How Cells Harvest Energy

CHAPTER 7
LECTURE
SLIDES

Respiration
• Organisms can be classified based on how they obtain energy:
• Autotrophs
– Able to produce their own organic molecules through photosynthesis

• Heterotrophs
– Live on organic compounds produced by other organisms

• All organisms use cellular respiration to extract energy from organic molecules

Cellular respiration
• Digestion – enzymes breaking down large macromolecules into smaller ones.
• Cellular respiration is a series of reactions
• Oxidations – loss of electrons
• Dehydrogenations – lost electrons are accompanied by protons
– A hydrogen atom is lost (1 electron, 1 proton)

Redox
• During redox reactions, electrons carry energy from one molecule to another
• Nicotinamide adenosine dinucleotide
(NAD+)
– Is an electron carrier
– NAD+ accepts 2 electrons and 1 proton to become NADH
– Reaction is reversible

NAD+ to NADH
• How may electrons does NAD+ need to be neutral? – 1 electron

• How many electrons and protons in a hydrogen atom?
– 1 proton
– 1 electron

• How many protons and electrons are needed to make NAD+ to NADH?

NAD+ + 2H → NADH + H+

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Cellular Respiration
• In overall cellular energy harvest
– Dozens or redox reactions take place
– Number of electron acceptors including NAD+

• In the end, high-energy electrons from initial chemical bonds have lost much of their energy
• Transferred to a final electron acceptor

Type of Metabolisms
• Generally speaking, the type of terminal
(final) electron acceptor determines the type or metabolism.
• Aerobic respiration
– Final electron receptor is oxygen (O2)

• Anaerobic respiration
– Final electron acceptor is an inorganic molecule (not O2)

• Fermentation
– Final electron acceptor is an organic molecule

Aerobic respiration
C6H12O6 + 6O2

6CO2 + 6H2O

∆G = -686kcal/mol of glucose
∆G can be even higher than this in a cell
• This large amount of energy must be released in small steps rather than all at once. NADH
NAD+
+

ATP
2e –

Controlled release of energy for synthesis of ATP

H+

2e –
H+

H2O

1
−
2

O2

Electron carriers
• Many types of carriers used
– Soluble, membrane-bound, move within membrane • All carriers can be easily oxidized and reduced • Some carry just electrons, some electrons and protons
• NAD+ acquires 2 electrons and a proton to become NADH

Oxidation
Dehydrogenase

NAD+

+ 2H
2 H+ + 2 e–

Reduction

NADH + H+
(carries
2 electrons)

Metabolism Harvests Energy in Stages
• Combustible reactions and reactions in biological systems are essentially the same thing. • Instead of one reaction where immense heat is generated (like burning a log), biological systems transfer electrons to intermediate electron carriers thus coupling reactions.
• The electron transport chain is a series of redox reactions produces potential energy in the form of an electrochemical gradient.

ATP
• Cells use ATP to drive endergonic reactions – ΔG = -7.3 kcal/mol

• 2 mechanisms for synthesis
1. Substrate-level phosphorylation
• Transfer phosphate group directly to ADP
• During glycolysis

2. Oxidative phosphorylation
• ATP synthase uses energy from a proton gradient

Substrate-level phosphorylation – phosphate groups are added to substrates in enzyme catalyzed reactions.
Enzyme

Enzyme
P

ADP
+

P
Substrate

P
Product

ATP

Oxidation of Glucose
The complete oxidation of glucose proceeds in stages:
1. Glycolysis
2. Pyruvate oxidation
3. Krebs cycle
4. Electron transport chain & chemiosmosis

Cellular respiration occurs in three main stages • Stage 1: Glycolysis
– Glycolysis begins respiration by breaking glucose (6 carbons) into 2 molecules of pyruvate (3 carbons)
– This stage occurs in the cytoplasm
 Stage 2: The citric acid cycle
– The citric acid cycle breaks down pyruvate into carbon dioxide and supplies the third stage of cellular respiration with electrons
– This stage occurs in the mitochondria
Copyright © 2009 Pearson Education, Inc.

