Chapter 9 Cellular Respiration: Harvesting Chemical Energy

Introduction

Energy for work enters as LIGHT

Once trapped in organic molecules, that energy is available to both producers and consumers

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1. Cellular Respiration/ Fermentation are energy yieldingCatabolic pathways:

Fermentation = no oxygen Cellular Respiration= oxygen Most of Cellular Respiration occurs

in MITOCHONDRIA:

Organic compounds + oxygen Carbon dioxide + water + E (ATP+HEAT)

1. Cellular Respiration/ Fermentation are energy yielding

Carbohydrates, Fats, Proteins can all be used for fuel, but glucose is commonly used to learn the process:

FORMULA:

C6H12O6 + 6O2 ---> 6CO2 + 6H2O + E (ATP + heat)

2. Cells Recycle ATP for WORK

•The price of cellular work is the exergonic reaction which converts ATP→ADP+Pi (inorganic phosphate)

•Cells can regenerate ATP from ADP and Pi by the catabolism of organic molecules.

2. Cells Recycle ATP for WORK

3.Redox reactions release energy

Oxidation-reductionOIL RIG (adding e- reduces + charge)

Oxidation is e- loss; reduction is e- gain

Reducing agent: e- donor

Oxidizing agent: e- acceptor

3. Redox reactions release energy

Why are they so important to understand?

Relocation of electrons releases stored energy

Not all redox reactions completely transfer electrons, some just change degree of sharing in bonds

Electrons release energy as they more from a less electronegative atom to a more electronegative atom

Relocation of e- closer to oxygen releases chemical energy for work!

4. Electrons “Fall” From Organic Compounds to Oxygen during C. Resp

Glucose/ Other fuel molecules are oxidized = release of energy

Molecules with lots of H atoms, lots of atoms that can be transferred to a more electronegative O

RICH RESERVOIR OF Hydrogen ELECTRONS=CARBS, FATS, need ENZYMES TO combine with O2

5. Fall of Electrons is gradual, via NAD+ and Electron Transport Chain

NAD+ (nicotinamide adenine dinucleotide) Removes electrons from food (series of reactions) NAD + is reduced to NADH Enzyme action: dehydrogenase Oxygen is the eventual e- acceptor

5. Fall of Electrons is gradual, via NAD+ and Electron Transport Chain

H- atoms are stripped from glucose, passed to coenzyme: NICOTINAMIDE ADENINE DINUCLEOTIDE

Dehydrogenase enzymes strip two hydrogens from fuel, pass two electrons and one proton to NAD+ and release H+

THIS MAKES REDUCED FORM= NADH NADH= Reducing agent, electrons lose very little energyand energy is“tapped” to synthesize ATP as electrons move from NADH to oxygen

5. “Fall” of Electrons is gradual, via NAD+ and Electron Transport Chain

Electron carrier molecules (membrane proteins)

Shuttles electrons that release energy used to make ATP

Sequence of reactions that prevents energy release in 1 explosive step

Electron route: food---> NADH --->

electron transport chain ---> oxygen

Cellular respiration: an overview

1. Glycolysis: cytosol; degrades glucose into pyruvate

2. Kreb’s Cycle: mitochondrial matrix; pyruvate into carbon dioxide

3.Electron Transport Chain: inner membrane of mitochondrion; electrons passed to oxygen

Cellular respiration: an overview

•Several steps in Glycolysis and the Krebs cycle transfer electrons from substrates to NAD+ forming NADH

•NADH passes these electrons to the electron transport chain.

The electron transport chain moves electrons from molecule to molecule until they combine with O and H ions and form water.

Cellular respiration: an overview

MAKING ATP:

Substrate-level phosphorylation: The formation of ATP by directly transferring a phosphate group to ADP from intermediate substrate in catabolism.

Oxidative Phosphorylation:

The production of ATP using energy derived from redox reactions of an electron transport chain

Glycolysis and Cellular Respiration

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Glycolysis: oxidation of glucose to pyruvate

GLUCOSE to 2 pyruvate molecules

1. Energy investment phase: cell uses 2 ATP to phosphorylate fuel

2. Energy payoff phase: ATP is produced by substrate-level phosphorylation and NAD+ is reduced to NADH by food oxidation

Glycolysis: oxidation of glucose to pyruvate

Net energy yield per glucose molecule: 2 ATP plus 2 NADH; no CO2 is released; occurs aerobically or anaerobically

If oxygen is present, pyruvate moves to Kreb’s cycle…

Each step in glycolysis is catalyzed by a specific enzyme!

