CHAPTER 7 CELLULAR RESPIRATION. WHERE IS THE ENERGY IN FOOD? Electrons pass from atoms or molecules to one another as part of many energy reactions. Oxidation

CHAPTER 7

CELLULAR RESPIRATION

WHERE IS THE ENERGY IN FOOD?

• Electrons pass from atoms or molecules to one another as part of many energy reactions.• Oxidation is when an atom or molecule loses an

electron.• Reduction is when an atom or molecule gains an

electron.• These reactions always occur together:

• Oxidation-reduction (redox) reactions


• Redox reactions involve transfers of energy because the electrons retain their potential energy.• The reduced form of an organic molecule has a higher level of

energy than the oxidized form.

e–

A B

+ –o o

+A* B*A + B

Loss of electron (oxidation)

Low energy

High energy

Gain of electron (reduction)


• The energy for living is obtained by breaking down the organic molecules originally produced in plants.• The ATP energy and reducing power invested in

building the organic molecules are stripped away as the chemical bonds are broken and used to make ATP.

• The oxidation of food stuffs to obtain energy is called cellular respiration.


• Cellular respiration is the harvesting of energy from breakdown of organic molecules produced by plants

• The overall process may be summarized as

6 CO2

carbondioxide

(heat or ATP)

+ energyC6H12O6

glucose

+ 6 O2

oxygen

+ 6 H2Owater


• Cellular respiration takes place in two stages:– Glycolysis

• Occurs in the cytoplasm.

• Does not require O2 to generate ATP.


– Krebs cycle• Occurs within the mitochondrion.• Harvests energy-rich electrons through a

cycle of oxidation reactions.• The electrons are passed to an electron

transport chain in order to power the production of ATP.

Glucose

Glycolysis

Pyruvate

NADH

NADH

NADH

Electron transport chain

H2O

CO2

CO2

NAD+ and ADF

Mitochondrial matrix

Cytoplasm

e–

Mitochondrion

Intermembranespace

FADH2

ATPKrebscycle

ATP

ATP

Pyruvateoxidation

Acetyl-CoA

Inner mitochondrialmembrane

USING COUPLED REACTIONS TO MAKE ATP

• Glycolysis is a sequence of reactions that form a biochemical pathway.• In ten enzyme-catalyzed reactions, the six-carbon

sugar glucose is broken into two three-carbon pyruvate molecules.


• The breaking of the bonds yields energy that is used to phosphorylate ADP to ATP.• This process is called substrate-level

phosphorylation.• In addition, electrons and hydrogen atoms are

donated to NAD+ to form NADH.

http://www.youtube.com/watch?v=3GTjQTqUuOw&list=FL9N_Px072WuVorSwDfqf-9w&index=4&feature=plpp

11

Glucose

Glycolysis

Phosphorylation ofglucose by ATP.

1

2–3

4–5

6

Rearrangement,followed by a secondATP phosphorylation.

The six-carbon moleculeis split into two three-carbonmolecules of G3P.

Krebscycle

Electron transportchain

Pyruvateoxidation

NADH NADH

NAD+

Glucose

ATP

ADP

Glucose 6-phosphate

Fructose 6-phosphate

Fructose 1,6-bisphosphate

Glyceraldehyde 3-phosphate (G3P)

Glyceraldehyde 3-phosphate (G3P)

ATP

ADP

1,3-bisphosphoglycerate (BPG)1,3-bisphosphoglycerate (BPG)

P

P

P P

P P

PPPP

1

2

3

6

4,5

Pi Pi

NAD+

Oxidation followed byphosphorylation producestwo NADH molecules andgives two molecules of BPG,each with one high-energyphosphate bond.

7

8–9

10

Removal of high-energyphosphate by two ADPmolecules produces twoATP molecules and givestwo 3PG molecules.

Removal of water givestwo PEP molecules, eachwith a chemically reactivephosphate bond.

Removal of high-energyphosphate by two ADPmolecules produces twoATP molecules and givestwo pyruvate molecules.

