Splash Contents Chapter Introduction An Overview of Respiration 5.1Metabolism and Cell Respiration...

Chapter Introduction

An Overview of Respiration

5.1 Metabolism and Cell Respiration

5.2 The Stages of Aerobic Respiration

The Reactions of Respiration

5.3 Glycolysis

5.4 Mitochondria and Respiration

5.5 The Krebs Cycle

5.6 The Electron Transport System

5.7 Oxygen, Respiration, and Photosynthesis

Respiration and Cellular Activities

5.8 The Krebs Cycle in Fat and Protein Metabolism

5.9 Respiration and Heat Production

5.10 Control of Respiration

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A Relate metabolism to cellular respiration.

Learning Outcomes

By the end of this chapter you will be able to:

B Distinguish among the three hydrogen-carrier molecules and their functions.

C Describe how the three stages of cellular respiration are related.

D Relate glycolysis and the Krebs cycle to compartmentalization in the cell.

E Explain the significance of the electron transport system to respiration.

F Discuss the importance of oxygen to respiration and photosynthesis.

G Explain the central role of the Krebs cycle in metabolism.


What adaptations help them overcome this problem?

Cell Respiration: Releasing Chemical Energy What problem do birds

and other small animals face in winter?

A robin (Erithacus rubecula) perched on a pine tree


Cell Respiration: Releasing Chemical Energy• All organisms require

energy to carry out their life functions.

• Evolution has produced a number of biochemical processes that enable organisms to obtain the energy stored in nutrients such as carbohydrates, fats, and proteins.

A robin (Erithacus rubecula) perched on a pine tree


• The chemical reactions in an organism, has two complementary parts—synthesis and decomposition.


5.1 Metabolism and Cell Respiration

• Synthesis reactions form compounds that aid in cell growth and maintenance.

• Decomposition reactions release energy.

• ATP is a major energy carrier.

• Because ATP is made during decomposition and used during biosynthesis, it links these two processes.


5.1 Metabolism and Cell Respiration (cont.)


Metabolism includes all decomposition and synthesis reactions in an organism. Cell respiration is part of decomposition. ATP mediates the transfer of energy in metabolism.


• Cell respiration is a decomposition pathway that provides the energy cells need to function.

• Respiration releases free energy by oxidizing sugars or other organic substrates.

• Some of this energy is conserved in ATP which in turn provides the energy to power most life processes.




• Cell respiration can occur in the presence or absence of oxygen.

– In aerobic respiration—occurring in the presence of oxygen—oxygen is the oxidizing agent that receives electrons from the decomposed substrates.



– In anaerobic respiration—occurring without oxygen—the substrate may be only partly decomposed, releasing less energy, or a nitrogen or sulfur compound may substitute for oxygen.


• The raw materials for aerobic respiration are carbohydrates, fats, and proteins.

• Glucose (C6H12O6) and glucose-phosphate (C6H11O6

—H3PO3) are important substrates for respiration.




• During aerobic respiration, a great deal of energy released as glucose gradually oxidizes and breaks down to carbon dioxide.

• The overall reaction is summarized in the following equation:




• Oxidizing one molecule of glucose releases much more energy than a single reaction needs.

• Cell respiration releases energy by oxidizing glucose in a series of small steps that lead to the production of one molecule of ATP.



The letters a through g represent intermediate compounds in the decomposition of glucose to carbon dioxide and water.


• Cell respiration provides both ATP and the carbon skeletons needed for biosynthesis.




Products of cell respiration

Click the image to view an animated version.


• The respiration of a simple carbohydrate such as glucose can be divided into three main stages:

5.2 The Stages of Aerobic Respiration

– glycolysis

– the Krebs cycle

– the electron transport system


• Each stage involves a series of chemical reactions catalyzed by enzymes.


5.2 The Stages of Aerobic Respiration (cont.)


Aerobic respiration occurs in three stages—glycolysis, the Krebs cycle, and the electron transport system. As glucose and other substrates are oxidized to carbon dioxide and water, NAD+ is reduced to NADH, and FAD is reduced to FADH2. These reduced compounds carry hydrogen ions (H+) and electrons (e–) to the electron transport system.


• Glycolysis is the initial breakdown of a carbohydrate, usually glucose, into smaller molecules at the beginning of cell respiration or fermentation.

• The Krebs cycle completes the breakdown of the intermediate products of glycolysis, releasing energy; also, a source of carbon skeletons for use in biosynthesis reactions.



• The electron transport system is the process in which electrons transfer from one carrier molecule to another in photosynthesis and in cell respiration. It results in storage of some of the energy in ATP molecules.