Cellular respiration occurs in three main stages • Stage 3: Oxidative phosphorylation
– During this stage, electrons are shuttled through the electron transport chain
– As a result, ATP is generated through oxidative phosphorylation associated with chemiosmosis
– This stage occurs in the inner mitochondrion membrane Copyright © 2009 Pearson Education, Inc.

Cellular respiration occurs in three main stages • During the transport of electrons, a concentration gradient of H+ ions is formed across the inner membrane into the intermembrane space
– The potential energy of this concentration gradient is used to make ATP by a process called chemiosmosis
– The concentration gradient drives H+ through
ATP synthases and enzymes found in the membrane, and ATP is produced
Copyright © 2009 Pearson Education, Inc.

NADH
Mitochondrion

High-energy electrons carried by NADH
NADH

FADH2 and OXIDATIVE

GLYCOLYSIS
Glucose

PHOSPHORYLATION
(Electron Transport and Chemiosmosis)

CITRIC ACID
CYCLE

Pyruvate

Cytoplasm

Inner mitochondrial membrane
CO2

CO2
ATP

ATP

Substrate-level phosphorylation Substrate-level phosphorylation ATP

Oxidative phosphorylation Glycolysis
• Converts 1 glucose (6 carbons) to 2 pyruvate
(3 carbons)
• 10-step biochemical pathway
• Occurs in the cytoplasm
• 2 ATP are used in the investment phase.
• 4 ATP are synthesized in the pay off phase.
• Net production of 2 ATP molecules by substrate-level phosphorylation
• 2 NADH produced by the reduction of NAD+

ENERGY INVESTMENT
PHASE

Glucose

ATP
Step

Steps 1 – 3 A fuel molecule is energized, using ATP.

1

ADP
P

Glucose-6-phosphate

P

Fructose-6-phosphate

P

Fructose-1,6-bisphosphate

2

ATP
3

ADP
P

Step 4 A six-carbon intermediate splits
Into two three-carbon intermediates.

4

P

Step 5 A redox reaction generates NADH.

Glyceraldehyde-3-phosphate
(G3P)

P

NAD+

NAD+

5

P

NADH

5

NADH

+ H+

ENERGY PAYOFF PHASE
P

+ H+
P

P
ADP

P

P 1,3-Bisphosphoglycerate

ADP
6

6

ATP

ATP
P

P 3-Phosphoglycerate

7

Steps 6 – 9 ATP and pyruvate are produced.

7

P

P
2-Phosphoglycerate
8

H2 O
P

P

ADP

Phosphoenolpyruvate
(PEP)

ADP
9

ATP

8

H2 O

9

ATP
Pyruvate

Glucose
ATP
Steps 1 – 3 A fuel molecule is energized, using ATP.

ENERGY
INVESTMENT
PHASE

Step
1
ADP
P

Glucose-6-phosphate

P

Fructose-6-phosphate

P

Fructose-1,6-bisphosphate

2

ATP
3
ADP
P
Step 4 A six-carbon intermediate splits
Into two three-carbon intermediates.

4

P

P

Glyceraldehyde-3phosphate (G3P)

Step 5 A redox reaction generates NADH.

Glyceraldehyde-3-phosphate
(G3P)

P

P

NAD+

NAD+

5
P

NADH

5

ENERGY PAYOFF PHASE
P

NADH

+ H+

+ H+
P

P

P

ADP

1,3-Bisphosphoglycerate

P

P

3-Phosphoglycerate

ADP
6

6

ATP

ATP
P
7

Steps 6 – 9 ATP and pyruvate are produced.

7

P

P
2-Phosphoglycerate
8

H2O

8
H2O

P

P

ADP

ADP
9

ATP

Phosphoenolpyruvate
(PEP)
9

ATP
Pyruvate

Review Glycolysis
• Overall:
– Prepare sugars for the Krebs Cycle.
– Generated some NADH for the electron transport chain.
– Make some ATP.

