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CELLULAR RESPIRATION: AEROBIC HARVEST OF FOOD ENERGY• Cellular respiration is:

– The main way that chemical energy is harvested from food and converted to ATP

– An aerobic process—it requires oxygen

© 2010 Pearson Education, Inc.

• Cellular respiration and breathing are closely related.

– Cellular respiration requires a cell to exchange gases with its surroundings.

– Cells take in oxygen gas.

– Cells release waste carbon dioxide gas.

– Breathing exchanges these same gases between the blood and outside air.


Breathing

Cellularrespiration

Musclecells

Lungs

CO2

CO2

O2

O2

Figure 6.3

Breathing

Cellularrespiration

Musclecells

Lungs

CO2

CO2

O2

O2

Figure 6.3a

C6H12O6 CO2O2 H2O

Glucose Oxygen Carbondioxide

Water

6 66

Reduction

Oxidation

Oxygen gains electrons (and hydrogens)

Glucose loses electrons(and hydrogens)

Figure 6.UN02


The Role of Oxygen in Cellular Respiration• Cellular respiration can produce up to 38 ATP molecules for each

glucose molecule consumed.

• During cellular respiration, hydrogen and its bonding electrons change partners.

– Hydrogen and its electrons go from sugar to oxygen, forming water.

– This hydrogen transfer is why oxygen is so vital to cellular respiration.


Redox Reactions• Chemical reactions that transfer electrons from one substance to

another are called:

– Oxidation-reduction reactions or

– Redox reactions for short


• The loss of electrons during a redox reaction is called oxidation.

• The acceptance of electrons during a redox reaction is called reduction.


• During cellular respiration glucose is oxidized while oxygen is reduced.


• Why does electron transfer to oxygen release energy?

– When electrons move from glucose to oxygen, it is as though the electrons were falling.

– This “fall” of electrons releases energy during cellular respiration.


• Cellular respiration is:

– A controlled fall of electrons

– A stepwise cascade much like going down a staircase


NADH and Electron Transport Chains• The path that electrons take on their way down from glucose to

oxygen involves many steps.


• The first step is an electron acceptor called NAD+.

– The transfer of electrons from organic fuel to NAD+ reduces it to NADH.

• The rest of the path consists of an electron transport chain, which:

– Involves a series of redox reactions

– Ultimately leads to the production of large amounts of ATP

Electrons from food

Stepwise releaseof energy usedto make

Hydrogen, electrons,and oxygen combineto produce water

Electron transport chain

NADHNAD

H

H

ATP

H2O

O2

2 2

2

2

21

e e

e e

e

e

Figure 6.5


An Overview of Cellular Respiration• Cellular respiration:

– Is an example of a metabolic pathway, which is a series of chemical reactions in cells

• All of the reactions involved in cellular respiration can be grouped into three main stages:

– Glycolysis

– The citric acid cycle

– Electron transport

Cytoplasm

Cytoplasm

Cytoplasm

Animal cell Plant cell

Mitochondrion

Mitochondrion

High-energyelectronscarriedby NADH

High-energyelectrons carriedmainly byNADH

CitricAcidCycle

ElectronTransport

Glycolysis

Glucose2

Pyruvicacid

ATP ATP ATP

Figure 6.6

CytoplasmMitochondrion

High-energyelectronscarriedby NADH

High-energyelectrons carriedmainly byNADH

CitricAcidCycle

ElectronTransport

Glycolysis

Glucose2

Pyruvicacid

ATP ATP ATP

Figure 6.6a


The Three Stages of Cellular Respiration• With the big-picture view of cellular respiration in mind, let’s

examine the process in more detail.


Stage 1: Glycolysis• A six-carbon glucose molecule is split in half to form two

molecules of pyruvic acid.

• These two molecules then donate high energy electrons to NAD+, forming NADH.

