Cellular Respiration: Harvesting Chemical Energy

Chapter 9

Life Is Work

• Living cells require energy from outside sources

• Plants E from ?

• Animals E from ?

Light energy

ECOSYSTEM

Photosynthesis in chloroplasts

CO2 + H2O

Cellular respiration in mitochondria

Organic molecules

+ O2

ATP powers most cellular work

Heat energy

ATP

Energy flows into

ecosystem as light

Energy leaves as heat

ATP powers work

• Photosynthesis

– Organelle = ?

– Generates O2 and organic molecules

• Cellular respiration

– Organelle = ?

– Uses organic molecules to generate ATP

Catabolic Pathway review

• Organic molecules have potential (chemical) energy

• Exergonic rxns break down organic molecules energy (and heat)

Cellular Respiration

• Aerobic respiration – Uses O2

– ATP produced

Anaerobic respiration Does not use O2

ATP produced

Cellular respiration

1. Glycolysis Occurs in cytoplasm

Anaerobic

Glucose + 2NAD+ + 2ATP 2 pyruvate+ 2NADH + 4ATP

• 1 glucose 2 ATP and 2 pyruvate

• Glucose oxidized to pyruvate (loses electron)

• NAD+ reduced to NADH (gains electron)

Substrate-level

phosphorylation

ATP

Cytosol

Glucose Pyruvate

Glycolysis

Electrons

carried

via NADH

Glycolysis

CYTOSOL

Electron donor

mitochondrion

Glycolysis

1. Energy investment phase uses 2 ATP

2. Energy payoff phase

– 4 ATP produced

– 2NAD+ reduced to 2NADH

– 1 glucose split to 2 pyruvate

Glucose + 2NAD+ + 2ATP 2 pyruvate+ 2NADH + 4ATP

Energy investment phase

Glucose

2 ADP + 2 P 2 ATP used

formed 4 ATP

Energy payoff phase

4 ADP + 4 P

2 NAD+ + 4 e– + 4 H+ 2 NADH + 2 H+

2 Pyruvate + 2 H2O

2 Pyruvate + 2 H2O Glucose

Net

4 ATP formed – 2 ATP used 2 ATP

2 NAD+ + 4 e– + 4 H+ 2 NADH + 2 H+

10 enzymatic

steps in

glycolysis

2. Citric acid cycle (Krebs cycle) • mt matrix

– Matrix is enclosed by the inner membrane

What’s in the matrix?

Enzymes (acetyl CoA)

mtDNA

Ribosomes

Citric acid cycle 2Pyruvate + NAD+ + FADH 2ATP + NADH + FADH2 + CO2 + H2O

Where did the pyruvate come from? How did it get into the mt matrix? # ATP generated? Waste product? Where does it go? NADH and FADH2 can donate electrons later What happened to the sugar? O2?

Mitochondrion

Substrate-level

phosphorylation

ATP

Cytosol

Glucose Pyruvate

Glycolysis

Electrons

carried

via NADH

Substrate-level

phosphorylation

ATP

Electrons carried

via NADH and

FADH2

Citric

acid

cycle

MITCHONDRION

citric acid cycle

1. Convert 2pyruvate to 2acetyl A (before cycle)

Acetyl CoA links

glycolysis to cycle

Pyruvate diffuses into mt matrix and is converted to acetyl CoA

Cellular Respiration: Bioflix animation

CYTOSOL MITOCHONDRION

NAD+ NADH + H+

2

1 3

Pyruvate

Transport protein

CO2 Coenzyme A

Acetyl CoA

• 2. The citric acid cycle

Pyruvate

NAD+

NADH

+ H+ Acetyl CoA

CO2

CoA

CoA

CoA

Citric acid cycle

FADH2

FAD

CO2 2

3

3 NAD+

+ 3 H+

ADP + P i

ATP

NADH

8 enzymatic steps

ATP:

For each pyruvate?

For each glucose?

For each turn of cycle?

