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How Cells Harvest Chemical Energy

Chapter 6

Cellular Respiration

Energy Flow and Chemical Cyclingin the Biosphere

Fuel molecules in food represent solar energy traced back to the sun

Animals depend on plants: to convert solar energy to

chemical energy In form of sugars and other

organic molecules

Gas Exchange in the Body

Cellular respiration and breathing are closely related Cellular respiration requires a

cell to exchange gases with its surroundings

Breathing exchanges these gases between blood and outside air

Cellular respiration

Cellular respiration is an exergonic process that transfers energy from the bonds in glucose to ATP– produces 38 ATP molecules from each

glucose molecule– Other foods (organic molecules) can be

used as a source of energy as well

Cellular Respiration

Release of energy from molecules accompanied by the use of this energy to synthesize ATP molecules

Metabolic pathway Main method that chemical energy is

harvested from food and converted to ATP

Aerobic Requires oxygen and gives off carbon

dioxide

Where Is the Energy in Food?

The process of aerobic respiration requires oxygen and carbohydrates

C6H12O6 + 6 O2 6 CO2 + 6 H2O + energy

The products are carbon dioxide, water, and energy (heat or ATP)

How do we get the energy??

Energy contained in the arrangement of electrons in chemical bonds in organic molecules

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

When the carbon-hydrogen bonds of glucose are broken, electrons are transferred to oxygen– Oxygen has a strong tendency to attract electrons

ATP

Adenosine triphosphate (ATP) Nucleotide with the base

adenine and the sugar ribose Main energy carrier in cells Formed during reactions that

breakdown organic compounds to CO2 and water

Requires ample oxygen Occurs within the

mitochondrion Hydrolyzes phosphates to

release energy form adenosine

diphosphate (ADP)

Redox Reaction (O-R)

Chemical reaction that transfers electrons from one substance to another electrons retain their

potential energy– Glucose loses its

hydrogen atoms and is ultimately converted to CO2

– O2 gains hydrogen atoms and is converted to H2O

Oxidation

Reduction

Redox Reaction

Electrons pass from atoms or molecules to one another as part of many energy reactions Oxidation

When an atom or molecule loses an electron Glucose is oxidized

Reduction When an atom or molecule gains an elections Oxygen is reduced

Other important players……

Enzymes are necessary to oxidize glucose and other foods– Dehydrogenase

– enzyme that removes hydrogen from an organic molecule

– requires a coenzyme called NAD+ – (nicotinamide adenine dinucleotide)– shuttle electrons– NAD+ can become reduced when it accepts electrons

and oxidized when it gives them up– Reduced to NADH

The Finale….

First step is transfer of electrons from organic molecule to NAD+

Other electron carrier molecules represent the electron transport chain Undergoes series of redox reactions Release energy to make ATP

ATPNAD+

NADH

H+

H+2e–

2e–

Electron transport

chain

Controlledrelease ofenergy forsynthesis

of ATP

+

O2

H2O

12

Stages of Cellular Respiration:

1. Glycolysis

2. Citric Acid Cycle

3. Oxidative Phosphorylation

Mitochondrion

CO2 CO2

NADH

ATP

High-energy electronscarried by NADH

NADH

CITRIC ACID

CYCLE

GLYCOLYSIS

PyruvateGlucose

andFADH2

Substrate-levelphosphorylation

Substrate-levelphosphorylation

OXIDATIVEPHOSPHORYLATION(Electron Transportand Chemiosmosis)

Oxidativephosphorylation

ATPATP

CytoplasmInnermitochondrialmembrane

1. Glycolysis

Occurs in the cytoplasm Does not require

oxygen to generate ATP

Then, enters aerobic or anaerobic reactions

Glycolysis

6 Carbon oxidizes glucose into 2 molecules Pyruvate 3 Carbon

breaking of the bond yields energy that is used to phosphorylate ADP to ATP

in addition, electrons and hydrogen are donated to NAD+ to form NADH

Glucose

NAD+

+2

2 ADP

NADH2

P2

2

ATP2 +

H+

2 Pyruvate

ADP

ATP

Substrate

Enzyme

Product

Enzyme

P

P

P

Anaerobic verse aerobic?

