Chapter 6 How Cells Harvest Chemical Energy In eukaryotes, cellular respiration –harvests energy from food, –yields large amounts of ATP, and –Uses ATP

Chapter 6 How Cells Harvest Chemical Energy

In eukaryotes, cellular respiration

– harvests energy from food,

– yields large amounts of ATP, and

– Uses ATP to drive cellular work.

A similar process takes place in many prokaryotic organisms.

© 2012 Pearson Education, Inc.

CELLULAR RESPIRATION: AEROBIC HARVESTING

OF ENERGY


6.1 Photosynthesis and cellular respiration provide energy for life

Life requires energy.

In almost all ecosystems, energy ultimately comes from the sun.

In photosynthesis,

– some of the energy in sunlight is captured by chloroplasts,

– atoms of carbon dioxide and water are rearranged, and

– glucose and oxygen are produced.


6.1 Photosynthesis and cellular respiration provide energy for life

In cellular respiration

– glucose is broken down to carbon dioxide and water and

– the cell captures some of the released energy to make ATP.

Cellular respiration takes place in the mitochondria of eukaryotic cells.


Figure 6.1

Sunlightenergy

ECOSYSTEM

Photosynthesis inchloroplasts Organic

moleculesCellularrespiration inmitochondria

ATP powers mostcellular work

Heatenergy

CO2 + H2O + O2

ATP

6.2 Breathing supplies O2 for use in cellular respiration and removes CO2

Respiration, as it relates to breathing, and cellular respiration are not the same.

– Respiration, in the breathing sense, refers to an exchange of gases. Usually an organism brings in oxygen from the environment and releases waste CO2.

– Cellular respiration is the aerobic (oxygen requiring) harvesting of energy from food molecules by cells.


Figure 6.2

Breathing

Lungs

BloodstreamCO2 O2

O2 CO2

Muscle cells carrying out

Cellular Respiration

Glucose O2 CO2 H2O ATP

6.3 Cellular respiration banks energy in ATP molecules

Cellular respiration is an exergonic process that transfers energy from the bonds in glucose to form ATP.

Cellular respiration

– produces up to 32 ATP molecules from each glucose molecule and

– captures only about 34% of the energy originally stored in glucose.

Other foods (organic molecules) can also be used as a source of energy.


Figure 6.3 Summary equation for cellular respiration

Glucose Oxygen WaterCarbondioxide

C6H12O6 O2 H2O ATP6 6 6

Heat

CO2

6.4 CONNECTION: The human body uses energy from ATP for all its activities

The average adult human needs about 2,200 kcal of energy per day.

– About 75% of these calories are used to maintain a healthy body.

– The remaining 25% is used to power physical activities.


6.4 CONNECTION: The human body uses energy from ATP for all its activities

A kilocalorie (kcal) is

– the quantity of heat required to raise the temperature of 1 kilogram (kg) of water by 1oC,

– the same as a food Calorie, and

– used to measure the nutritional values indicated on food labels.


Figure 6.4 Energy consumed by various activitiesFigure 6.4-0

Activity

Running (8–9 mph)

Dancing (fast)

Bicycling (10 mph)

Swimming (2 mph)

Walking (4 mph)

Walking (3 mph)

Dancing (slow)

Driving a car

Sitting (writing)

kcal consumed per hourby a 67.5-kg (150-lb) person*

*Not including kcal needed forbody maintenance

979

510

490

408

341

245

204

61

28

6.5 Cells tap energy from electrons “falling” from organic fuels to oxygen

The energy necessary for life is contained in the arrangement of electrons in chemical bonds in organic molecules.

An important question is how do cells extract this energy?



When the carbon-hydrogen bonds of glucose are broken, electrons are transferred to oxygen.

– Oxygen has a strong tendency to attract electrons.

– An electron loses potential energy when it “falls” to oxygen.



Energy can be released from glucose by simply burning it.

The energy is dissipated as heat and light and is not available to living organisms.



On the other hand, cellular respiration is the controlled breakdown of organic molecules.

Energy is

– gradually released in small amounts,

– captured by a biological system, and

– stored in ATP.



