Cellular Respiration:Harvesting Chemical Energy

AP Biology

Ms. Haut

Energy Flow

• Energy flows into an ecosystem as sunlight and leaves as heat

• Photosynthesis generates oxygen and organic molecules, which are used in cellular respiration

• Cells use chemical energy stored in organic molecules to regenerate ATP, which powers work

Catabolic Pathways and Production of ATP

• The breakdown of organic molecules is exergonic

• Fermentation is a partial degradation of sugars that occurs without oxygen

• Cellular respiration consumes oxygen and organic molecules and yields ATP

• Although carbohydrates, fats, and proteins are all consumed as fuel, it is helpful to trace cellular respiration with the sugar glucose:

C6H12O6 + 6O2 6CO2 + 6H2O + Energy (ATP + heat)

Cellular Respiration

• ATP-producing catabolic process in which the ultimate electron acceptor is an inorganic compound, Oxygen

• Most efficient catabolic pathway• Is an exergonic process (ΔG = -686kcal/mol)

CC66HH1212OO66 + 6O + 6O22 6CO 6CO22 + 6H + 6H22O + Energy (ATP + Heat)O + Energy (ATP + Heat)

Oxidation

Reduction

Electrons “fall” from Organic Molecules to Oxygen during Cellular Respiration

The Stages of Cellular Respiration: A Preview

• Cellular respiration has three stages:– Glycolysis (breaks down glucose into two

molecules of pyruvate)– The Krebs Cycle (citric acid cycle) (completes the

breakdown of glucose)– Oxidative phosphorylation (accounts for most of the

ATP synthesis)• The process that generates most of the ATP is

called oxidative phosphorylation because it is powered by redox reactions

Glycolysis

• Catabolic pathway• Occurs in the cytosol• Partially oxidizes

glucose (6C) into two pyruvate (3C) molecules Mitochondrion

Glycolysis

PyruvateGlucose

Cytosol

ATP

Substrate-levelphosphorylation

Krebs Cycle

• Catabolic pathway• Occurs in

mitochondrial matrix• Completes glucose

oxidation by breaking down a acetyl-CoA into CO2

Mitochondrion

Glycolysis

PyruvateGlucose

Cytosol

ATP

Substrate-levelphosphorylation

ATP

Substrate-levelphosphorylation

Citricacidcycle

Glycolysis and Krebs Cycle

Together produce:

• Small amount of ATP by substrate-level phosphorylation

• NADH by transferring electrons from substrate to NAD+

• Krebs Cycle also produces FADH2 by transferring electrons to FAD+

Electron Transport Chain

• Located near inner membrane of mitochondrion

• Accepts energized electrons from reduced coenzymes (NADH and FADH2) that are harvested during glycolysis and Krebs Cycle

– Oxygen pulls electrons down ETC to a lower energy state

Mitochondrion

Glycolysis

PyruvateGlucose

Cytosol

ATP

Substrate-levelphosphorylation

ATP

Substrate-levelphosphorylation

Citricacidcycle

ATP

Oxidativephosphorylation

Oxidativephosphorylation:electron transport

andchemiosmosis

Electronscarried

via NADH

Electrons carriedvia NADH and

FADH2

Oxidative Phosphorylation

• Accounts for almost 90% of the ATP generated by cellular respiration

• Energy released at each step of the ETC is stored in a form the mitochondrion can use to make ATP– Powered by redox reactions that transfer electrons

from food to oxygen

• Small amount of ATP is produced directly by the enzymatic transfer of phosphate from an intermediate substrate in catabolism to ADP (substrate-level phosphorylation)

Glycolysis

• Glycolysis (“splitting of sugar”) breaks down glucose into two molecules of pyruvate

• Two major phases:– Energy investment

phase– Energy payoff phase

Glycolysis

Glycolysis

Glycolysis

Glycolysis

Conversion of Pyruvate to Acetyl CoA:Pyruvate Oxidation

LE 9-12_1

ATP ATP ATP

Glycolysis Oxidationphosphorylation

Citricacidcycle

Citricacidcycle

Citrate

Isocitrate

Oxaloacetate

Acetyl CoA

H2O

LE 9-12_2

ATP ATP ATP

Glycolysis Oxidationphosphorylation

Citricacidcycle

Citricacidcycle

Citrate

Isocitrate

Oxaloacetate

Acetyl CoA

H2O

CO2

NAD+

NADH

+ H+

-Ketoglutarate

CO2NAD+

NADH

+ H+SuccinylCoA

LE 9-12_3

ATP ATP ATP

Glycolysis Oxidationphosphorylation

Citricacidcycle

Citricacidcycle

Citrate

Isocitrate

Oxaloacetate

Acetyl CoA

H2O

CO2

NAD+

NADH

+ H+

-Ketoglutarate

CO2NAD+

NADH

+ H+SuccinylCoA

Succinate

GTP GDP

ADP

ATP

FAD

FADH2

P i

Fumarate

LE 9-12_4

ATP ATP ATP

Glycolysis Oxidationphosphorylation

Citricacidcycle

Citricacidcycle

Citrate

Isocitrate

Oxaloacetate

Acetyl CoA

H2O

CO2

NAD+

NADH

+ H+

-Ketoglutarate

CO2NAD+

NADH

+ H+SuccinylCoA

Succinate

GTP GDP

ADP

ATP

FAD

FADH2

P i

Fumarate

H2O

Malate

NAD+

NADH

+ H+

Net ProductsGlycolysis:

2 pyruvate2 NADH2 ATP

Pyruvate Oxidation:2 NADH

Krebs Cycle:6 NADH

2 FADH2

2 ATP

During oxidative phosphorylation, chemiosmosis couples electron transport to

ATP synthesis

• Following glycolysis and the citric acid cycle, NADH and FADH2 account for most of the energy extracted from food

• These two electron carriers donate electrons to the electron transport chain, which powers ATP synthesis via oxidative phosphorylation

The Pathway of Electron Transport

• The electron transport chain is in the cristae of the mitochondrion

• The carriers alternate reduced and oxidized states as they accept and donate electrons

• Electrons lose energy as they go down the chain and are finally passed to O2, forming water

• The electron transport chain generates no ATP

LE 9-13

ATP ATP ATP

GlycolysisOxidative

phosphorylation:electron transportand chemiosmosis

Citricacidcycle

NADH

50

FADH2

40 FMN

Fe•S

I FAD

Fe•S II

IIIQ

Fe•S

Cyt b

30

20

Cyt c

Cyt c1

Cyt a

Cyt a3

IV

10

0

Multiproteincomplexes

Fre

e en

erg

y (G

) re

lati

ve t

o O

2 (k

cal/m

ol)

H2O

O22 H+ + 1/2

Chemiosmosis

• Electron transfer in the electron transport chain causes proteins to pump H+ from the mitochondrial matrix to the intermembrane space

• H+ then moves back across the membrane, passing through channels in ATP synthase

• ATP synthase uses the exergonic flow of H+ to drive phosphorylation of ATP

• This is an example of chemiosmosis, the use of energy in a H+ gradient to drive cellular work

LE 9-14

INTERMEMBRANE SPACE

H+ H+

H+H+

H+

H+

H+

H+

ATP

MITOCHONDRAL MATRIX

ADP+

Pi

A rotor within the membrane spins as shown when H+ flows past it down the H+ gradient.

A stator anchored in the membrane holds the knob stationary.

A rod (or “stalk”) extending into the knob also spins, activating catalytic sites in the knob.

Three catalytic sites in the stationary knob join inorganic phosphate to ADP to make ATP.

• The energy stored in a H+ gradient across a membrane couples the redox reactions of the electron transport chain to ATP synthesis

• The H+ gradient is referred to as a proton-motive force, emphasizing its capacity to do work

1 NADH = 3 ATP1 FADH2 = 2 ATP

10 NADH = 30 ATP2 FADH2 = 4 ATP

34 ATP

Aerobic: existing in the presence of oxygen

Anaerobic: existing in the absence of oxygen

Fermentation

• Cellular respiration requires O2 to produce ATP

• Glycolysis can produce ATP with or without O2 (in aerobic or anaerobic conditions)

• In the absence of O2, glycolysis couples with fermentation to produce ATP

Fermentation

• Anaerobic catabolism of organic nutrients

• After pyruvate is produced in glycolysis, it is reduced, and NAD+ is regenerated– Prevents cell from depleting the pool of

NAD+, needed in glycolysis– No additional ATP is produced

Organisms Classified by Oxygen Requirements

• Strict (obligate) aerobes —require O2 for growth and metabolism

• Strict (obligate) anaerobes —only grow in the absence of O2 (O2 is toxic)

• Facultative anaerobes —capable of growing in either aerobic or anaerobic conditions

1. NADH is oxidized to NAD+ and pyruvate is reduced to lactate (lactic acid)

•Commercially important products: cheese & yogurt•Human muscle cells switch to lactic acid fermentation when O2 is scarce. Lactate accumulates, slowly carried to liver and converted back to pyruvate when O2 is available

1. Pyruvate loses CO2 and is converted to the acetylaldehyde (2C)

2. NADH is oxidized to NAD+ and acetylaldehyde is reduced to ethanol (EtOH)

•Many bacteria and yeast carry out alcohol fermentation under anaerobic conditions

Alcohol Fermentation

Yeast during brewing process

http://www.langhambrewery.co.uk/content/fermentation-yeast.jpg

Actively fermenting beer. The yeast mass is converting the sugars to alcohol and carbon dioxide. www.berkshirebrewingcompany.com/.../page4.html

Versatility of Catabolism• Starchglucose in

digestive tract• Liver converts

glycogenglucose• Excess amino

acidspyruvate, acetyl CoA, and α-ketoglutarate

• Fatsglycerol + fatty acids• Glycerolglyceraldehyde

phosphate• Fatty acidsacetyl CoA

(beta oxidation)

Biosynthesis

• Some organic molecules of food provide carbon skeletons or raw materials for making new macromolecules

• Some organic molecules from digestion used directly in anabolic pathways

• Some precursors come from glycolysis and Krebs Cycle

• Anabolic pathways require ATP produced from catabolic pathways

Feedback Mechanism of Control

• Ratio of ATP:ADP and AMP reflects energy status and phosphofructokinase is sensitive to changes in the ratio

• Citrate and ATP = allosteric inhibitors

• ADP and AMP = allosteric activators