Energy investment phase

Carbon atomPhosphategroupHigh-energyelectron

Key

Glucose

2 ATP 2 ADP

INPUT OUTPUT

Figure 6.7-1



Key

Glucose

2 ATP 2 ADP

INPUT OUTPUT

Energy harvest phase

NADH

NADHNAD

NAD

Figure 6.7-2



Key

Glucose

2 ATP 2 ADP

INPUT OUTPUT

Energy harvest phase

NADH

NADHNAD

NAD 2 ATP

2 ATP

2 ADP

2 ADP

2 Pyruvic acid

Figure 6.7-3


• Glycolysis:

– Uses two ATP molecules per glucose to split the six-carbon glucose

– Makes four additional ATP directly when enzymes transfer phosphate groups from fuel molecules to ADP

• Thus, glycolysis produces a net of two molecules of ATP per glucose molecule.

ADPATP

P

P P

Enzyme

Figure 6.8


Stage 2: The Citric Acid Cycle• The citric acid cycle completes the breakdown of sugar.


• In the citric acid cycle, pyruvic acid from glycolysis is first “prepped.”

(from glycolysis) (to citric acid cycle)Oxidation of the fuelgenerates NADH

Pyruvic acidloses a carbonas CO2

Acetic acidattaches tocoenzyme APyruvic acid

Acetic acidAcetyl CoA

Coenzyme A

CoA

CO2

NAD NADH

INPUT OUTPUT

Figure 6.9


• The citric acid cycle:

– Extracts the energy of sugar by breaking the acetic acid molecules all the way down to CO2

– Uses some of this energy to make ATP

– Forms NADH and FADH2

Blast Animation: Harvesting Energy: Krebs Cycle

3 NAD

ADP P

3 NADH

FADH2FAD

Aceticacid

Citricacid

Acceptormolecule

CitricAcidCycle

ATP

2 CO2

INPUT OUTPUT

Figure 6.10


Stage 3: Electron Transport• Electron transport releases the energy your cells need to make the

most of their ATP.


• The molecules of the electron transport chain are built into the inner membranes of mitochondria.

– The chain functions as a chemical machine that uses energy released by the “fall” of electrons to pump hydrogen ions across the inner mitochondrial membrane.

– These ions store potential energy.


• When the hydrogen ions flow back through the membrane, they release energy.

– The hydrogen ions flow through ATP synthase.

– ATP synthase:

– Takes the energy from this flow

– Synthesizes ATP

Space betweenmembranes

Innermitochondrialmembrane

Electroncarrier

Proteincomplex

Electronflow

Matrix Electron transport chain ATP synthase

NADHNAD

FADH2 FAD

ATPADP

H2OO2

H

12

HHH

H

H

H

H

H

H

H

HH

HH

HH

HH

H 2

P

Figure 6.11

12

Spacebetweenmembranes

Innermitochondrialmembrane

Electroncarrier

Proteincomplex

Electronflow

Matrix Electron transport chain ATP synthase

NADHNAD

FADH2 FAD

ATPADP

H2OO2

HHHH

H

H

H

H

H

H

H

HH

HH

HH

HH

H 2

P

Figure 6.11a


The Versatility of Cellular Respiration• In addition to glucose, cellular respiration can “burn”:

– Diverse types of carbohydrates

– Fats

– Proteins

Food

Polysaccharides Fats Proteins

Sugars Glycerol Fatty acids Amino acids

Glycolysis AcetylCoA

CitricAcidCycle

ElectronTransport

ATP

Figure 6.12


Adding Up the ATP from Cellular Respiration• Cellular respiration can generate up to 38 molecules of ATP per

molecule of glucose.

Cytoplasm

Mitochondrion

NADH

CitricAcidCycle

ElectronTransport

Glycolysis

Glucose2

Pyruvicacid

2ATP

2ATP

NADHNADH

FADH2

Maximumper

glucose:

2AcetylCoA

About34 ATP

by directsynthesis

by directsynthesis by ATP

synthase

2 2

2

6

About38 ATP

Figure 6.13


FERMENTATION: ANAEROBIC HARVEST OF FOOD ENERGY• Some of your cells can actually work for short periods without

oxygen.

• Fermentation is the anaerobic (without oxygen) harvest of food energy.


Fermentation in Human Muscle Cells• After functioning anaerobically for about 15 seconds:

– Muscle cells will begin to generate ATP by the process of fermentation

• Fermentation relies on glycolysis to produce ATP.