ADP

ATP

ADP

ATP

ADP

ATP

ADP

ATP

3-phosphoglycerate(3PG)




Phosphoenolpyruvate(PEP)

Phosphoenolpyruvate(PEP)

Pyruvate Pyruvate

P P

PP

P P

7

8

9

10


• Glycolysis yields only a small amount of ATP.• Only two ATP are made for each molecule of glucose.• This is the only way organisms can derive energy from

food in the absence of oxygen.• All organisms are capable of carrying out glycolysis.• This biochemical process was probably one of the

earliest to evolve.

HARVESTING ELECTRONS FROM CHEMICAL BONDS

• In the presence of oxygen, the first step of oxidative respiration in the mitochondrion is the oxidation of pyruvate.• Pyruvate still contains considerable stored energy at

the end of glycolysis.• Pyruvate is oxidized to form acetyl-CoA.

ACETYL-COA

• When pyruvate is oxidized, one of its three carbons is cleaved.• This carbon leaves as

part of a CO2 molecule.• In addition, a hydrogen

and electrons are removed from pyruvate and donated to NAD+ to form NADH.

• The remaining two-carbon fragment of pyruvate is joined to a cofactor called coenzyme A (CoA).

• The final compound is called acetyl-CoA.

Glycolysis

Pyruvate

CO2

NAD+

CoenzymeA

Lipid

NADH

Protein

Fat ATP

CoA–Acetyl–CoA

H

H

H + e–

++

H

H

321

NADH then diffuses away and isavailable to donate the hydrogen toother molecules.

NAD+

NAD+ NAD+ NAD+

NAD+

SubstrateProduct

Enzymes that harvest hydrogen atomshave a binding site for NAD+ locatednear the substrate binding site.

In an oxidation-reduction reaction, thehydrogen atom and an electron aretransferred to NAD+, forming NADH.

e– e–

• NADH and NAD+ are used by cells to carry hydrogen atoms and energetic electrons.

KEY BIOLOGICAL PROCESS: TRANSFER OF H ATOMS


• The fate of acetyl-CoA depends on the availability of ATP in the cell.• If there is insufficient ATP, then the acetyl-CoA

heads to the Krebs cycle.• If there is plentiful ATP, then the acetyl-CoA is

diverted to fat synthesis for energy storage.

KREBS CYCLE

• The second step of oxidative respiration is called the Krebs cycle.

• The Krebs cycle is a series of 9 reactions that can be broken down into three stages:1. Acetyl-CoA enters the cycle and binds to a four-

carbon molecule, forming a 6-C 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.

http://www.youtube.com/watch?v=-cDFYXc9Wko

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.

Pyruvate

CO2

NAD+

CoA–

Oxidation of pyruvateGlucose

Glycolysis

Pyruvateoxidation

Krebscycle


Coenzyme A

Acetyl-CoA

NADH

CO2

CO2

Krebs cycle

(4 C) Oxaloacetate Citrate (6 C)

Isocitrate (6 C)

-Ketoglutarate (5 C)

NAD+

NAD+

S CoA-SH

(4 C) Succinate

CoA-SH

(4 C) Malate

H2O

(4 C) Fumarate

NAD+

The cycle begins whena C2 unit reacts with a C4

molecule to give citrate(C6).

1

2-4

5

6-7

8-9

8

1

2

3

4

6

7

9

5

Mitochondrialmembrane

CoA

NADH

NADH

Succinyl-CoA (4 C)

ADPATP

FAD

FADH2

The dehydrogenationof malate produces athird NADH, and thecycle returns to itsstarting point.

A molecule of ATP isproduced and the oxidationof succinate produces FADH2.

NADH

A second oxidativedecarboxylation producesa second NADH with therelease of a second CO2.

Oxidative decarboxylationproduces NADH with therelease of CO2.

CoA


• In the process of cellular respiration, the glucose is entirely consumed.• The energy from its chemical bonds has been

transformed into:• 4 ATP molecules.• 10 NADH electron carriers.

• 2 FADH2 electron carriers.