Kreb’s Cycle If molecular oxygen is

present……. Each pyruvate is converted into

acetyl CoA (begin w/ 2):1 A carboxyl group is removed as

CO22. A pair of electrons is transferred

from remaining 2- carbon fragment to NAD+ to form NADH

3. Oxidized fragment acetate, combines with coenzyme A to form acetyl CoA.

Kreb’s Cycle

From this point, each turn 2 C atoms enter (pyruvate) and 2 exit (carbon dioxide)

•This cycle begins when acetate from acetyl CoA combines with oxaloacetate to form citrate.

•Ultimately, the oxaloacetate is recycled and the acetate is broken down to CO2.

For each pyruvate that enters: 3 NAD+ reduced to NADH; 1 FAD+ reduced to FADH2

(riboflavin, B vitamin); 1 ATP molecule

Kreb’s Cycle

•Kreb’s Cycle consists of 8 steps

•The conversion of pyruvate and the Kreb’s cycle produces large quantities of ELECTRON CARRIERS

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Electron transport chain THE BEGINNING: Most components of the chain are

proteins that are bound with prosthetic groups:

alternate between reduced and oxidized states as they accept and donate electrons.

•Electrons drop in free energy as they pass down the electron transport chain.

•Electrons carried by NADH are transferred to the first molecule in the electron transport chain, flavoprotein.

The electrons continue along the chain that includes several cytochrome proteins and one lipid carrier.

1000s of copies of the electron transport chain are found in the extensive surface of the cristae, the inner membrane of the mitochondrion.

Electron transport chain Electrons carried by FADH2 =

lower free energy, added to a later point in the chain.

For every two electron carriers (four e), 1 O2 molecule is reduced to 2 molecules of water.

No generation of ATP directly. Function: break the large free

energy drop from food to oxygen into a series of smaller steps •Electrons from NADH or

FADH2 ultimately pass to oxygen.

Electron transport chain

The movement of electrons along the electron transport chain does contribute to chemiosmosis and ATP synthesis.

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Electron transport chain

ATP synthase, in the cristae, actually makes ATP from ADP and Pi.•ATP uses the energy of proton gradient to power ATP synthesis.• Proton gradient develops between the intermembrane space and the matrix.•The proton gradient is produced by the movement of electrons along the electron transport chain.•Several chain molecules can use the exergonic flow of electrons to pump H+ from the matrix to the intermembrane space.•This concentration of H+ is the proton-motive force.

ETC: ATP synthase ATP synthase molecules: only place that will allow H+ to diffuse back to the matrix.• Exergonic flow of H+ is used by the enzyme to generate ATP.•Coupling of the redox reactions of the electron transport chain to ATP synthesis is called chemiosmosis.

Electron Transport: Making ATP

As hydrogen ions flow down their gradient, they cause the cylinder portion and attached rod of ATP synthase to rotate.2.The spinning rod causes a conformational change in the knob region, activating catalytic sites where ADP and inorganic phosphate combine to make ATP

Review: Cellular Respiration Glycolysis:

2 ATP (substrate-level phosphorylation)

Kreb’s Cycle: 2 ATP (substrate-level

phosphorylation) Electron transport &

oxidative phosphorylation: 2 NADH (glycolysis) =

6ATP 2 NADH (acetyl CoA) = 6ATP

6 NADH (Kreb’s) = 18 ATP 2 FADH2 (Kreb’s) = 4 ATP 38 TOTAL ATP/glucose

Related metabolic processes…•Glycolysis :2 ATP whether oxygen is present

or not .•Anaerobic catabolism of sugars:

FERMENTATION•Fermentation can generate ATP from

glucose by substrate-level phosphorylation as long as there is a supply of NAD+ to accept electrons.

•If NAD+ pool is exhausted, glycolysis shuts down.

aerobic conditions: NADH transfers its electrons to the electron transfer chain, recycling NAD+.

anaerobic conditions: various fermentation pathways generate ATP by glycolysis and recycle NAD+ by transferring electrons from NADH to pyruvate or derivatives of pyruvate

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