Chapter 9~

Cellular Respiration: Harvesting Chemical Energy

Principles of Energy Harvest

Catabolic pathway to break down C 6H 12O 6

Exergonic: energy released is used to produce ATP

Anaerobic pathway à Fermentation (forms lactic acid (animals) or ethyl alcohol and CO 2)

Aerobic pathway à

Cellular Respiration (forms CO 2 and H 2O

C 6H 12O 6 + 6O 2 à 6CO 2 + 6H 2O + E (ATP + heat)

They are processes that produce energy or forms of energy through a series of rxns of shuffling electrons.

These rxns are called redox reactions.

Oxidation-reduction (redox) rxns involve a complete or partial transfer of electrons from one reactant to another.

Redox rxns release energy when electrons move closer to electronegative atoms.

Oxidation – loss of electrons

Reduction - gain of electrons

Generalized Redox Rxn:

Electron transfer requires both a donor and an acceptor, so when one reactant is oxidized, the other is reduced.

oxidation

Xe - + Y à X + Ye -

reduction

oxidation

Xe - + Y à X + Ye -

reduction

X = substance is being oxidized. Acts as a reducing agent because it reduces Y.

Y = substance is being reduced. Acts as an oxidizing agent because it oxidizes X. One of the most powerful is oxygen.

Redox reactions

Oxidation-reduction

OIL RIG (adding e- reduces + charge)

Oxidation is e- loss; reduction is e- gain

Reducing agent: e- donor

Oxidizing agent: e- acceptor

Cellular Respiration

Anaerobic (No oxygen)

Glycolysis

Aerobic (Oxygen present)

Formation of acetyl coA

Citric Acid Cycle (Krebs)

Electron transport chain (ETC)

Oxidizing agent in respiration

NAD + (nicotinamide adenine dinucleotide)

Removes electrons from food (series of reactions)

NAD + is reduced to NADH + H +

Enzyme action: dehydrogenase

Oxygen is the eventual e- acceptor

FAD + is reduced to FADH 2

Electron transport chains

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 à NADH à electron transport chain à oxygen

Cellular respiration

Glycolysis: cytosol; degrades glucose into pyruvate

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

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

Glycolysis:

Is a catabolic pathway where a 6C molecule of glucose is split into 2 3C sugars.

These 3C sugars are then oxidized and rearranged into 2 molecule of pyruvate (pyruvic acid).

Glycolysis occurs in the cytoplasm of the cell. Specific enzymes in the cytoplasm catalyze each rxn.

No CO 2 is produced.

It is an anaerobic process; will occur whether or not oxygen is present.

Cellular respiration

Glycolysis: uses glucose

Produces pyruvate (to Krebs cycle)

Produces NADH+H + (to OP & ETC)

Produces 2ATP

Krebs cycle (transition reaction): uses pyruvate

Produces CO2 (to lungs via blood)

Produces NADH+H + and FADH 2 (to OP & ETC)

Produces 2ATP

Glycolysis

1 Glucose à 2 pyruvate molecules

Energy investment phase : cell uses ATP to phosphorylate fuel

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

Net energy yield per 1 glucose : 2 ATP plus 2 NADH + H +; no CO 2 is released; occurs aerobically or anaerobically

Glycolysis : 1.Energy investment phase

The cell uses (consumes) 2 ATP molecules to phosphorylate the intermediates of glycolysis.

Glucose 1 st converts to a single molecule of Fructose 1,6-biphosphate

Fructose 1,6-biphosphate is then split into 2 molecules of 3C sugars: glyceraldehyde phosphate (G3P).

Glycolysis : 2.Energy Yielding Phase

G3P is oxidized by the transfer of electrons (H plus NAD +, forming NADH)

Transferring the phosphate groups from G3P to ADP à ATP.

This is called substrate level phosphorylation.

NAD +

Nicotinamide adenine dinucleotide

It f(x)’s as a coenzyme in the redox rxns of metabolism.

Found in all cells.

Assists enzymes in electron transfer during redox rxns.

Summary of Glycolysis

Since there was an investment of 2 ATP’s and there was a gain of 4 ATP’s, the NET gain is 2 ATP’s for every molecule of glucose.

Pyruvate IS where glycolysis stops.

