09 Cellular Respiration

© 2011 Pearson Education, Inc.

Cellular Respiration: Key Concepts

In cells, the endergonic reactions needed for life are paired with exergonic reactions requiring ATP.

Cellular respiration produces ATP from molecules with high potential energy – often glucose.

Cellular respiration has four components:

1. Glycolysis

2. Pyruvate processing

3. The citric acid cycle

4. Electron transport and chemiosmosis


Cellular Respiration: Key ConceptsRespiration and fermentation are carefully regulated.

Fermentation pathways allow glycolysis to continue when the lack of an electron acceptor shuts down electron transport chains.


Introducing ATP

• ATP (adenosine triphosphate) is the cellular currency for energy – it provides the fuel for most cellular activities.

• ATP has high potential energy and allows cells to do work.

• ATP works by phosphorylating (transferring a phosphate group) target molecules.


The Nature of Chemical Energy and Redox Reactions

• In cells, electrons are the most important source of chemical potential energy.

• The amount of potential energy in an electron is based on its position relative to positive and negative charges.

– Electrons closer to negative charges (from other electrons) and farther from positive charges (in nuclei of nearby atoms), have higher potential energy.

• In general, a molecule’s potential energy is a function of its electrons’ configuration and position.


Structure and Function of ATP

• The electrons in ATP have high potential energy because the four negative charges in its three phosphate groups repel each other.

• Hydrolysis of the bond between the two outermost phosphate groups results in formation of ADP and Pi (inorganic phosphate, H2PO4

−) in a highly exergonic reaction.

– The released phosphate group is transferred to a protein.





ATP Hydrolysis and Protein Phosphorylation

• Hydrolysis of ATP is exergonic because the entropy of the product molecules is much higher than that of the reactants.

• Energy released during ATP hydrolysis is transferred to a protein during phosphorylation.

– This phosphorylation usually causes a change in the protein’s shape.



How Does ATP Drive Endergonic Reactions?

When a protein is phosphorylated, the exergonic phosphorylation reaction is paired with an endergonic reaction in a process called energetic coupling.

• In cells, endergonic reactions become exergonic when the substrates or enzymes involved are phosphorylated.



What Is a Redox Reaction?

• Reduction–oxidation reactions (redox reactions) are chemical reactions that involve electron transfer.

– Redox reactions drive ATP formation.

• When an atom or molecule gains an electron, it is reduced.

• When an atom or molecule loses an electron, it is oxidized.

• Oxidation and reduction events are always coupled—if one atom loses an electron, another has to gain it.

– Electron donors are always paired with electron acceptors.


The Gain or Loss of an Electron Can Be Relative

• During a redox reaction, electrons can be transferred completely from one atom to another, or they can simply shift their position in covalent bonds.



Electrons Are Usually Accompanied by Protons

• Each electron transferred from one molecule to another during a redox reaction is usually accompanied by a proton (H+).

– The reduced molecule gains a proton and has higher potential energy.

– The oxidized molecule loses a proton and has lower potential energy.

• Nicotinamide adenine dinucleotide (NAD) is reduced to form NADH.

– NADH readily donates electrons to other molecules and is thus called an electron carrier and has “reducing power.”



What Happens When Glucose Is Oxidized?

• The carbon atoms of glucose are oxidized to form carbon dioxide, and the oxygen atoms in oxygen are reduced to form water:

C6H12O6 + 6 O2 6 CO2 + 6 H2O + energy

glucose oxygen carbon water dioxide

In cells, glucose is oxidized through a long series of carefully controlled redox reactions. The resulting change in free energy is used to synthesize ATP from ADP and Pi. Together, these reactions comprise cellular respiration.



An Overview of Cellular Respiration

• All organisms use glucose to build fats, carbohydrates, and other compounds; cells recover glucose by breaking down these molecules.

– Glucose is used to make ATP through either cellular respiration or fermentation.

Cellular respiration produces ATP from a molecule with high potential energy – usually glucose. Each of the four steps of cellular respiration consists of a series of chemical reactions, and a distinctive starting molecule and characteristic set of products.



The Steps of Cellular Respiration

• Cellular respiration is any suite of reactions that produces ATP in an electron transport chain.

• Cellular respiration has four steps:

1. Glycolysis – glucose is broken down to pyruvate.

2. Pyruvate processing – pyruvate is oxidized to form acetyl CoA.

3. Citric acid cycle – acetyl CoA is oxidized to CO2.

4. Electron transport and chemiosmosis – compounds that were reduced in steps 1–3 are oxidized in reactions leading to ATP production.



Glycolysis: Processing Glucose to Pyruvate

• Glycolysis, a series of 10 chemical reactions, is the first step in glucose oxidation.

• All of the enzymes needed for glycolysis are found in the cytosol.

• In glycolysis, glucose is broken down into two 3-carbon molecules of pyruvate, and the potential energy released is used to phosphorylate ADP to form ATP.


The Glycolysis Reactions

• Glycolysis consists of an energy investment phase and an energy payoff phase.

• In the energy investment phase, two molecules of ATP are consumed, and glucose is phosphorylated twice, forming fructose-1,6-bisphosphate.

