Cellular Respiration: Harvesting Chemical Energy

Energy Flows through the Ecosystem

CO2

H2O

Glucose

O2

ATP

ECOSYSTEM

Sunlight energy

Photosynthesis in chloroplasts

Cellular respiration in mitochondria

(for cellular work)

Heat energy

+! +!

Energy enters the ecosystem in form of solar energy

Photosynthesis converts solar energy to chemical energy. Glucose and O2 are produced from CO2 and H2O Cellular respiration converts chemical energy from food to ATP. Glucose is oxidized to CO2 and H2O

With each conversion of energy, some energy gets lost as heat

Redox Reactions (oxidation and reduction simultaneously occur in a chemical reaction)

The human body uses energy from ATP for all its activities

Cells need ATP

!  ATP powers •  active transport •  Mechanical work (movement) •  Anabolic (endergonic) reactions

Cellular Respiration

Oxidative phosphorylation

Substrate-level phosphorylation

Substrate-level phosphorylation

Glycolysis Glucose and other

fuel molecules

Pyruvate

Pyruvate oxidation

Acetyl-CoA

Citric acid cycle

Electrons carried by NADH and FADH2

Electron transfer system and oxidative

phosphorylation

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Cellular Respiration occurs in three stages

Citric acid cycle

Glycolysis

Pyruvate oxidation

Oxidative phosphorylation

ATP

ATP

ATP

Cytosol

Glucose

2 Pyruvate

Oxidation

In glycolysis, glucose is oxidized to pyruvate.

The electrons from that reaction reduce NAD+ to NADH.

NADH carries the electrons to be used in the oxidative phosphorylation

In Glycolysis and the Citric Acid Cycle, ATP is produced by Substrate Level Phosphorylation

Enzyme

ADP

P

Substrate

Product

Enzyme

ATP +

Substrate

Product

The Electron Carrier NAD+ can pick up 2 electrons

Pyruvate is transported into the mitochondrial matrix as Acetyl-CoA

Acetyl CoA (acetyl coenzyme A)

Coenzyme A

Pyruvate

CO2

NAD+ NADH H+

CoA

+

Glycolysis

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

Summary Glycolysis

!  Converts 1 glucose (6 carbons) to 2 pyruvate (3 carbons)

!  10-step biochemical pathway !  Occurs in the cytoplasm !  Net production of 2 ATP molecules by substrate-

level phosphorylation !  2 NADH produced by the reduction of NAD+

The Pyruvate needs “grooming” for the next step

Citric acid cycle

Glycolysis

Pyruvate oxidation

Oxidative phosphorylation

Mitochondrial matrix

Pyruvate is transported into the mitochondrial matrix as Acetyl-CoA

Pyruvate

Acetyl-CoA

Mitochondria, Sites of Cellular Respiration Acetyl-CoA

Summary of Krebs Cycle (Citric Acid Cycle) Reactions

!  For each Acetyl-CoA entering the Krebs Cycle: •  2 molecules of CO2 are released

•  3 NAD+ are reduced to 3 NADH

•  1 FAD (electron carrier) is reduced to FADH2 •  •  1 ATP is produced

•  The initial acceptor molecule Oxaloacetate is regenerated

Pyruvate

Acetyl-CoA

Citric acid cycle

The last step of Cellular Respiration: Oxidative Phosphorylation

Citric acid cycle

Glycolysis

Pyruvate oxidation

Oxidative phosphorylation

Inner mitochondrial membrane

Oxidative Phosporylation consists of the Electron transport chain and Chemiosmosis

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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

Why Electron Transport?

2 H+ + 2 e–

2 H

(from food via NADH)

Controlled release of energy for

synthesis of ATP

ATP

ATP

ATP

2 H+

2 e–

H2O

+ 1/2 O2

1/2 O2

Cellular respiration

Free

ene

rgy,

G Electron transport chain

1/2 O2 H2

+

H2O

Explosive release of

heat and light energy

Uncontrolled reaction

Free

ene

rgy,

G

H2 + ! O2 2 H + ! O2

2 H+

2 e!

2 e! 2 H+ +

H2O

! O2

Controlled release of

energy

Electron transport

chain

ATP

ATP

ATP

Explosive release

Cellular respiration Uncontrolled reaction (a) (b)

Free

ene

rgy,

G

Free

ene

rgy,

G

H2O

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

Overview Cellular Respiration Inhibitors of the Electron Transport Chain

H+

H+

H+

H+

H+

H+ H+ H+ H+

H+

H+

H+ H+

O2

H2O P ATP

NADH NAD+

FADH2 FAD

ATP Synthase

+! 2

ADP +!

Electron Transport Chain Chemiosmosis

1 2

Rotenone Cyanide, carbon monoxide

Oligomycin

DNP

Summary: Glycolysis, Citric Acid Cycle and Oxidative Phosphorylation

CYTOSOL Electron shuttles span membrane 2 NADH

or

2 FADH2

MITOCHONDRION

Oxidative phosphorylation: electron transport

and chemiosmosis

2 FADH2 2 NADH 6 NADH

Citric acid

cycle

2 Acetyl CoA

2 NADH

Glycolysis

Glucose 2

Pyruvate

+ 2 ATP

by substrate-level phosphorylation

+ 2 ATP

by substrate-level phosphorylation

+ about 32 or 34 ATP

by oxidation phosphorylation, depending on which shuttle transports electrons form NADH in cytosol

About 36 or 38 ATP Maximum per glucose:

The actual yield is about 30 ATP per molecule of glucose due to leaky membranes

The Fate of Pyruvate is Dependent on Oxygen Availability

Fermentation occurs when oxygen is not available

Glucose

Pyruvate

Glycolysis

Lactate

Alcoholic Fermentation

Glucose

Pyruvate

Glycolysis

Ethanol

Fermentation replenishes NAD+ to further drive Glycolysis

Cells use many kinds of organic molecules as fuel for cellular respiration (catabolic processes)

OXIDATIVE PHOSPHORYLATION

(Electron Transport and Chemiosmosis)

Food, such as peanuts

Carbohydrates Fats Proteins

Sugars Glycerol Fatty acids Amino acids

Amino groups

Glucose G3P Pyruvate Acetyl CoA

CITRIC ACID

CYCLE

ATP

GLYCOLYSIS

Food molecules provide raw materials for biosynthesis (anabolic processes)

ATP needed to drive biosynthesis

ATP

CITRIC ACID

CYCLE

GLUCOSE SYNTHESIS

Acetyl CoA

Pyruvate G3P Glucose

Amino groups

Amino acids Fatty acids

Glycerol Sugars

Carbohydrates Fats Proteins

Cells, tissues, organisms