chapter 9 cell respiration p147. the principle of redox redox reactions –transfer electrons from...

Chapter 9

Cell Respiration p147

The Principle of Redox

• Redox reactions– Transfer electrons from one reactant to another

by oxidation and reduction

• Redox reactions also occur when the movement of electrons is not complete but involve a change in the degree of electron sharing in covalent bonds.

• In the combustion of methane to form water and carbon dioxide, the nonpolar covalent bonds of methane (C-H) and oxygen (O=O) are converted to polar covalent bonds (C=O and O-H).

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 9.3

Oxidation of Organic Fuel Molecules During Cellular

Respiration• During cellular respiration

– Glucose is oxidized and oxygen is reduced

C6H12O6 + 6O2 6CO2 + 6H2O + Energy

becomes oxidized

becomes reduced

Stepwise Energy Harvest via NAD+ and the Electron Transport Chain

• Cellular respiration– Oxidizes glucose in a series of steps

• Electrons from organic compounds– Are usually first transferred to NAD+, a

coenzyme

NAD+

H

O

O

O O–

O

O O–

O

O

O

P

P

CH2

CH2

HO OHH

HHO OH

HO

H

H

N+

C NH2

HN

H

NH2

N

N

Nicotinamide(oxidized form)

NH2+ 2[H]

(from food)

Dehydrogenase

Reduction of NAD+

Oxidation of NADH

2 e– + 2 H+

2 e– + H+

NADH

OH H

N

C +

Nicotinamide(reduced form)

N

Figure 9.4

• NADH, the reduced form of NAD+

– Passes the electrons to the electron transport chain


Fig. 9.5

• Unlike the explosive release of heat energy that would occur when H2 and O2 combine, cellular respiration uses an electron transport chain to break the fall of electrons to O2 into several steps.

• ATP, adenosine triphosphate, is the pivotal molecule in cellular energetics.

• It is the chemical equivalent of a loaded spring.– The close packing of three negatively-charged

phosphate groups is an unstable, energy-storing arrangement.

– Loss of the end phosphate group “relaxes” the “spring”.

• The price of most cellular work is the conversion of ATP to ADP and inorganic phosphate (Pi).

• An animal cell regenerates ATP from ADP and Pi by the catabolism of organic molecules.

2. Cells recycle the ATP they use for work


High Energy Bonds

2 High energy bonds between phosphate groups

2 High energy bonds between phosphate groups

Breaking a bond releases energy

Breaking a bond releases energy

Catabolism breaks down molecules

ex: aerobic respiration

Anabolism builds up molecules

ex: muscle building

• The transfer of the terminal phosphate group from ATP to another molecule is phosphorylation. – This changes the shape of the receiving molecule,

performing work (transport, mechanical, or chemical).

– When the phosphate groups leaves the molecule, the molecule returns to its alternate shape.


Fig. 9.2

The Stages of Cellular Respiration: A Preview

• Respiration is a cumulative function of three metabolic stages– Glycolysis– The citric acid cycle– Oxidative phosphorylation

• Glycolysis– Breaks down glucose into two molecules of

pyruvate

• The citric acid cycle– Completes the breakdown of glucose

• Oxidative phosphorylation– Is driven by the electron transport chain– Generates ATP

• An overview of cellular respiration

Figure 9.6

Electronscarried

via NADH

GlycolsisGlucos

ePyruvate

ATP

Substrate-levelphosphorylation

Electrons carried via NADH and

FADH2

Citric acid cycle

Oxidativephosphorylation:

electron transport and

chemiosmosis

ATPATP

Substrate-levelphosphorylation

Oxidativephosphorylation

MitochondrionCytosol

• Both glycolysis and the citric acid cycle– Can generate ATP by substrate-level

phosphorylation

Figure 9.7

Enzyme Enzyme

ATP

ADP

Product

SubstrateP

+

Transparency 7A-1

Transparency 7A-2

Transparency 7A-3

Transparency 7A-4

Transparency 7A-5

Transparency 7A-6

Oxidative Phosphorylation

Transparency 7A-6

1. What is the source of carbon for cellular respiration?

Glucose (C6H

12O

6) is the carbon source.


Transparency 7A-7

2. Where do Stages 2 and 3 of cellular respiration take place?

in mitochondria



Fig. 9.9a

Feedback

Inhibition

Feedback

Inhibition

• The net yield from glycolysis is 2 ATP and 2 NADH per glucose.– No CO2 is produced during glycolysis.

• Glycolysis occurs whether O2 is present or not.

– If O2 is present, pyruvate moves to the Krebs cycle and the energy stored in NADH can be converted to ATP by the electron transport system and oxidative phosphorylation.


Glycolysis Animation

• As pyruvate enters the mitochondrion, a multienzyme complex modifies pyruvate to acetyl CoA which enters the Krebs cycle in the matrix.– A carboxyl group is removed as CO2.