2 possible destinations of pyruvate:

1. If oxygen is present, can enter the mitochondria to begin the next step of cellular respiration, the Kreb’s Cycle.

Under aerobic conditions, pyruvate will continue to be oxidized.

2 possible destinations of pyruvate:

2. If no oxygen is present, it will enter continue anaerobic respiration (fermentation).

Under anaerobic conditions, pyruvate will be reduced.

2 Pathways for fermentation:

1. Ethyl alcohol fermentation.

2. Lactic Acid fermentation.

Fermentation is the anaerobic catabolism of organic nutrients.

Fermentation:

If Pyruvate is reduced, then something has to be oxidized.

NADH is oxidized to form NAD +.

NO additional ATP is produced.

Ethyl alcohol fermentation

Pyruvate is reduced into a molecule of ethyl alcohol.

CO 2 gas is released.

NADH is consumed.

Important products: beer, wine, bread rising from yeast

Lactic Acid fermentation

Pyruvate is reduced into a molecule of lactic acid.

No CO 2 gas is released.

NADH is consumed.

Important products: cheese, yogurt, makes muscles cells sore

During strenuous exercise, your body can not get enough oxygen to the muscle cells.

When this happens, pyruvate will NOT enter the mitochondria and instead will undergo anaerobic fermentation.

Lactic acid is toxic to cells.

When lactic acid accumulates, it will be gradually carried to the liver.

Pyruvate’s destination w/oxygen:

It enters the mitochondria so that aerobic respiration can occur.

It will continue to be oxidized.

Upon entering the mitochondria, it will be converted to a 2C molecule called acetyl coenzyme A (acetyl coA).

Formation of acetyl coA

Occurs in the mitochondria.

1. Pyruvate’s carboxyl group is removed and released as CO 2. This changes pyruvate from a 3C molecule to a 2C molecule called acetyl coA.

2. The 2C molecule acetate is oxidized; therefore NAD + is reduced à NADH

So far, for every 1 molecule of glucose:

2 ATP’s (net gain) were produced from glycolysis.

4 NADH’s have been produced.

2 H 2O

2 CO 2

Krebs Cycle (citric acid cycle):

The catabolic pathway was discovered by Hans Krebs in the 1930’s.

The acetyl coA that was produced from pyruvate enters the Krebs Cycle.

Energy released in the Krebs Cycle results in the production of more NADH & FADH 2

FAD = flavin adenine dinucleotide (from riboflavin, a B vitamin)

Krebs Cycle

All steps take place entirely inside the mitochondrial matrix.

For every complete turn of the Krebs Cycle:

Two carbons enter in the acetyl fragment of acetyl coA. The coA is released.

Two different carbons are oxidized and leave as CO 2 gas.

3 NADH & one FADH 2 are produced.

One ATP molecule is produced by substrate-level phosphorylation.

Oxaloacetate is regenerated.

For every glucose molecule split by glycolysis:

2 pyruvates are produced à

2 acetyl coA are produced à

Therefore, it takes 2 turns of the Krebs Cycle to complete the oxidation of glucose

Kreb’s Cycle

If molecular oxygen is present…….

Transition reaction:

Each pyruvate is converted into acetyl CoA (2 pyruvates from 1 glucose); CO 2 is released; NAD + à NADH + H +; coenzyme A (from B vitamin) makes molecule very reactive

Acetyl CoA (2C) enters cycle and 2 CO 2 molecules exit

Oxaloacetate is regenerated (the “cycle”)

For each pyruvate that enters: 3 NAD+ reduced to NADH; 1 FAD+ reduced to FADH2; (riboflavin, B vitamin); 1 ATP molecule

Kreb’s Cycle

Acetyl CoA (2C) enters cycle, combines with oxaloacetic acid (4C) to form citric acid (6C)

Oxaloacetate is regenerated at the end of the “cycle”

For each acetyl CoA that enters: 3 NAD + reduced to NADH+H +; 1 FAD + reduced to FADH 2 (riboflavin, B vitamin); 1 ATP molecule is produced

2 CO 2 are released

Intermediates in Krebs cycle are keto acids.

Kreb’s Cycle

Electron transport chain

Cytochromes carry electron carrier molecules (NADH +H +& FADH 2) down to oxygen

Chemiosmosis : energy coupling mechanism

ATP synthase : produces ATP by using the H + gradient (proton-motive force) pumped into the inner membrane space from the electron transport chain; this enzyme harnesses the flow of H+ back into the matrix to phosphorylate ADP to ATP (oxidative phosphorylation)

Electron Transport

For every complete turn of the Krebs Cycle:

Two carbons enter in the acetyl fragment of acetyl coA. The coA is released.