• During glycolysis, enzymes partially oxidize glucose and split it into two 3-carbon molecules releasing enough energy to form a small amount of ATP.

• An enzyme releases a molecule of carbon dioxide from each 3-carbon molecule that was produced in glycolysis.

• The resulting 2-carbon molecules are oxidized completely to carbon dioxide in the second stage, called the Krebs cycle, producing additional ATP molecules.




• Whenever one substance is oxidized, another must be reduced.

• As glucose is oxidized, electrons and protons, are passed to NAD+ (nicotinamide adenine dinucleotide), reducing it to form NADH.




• In the electron transport system, NADH is oxidized as it donates protons and electrons, regenerating the supply of NAD+.

• The protons and electrons release energy to form ATP as the electron transport system transfers them to oxygen, forming water.




• Most of the ATP is synthesized by the electron transport system.

• In one step in the Krebs cycle, two hydrogen atoms derived from glucose reduce a second hydrogen-carrier molecule, FAD (flavin adenine dinucleotide), instead of NAD+.



• NADH, NADPH, and FADH2 all carry hydrogen in cells.


• This process is called aerobic respiration because oxygen must accept the electrons at the end of the electron transport system.



• The energy released in this reaction is used to synthesize ATP.


• Both aerobic and anaerobic respiration begin with glycolysis.


5.3 Glycolysis

• Three important things happen during glycolysis:

1. the glucose molecule breaks into two pieces

2. some ATP forms

3. some NAD+ is reduced to form NADH


Step A: Glycolysis begins when an enzyme converts a molecule of glucose to glucose-6-phosphate.

5.3 Glycolysis (cont.)



Step B: A molecule of ATP provides the phosphate and the energy to power the reaction. Another enzyme rearranges the glucose-6-phosphate, and a second ATP molecule donates another phosphate group.




Step C: The resulting molecule splits into two 3-carbon sugar-phosphates.




Step D: Other enzymes catalyze the rearrangement and partial oxidation of these molecules to form the 3-carbon compound pyruvic acid.




• In plant cells, starch and sucrose break down to glucose or glucose-1-phosphate, which can enter glycolysis directly at step a.

• Three-carbon sugar-phosphates formed in photosynthesis can enter the process at step c.




• At the end of glycolysis, the fate of pyruvate depends on whether oxygen is present.

– If insufficient oxygen is present, animal cells convert NADH and pyruvate into NAD+

and lactate.



– NAD+ cycles back to glycolysis, in an anaerobic pathway known as lactic-acid fermentation.

– If sufficient oxygen is present, pyruvate enters the Krebs cycle.


• Mitochondria are the organelles in prokaryotes in which the Krebs cycle and the electron transport system occur.

5.4 Mitochondria and Respiration

• Most ATP is synthesized in the mitochondria.



• A cell may contain anywhere from ten to several thousand mitochondria depending on its energy needs.

• Each mitochondrion is usually only 2–3 µm long and about 1 µm thick.

5.4 Mitochondria and Respiration (cont.)


This transmission electron micrograph shows a mitochondrion in a human liver cell (x80,000; color added).


• A mitochondrion has outer and inner membranes.


5.4 Mitochondria and Respiration (cont.)

• The inner membrane holds the enzymes of the electron transport system and the enzymes for ATP formation.

• Most of the enzymes of the Krebs cycle are within the fluid-filled interior matrix.

• The outer membrane regulates the movement of molecules into and out of the mitochondrion.


• The Krebs cycle completes the decomposition and oxidation of glucose to carbon dioxide.

5.5 The Krebs Cycle

• As the breakdown products of glucose are oxidized, NAD+ and FAD are reduced, and a small amount of energy is saved as ATP.



Step A: pyruvate is transported into the mitochondria where enzymes release a molecule of carbon dioxide from each pyruvate molecule, leaving a molecule of acetate.

A carrier molecule, coenzyme A (CoA), binds to the acetate and delivers the acetate to the Krebs cycle.

5.5 The Krebs Cycle (cont.)



Step B: Acetate enters the Krebs cycle, as an enzyme combined the acetate group of acetyl CoA with a 4-carbon acid (oxaloacetate) to form a 6-carbon acid (citrate).



Coenzyme A is released and recycled to deliver more acetate.


Steps C and D: Other enzymes catalyze the rearrangement and oxidation of citrate. Two of the carbon atoms in citrate are oxidized to carbon dioxide.



The hydrogen atoms that these carbon atoms lose reduce two molecules of NAD+.


Steps E and F: A 4-carbon organic acid rearranged and further oxidized.



The result is a new molecule of oxaloacetate that begins another round of the cycle.