The movement of electrons from one molecule to another is an oxidation-reduction reaction, or redox reaction. In a redox reaction,

– the loss of electrons from one substance is called oxidation,

– the addition of electrons to another substance is called reduction,

– a molecule is oxidized when it loses one or more electrons, and

– reduced when it gains one or more electrons.



A cellular respiration equation is helpful to show the changes in hydrogen atom distribution.

Glucose

– loses its hydrogen atoms and

– becomes oxidized to CO2.

Oxygen

– gains hydrogen atoms and

– becomes reduced to H2O.


Figure 6.5A Rearrangement of hydrogen atoms (with their electrons) in the redox reactions of cellular respiration

Loss of hydrogen atoms(becomes oxidized)

Gain of hydrogen atoms(becomes reduced)

(Glucose)

C6H12O6 + 6 O2 6 CO2 + 6 H2O + ATP + Heat


Enzymes are necessary to oxidize glucose and other foods.

NAD+

– is an important coenzyme in oxidizing glucose,

– accepts electrons, and

– becomes reduced to NADH.


Figure 6.5B A pair of redox reactions occuring simultaneously

Becomes oxidized

Becomes reduced

2H

2HNAD NADH

H

H

2 2(carries

2 electrons)


There are other electron “carrier” molecules that function like NAD+.

– They form a staircase where the electrons pass from one to the next down the staircase.

– These electron carriers collectively are called the electron transport chain.

– As electrons are transported down the chain, ATP is generated.


Figure 6.5c

NAD+

H+

NADHEnergy releasedand availablefor making

2

2O2

2

H2O

−21

ATP

STAGES OF CELLULAR RESPIRATION


6.6 Overview: Cellular respiration occurs in three main stages

Cellular respiration consists of a sequence of steps that can be divided into three stages.

– Stage 1 – Glycolysis

– Stage 2 – Pyruvate oxidation and citric acid cycle

– Stage 3 – Oxidative phosphorylation



Stage 1: Glycolysis

– occurs in the cytoplasm,

– begins cellular respiration, and

– breaks down glucose into two molecules of a three-carbon compound called pyruvate.



Stage 2: The citric acid cycle

– takes place in mitochondria,

– oxidizes pyruvate to a two-carbon compound, and

– supplies the third stage with electrons.



Stage 3: Oxidative phosphorylation

– involves electrons carried by NADH and FADH2,

– shuttles these electrons to the electron transport chain embedded in the inner mitochondrial membrane,

– involves chemiosmosis, and

– generates ATP through oxidative phosphorylation associated with chemiosmosis.


Figure 6.6

Electrons carried by NADH FADH2

ATPATP

ATP

Glycolysis

Glucose PyruvatePyruvateOxidation

Citric AcidCycle

Substrate-levelphosphorylation

Substrate-levelphosphorylation

Oxidativephosphorylation

OxidativePhosphorylation

(Electron transportand chemiosmosis)

CYTOSOL MITOCHONDRION

6.7 Glycolysis harvests chemical energy by oxidizing glucose to pyruvate

In glycolysis,

– a single molecule of glucose is enzymatically cut in half through a series of steps,

– two molecules of pyruvate are produced,

– two molecules of NAD+ are reduced to two molecules of NADH, and

– a net of two molecules of ATP is produced.


Figure 6.7A An overview of glycolysis

Glucose

2 Pyruvate

2 ADP

2 P2 NAD

2 NADH

2 HATP2

6.7 Glycolysis harvests chemical energy by oxidizing glucose to pyruvate

ATP is formed in glycolysis by substrate-level phosphorylation during which

– an enzyme transfers a phosphate group from a substrate molecule to ADP and

– ATP is formed.

The compounds that form between the initial reactant, glucose, and the final product, pyruvate, are called intermediates.


Figure 6.7B Substrate-level phosphorylation: transfer of a phosphate group from a substrate to ADP, producing ATP

Enzyme Enzyme

P

P

ADP

P

ATP

Substrate Product

6.8 Pyruvate is oxidized prior to the citric acid cycle

The pyruvate formed in glycolysis is transported from the cytoplasm into a mitochondrion where

– the citric acid cycle and

– oxidative phosphorylation will occur.


6.8 Pyruvate is oxidized prior to the citric acid cycle

Two molecules of pyruvate are produced for each molecule of glucose that enters glycolysis.