USING THE ELECTRONS TOMAKE ATP

• NADH and FADH2 transfer their electrons to a series of membrane-associated molecules called the electron transport chain.• Some protein complexes in the electron transport

chain act as proton pumps.• The last transport protein donates the electrons to

hydrogen and oxygen in order to form water.• The supply of oxygen able to accept electrons makes

oxidative respiration possible.

http://www.youtube.com/watch?v=kN5MtqAB_Yc&list=FL9N_Px072WuVorSwDfqf-9w&index=2&feature=plpp

THE ELECTRON TRANSPORT CHAIN

Intermembrane space

Mitochondrial matrix

NAD+ H2O+ H+ 2H+ + – O2

H+ H+ H+

Glucose

Glycolysis

e–

e– e–

Innermitochondrialmembrane

Proteincomplex III

Proteincomplex II

Proteincomplex I

1

2


Krebs

Krebscycle

Pyruvateoxidation

FADH2

NADH

USING THE ELECTRONS TOMAKE ATP

• Chemiosmosis is integrated with electron transport.• Electrons harvested from reduced carriers (NADH and

FADH2) are used to drive proton pumps and concentrate protons in the intermembrane space.

• The re-entry of the protons into the matrix across ATP synthase drives the synthesis of ATP by chemiosmosis.

H+

H+H+

H+

H+

O2

34

2

H2O

CO2

NADH

NADH

Acetyl-CoA

12

3

4

+

e–

Pyruvate fromcytoplasm

Innermitochondrialmembrane

Intermembranespace

Electrontransportchain

Electrons are harvested and carried to the transport chain.

Electrons provideenergy to pump protonsacross the membrane.

Oxygen joins withprotons and electronsto form water.

Protons diffuse back in downtheir concentration gradient,driving the synthesis of ATP.

Mitochondrialmatrix

e–

e–

e–

FADH2

Krebscycle

ATP

2 H+

ATPATPsynthase

– O21

2

26

CELLS CAN METABOLIZE FOOD WITHOUT OXYGEN

• In the absence of oxygen, organisms must rely exclusively on glycolysis to produce ATP.

• In a process called fermentation, the hydrogen atoms from the NADH generated by glycolysis are donated to organic molecules, and NAD+ is regenerated.

• With the recycling of NAD+, glycolysis is allowed to continue.

FERMENTATION

• Bacteria can perform more than a dozen different kinds of fermentation.

• Eukaryotic cells are only capable of a few types of fermentation.

FERMENTATION

• In yeasts (single-celled fungi), pyruvate is converted into acetaldehyde, which then accepts a hydrogen from NADH, producing NAD+ and ethanol.

• In animals, such as ourselves, pyruvate accepts a hydrogen atom from NADH, producing NAD+ and lactate.

C O

O–

C O

CH3

CH OH

H

CH3

OC

H

CH3

2 NADH

CO2

Ethanol fermentation in yeast

Glucose

2ATP

2ADPGLYCOLYS IS

2NAD+

2 Pyruvate 2 Acetadehyde

2 Ethanol

C O

O–

CH3

CH OH

Lactic acid fermentation in muscle cells

Glucose

2 ADP

2 ATP

C O

O–

C O

CH3

2 Pyruvate

GLYCOLYS IS

2 Lactate

2 NADH

2NAD+

GLUCOSE IS NOT THE ONLYFOOD MOLECULE

• Cells also get energy from foods other than sugars.

• These complex molecules are first digested into simpler subunits, which are then chemically modified into intermediates.• These intermediates enter cellular respiration at

different steps.

H2O

Macro-molecule

degradation

Cellbuildingblocks

Oxidativerespiration

Ultimatemetabolicproducts

CO2NH3

Nucleicacids Proteins Polysaccharides

Lipidsand fats

Nucleotides Amino acids Sugars Fatty acids

β-oxidation

Krebscycle

Acetyl-CoA

Pyruvate

GlycolysisDeamination

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CHAPTER 7 CELLULAR RESPIRATION. WHERE IS THE ENERGY IN FOOD? Electrons pass from atoms or molecules to one another as part of many energy reactions. Oxidation