ATP Citrate Acetyl CoA

CITRIC ACID Figure 9.20—The CYCLE control of cellular respiration Oxidative phosphorylation