• In the energy payoff phase:

– Sugar is split to form two pyruvate molecules.

– Two molecules of NAD+ are reduced to NADH.

– Four molecules of ATP are formed by substrate-level phosphorylation (net gain of 2 ATP).




Methods of Producing ATP

• Substrate-level phosphorylation occurs when ATP is produced by the enzyme-catalyzed transfer of a phosphate group from an intermediate substrate to ADP.

– This is how ATP is produced in glycolysis and the citric acid cycle.

• In an electron transport chain a proton gradient provides energy for ATP production; the membrane protein ATP synthase uses this energy to phosphorylate ADP to form ATP. This process is called oxidative phosphorylation.



Feedback Inhibition

• Feedback inhibition occurs when an enzyme in a pathway is inhibited by the product of that pathway.

– Cells that are able to stop glycolytic reactions when ATP is abundant can conserve their stores of glucose for times when ATP is scarce.



Feedback Inhibition Regulates Glycolysis

• During glycolysis, high levels of ATP inhibit the enzyme phosphofructokinase, which catalyzes one of the early reactions.

• Phosphofructokinase has two binding sites for ATP:

1. The active site, where ATP phosphorylates fructose-6-phosphate, resulting in the synthesis of fructose-1,6-bisphosphate

2. A regulatory site

High ATP concentrations cause ATP to bind at the regulatory site, changing the enzyme’s shape and dramatically decreasing the reaction rate at the active site.



The Remaining Reactions Occur in the Mitochondria

• Pyruvate produced during glycolysis is transported from the cytosol into the mitochondria.

• Mitochondria have both inner and outer membranes.

• Layers of sac-like structures called cristae fill the interior of the mitochondria, and are connected to the inner membrane by short tubes.

• The mitochondrial matrix is inside the inner membrane but outside the cristae.



Pyruvate Processing

• Pyruvate processing is the second step in glucose oxidation. It is catalyzed by the enzyme pyruvate dehydrogenase in the mitochondrial matrix.

• In the presence of O2, pyruvate undergoes a series of reactions that results in the product molecule acetyl coenzyme A (acetyl CoA).

– During these reactions, another molecule of NADH is synthesized, and one of the carbon atoms in pyruvate is oxidized to CO2.



Pyruvate Processing Regulation

Pyruvate processing is under both positive and negative control. Abundant ATP reserves inhibit the enzyme complex; large supplies of reactants, such as acetyl CoA and NADH, and low supplies of products, such as ATP, stimulate it.


The Citric Acid Cycle

• During the third step of glucose oxidation, the acetyl CoA produced by pyruvate processing enters the citric acid cycle, located in the mitochondrial matrix.

– Each acetyl CoA is oxidized to two molecules of CO2.

• Some of the potential energy released is used to

1. Reduce NAD+ to NADH.

2. Reduce flavin adenine dinucleotide (FAD) to FADH2 (another electron carrier).

3. Phosphorylate GDP to form GTP (later converted to ATP).


The Substrates of the Citric Acid Cycle

• A series of carboxylic acids is oxidized and recycled in the citric acid cycle.

• Citrate (the first molecule in the cycle) is formed from pyruvate and oxaloacetate (the last molecule in the cycle).

• The citric acid cycle completes glucose oxidation. The energy released by the oxidation of one acetyl CoA molecule is used to produce 3 NADH, 1 FADH2, and 1 GTP, which is then converted to ATP.



The Citric Acid Cycle Regulation and Summary

The citric acid cycle can be turned off at multiple points, via several different mechanisms of feedback inhibition.

To summarize, the citric acid cycle starts with acetyl CoA and ends with CO2. The potential energy that is released is used to produce NADH, FADH2, and ATP. When energy supplies are high, the cycle slows down.



Harvesting Energy: Krebs Cycle

BLAST Animation: Harvesting Energy: Krebs Cycle


Glucose Oxidation Summary

• Glucose oxidation produces ATP, NADH, FADH2, and CO2.

• Glucose is completely oxidized to carbon dioxide via glycolysis, the subsequent oxidation of pyruvate, and then the citric acid cycle.

• In eukaryotes, glycolysis occurs in the cytosol; pyruvate oxidation and the citric acid cycle take place in the mitochondrial matrix.



Free Energy Changes, NADH, and FADH2

• For each glucose molecule that is oxidized to 6 CO2, the cell reduces 10 molecules of NAD+ to NADH and 2 molecules of FAD to FADH2, and produces 4 molecules of ATP by substrate-level phosphorylation.

• The ATP can be used directly for cellular work.

• However, most of glucose’s original energy is contained in the electrons transferred to NADH and FADH2, which then carry them to oxygen, the final electron acceptor.



The Electron Transport Chain

• During the fourth step in cellular respiration, the high potential energy of the electrons carried by NADH and FADH2 is gradually decreased as they move through a series of redox reactions.

• The proteins involved in these reactions make up what is called an electron transport chain (ETC).

• O2 is the final electron acceptor. The transfer of electrons along with protons to oxygen forms water.