Absence of oxygen Fermentation Make lactate or ethanol

Presence of oxygen Oxidative respiration Pyruvate transported to mitochondria Oxidize pyruvate to form acetyl-coA

When pyruvate is oxidized:

A single carbon cleaved off by the enzyme pyruvate dehydrogenase This carbon leaves as part of a CO2

molecule hydrogen and electrons are removed

from pyruvate donated to NAD+ to form NADH

Remaining two-carbon fragment of pyruvate is joined to a cofactor called coenzyme A (CoA)

Final compound called acetyl-CoA

Acetyl-CoA

The fate of acetyl-CoA depends on the availability of ATP in the cell Insufficient ATP

The acetyl-CoA heads to the Krebs cycle Plentiful ATP

The acetyl-CoA is diverted to fat synthesis for energy storage

2. Citric Acid Cycle

“Krebs Cycle” occurs within the

mitochondrion Breaks down

pyruvate into carbon dioxide

electrons passed to an electron transport chain in order to power the production of ATP

Stages of Citric Acid Cycle

acetyl (two-carbon) compound enters the citric acid cycle1. Acetyl-CoA enters the cycle and binds to a four-carbon

molecule, forming a six-carbon molecule

2. Two carbons are removed as CO2 and their electrons donated to NAD+

In addition, an ATP is produced

3. The four-carbon molecule is recycled and more electrons are extracted, forming NADH and FADH2

The Krebs cycle

Note: a single glucose molecule produces two turns of the cycle, one for each of the two pyruvate molecules generated by glycolysis

CITRIC ACID CYCLE

NAD+

NADH

3 H+

CO2

3

3

2

CoA

CoA

Acetyl CoA

PADP +ATP

FADH2

FAD

Cytoplasm

Glucose

FADH2

Mitochondrion

Maximum per glucose:


CITRIC ACIDCYCLE

Electron shuttleacross membrane

2 NADH

2 NADH

2 NADH

6 NADH 2(or 2 FADH2)

2 AcetylCoA

GLYCOLYSIS2

Pyruvate

About38 ATP

about 34 ATP

by substrate-levelphosphorylation

by oxidative phosphorylation

2 ATP


2 ATP


Electron transport chain Shuttle molecules NADH and FADH take electrons

to oxygen Final acceptor

Forms H2O Carriers bind and release electrons in redox

reactions Pass electrons down the “energy staircase” Use energy released from the transfers to transport H+

ATP

H+

Intermembranespace

O2

H2O

12

Innermitochondrialmembrane

H+NAD+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

Mitochondrialmatrix

Electronflow

Electroncarrier

Proteincomplexof electroncarriers

NADH

FADH2FAD

ATPsynthase

PADP +

Chemiosmosis

+ 2

OXIDATIVE PHOSPHORYLATION

Electron Transport Chain


Chemiosmosis Uses energy stored in a hydrogen ion

gradient to drive ATP synthesis H+ concentration gradient stores potential

energy ATP synthase drives hydrogen ions

through Generates ATP

Cytoplasm

Glucose

FADH2

Mitochondrion

Maximum per glucose:


CITRIC ACIDCYCLE

Electron shuttleacross membrane

2 NADH

2 NADH

2 NADH

6 NADH 2(or 2 FADH2)

2 AcetylCoA

GLYCOLYSIS2

Pyruvate

About38 ATP

about 34 ATP


by oxidative phosphorylation

2 ATP


2 ATP

Fermentation

Occurs when O2 is not available

Animal cells and bacteria convert pyruvate to lactate

Other organisms convert pyruvate to alcohol and CO2

Glucose Is Not the Only Food Molecule

Cells also get energy from foods other than sugars

The other organic building blocks undergo chemical modifications that permit them to enter cellular respiration

Food, such aspeanuts

ProteinsFatsCarbohydrates

Glucose


CITRICACID

CYCLE

AcetylCoA

GLYCOLYSIS

Pyruvate

Amino acidsGlycerolSugars Fatty acids

Amino groups

G3P

ATP

Do plants perform cellular respiration??