How Cells Harvest Energy

Chapter 7

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MAIN IDEA

• All cells derive chemical energy form organic molecules and use it to convert that energy to ATP

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Respiration

Organisms can be classified based on how they obtain energy:

autotrophs: are 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.

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heterotrophs

• 95% of all organisms on Earth are heterotrophs

• They include all animals, fungi, most protists and prokaryotes

• They do not include plants (which use sunlight to synthesize organic compounds

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Respiration

Cellular respiration is a series of reactions that:

-are oxidations – loss of electrons

-are also dehydrogenations – lost electrons are accompanied by hydrogen

Therefore, what is actually lost is a hydrogen atom (1 electron, 1 proton).

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Summary and purpose

• Food (carbs & fats) C-H & C-O bonds are broken down into smaller molecules (digestion)

• Other enzymes break C-H bonds and harvest energy (oxidation)

• Redox – transfer of electrons

• Energy from food converted to ATP

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Respiration

During redox reactions, electrons carry energy from one molecule to another.

NAD+ is an electron carrier (cofactor)

-NAD accepts 2 electrons and 1 proton to become NADH

-the reaction is reversible

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ELECTRON SHUTTLE

• Dinucleotides linked by a phosphate bridge

• Nicotinamide monophosphate (NMP) is the active site of the reaction

• Adenine monophosphate (AMP) is the core that gives the molecule its shape

• NADH supplies fatty acid with high energy electrons to form fat stores of energy

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Respiration

During respiration, electrons are shuttled through electron carriers to a final electron acceptor.

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

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HOW ELECTRON TRANSPORT WORKS

• Electrons in the C-H bonds are stripped off in stages in a series of enzyme-catalyzed reactions

• Not all of the energy is released at once

• Located mitochondrial inner membrane

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Respiration

Aerobic respiration:

C6H12O6 + 6O2 6CO2 + 6H2O

G = -686kcal/mol of glucose G can be even higher than this in a cellThis large amount of energy must be

released in small steps rather than all at once.

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Respiration

The goal of respiration is to produce ATP.

-energy is released from an oxidation reaction in the form of electrons

-electrons are shuttled by electron carriers (e.g. NAD+) to an electron transport chain

-electron energy is converted to ATP at the electron transport chain

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Oxidation of Glucose

Cells are able to make ATP via:

1. substrate-level phosphorylation – transferring a phosphate directly to ADP from another molecule

2. oxidative phosphorylation – use of ATP synthase and energy derived from a proton (H+) gradient to make ATP

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SUBSTRATE-LEVEL PHOSPHORYLATION

• When PEP’s phosphate group is transferred enzymaticaly to ADP, the energy in the bond is conserved and ATP is created.

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Oxidation of Glucose

The complete oxidation of glucose proceeds in stages:

1. glycolysis

2. pyruvate oxidation

3. Krebs cycle

4. electron transport chain & chemiosmosis

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Glycolysis

Glycolysis converts glucose to pyruvate.

-a 10-step biochemical pathway

-occurs in the cytoplasm

-2 molecules of pyruvate are formed

-net production of 2 ATP molecules by substrate-level phosphorylation

-2 NADH produced by the reduction of NAD+

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Glycolysis

For glycolysis to continue, NADH must be recycled to NAD+ by either:

1. aerobic respiration – occurs when oxygen is available as the final electron acceptor

2. fermentation – occurs when oxygen is not available; an organic molecule is the final electron acceptor

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Glycolysis

The fate of pyruvate depends on oxygen availability.

When oxygen is present, pyruvate is oxidized to acetyl-CoA which enters the Krebs cycle

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

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Net reaction of glycolytic sequence

• Glucose + 2 ADP + Pi + 2 NAD+

• 2 pyruvate + 2 ATP + 2 NADH + H+ +H20

• Steps of process:

• 1. Start with 6-carbon glucose

• 2. 2 phosphates (from 2 ATP) added by phosphorylation

• 3. split – forming 2 3-carbon sugar phosphates

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Process, cont.

• 4. Oxidation reaction converts the 2 sugar phosphates into intermediates that can transfer a P to ADP to form ATP

• 5. 2 NAD + is Phosphorylated to yield 2 NADH

• 6. because each glucose molecule is split into 2 G3P molecules, the overall reaction has a net yield of 2 ATP & 2NADH & 2 pyruvate.

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Net Yield

• 4 ATP ( 2 ATP for each of the 2 G3P molecules)• - 2 ATP (used in the 2 reactions, #1 & 2)• __________________________________• 2 ATP (net yield for entire process)

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Pyruvate Oxidation

In the presence of oxygen, pyruvate is oxidized.