• The oxidation of glucose in glycolysis and the Krebs cycle reduces NAD+ to NADH and FAD to FADH2 which carry hydrogen atoms to the electron transport system.

5.6 The Electron Transport System

• The electron transport system consists of a series of enzymes and other proteins known as cytochromes that are embedded in the inner membranes of mitochondria.



• The electron transport system separates hydrogen atoms into electrons and protons.

• The cytochromes transfer the electrons step by step through the system.

• The last, or terminal, cytochrome is an enzyme that combines the electrons and protons with oxygen, forming water.

5.6 The Electron Transport System (cont.)



The mitochondrial electron transport system



• At each transfer in the electron transport chain, the electrons release free energy that enables enzymes in the inner mitochondrial membrane to actively transport protons from the matrix to the intermembrane space.

• As protons become highly concentrated they tend to diffuse back into the matrix of the mitochondrion passing through the ATP-synthetase enzyme complex, where ATP is synthesized.




As glucose is oxidized in glycolysis and the Krebs cycle, NAD+ and FAD are reduced to NADH and FADH2. These carriers pass electrons to the electron transport system. ATP forms as these electrons lose energy in reducing oxygen. Each molecule of NADH generates three ATP molecules, and each molecule of FADH2 generates two ATP molecules. The resulting oxidized NAD+ and FAD are recycled as more glucose is oxidized.


• Bacteria do not have mitochondria. Their cell membranes contain their electron transport systems.

• In some bacteria, in a process called anaerobic respiration, electrons flow through the system to oxidizers other than oxygen, such as sulfate (SO4

–2) or nitrate (NO3–) .




• Bacteria that can survive for long periods with or without oxygen, switching between fermentation and aerobic respiration, are called facultative aerobes.

• Bacteria that are poisoned by oxygen and generate ATP entirely from fermentation or anaerobic respiration are called obligate anaerobes.



• Most organisms, such as animals and plants, are obligate aerobes; they cannot survive for long without oxygen.


• Oxygen is needed to oxidize glucose.

5.7 Oxygen, Respiration, and Photosynthesis

– Without oxygen, cells must ferment glucose, forming only two ATP molecules per glucose molecule.

– With oxygen present, organisms gain much more energy from their food.



• In general, the products of photosynthesis—oxygen and carbohydrates—are the raw materials for cell respiration.

• Cell respiration, in turn, provides the raw materials for photosynthesis (carbon dioxide and water).

5.7 Oxygen, Respiration, and Photosynthesis (cont.)



5.7 Oxygen, Respiration, and Photosynthesis (cont.)


Respiration releases chemical energy by using the reduction of oxygen to water to drive the oxidation of sugar to carbon dioxide. Photosynthesis stores chemical energy by using the oxidation of water to oxygen to drive the reduction of carbon dioxide to sugar.


• The release of energy from fats and proteins also involves the Krebs cycle.


5.8 The Krebs Cycle in Fat and Protein Metabolism

• When cells use the fatty acids of fats for energy, enzymes in the mitochondria break down the fatty acids to acetate which coenzyme A transfers to the Krebs cycle.

• Without oxygen, most of the energy in fat cannot be transferred to ATP.


• When cells use proteins in respiration, digestive enzymes first break down the proteins to amino acids.

5.8 The Krebs Cycle in Fat and Protein Metabolism (cont.)

• Other enzymes remove the amino groups and convert the ammonia this produces to safer nitrogen compounds.

• The carbon skeletons remaining from some amino acids can undergo reactions that form 4- or 5-carbon acids (oxaloacetate or ketoglutarate), which can enter the Krebs cycle.



The reactions of cell respiration and particularly of the Krebs cycle contribute to both the decomposition and biosynthesis of carbohydrates, fats, and proteins. Certain amino acids can be synthesized from the carbon skeletons by adding amino groups (—NH2) derived from ammonia (NH3). Carbon skeletons can be formed from amino acids by removing the amino groups. Most organisms cannot convert fat to carbohydrate.


• The Krebs cycle and glycolysis also provide building blocks for biosynthesis.

– In autotrophs, these pathways, along with the Calvin cycle, lead to the synthesis of every organic compound the organism needs.

– In heterotrophs, these pathways lead to the synthesis of most, but not all, of the necessary organic compounds.




• Most synthesis pathways are not the reverse of decomposition pathways.

• Separate enzymes and pathways for synthesis and decomposition help cells operate efficiently and control the activities of these pathways.

• Most biological decompositions involve hydrolysis, a type of decomposition that inserts the components of water (H and OH) into a bond to break it.




In each example of a hydrolysis reaction, bonds are broken in a water molecule and another molecule.


• Cell respiration releases heat energy, which helps many organisms keep warm.