• Starting material – (1) Glucose – 6 carbon molecule. • Ending material – (2) Pyruvate – 3 carbon molecule. Glycolysis
• Does not require oxygen.
• Net balance:
– 2 ATP
– 2 NADH http://www.youtube.com/watch?v=x-stLxqPt6E http://glencoe.mcgrawhill.com/sites/9834092339/student_view0/chapter7/how_glycolysis_works.html

NADH must be recycled
• For glycolysis to continue, NADH must be recycled to NAD+ by either:
1.Aerobic respiration
– Oxygen is available as the final electron acceptor – Produces significant amount of ATP

2.Fermentation
– Occurs when oxygen is not available
– Organic molecule is the final electron acceptor Fate of pyruvate
• Depends on oxygen availability.
– When oxygen is present, pyruvate is oxidized to acetyl-CoA which enters the Krebs cycle
• Aerobic respiration

– Without oxygen, pyruvate is reduced in order to oxidize NADH back to NAD+
• Fermentation

Pyruvate Oxidation
• In the presence of oxygen, pyruvate is oxidized – Occurs in the mitochondria in eukaryotes
• multienzyme complex called pyruvate dehydrogenase catalyzes the reaction

– Occurs at the plasma membrane in prokaryotes Pyruvate Oxidation - For each 3 carbon pyruvate molecule:
NAD+

NADH

+ H+

2
CoA
Pyruvate

Acetyl coenzyme A

1
3
CO2
Coenzyme A

– 1 CO2
• Decarboxylation by pyruvate dehydrogenase
– 1 NADH
– 1 acetyl-CoA which consists of 2 carbons from pyruvate attached to coenzyme A
• Acetyl-CoA proceeds to the Krebs cycle

Oxidation of Pyruvate
• Overview:
– To move pyruvate from the cytosol to the mitochondrial matrix.
– To turn pyruvate into acetyl-CoA for the Krebs
Cycle.
– To make some NADH for the electron transport chain.

• Net balance:
– 2 NADH per glucose or 1 NADH per pyruvate.

Krebs Cycle
• Oxidizes the acetyl group from pyruvate
• Occurs in the matrix of the mitochondria
• Biochemical pathway of 9 steps in three segments 1. Acetyl-CoA + oxaloacetate → citrate
2. Citrate rearrangement and decarboxylation
3. Regeneration of oxaloacetate

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Krebs Cycle
• For each Acetyl-CoA entering:
– Release 2 molecules of CO2
– Reduce 3 NAD+ to 3 NADH
– Reduce 1 FAD (electron carrier) to FADH2
– Produce 1 ATP
– Regenerate oxaloacetate

Krebs (Citric Acid) Cycle
• Purpose:
– Generate a lot of NADH for the electron transport chain.
– Generate some FADH2 for the electron transport chain.
– Generate some ATP.

• Starting material – (2) Acetyl-CoA
• Ending material – (2) Oxaloacetate

Krebs (Citric Acid) Cycle
• Requires oxygen
• Net balance:
– 2 ATP
– 6 NADH
– 2 FADH2
• http://www.youtube.com/watch?v=aCypoN3X7K
Q
• http://glencoe.mcgrawhill.com/sites/9834092339/student_view0/chapte r7/how_the_krebs_cycle_works.html At this point
• Glucose has been oxidized to:
– 6 CO2
– 4 ATP
– 10 NADH
– 2 FADH2

These electron carriers proceed to the electron transport chain

• Electron transfer has released 53kcal/mol of energy by gradual energy extraction
• Energy will be put to use to manufacture
ATP

Electron Transport Chain
• ETC is a series of membrane-bound electron carriers
• Embedded in the inner mitochondrial membrane • Electrons from NADH and FADH2 are transferred to complexes of the ETC
• Each complex
– A proton pump creating proton gradient
– Transfers electrons to next carrier http://glencoe.mcgrawhill.com/sites/9834092339/student_view0/chapter7/how_the_nad__works.html Intermembrane space Protein complex of electron carriers H+