Oxidative Phosphorylation

• The energy released as electrons move through the ETC is used to pump protons across the plasma membrane into the intermembrane space, forming a strong electrochemical gradient.

• The protons then move through the enzyme ATP synthase, driving the production of ATP from ADP and Pi.

• Because this mode of ATP production links the phosphorylation of ADP with NADH and FADH2 oxidation, it is called oxidative phosphorylation.


Electron Transport and Chemiosmosis

• Most of the ETC molecules are proteins containing chemical groups that facilitate redox reactions. All but one of these proteins are embedded in the inner mitochondrial membrane.

– In contrast, the lipid-soluble ubiquinone (Q) can move throughout the membrane.

• During electron transport, NADH donates electrons to a flavin-containing protein at the top of the chain, but FADH2 donates electrons to an iron-sulfur protein that passes electrons directly to Q.



The Chemiosmotic Hypothesis

• The ETC pumps protons from the mitochondrial matrix to the intermembrane space. The proton-motive force from this electrochemical gradient can be used to make ATP in a process known as chemiosmosis.






How Is the Electron Transport Chain Organized?

• ETC proteins are organized into four large multiprotein complexes (called complex I–IV) and cofactors. Protons are pumped into the intermembrane space from the mitochondrial matrix by complexes I and IV.

• Q and the protein cytochrome c transfer electrons between complexes.

– Q also carries protons across the membrane.



ATP Synthase Structure

• ATP synthase is an enzyme complex consisting of two components:

– An ATPase “knob” (F1 unit)

– A membrane-bound, proton-transporting base (F0 unit)

• The units are connected by a rotor, which spins the F1 unit, and a stator, which interacts with the spinning F1 unit.

• Protons flowing through the F0 unit spin the rotor.

• As the F1 unit spins, its subunits change shape, and catalyze the phosphorylation of ADP to ATP.





ATP Yield from Cellular Respiration

• The vast majority of the “payoff” from glucose oxidation occurs via oxidative phosphorylation; ATP synthase produces 25 of the 29 ATP molecules produced per glucose molecule during cell respiration.



Aerobic and Anaerobic Respiration

• All eukaryotes and many prokaryotes use oxygen as the final electron acceptor of electron transport chains in the process of aerobic respiration.

• Some prokaryotes, especially those in oxygen-poor environments, use other electron acceptors in the process of anaerobic respiration.


Oxygen as a Final Electron Acceptor

• Oxygen is the most effective electron acceptor because it is highly electronegative. There is a large difference between the potential energy of NADH and O2 electrons which allows the generation of a large proton-motive force for ATP production.

• Cells that do not use oxygen as an electron acceptor cannot generate such a large potential energy difference. Thus, they make less ATP than cells that use aerobic respiration.


Fermentation

• In most organisms, cellular respiration cannot occur without oxygen. Fermentation, a metabolic pathway that regenerates NAD+ from stockpiles of NADH, allows glycolysis to continue producing ATP in the absence of oxygen.

• Fermentation occurs when pyruvate or a molecule derived from pyruvate accepts electrons from NADH.

• This transfer of electrons oxidizes NADH to NAD+.

– With NAD+ present, glycolysis can continue to produce ATP via substrate-level phosphorylation.



Different Fermentation Pathways

• In lactic acid fermentation, pyruvate produced by glycolysis accepts electrons from NADH. Lactate and NAD+ are produced.

– Lactic acid fermentation occurs in muscle cells.

• In alcohol fermentation, pyruvate is enzymatically converted to acetaldehyde and CO2. Acetaldehyde accepts electrons from NADH. Ethanol and NAD+ are produced.

– Alcohol fermentation occurs in yeast.





Fermentation and Cellular Respiration Efficiency

• Fermentation is extremely inefficient compared with cellular respiration.

– Fermentation produces just two ATP molecules per glucose molecule, compared with about 29 ATP molecules per glucose molecule in cellular respiration.

– Consequently, organisms never use fermentation if an appropriate electron acceptor is available for cellular respiration.


Cellular Respiration Interacts with Metabolic Pathways

• Energy and carbon are cells’ two fundamental requirements.

– They need high-energy electrons for generating chemical energy in the form of ATP, and a source of carbon-containing molecules for synthesizing macromolecules.

• Metabolism includes thousands of different chemical reactions.

– Catabolic pathways involve the breakdown of molecules and the production of ATP.

– Anabolic pathways result in the synthesis of larger molecules from smaller components.



Processing Proteins and Fats as Fuel

• Proteins, carbohydrates, and fats can all furnish substrates for cellular respiration.

– Enzymes routinely break down fats to form glycerol, which enters the glycolytic pathway, and acetyl CoA, which enters the citric acid cycle.

– Enzymes remove the amino groups from proteins; the remaining carbon compounds are intermediates in glycolysis and the citric acid cycle.

• For ATP production, cells first use carbohydrates, then fats, and finally proteins.



Anabolic Pathways Synthesize Key Molecules

• Molecules found in carbohydrate metabolism are used to synthesize macromolecules such as RNA, DNA, glycogen or starch, amino acids, fatty acids, and other cell components.