5.9 Respiration and Heat Production

• Some mammals have brown fat which contains more mitochondria than any other body tissue and is adapted for rapid production of thermal energy.


The arctic ground squirrel, Spermophilus parryi, spends its summer on the Arctic tundra. In the winter, it hibernates in its nest. Brown fat enables the ground squirrel to quickly elevate its body temperature at the end of hibernation.


• Many plants have also evolved a form of respiration that produces a great deal of heat energy using an alternate branch of the electron transport system.

• In this pathway, some of the energy of electron flow results in the production of more heat energy and less ATP than in normal respiration.

5.9 Respiration and Heat Production (cont.)


Skunk cabbage, Symplocarpus foetidus, releases heat energy that melts snow as it emerges from the ground.


• Organisms must control their rate of respiration in order to direct energy and carbon skeletons accurately to where they are needed.

5.10 Control of Respiration

• Control is critical to organization, and cells must be organized to survive.

• The mechanisms that control whether glucose is broken down in respiration or converted to starch or fat operate by supply and demand.



Energy regulation in animals



Summary

• Cell respiration involves reactions that oxidize carbohydrates, fats, or amino acids, with a release of energy that is conserved as ATP.

• Cell respiration also provides carbon skeletons for the biosynthesis of macromolecules the cell requires.

• Supply and demand in the cell determine whether carbon skeletons in this pathway are oxidized or used in biosynthesis.

• Three stages of aerobic respiration are: glycolysis, the Krebs cycle, and the electron transport system.

• Metabolism consists of all the chemical reactions in an organism, including biosynthesis and degradation.


Summary (cont.)

• In the absence of oxygen, fermentation occurs.

• Fermentation results in the net synthesis of only two molecules of ATP per molecule of glucose.

• In the presence of oxygen, aerobic respiration occurs.

• Pyruvate from glycolysis is transported into a mitochondrion (except in bacteria).

• Glycolysis, the first stage of both aerobic respiration and fermentation, produces a small quantity of ATP and NADH.


Summary (cont.)

• Hydrogen atoms carried by NADH and FADH2 are used to synthesize ATP.

• The complete aerobic respiration of a molecule of glucose forms 6 molecules of carbon dioxide and a maximum of 38 molecules of ATP.

• Cell respiration and energy production are closely regulated.

• Some plants and animals have special forms of cell respiration that release larger quantities of heat energy and produce less ATP.

• Coenzyme A carries the acetyl group to the Krebs-cycle enzymes and produces ATP, NADH, and FADH2.


Reviewing Key TermsMatch the term on the left with the correct description.

___ facultative aerobes

___ hydrolysis

___ coenzyme A

___ cytochromes

___ obligate anaerobes

a. an electron carrying pigment in electron transport systems

b. bacteria that can survive with or without oxygen

c. a small molecule that delivers acetate to the Krebs cycle

d. organisms that cannot survive in the presence of oxygen

e. the splitting of a molecule by reaction with water

b

e

c

a

d


Reviewing Ideas1. Which is more efficient, aerobic or anaerobic

respiration? Explain.

Aerobic respiration is much more efficient than anaerobic respiration. Aerobic respiration generates up to 38 ATP molecules from a single glucose molecule. Anaerobic respiration generates only two ATP molecules per glucose molecule.


Reviewing Ideas2. How are photosynthesis and respiration

symbiotically related?

The products of photosynthesis—oxygen and carbohydrates—are the raw materials for cell respiration. Cell respiration, in turn, provides the raw materials—carbon dioxide and water—for photosynthesis.


Using Concepts3. Why is oxidizing glucose a in a series of small

steps important?

Oxidizing one molecule of glucose releases much more energy than a single reaction needs, however, so glucose is not useful as a direct source of energy. To release all the energy in glucose at once would waste most of the energy and could heat the organism until it cooked itself.


Using Concepts4. Premature human infants often lack a normal layer of

brown fat. What complications can might this cause? Why?

The infant may have trouble regulating its body temperature. Brown fat contains more mitochondria than any other body tissue and is adapted for the production of thermal energy. Respiration of stored fat in brown fat cells produces much heat energy and little ATP.


Synthesize5. What similarities exist between photosynthesis

and respiration?

The process of evolution has organized cell respiration and photosynthesis in similar ways. As in photosynthesis, the electron transport system of cell respiration is embedded in a membrane. Just as chloroplasts contain and organize the enzymes of photosynthesis in nonbacterial cells, specialized structures called mitochondria provide efficiency and organization to cell respiration. Like a chloroplast, a mitochondrion has an outer and an inner membrane.


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Chapter Animations

Products of cell respiration

The mitochondrial electron transport system

Energy regulation in animals

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