Cellular Respiration: Obtaining Energy from Food

© 2010 Pearson Education, Inc.

Lectures by Chris C. Romero, updated by Edward J. Zalisko

PowerPoint® Lectures for Campbell Essential Biology, Fourth Edition – Eric Simon, Jane Reece, and Jean Dickey

Campbell Essential Biology with Physiology, Third Edition – Eric Simon, Jane Reece, and Jean Dickey

Chapter 6

Cellular Respiration: Obtaining Energy from Food

ENERGY FLOW AND CHEMICAL CYCLING IN THE BIOSPHERE

• Animals depend on plants to convert solar energy to: – Chemical energy of sugars – Other molecules we consume as food


• Photosynthesis:

– Uses light energy from the sun to power a chemical process that makes organic molecules.


Producers and Consumers

Producers Plants and other autotrophs (self-feeders)

Make their own organic matter (carbohydrates, proteins, fats and nucleic acids) from nutrients that are entirely inorganic (CO2, H2O, minerals from soil). Autotrophs are producers because ecosystems depend upon them for food.

Consumers They eat plants or other animals.

Cannot make their own organic molecules from inorganic ones. Heterotrophs are consumers because they eat plants or other animals.

Chemical Cycling between Photosynthesis and Cellular Respiration

• The ingredients for photosynthesis are carbon dioxide and water.

– CO2 is obtained from the air by a plant’s leaves. – H2O is obtained from the damp soil by a plant’s roots.


• Chloroplasts in the cells of leaves:

– Use light energy to rearrange the atoms of CO2 and H2O, which produces

– Sugars (such as glucose)

– Other organic molecules

– Oxygen


• Plant and animal cells perform cellular respiration, a chemical process that:

– Primarily occurs in mitochondria – Harvests energy stored in organic molecules – Uses oxygen – Generates ATP

• The waste products of cellular respiration are:

– CO2 and H2O

– Used in photosynthesis


• Animals perform only cellular respiration. • Plants perform:

– Photosynthesis and – Cellular respiration

Sunlight energy enters ecosystem

Photosynthesis

Cellular respiration

C6H12O6

Glucose

O2

Oxygen

CO2

Carbon dioxide

H2O

Water

drives cellular work

Heat energy exits ecosystem

ATP

Figure 6.2

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


• 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

Cellular respiration

Muscle cells

Lungs

CO2

CO2

O2

O2

Figure 6.3


The Overall Equation for Cellular Respiration

• A common fuel molecule for cellular respiration is glucose. • The overall equation for what happens to glucose during

cellular respiration:

C6H12O6

C6H12O6 CO2 O2 H2O

Glucose Oxygen Carbon dioxide

Water

+ 6 + 6 6

Reduction

Oxidation

Oxygen gains electrons (and hydrogens)

Glucose loses electrons (and hydrogens)

Figure 6.UN02

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

OIL RIG: Oxidation is losing; Reduction is gaining


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. (OIL)

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

• 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.

Release of heat energy

1 2

H2 O2

H2O

Figure 6.4


• 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+ (nicotinamide adenine dinucleotide).

– 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

– Oxygen, “the final electron grabber”, is needed to drive cellular resporation

Electrons from food

Stepwise release of energy used to make

Hydrogen, electrons, and oxygen combine to produce water

NADH NAD+

H+

H+

ATP

H2O

O2

2 2

2

2

2 1

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

Citric Acid Cycle

Electron Transport Glycolysis

ATP ATP ATP

Figure 6.UN03

Cytoplasm

Cytoplasm

Cytoplasm

Animal cell Plant cell

Mitochondrion

Mitochondrion

High-energy electrons carried by NADH

High-energy electrons carried mainly by NADH

Citric Acid Cycle

Electron Transport

Glycolysis

Glucose 2

Pyruvic acid

ATP ATP ATP

Figure 6.6


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 atom

Phosphate group

High-energy electron

Key

Glucose

2 ATP 2 ADP

INPUT OUTPUT

Figure 6.7-1


Carbon atom

Phosphate group


Key

Glucose

2 ATP 2 ADP

INPUT OUTPUT

Energy harvest phase

NADH

NADH

NAD+

NAD+

Figure 6.7-2


Carbon atom

Phosphate group


Key

Glucose

2 ATP 2 ADP

INPUT OUTPUT

Energy harvest phase

NADH

NADH

NAD+

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.