-occurs in the mitochondria in eukaryotes

-occurs at the plasma membrane in prokaryotes

-in mitochondria, a multienzyme complex called pyruvate dehydrogenase catalyzes the reaction

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Pyruvate Oxidation

The products of pyruvate oxidation include:

-1 CO2 -1 NADH-1 acetyl-CoA which consists of 2 carbons

from pyruvate attached to coenzyme A

Acetyl-CoA proceeds to the Krebs cycle.

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Summary of Reaction

• Formula

• Pyruvate + NAD+ + CoA- acetyl-CoA +

• NADH + CO2 + H+

• NADH used later to produce ATP

• Acetyl group is fed into the Krebs cycle, with CoA being recycled (p.128)

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Krebs Cycle

The Krebs cycle oxidizes the acetyl group from pyruvate.

-occurs in the matrix of the mitochondria

-biochemical pathway of 9 steps

-Segment A-first reaction:

acetyl group + oxaloacetate citrate

(2 carbons) (4 carbons) (6 carbons)

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Krebs cycle segments

• Segment B – citrate rearrangement and decarboxylation-Reactions 2-6– a. citrate is reduced by decarboxylation to a 5-

carbon intermediate and then to 4-carbon succinate

– B. 2 NADH and 1 ATP are producedSegment C – Regeneration of oxaloacetate-a. succinate 3 reactions(7-9) to become

oxaloacetate again

-b. 1 NADH and FAD reduced to FADH2

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Krebs Cycle

The remaining steps of the Krebs cycle:

-release 2 molecules of CO2 –absorbed for energy

-reduce 3 NAD+ to 3 NADH

-reduce 1 FAD (electron carrier) to FADH2

-produce 1 ATP

-regenerate oxaloacetate

THIS IS 1 TURN, IT TAKES 2 TO PROVIDE THE ETC WITH ENOUGH ELECTRONS AND PROTONS TO FUNCTION.

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Extraction of electrons

• 9 reactions from p. 130

• Takes place within the matrix of the mitochondria

• To complete the breakdown of glucose

• 2 acetyl-CoA + pyruvate oxidation each make a trip around the Krebs Cycle

• Glucose is consumed entirely in aerobic respiration

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Krebs Cycle

After glycolysis, pyruvate oxidation, and the Krebs cycle, glucose has been oxidized to:

- 6 CO2

- 4 ATP

- 10 NADH

- 2 FADH2

These electron carriers proceedto the electron transport chain.

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Electron Transport Chain

The electron transport chain (ETC) is a series of membrane-bound electron carriers.

-embedded in the mitochondrial inner membrane

-electrons from NADH and FADH2 are transferred to complexes of the ETC

-each complex transfers the electrons to the next complex in the chain

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Electron Transport Chain

As the electrons are transferred, some electron energy is lost with each transfer.

This energy is used to pump protons (H+) across the membrane from the matrix to the inner membrane space.

A proton gradient is established.

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Electron Transport Chain

The higher negative charge in the matrix attracts the protons (H+) back from the intermembrane space to the matrix.

The accumulation of protons in the intermembrane space drives protons into the matrix via diffusion.

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Electron Transport Chain

Most protons move back to the matrix through ATP synthase.

ATP synthase is a membrane-bound enzyme that uses the energy of the proton gradient to synthesize ATP from ADP + Pi.

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Energy Yield of Respiration

theoretical energy yields

- 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 and use of the proton gradient for purposes other than ATP synthesis

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Regulation of Respiration

Regulation of aerobic respiration is by feedback inhibition.

-a step within glycolysis is allosterically inhibited by ATP and by citrate

-high levels of NADH inhibit pyruvate dehydrogenase

-high levels of ATP inhibit citrate synthetase

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Oxidation Without O2

Respiration occurs without O2 via either:

1. anaerobic respiration

-use of inorganic molecules (other than O2) as final electron acceptor

2. fermentation

-use of organic molecules as final electron acceptor

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Oxidation Without O2

Anaerobic respiration by methanogens

-methanogens use CO2

-CO2 is reduced to CH4 (methane)

Anaerobic respiration by sulfur bacteria

-inorganic sulphate (SO4) is reduced to hydrogen sulfide (H2S)

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Oxidation Without O2

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

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Catabolism of Protein & Fat

Catabolism of proteins:

-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

-for example:

alanine is converted to pyruvate

aspartate is converted to oxaloacetate

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Catabolism of Protein & Fat

Catabolism of fats:

-fats are broken down to fatty acids and glycerol

-fatty acids are converted to acetyl groups by -oxidation

The respiration of a 6-carbon fatty acid yields 20% more energy than glucose.

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Evolution of Metabolism

A hypothetical timeline for the evolution of metabolism:

1. ability to store chemical energy in ATP2. evolution of glycolysis

3. anaerobic photosynthesis (using H2S)

4. use of H2O in photosynthesis (not H2S)5. evolution of nitrogen fixation6. aerobic respiration evolved most recently