Section 3

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Essential Questions

I What are the stages of cellularrespiration?

I What isthe role of electron carriersin each stage:of cellular respiration?

I Whatare the similarities betweenalcoholicfermentation and lactic acidfermentation?

Review Vocabularycyanobacterium: a type ofbacterium that is a photosyntheticautotroph

New Vocabularyanaerobic processaerobic respirationaerobic processglycolysisKrebs cyclefermentation

Cellular Respiration

AIN ~ Living organisms obtain energy by breakjng downorganic molecules during cellular respiration.

Real-World Reading Link Monarch butterflies must constantly feed onnectar from flowers to provide energy to sustain themselves during their wintermigration to parts of Mexico and California each year. Similarly, humans andother living organisms need reliable food sources to supply energy to surviveand grow.

Overview of Cellular RespirationRecall that organisms obtain energy in a process called cellular respiration. The function of cellular respiration is to harvest electrons fromcarbon compounds, such as glucose, and use that energy to make ATP.ATP is used to provide energy for cells to do work. The overall chemical equation for cellular respiration is shown below. Notice the equationfor cellular respiration is the opposite of the equation for photosynthesis.

C6H1206 + 602 —* 6C02 + 6H20 + Energy

Multilingual eGlossary Cellular respiration occurs in two main parts: glycolysis and aerobicrespiration- The first stage, glycolysis, is an anaerobic process. Anaerobicprocesses do not require oxygen. Aerobic respiration includes theKrebs cycle and electron transport and is an aerobic process. Aerobicprocesses require oxygen. Cellular respiration with aerobic respirationis summarized in Figure 11.

Figure 11 Cellular respiration occurs in the mitochondria, the energy powerhouseorganelles of a cell.

Golgiapparatus

AD~J

GlycolysisTransitionreaction

ADP

Electrontransportchain

Mitochondrion

ADP

Animalcell

Nucleus

Plasmamembrane

Mitochondrion

Ribosomes

C

Endoplasmicreticulum

4

Krebscycle

C

eNADH NADH FADH

NADH

228 chapter 8 • Cellular Energy

Figure 12 Glucose is broken down duringGlucose glycolysis inside the cytoplasm of cells.

Summarize the reactants and prodf glycolysis.~

Re •ew Personal Tutor

... ...2 ADP

DCNAD~

NADHNADH

Pyruvate Pyruvate... ...

GIycoIys~~ VOCABULARYlucose is broke9~pwn in the cytoplasm through the process of WORD ORIGIN

Ic glycolysis. Two molecules of ATP and two molecules of NADH are Glycolysisormed for each niolecule of glucose that is broken down. Follow : comes from the Greek words glykys,long with Figure 11 as you read about the steps of glycolysis. meaning sweet

• ri! and lysis, meaning to rupture orFirst, two phosphate groups, derived from two molecules of ATP, breakrejoined to glucoée. Notice that some energy, two ATP, is required totart the reactibilsthat will produce energy for the cell. The 6-carbon

molecule is then!I~roken down into two 3-carbon compounds. Next,two phosphates ar~added and electrons and hydrogen ions (H ) corn FOLDABLES

me with two NAD molecules to form two NADH molecules. NAD Incorporate informationfrom this section intoimilar to NADP,ian electron carrier used during photosynthesis. your Foldable.

st, the two 3-c~rbon compounds are converted into two moleculespyruvate. At-the sametime, four molecules of ATP are produced.

Reading Check Explain why there is a net yield of two, not four, ATPmolecules inglycolysis.

arebs Cyçlé

lYcolysis has ainet result of two ATP and two pyruvate molecules.lost of the energy from the glucose is still contained in the pyruvate. In

Presence ofpxygen, pyruvate is transported into the mitochondrialtrix, where it-is eventually converted to carbon dioxide. The series ofCtions in which pyruvate is broken down into carbon dioxide is calledICrebs Cyclefbr. the tricarboxylic acid (TCA) cycle. This cycle also is

ferred to as tile citric acid cycle.

Section 3 • Cellular Respiration 229

j

Jtz4 TipClarifying Statement Workwith a partner to read the text anddiscuss unfamiliar words and difficultconcepts. Write a clarifying statementto summarize the Krebs cycle.

CARFERS IN BIOLOGY

Bioenergeticist A researcher whostudies energy transfers in cells is abioenergeticist. Some bioenergeticists study mitochondria and theirrelationship to aging and disease.