H+

H+

H+

H+

H+

H+

Electron carrier H+

H+

ATP synthase Inner mitochondrial membrane
FADH2

Electron flow NADH
Mitochondrial
matrix

FAD

NAD+

H+

1
−
2

O2 + 2 H+

H+
H+

H2O

Electron Transport Chain
OXIDATIVE PHOSPHORYLATION

ADP + P

ATP

H+

Chemiosmosis

Chemiosmosis
• Accumulation of protons in the intermembrane space drives protons into the matrix via diffusion
• Membrane relatively impermeable to ions
• Most protons can only reenter matrix through ATP synthase
– Uses energy of gradient to make ATP from
ADP + Pi

51

Electron Transport Chain
Videos and ATP Synthase http://vcell.ndsu.edu/animations/etc/movie.htm http://glencoe.mcgrawhill.com/sites/9834092339/student_view0/chapter7/electron_transport_system_and_atp
_synthesis.html

http://vcell.ndsu.edu/animations/atpgradient/movie.htm

Electron Transport Chain
• Purpose– To pump H+ into the inner membrane space of the mitochondria to generate a chemical gradient (chemiosmosis).
– As the H+ ions move back into the mitochondrial matrix, through ATP synthase, it generates ATP.
– The main purpose of this is to generate a whole bunch of ATP.
– NADH and FADH2 are electron donors to this.

• Net balance:
– ~32-34 ATP

Energy Yield of Respiration
• Theoretical energy yield
– 38 ATP per glucose for bacteria
– 36 ATP per glucose for eukaryotes

• Actual energy yield
– 30 ATP per glucose for eukaryotes
– Reduced yield is due to
• “Leaky” inner membrane
• Use of the proton gradient for purposes other than
ATP synthesis

Regulation of Respiration
• Example of feedback inhibition
• 2 key control points
1. In glycolysis
• Phosphofructokinase is allosterically inhibited by
ATP and/or citrate

2. In pyruvate oxidation
• Pyruvate dehydrogenase inhibited by high levels of
NADH
• Citrate synthetase inhibited by high levels of ATP

Oxidation Without O2
1. Anaerobic respiration
– Use of inorganic molecules (other than O2) as final electron acceptor
– Many prokaryotes use sulfur, nitrate, carbon dioxide or even inorganic metals

2. Fermentation
– Use of organic molecules as final electron acceptor Anaerobic respiration
• Methanogens
– CO2 is reduced to CH4 (methane)
– Found in diverse organisms including cows

• Sulfur bacteria
– Inorganic sulphate (SO4) is reduced to hydrogen sulfide (H2S)
– Early sulfate reducers set the stage for evolution of photosynthesis

Fermentation
• Reduces organic molecules in order to regenerate NAD+
1.Ethanol fermentation occurs in yeast
– CO2, ethanol, and NAD+ are produced

2.Lactic acid fermentation
– Occurs in animal cells (especially muscles)
– Electrons are transferred from NADH to pyruvate to produce lactic acid

Catabolism of Protein
• Amino acids undergo deamination to remove the amino group
• Remainder of the amino acid is converted to a molecule that enters glycolysis or the
Krebs cycle
– Alanine is converted to pyruvate
– Aspartate is converted to oxaloacetate

Catabolism of Fat
• Fats are broken down to fatty acids and glycerol – Fatty acids are converted to acetyl groups by β-oxidation – Oxygen-dependent process

• The respiration of a 6-carbon fatty acid yields 20% more energy than 6-carbon glucose. http://nutrition.jbpub.com/re sources/animations.cfm?id= 23&debug=0

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Food, such as peanuts Carbohydrates

Fats

Glycerol

Sugars

Proteins

Fatty acids

Amino acids
Amino
groups

Glucose

G3P

GLYCOLYSIS

Pyruvate

Acetyl
CoA

ATP

CITRIC
ACID
CYCLE

OXIDATIVE
PHOSPHORYLATION
(Electron Transport and Chemiosmosis)