– A pair of electrons is transferred from the remaining two-carbon fragment to NAD+ to form NADH.

– The oxidized fragment, acetate, combines with coenzyme A to form acetyl CoA.

Fig. 9.10

Bridge to the Krebs Cycle

Bridge to the Krebs Cycle

Krebs Cycle

2 NADH

2 NADH

6 NADH2 FADH2

(pyruvate)

Proteins of the Electron Transport Chain (ETC) are located in the inner mitochondial membrane called the cristae

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

along the chain which includes several cytochrome proteins and one lipid carrier.

• The electrons carried by FADH2 have lower free energy and are added to a later point in the chain.

Fig. 9.13

The flow of electrons in the ETC provide energy to pump protons to the outer compartment –forming a proton gradient

(inner Matrix)

• A protein complex, ATP synthase, in the cristae actually makes ATP from ADP and Pi.

• ATP used the energy of

an existing proton gradient to power ATP synthesis.– This proton gradient

develops between the intermembrane space and the matrix.


Fig. 9.14

• 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.


H+H+

H+

H+

H+

Proton pumps make a chemical and electrical gradient that is used to make ATP – this is chemiosmosis

• The ATP synthase molecules are the only place that will allow H+ to diffuse back to the matrix.

• This exergonic flow of H+ is used by the enzyme to generate ATP.

• This coupling of the redox reactions of the electron transport chain to ATP synthesis is called chemiosmosis.


• The mechanism of ATP generation by ATP synthase is still an area of active investigation.– As hydrogen ions flow

down their gradient, they cause the cylinder portion and attached rod of ATP synthase to rotate.

– The spinning rod causes a conformational change in the knob region, activating catalytic sites where ADP and inorganic phosphate combine to make ATP.Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 9.14

• Chemiosmosis is an energy-coupling mechanism that uses energy stored in the form of an H+ gradient across a membrane to drive cellular work.– In the mitochondrion, chemiosmosis generates ATP.– Chemiosmosis in chloroplasts also generates ATP,

but light drives the electron flow down an electron transport chain and H+ gradient formation.

– Prokaryotes generate H+ gradients across their plasma membrane.• They can use this proton-motive force not only to generate

ATP but also to pump nutrients and waste products across the membrane and to rotate their flagella.


Mitchell Hypothesis

Fig. 9.16

• How efficient is respiration in generating ATP?– Complete oxidation of glucose releases 686 kcal per

mole.– Formation of each ATP requires at least 7.3

kcal/mole.– Efficiency of respiration is 7.3 kcal/mole x 38

ATP/glucose/686 kcal/mole glucose = 40%.– The other approximately 60% is lost as heat.

• Cellular respiration is remarkably efficient in energy conversion.


2 NADH

2 NADH

6 NADH

2 FADH2

2 ATP

2 ATP

34 ATP

2 CO2

4 CO2

Electron Transport System

Fermentation

Glycolysis

Krebs Cycle

½ O2

• Oxidation refers to the loss of electrons to any electron acceptor, not just to oxygen.– In glycolysis, glucose is oxidized to two pyruvate

molecules with NAD+ as the oxidizing agent, not O2.– Some energy from this oxidation produces 2 ATP

(net).– If oxygen is present, additional ATP can be generated

when NADH delivers its electrons to the electron transport chain.

• Glycolysis generates 2 ATP whether oxygen is present (aerobic) or not (anaerobic).

1. Fermentation enables some cells to produce ATP without the help of oxygen


The ETC regenerates NAD+ to be recycled back to Glycolysis. The ETC can only do this if O2 is available

Without NAD+, Glycolysis cannot occur.

NADH NAD+

NAD+ must be regenerated for glycolysis to continue

34

38

NAD+ recycled

If no O2 is available NAD+ is made by having NADH dump its electrons on to pyruvate.

NAD+ recycled

NAD+ is recycled back to glycolysis

Because NAD+ is recycled most of the energy from Glycolysis is lost

• In alcohol fermentation, pyruvate is converted to ethanol in two steps.– First, pyruvate is converted to a two-carbon

compound, acetaldehyde by the removal of CO2.

– Second, acetaldehyde is reduced by NADH to ethanol.

– Alcohol fermentation by yeast is used in brewing and winemaking.