Steps of the Krebs cycle Prior to the Krebs cycle, pyruvate firstreacts with coenzyme A (CoA) to form a 2-carbon intermediate calledacetyl CoA. At the same time, carbon dioxide is released and NAD isconverted to NADH. Acetyl CoA then moves to the mitochondrialmatrix. The reaction results in the production of two carbon dioxidemolecules and two NADH molecules. Follow along in Figure 13 asyou continue reading about the steps of the Krebs cycle. ~

as citric acid.

• Citric acid is then broken down in the next series of stepsjreleasing two molecules of carbon dioxide and generating one ATP,three NADH, and one FADH2. FAD is another electron darnersimilar to NAD~ and NADP

• Finally, acetyl CoA and citric acid are generated and the éfdecontinues.

Recall that two molecules of pyruvate are formed during glycolySisresulting in two “turns” of the Krebs cycle for each glucose molçcule.The net yield from the Krebs cycle is six carbon dioxide molecules, twoATP, eight NADH, and two FADH2. Ten NADH and two FADH2 moveon to play a significant role in the next stage of aerobic respjration.

Electron TransportIn aerobic respiration, electron transport is the final step in the breakdown of glucose. It also is the point at which most of the ATP is produced. High-energy electrons and hydrogen ions from NADH andFADH2 produced in the Krebs cycle are used to convert ADP to ATP

Figure 13 Pyruvate is broken down intocarbon dioxide during the Krebs cycle inside themitochondria of cells.Trace Follow the path of carbonmolecules that enter and leave theKrebs cycle.

Concepts in Motion

Anlmati n

Pyruvate(from glycolysis)

0 _

I1~r~n1~CoA’%~

NADH

Acetyl CoA••— CoA

coAFour-carbon Citric acidcompound

eKrebsCycle

IIN4DHI ~NA~DH~

Five-carboncompound

S

• The Krebs cycle begins with acetyl CoA combining with4-carbon compound to form a 6-carbon compound known

230 Chapter 8 • Cellular Energy

intermem~a~ spaceElectron transport chain

Mitochondrial matrix a’

As shown in Figure 14, electrons move along the mitochondrialmembrane from one protein to another. As NADH and FADH2 releaseIectrofls, the energy carriers are converted to NAD~ and FAD, and Hns are released into the mitochondrial matrix. The H ions are pumped

nto the mitochondrial matrix across the inner mitochondrial membrane.fi~I H ions then diffuse down their concentration gradient back across the

:alld membrane and into the matrix through ATP synthase molecules in+ hemiosmosis. Electron transport and chemiosmosis in cellular respira

i n are similar to these processes in photosynthesis. Oxygen is the final

ide e tron acceptor in the electron transport system in cellular respiration.as Protons and electrons are transferred to oxygen to form water.

Overall, electron transport produces 32 ATP. Each NADH moleculeroduces three ATP and each group of three FADH2 produces two ATP. Inkaryotes, one molecule of glucose yields 36 ATP under ideal conditions.

Prokaryotic cellular respiration Some prokaryotes also undergoerobic respiration. Because prokaryotes do not have mitochondria,here are differences in the process. The main difference involves theeof the prokaryotic cellular membrane as the location of electronnsport. In eukaryotic cells, pyruvate is transported to the mitochon

na. In prokaryotes, this movement is unnecessary, saving the prokaryI cell two ATP, and increasing the net total of ATP produced to 38.

Anaerobic Respirationme cells can function for a short time when oxygen levels are low. Somekaryotes are anaerobic organisms—they grow and reproduce without

ygen. In some cases these cells continue to produce ATP through glycolHowever, there are problems with solely relying on glycolysis for

Glycolysis provides only two net ATP for each molecule of glucose,

AD~ has a limited amount ofNAD. Glycolysis will stop when all theis used up if there is not a process to replenish NAD~. The anaerobicWay that follows glycolysis is anaerobic respiration, or fermentation.

fltation occurs in the cytoplasm and regenerates the cell’s supply ofWhile producing a small amount of ATP. The two main types oftation are lactic acid fermentation and alcohol fermentation.

Figure 14 Electron transport occurs alongthe mitochondrial membrane.Compare and contrast electrontransport in cellular respiration andphotosynthesis.

SCIENCE USAGE V. COMMON USAGEConcentrationScience usage: the relative amount ofa substance dissolved in anothersubstanceThe concentration of hydrogen ions isgreater on one side of the membranethan on the other.

Common usage: the directing ofclose, undivided attentionThe student’s concentration was

focused on the exam.