• Glycolysis:

– Does not require oxygen

– Produces two ATP molecules for each glucose broken down to pyruvic acid


• Pyruvic acid, produced by glycolysis, is

– Reduced by NADH, producing NAD+, which keeps glycolysis going.

• In human muscle cells, lactic acid is a by-product.

Animation: Fermentation Overview

Glucose

2 ATP

2 NAD 2 NADH 2 NADH 2 NAD

2

2 ADP

2 Pyruvicacid 2 Lactic acid

Glycolysis

INPUT OUTPUT

2 P

H

Figure 6.14a


• Observation: Muscles produce lactic acid under anaerobic conditions.

• Question: Does the buildup of lactic acid cause muscle fatigue?

• Hypothesis: The buildup of lactic acid would cause muscle activity to stop.

• Experiment: Tested frog muscles under conditions when lactic acid could and could not diffuse away.

The Process of Science: Does Lactic Acid Buildup Cause Muscle Burn?

Battery

Forcemeasured

Battery

Forcemeasured

Frog musclestimulated byelectric current

Solution preventsdiffusion of lactic acid

Solution allowsdiffusion of lactic acid;

muscle can work fortwice as long

Figure 6.15


• Results: When lactic acid could diffuse away, performance improved greatly.

• Conclusion: Lactic acid accumulation is the primary cause of failure in muscle tissue.

• However, recent evidence suggests that the role of lactic acid in muscle function remains unclear.


Fermentation in Microorganisms• Fermentation alone is able to sustain many types of

microorganisms.

• The lactic acid produced by microbes using fermentation is used to produce:

– Cheese, sour cream, and yogurt dairy products

– Soy sauce, pickles, olives

– Sausage meat products


• Yeast are a type of microscopic fungus that:

– Use a different type of fermentation

– Produce CO2 and ethyl alcohol instead of lactic acid

• This type of fermentation, called alcoholic fermentation, is used to produce:

– Beer

– Wine

– Breads

Glucose

2 ATP


2

2 P

2 Pyruvicacid 2 Ethyl alcohol

Glycolysis

INPUT OUTPUT

2 CO2 released

Bread with airbubbles producedby fermenting yeast Beer

fermentation

2 ADP

H

Figure 6.16

Glucose

2 ATP


2

2 ADP

2 Pyruvicacid 2 Ethyl alcohol

Glycolysis

INPUT OUTPUT

2 CO2 released 2 P

H

Figure 6.16a

C6H12O6 CO2ATPO2 H2O


Water Energy

6 66

Figure 6.UN01

C6H12O6 CO2O2 H2O


Water

6 66

Reduction

Oxidation

Oxygen gains electrons (and hydrogens)

Glucose loses electrons(and hydrogens)

Figure 6.UN02

CitricAcidCycle

ElectronTransportGlycolysis

ATP ATPATP

Figure 6.UN03

CitricAcidCycle


ATP ATPATP

Figure 6.UN04

CitricAcidCycle


ATP ATPATP

Figure 6.UN05

C6H12O6

CO2 H2O

ATPO2

Heat

Photosynthesis

Sunlight

Cellularrespiration

Figure 6.UN06

C6H12O6 CO2 ATPO2 H2O 6 66 Approx. 38

Figure 6.UN07

C6H12O6 CO2

ATP

O2 H2O

OxidationGlucose loses electrons(and hydrogens)

ReductionOxygen gainselectrons (andhydrogens)

Electrons(and hydrogens)

Figure 6.UN08

NADH

CitricAcidCycle

ElectronTransport

Glycolysis

Glucose2

Pyruvicacid

2ATP

2ATP

NADHNADH

FADH2

2AcetylCoA

About34 ATPby direct

synthesisby directsynthesis

by ATPsynthase

2 2

2

6

About38 ATP

2 CO2 CO2

O2

H2O4

Mitochondrion

Figure 6.UN09


Sunlight energyenters ecosystem

Photosynthesis

Cellular respiration

C6H12O6

Glucose

O2

Oxygen

CO2

Carbon dioxide

H2OWater

drives cellular work

Heat energy exits ecosystem

ATP

Figure 6.2