ADP ATP

P

P P

Enzyme

Figure 6.8

ATP synthesis by direct phosphate transfer


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.”

• 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

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

Pyruvic acid loses a carbon as CO2

Acetic acid attaches to coenzyme A Pyruvic acid

Acetic acid Acetyl CoA

Coenzyme A

CoA

CO2

NAD+ NADH

INPUT OUTPUT

Figure 6.9

3 NAD+

ADP + P

3 NADH

FADH2 FAD

Acetic acid

Citric acid

Acceptor molecule

Citric Acid Cycle

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 between membranes

Inner mitochondrial membrane

Electron carrier

Protein complex

Electron flow

Matrix Electron transport chain ATP synthase

NADH NAD+

FADH2 FAD

ATP ADP +

H2O O2

H+

1

2

H+ H+ H+

H+

H+

H+

H+

H+

H+

H+

H+ H+

H+ H+

H+ H+

H+ H+

H+ + 2

P

Figure 6.11


• Cyanide is a deadly poison that: – Binds to one of the protein complexes in the electron transport

chain – Prevents the passage of electrons to oxygen – Stops the production of ATP


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 Acetyl CoA

Citric Acid Cycle

Electron Transport

ATP

Figure 6.12

Cytoplasm

Mitochondrion

NADH

Citric Acid Cycle

Electron Transport

Glycolysis

Glucose 2

Pyruvic acid

2 ATP

2 ATP

NADH NADH

FADH2

Maximum per

glucose:

2 Acetyl CoA

About 34 ATP

by direct synthesis

by direct synthesis

by ATP synthase

2 2

2

6

About 38 ATP

Figure 6.13

Adding Up the ATP from Cellular Respiration

• Cellular respiration can generate up to 38 molecules of ATP per molecule of glucose.


Adding up ATP

• Glycolysis: 2 ATP

• Citric acid cycle: 2 ATP

• Electron transport chain: 34 ATP

– Each electron pair dropped from NADH powers the synthesis of 3 ATP

– Each electron pair dropped from FADH2 is worth up to 2 ATP


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.

Glucose

2 ATP

2 NAD+ 2 NADH 2 NADH 2 NAD+

+ 2 H+

2 ADP

2 Pyruvic acid

2 Lactic acid

Glycolysis

INPUT OUTPUT

+ 2 P

Figure 6.14


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 NAD+ 2 NADH 2 NADH

2 NAD+

+ 2

+ 2 P

2 Pyruvic acid 2 Ethyl alcohol

Glycolysis

INPUT OUTPUT

2 CO2 released

Bread with air bubbles produced by fermenting yeast

Beer fermentation

2 ADP

H+

Figure 6.16

Evolution Connection: Life before and after Oxygen

• Glycolysis could be used by ancient bacteria to make ATP when little oxygen was available, and before organelles evolved.


• Today, glycolysis:

– Occurs in almost all organisms

– Is a metabolic heirloom of the first stage in the breakdown of organic molecules

O2 p

rese

nt

in

Ea

rth

’s a

tmo

sp

he

re

First eukaryotic organisms

Origin of Earth

Atmospheric oxygen reaches 10% of modern levels

Atmospheric oxygen first appears

Oldest prokaryotic fossils B

illi

on

s o

f ye

ars

ag

o

2.1

0

2.7

4.5

2.2

3.5

Figure 6.17

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Cellular Respiration: Obtaining Energy from Food

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