Innermitochondrial a’pyruvate membrane

111)1)1~ppyyyOUUl) 111111

NADH a—fle

NADH

FADH2

Krebscycle

(If, (Ie PPpp

02

e 1/21 —+

H20 2H

NATP ~_32 ATP synthase

ADP + P

Section 3 • Cellular Respiration 231

Lactic Acid Fermentation Alcohol Fermentation

Based On Real Data*

Lactic acid fermentation In lactic acidfermentation, enzymes convert the pyruvate made during glycolysis tolactic acid, as shown in Figure 15. This involves the transfer of high.energy electrons and protons from NADH. Skeletal muscles producelactic acid when the body cannot supply enough oxygen, such as duringperiods of strenuous exercise. When lactic acid builds up in muscle cellmuscles become fatigued and might feel sore. Lactic acid also is produced by several microorganisms that often are used to produce manyfoods, including cheese, yogurt, and sour cream.

Alcohol fermentation Alcohol fermentation occurs in yeast andsome bacteria. Figure 15 shows the chemical reaction that occurs during alcohol fermentation when pyruvate is converted to ethyl alcoholand carbon dioxide. Similar to lactic acid fermentation, NADH donateelectrons during this reaction and NAD is regenerated.

YSI LABIcU

ntr e ataHow does viral infection affect cellular respiration? Infection by viruses can significantlyaffect cellular respiration and the ability of cellsto produce ATP. To test the effect of viral infection on the stages of cellular respiration, cellswere infected with a virus, and the amount oflactic acid and ATP produced were measured.

Think Critically1. Analyze how the virus affected lactic acid

production in the cells.2. calculate After 8 hours, by what percent

age was the lactic acid higher in the virusgroup than in the control group? By whatpercentage was Al? production decreased?

Data and Observations

n ectionon Lactic Acid Production

0.8 _____

0.6 0.8.2 ~ 0.6

0.4 5~0 .0.4

______ ‘~ 02~2 4 10 0

lime (hours)

3. Infer why having a virus such as the flumight make a person feel tired.

Data obtained from: El-Bacha. T., et al. 2004. Mayaro virus infection alters gImetabolism in cultured cells through activation ol the enzyme 6.phosphOfWd1 -kinase. Molecular and Cellular Biocliemist’y 266191 198.

Glucose

G 2 Lactic aciden2ADP C NAD

o CNADHD

2 Pyruvateen

Glucose

6YI.2 ADP —_~ C r NAD~0

TP NADH

2 Pyruvateen

2 Ethanol

2 Acetaldehyde

2CO~

Figure 15 When oxygen is absent or inlimited supply, fermentation can occur.compare and contrast lactic acidfermentation and alcohol fermentation.

Inquiry Virtual Lab

DATA AN

Effect of Viral Infectionon ATP Production

c~t~ViT,J5

Control Virus

232 chapter 8 • Cellular Energy

Figure 16 Photosynthesis and4unl ght.- cellular respiration form a cycle in which

the products of one metabolic pathwayform the reactants of the other metabolicpathway.

Photo- Photo- FlrFihI[iisystem system

II I i_i‘‘2 ___________

AD NADP AD NADHOP

Glucose Glycolysis ruvate?

hloroplast Mitochondrion

ATP

Photosynthesis andCellular Respiration Video BrainPOPAs you have learned, photosynthesis and cellular respiration aretwo important processes that cells use to obtain energy. They aremetabolic pathways that produce and break down simple carbohy

tes drates. Figure 16 shows how these two processes are related. Recallthat the products of photosynthesis are oxygen and glucose, thereactants needed for cellular respiration. The products of cellularrespiration, which are carbon dioxide and water, are the reactantsfor photosynthesis.

ection.: AssessmentSection Summary Understand Main Ideas) Many living organisms use cellular respira- 1. 4 Summarize the stages of cellular respiration.

tion to break down glucose. 2. Identify how many carbons from one glucose molecule enter one roundb NADH and FADH2 are important electron of the Krebs cycle.

carriers for cellular respiration. : 3. Explain how high-energy electrons are used in electron transport.I In the absence of oxygen, cells can sustain 4. Describe the role of fermentation in maintaining ATP and NAD levels.

glycolysis by fermentation. :Think Critically

MATH n Biology5. How manyATP, NADH, and FADH2 are produced in each step of cellular

respiration? How is the number of ATP produced different from the net ATPavailable?

6. Compare and contrast the two types of fermentation.

Assessment Online Quiz Section 3 • Cellular Respiration 233