Fig. 9.17a

• During lactic acid fermentation, pyruvate is reduced directly by NADH to form lactate (ionized form of lactic acid). – Lactic acid fermentation by some fungi and bacteria

is used to make cheese and yogurt.– Muscle cells switch from aerobic respiration to

lactic acid fermentation to generate ATP when O2 is scarce.• The waste product,

lactate, may cause muscle fatigue, but ultimately it is converted back to pyruvate in the liver.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin CummingsFig. 9.17b

Different food molecules can enter and exit at different levels in the aerobic pathway

ATP acts as negative modulator inactivating the first enzyme

Feedback inhibition

Regulation such as feedback inhibition controls how much ATP is produced

1/2 second1/2 second

<1 Minute<1 Minute

1-3 Minutes1-3 Minutes

>3 Minutes>3 Minutes Other molecules besides glucose can be used in the aerobic pathway

Other molecules besides glucose can be used in the aerobic pathway

Chapter 9

Cellular Respiration: Harvesting Chemical Energy

Respiration

Occurs 2 waysWithout oxygen

Has two stages

Which produces Which produces waste products

Has 3 stages

2nd

1st1st

2nd

3rd

With oxygen

• When electrons flow along the electron transport chains of mitochondria, which ofthe following changes occur?– The pH of the matrix increases.– ATP synthase pumps protons by active transport.– The electrons gain free energy.– The cytochromes of the chain phosphorylate ADP

to form ATP.– NAD+ is oxidized.

• In the presence of a metabolic poison that specifically and completely inhibit the function of mitochondrial ATP synthase, which of the following would you expect?– a decrease in the pH difference across the inner

mitochondrial membrane– an increase in the pH difference across the inner

mitochondrial membrane– increased synthesis of ATP– oxygen consumption to cease– proton pumping by the electron transport chain to

cease

• In the 1940s, some physicians prescribed low doses of a drug called dinitrophenol (DNP) to help patients lose weight. This unsafe method was abandoned after a few patients died. DNP uncouples the chemiosmotic machinery by making the lipid bilayer of the inner mitochondrial membrane leaky to H+. What impact does this have on ATP production? *– reduces substrate level phosphorylations– increases substrate level phosphorylations– reduces oxidative level phosphorylations– increase oxidative level phosphorylations– This would have no impact on ATP production.

• Cyanide is a poison that blocks the passage of electrons along the electron transport chain. Which of the following is a metabolic effect of this poison?– The lower pH of the intermembrane space is much lower

than normal.– Electrons are passed directly to oxygen, causing cells to

explode.– Alcohol would build up in the cells.– NADH supplies would be exhausted, and ATP synthesis

would cease.– No proton gradient would be produced, and ATP synthesis

would cease.

• Which kind of metabolic poison would most directly interfere with glycolysis?– an agent that reacts with oxygen and depletes

its concentration in the cell– an agent that binds to pyruvate and inactivates

it– an agent that closely mimics the structure of

glucose but is not metabolized– an agent that reacts with NADH and oxidizes it

to NAD+

– an agent that inhibits the formation of acetyl coenzyme A

• Glucose, made from six radioactively labeled carbon atoms, is fed to yeast cells in the absence of oxygen. How many molecules of radioactive alcohol (C2H5OH) are formed from each molecule of glucose?– 0– 1– 2– 3– 6

• A young relative of yours has never had much energy. He goes to a doctor for help and is sent to the hospital for some tests. There they discover his mitochondria can use only fatty acids and amino acids for respiration, and his cells produce more lactate than normal. Of the following, which is the best explanation of his condition? – His mitochondria lack the transport protein that moves pyruvate across

the outer mitochondrial membrane.– His cells can not move NADH from glycolysis into the mitochondria.– His cells contain something that inhibits oxygen use in his

mitochondria.– His cells lack the enzyme in glycolysis that forms pyruvate.– His cells have a defective electron transport chain, so glucose goes to

lactate instead of to acetyl CoA.

• You have a friend who lost 15 pounds of fat on a diet. Where did the fat go (how was it lost)? *– It was released as CO2 and H2O.– Chemical energy was converted to heat and then

released.– It was converted to ATP, which weighs much less than

fat.– It was broken down to amino acids and eliminated

from the body.– It was converted to urine and eliminated from the

body.

Review Questions Links

• Link 1

• Link 2

• Link 3

• Link 4

WORKSHEET ELODEA/FISH1. Write the equation of the reaction of CO2 and water.

2. Bromothymol blue (BTB) is colored blue when the pH is __________.3. BTB is colored green when the pH is ____________________.4. BTB is colored yellow when the pH is _______________.5. A high conc. of CO2 will cause the pH of the water to be______.

What color will the BTB be?6. The gas that is released by consumers is ___________.7. The gas that is produced by producers is ___________.8. The gas that is absorbed by producers is ___________.9. A low level of CO2 will cause the pH of water to be _______.

What color will the BTB be?10. Explain the color of the BTB in the bowl 2 that just had the fish (Use the terms pH, CO2 and acid).

11. Was the fish healthy in bowl 2? Why?12. Explain the color of the BTB in the bowl 3 that just had the plant.13. Is the plant healthy in bowl 3? Why?14. Explain the color of the bowl 4 that had both the fish and the plant. (Use the terms pH, acid, CO2, photosynthesis, respiration).

15. Do you think the plant and fish were healthy in bowl 4? Why?BOWL 1: WATER + BTBBOWL 2: WATER + BTB + FISH BOWL 3: WATER + BTB + PLANTBOWL 4: WATER + BTB + FISH + PLANT

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