Pyruvate does not enter the citric acid cycle, but undergoes some chemical grooming in which

– a carboxyl group is removed and given off as CO2,

– the two-carbon compound remaining is oxidized while a molecule of NAD+ is reduced to NADH,

– coenzyme A joins with the two-carbon group to form acetyl coenzyme A, abbreviated as acetyl CoA, and

– acetyl CoA enters the citric acid cycle.© 2012 Pearson Education, Inc.

Figure 6.8 The link between glycolysis and the citric acid cycle

Pyruvate

Coenzyme A

Acetyl coenzyme A

NAD NADH H

CoA

CO2

3

2

1

6.9 The citric acid cycle completes the oxidation of organic molecules, generating many NADH and FADH2 molecules

The citric acid cycle

– is also called the Krebs cycle (after the German-British researcher Hans Krebs, who worked out much of this pathway in the 1930s),

– completes the oxidation of organic molecules, and

– generates many NADH and FADH2 molecules.


Figure 6.9A An overview of the citric acid cycleAcetyl CoA

Citric Acid Cycle

CoA

CoA

CO22

3

3

NAD

3 H

NADH

ADPATP P

FAD

FADH2


During the citric acid cycle

– the two-carbon group of acetyl CoA is added to a four-carbon compound, forming citrate,

– citrate is degraded back to the four-carbon compound,

– two CO2 are released, and

– 1 ATP, 3 NADH, and 1 FADH2 are produced.



Remember that the citric acid cycle processes two molecules of acetyl CoA for each initial glucose.

Thus, after two turns of the citric acid cycle, the overall yield per glucose molecule is

– 2 ATP,

– 6 NADH, and

– 2 FADH2.


6.10 Most ATP production occurs by oxidative phosphorylation

Oxidative phosphorylation

– involves electron transport and chemiosmosis and

– requires an adequate supply of oxygen.


6.10 Most ATP production occurs by oxidative phosphorylation

Electrons from NADH and FADH2 travel down the electron transport chain to O2.

Oxygen picks up H+ to form water.

Energy released by these redox reactions is used to pump H+ from the mitochondrial matrix into the intermembrane space.

In chemiosmosis, the H+ diffuses back across the inner membrane through ATP synthase complexes, driving the synthesis of ATP.


Figure 6.10a

OUTER MITOCHONDRIAL MEMBRANE

Q

FADH2 FAD

ATPPADP +

H+

NAD+NADH

H2O

O2

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

Cyt c

−21 + 2

II

III

I

ATPsynthase

Proteincomplexof electroncarriers

MobileelectroncarriersIntermem-

branespace

Inner mito-chondrialmembrane

Mito-chondrialmatrix

Electronflow

Electron Transport Chain

Oxidative Phosphorylation

Chemiosmosis

IV

6.11 CONNECTION: Interrupting cellular respiration can have both harmful and beneficial effects

Three categories of cellular poisons obstruct the process of oxidative phosphorylation. These poisons

1. block the electron transport chain (for example, rotenone, cyanide, and carbon monoxide),

2. inhibit ATP synthase (for example, the antibiotic oligomycin), or

3. make the membrane leaky to hydrogen ions (called uncouplers, examples include dinitrophenol (weight loss pill that caused fatalities)).


6.12 Review: Each molecule of glucose yields many molecules of ATP

Recall that the energy payoff of cellular respiration involves

1. glycolysis,

2. alteration of pyruvate,

3. the citric acid cycle, and

4. oxidative phosphorylation.


6.12 Review: Each molecule of glucose yields many molecules of ATP

The total yield is about 32 ATP molecules per glucose molecule.

This is about 34% of the potential energy of a glucose molecule.

In addition, water and CO2 are produced.


Figure 6.12

NADH FADH2

CO2

Maximumper glucose:

+ 2ATP

Glycolysis

Glucose2

Pyruvate

PyruvateOxidation2 Acetyl

CoA

Citric AcidCycle

by substrate-levelphosphorylation

by substrate-levelphosphorylation

by oxidativephosphorylation


(electron transportand chemiosmosis)

CYTOSOLMITOCHONDRION

2NADH2 NADH6 + 2

+ 2ATP

+ about28 ATP About

32 ATP

O2

H2O

FERMENTATION: ANAEROBIC HARVESTING OF ENERGY


6.13 Fermentation enables cells to produce ATP without oxygen

Fermentation is a way of harvesting chemical energy that does not require oxygen. Fermentation

– takes advantage of glycolysis,

– produces two ATP molecules per glucose, and

– reduces NAD+ to NADH.