Two different carbons are oxidized and leave as CO 2 gas.

3 NADH & one FADH 2 are produced.

One ATP molecule is produced by substrate-level phosphorylation.

Oxaloacetate is regenerated.

For every glucose molecule split by glycolysis:

2 pyruvates are produced à

2 acetyl coA are produced à

Therefore, it takes 2 turns of the Krebs Cycle to complete the oxidation of glucose

Steps of the Krebs:

1. Acetyl coA breaks. The 2C acetyl group bonds to a 4C oxaloacetate to form a 6C molecule of citrate (citric acid).

2. Citrate gets converted to isocitrate.

3. Isocitrate then loses CO 2, leaving a 5C molecule. The 5C compound is then oxidized and NAD + is reduced forming NADH.

4. A multienzyme complex then catalyzes the removal of CO 2 to form a 4C compound.

5. The 4C molecule continues to become oxidized in a series of rxns. At the end of the rxns, this 4C compound is converted to oxaloacetate. In doing this, ATP, FADH 2, and NADH.

Products of Krebs Cycle: (2 turns per glucose)

4 CO 2

6 NADH

2 FADH 2

2 ATP

So far…

Cellular respiration produces energy. But glycolysis & the Krebs Cycle only produce a net of 4 ATP.

Substrate-level phosphorylation is the method by which this occurs.

The molecules of NADH & FADH 2 account for most of the energy extracted from food.

It is theses electron shufflers (NADH & FADH 2) that link glycolysis and the Krebs Cycle to the ETC.

Oxidative phosphorylation occurs in the ETC & it is a much more efficient process.

THE ETC:

The ETC is powered by the rxns preceding it. NADH and FADH 2 can both enter the ETC for the production of ATP.

ETC occurs across the cristae of the mitochondria. Why is the inner membrane (cristae) so folded?

The proteins embedded in the surface of the inner membrane make up the the chain that shuffles electrons.

Most of the proteins are called cytochromes.

Each successive carrier protein is more electronegative than the previous one, so the electrons are pulled “downhill” towards oxygen.

Oxygen is the final electron acceptor.

Both NADH and FADH 2 can enter the ETC.

NADH produces more ATP because it enters the ETC at a higher energy level than FADH 2

The ETC does not make ATP directly.

It generates a proton gradient across the inner mitochondrial membrane, which stores potential energy that can be used to make ATP.

Chemiosmosis:

The coupling of exergonic electron flow down an electron transport chain to endergonic ATP production by the creation of a proton gradient across a membrane.

The proton gradient drives ATP synthesis as protons diffuse back across the membrane.

The term chemiosmosis emphasizes a coupling between:

1. Chemical rxns (phosphorylation)

2. Transport processes (proton transport)

ATP synthase is an enzyme that makes ATP.

It uses the existing proton gradient across the inner membrane to power ATP synthesis.

When the ETC is working in the mitochondria:

The pH in the intermembrane space is one or two pH units lower than in the matrix.

Organisms:

Can be classified based upon the effect oxygen has on growth & metabolism.

Strict (obligate) aerobe: requires oxygen

Strict (obligate anaerobe): Can only live in the absence of oxygen

Facultative anaerobe: can live either with or without oxygen

Photosynthesis

Process of using light

energy to make glucose.

Pigment that traps light is

called chlorophyll. It also gives leaves their green color.

Chlorophyll is located in chloroplasts.

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

Fermentation: alcohol~ pyruvate to ethanol lactic acid~ pyruvate to lactate

Facultative anaerobes (yeast/bacteria)

Beta-oxidation lipid catabolism

Connections

Proteins: Deaminated - lose NH 2 groups and enter carbohydrate pathway as keto acids or pyruvate. NH 2 groups form ammonia which is transformed to urea in liver and excreted by kidneys.

Lipids (triglycerides): Glycerol is converted to glyceraldehyde phosphate, enters glycolysis (3C) and yields energy equivalent to about ½ glucose. Fatty acids are cleaved into 2C compounds (beta oxidation) and enter Krebs cycle as acetyl CoA.