Fermentation provides an anaerobic path for recycling NADH back to NAD+.



Your muscle cells and certain bacteria can oxidize NADH through lactic acid fermentation, in which

– NADH is oxidized to NAD+ and

– pyruvate is reduced to lactate.



Lactate is carried by the blood to the liver, where it is converted back to pyruvate and oxidized in the mitochondria of liver cells.

The dairy industry uses lactic acid fermentation by bacteria to make cheese and yogurt.

Other types of microbial fermentation turn

– soybeans into soy sauce and

– cabbage into sauerkraut.


Figure 6.13A Lactic acid fermentation: NAD+ is regenerated as pyruvate is reduced to lactate.

2 NAD

2 NADH

2 NAD

2 NADH

2 Lactate

2 Pyruvate

Glucose

2 ADP

2 ATP

2 P

Gly

coly

sis


The baking and winemaking industries have used alcohol fermentation for thousands of years.

In this process yeasts (single-celled fungi)

– oxidize NADH back to NAD+ and

– convert pyruvate to CO2 and ethanol.


Figure 6.13B Alchohol fermentation: NAD is regenerated as pyruvate is broken down to CO2 and ethanol.

2 NAD

2 NADH

2 NAD

2 NADH

2 Ethanol

2 Pyruvate

Glucose

2 ADP

2 ATP

2 P

Gly

coly

sis

2 CO2


Obligate anaerobes

– are poisoned by oxygen, requiring anaerobic conditions, and

– live in stagnant ponds and deep soils.

Facultative anaerobes

– include yeasts and many bacteria, and

– can make ATP by fermentation or oxidative phosphorylation.


6.14 EVOLUTION CONNECTION: Glycolysis evolved early in the history of life on Earth

Glycolysis is the universal energy-harvesting process of life.

The role of glycolysis in fermentation and respiration dates back to

– life long before oxygen was present,

– when only prokaryotes inhabited the Earth,

– about 3.5 billion years ago.


6.14 EVOLUTION CONNECTION: Glycolysis evolved early in the history of life on Earth

The ancient history of glycolysis is supported by its

– occurrence in all the domains of life and

– location within the cell, using pathways that do not involve any membrane-bounded organelles.


CONNECTIONS BETWEEN METABOLIC PATHWAYS


6.15 Cells use many kinds of organic molecules as fuel for cellular respiration

Although glucose is considered to be the primary source of sugar for respiration and fermentation, ATP is generated using

– carbohydrates,

– fats, and

– proteins.


6.15 Cells use many kinds of organic molecules as fuel for cellular respiration

Fats make excellent cellular fuel because they

– contain many hydrogen atoms and thus many energy-rich electrons and

– yield more than twice as much ATP per gram than a gram of carbohydrate or protein.


Figure 6.15

Food, such aspeanuts

Sugars Glycerol Fatty acids Amino acids

Aminogroups


CitricAcidCycle

PyruvateOxidation

Acetyl CoA

ATP

Glucose G3P PyruvateGlycolysis

Carbohydrates Fats Proteins

6.16 Food molecules provide raw materials for biosynthesis

Cells use intermediates from cellular respiration for the biosynthesis of other organic molecules.



Glucose G3P Pyruvate

GlycolysisAcetylCoA

CitricAcidCycle

Figure 6.15 Biosynthesis of large organic molecules from intermediates of cellular respiration

Cells, tissues,organisms

ATP neededto drivebiosynthesis

CarbohydratesFatsProteins

SugarsGlycerolFattyacids

Amino acids

Aminogroups

Pyruvate G3P GlucoseGlucose SynthesisAcetyl

CoA

CitricAcidCycle

ATP

Documents

Chapter 6 How Cells Harvest Chemical Energy In eukaryotes, cellular respiration –harvests energy from food, –yields large amounts of ATP, and –Uses ATP