Summary of citric acid cycle • Per molecule glucose =2 pyruvate

– NADH and FADH 2 electron donors

– 2 ATP (1 per turn) per glucose

• CO 2 produced (2 per turn) out

• mt matrix

Text Activity: The Citric Acid Cycle

BIO 231 TCA cycle animation: Acetyl CoA formation

3. oxidative phosphorylation in mt cristae Cristae compartmentalize mt inner membrane = more surface area

What happens?

NADH and FADH 2 donate electrons in the series of steps

Oxygen accepts electrons water

H+ proton gradient

ADP + P ATP

34 ATP produced

Add up the ATP yield per glucose:

Glycolysis + Citric acid cycle + Ox Phos =

Mitochondrion

Substrate-level

phosphorylation

ATP

Cytosol

Glucose Pyruvate

Glycolysis

Electrons

carried

via NADH

Substrate-level

phosphorylation

ATP

Electrons carried

via NADH and

FADH2

Oxidative

phosphorylation

ATP

Citric

acid

cycle

Oxidative

phosphorylation:

electron transport

and

chemiosmosis

Oxidative phosphorylation: 34 ATP

Stepwise Energy Harvest via Electron Transport Chain

1. Controlled rxns

(a) Uncontrolled reaction

H2 + 1/2 O2

Explosive release of heat and

light energy

(b) Cellular respiration

Controlled release of energy for synthesis

of ATP

2 H+ + 2 e–

2 H 1/2

O2 (from food via NADH)

1/2

O2

• 2. electron transport is a fall in energy during each step to control release of fuel energy

NADH

NAD+ 2

FADH2

2 FAD

Multiprotein

complexes FAD

Fe•S

FMN

Fe•S

Q

Fe•S

Cyt b

Cyt c1

Cyt c

Cyt a

Cyt a3

IV

50

40

30

20

10 2

(from NADH

or FADH2)

0 2 H+ + 1/2 O2

H2O

e–

e–

e–

powered by redox reactions

Electron Transport Chain

BIO 231 Electron transport animation Watch the electrons

Overview: Wiley

Electron Transport: Wiley

Watch the electrons

• In addition to electron transfer…….

• 3. H+ ions pumped out

H+ gradient, a proton force • ET chain e- pumps H+ across mt membrane

• H+ gradient drives ATP production

• Interactive concepts

• Watch the H+ ions

• Mcgraw hill electron transport

• Watch the H+, no audio

Chemiosmosis couples energy of electron transport to ATP synthesis

• ATP synthase

– H+ ion enters for one turn

– ADP + P ATP

INTERMEMBRANE SPACE

Rotor

H+ Stator

Internal rod

Catalytic knob

ADP +

P ATP i

MITOCHONDRIAL MATRIX

Virtual Cell: Electron

Transport Chain animation

Protein complex of electron carriers

H+

H+ H+

Cyt c

Q

V

FADH2 FAD

NAD+

NADH

(carrying electrons from food)

Electron transport chain

2 H+ + 1/2O2 H2O

ADP + P i

Chemiosmosis

Oxidative phosphorylation

H+

H+

ATP

synthase

ATP

2 1

An Accounting of ATP Production by Cellular Respiration

• Most energy:

glucose NADH electron transport chain proton-motive force ATP

= ~38 ATP total

Maximum per glucose: About

36 or 38 ATP

+ 2 ATP +2ATP + about 32 or 34 ATP

Oxidative phosphorylation: electron transport

and chemiosmosis

Citric acid cycle

2 Acetyl CoA

Glycolysis

Glucose 2

Pyruvate

2 NADH 2 NADH 6 NADH 2 FADH2

2 FADH2

2 NADH CYTOSOL Electron shuttles span membrane

or

MITOCHONDRION

Glycolysis Citric Acid Cycle Ox. Phos.

Cytosol mt mt

Anaerobic respiration (no O2)

Anaerobic respiration (cytoplasm) Prokaryotes

Eukaryotes

Generate ATP without O2

1. Glycolysis

2. Fermentation

Fermentation

No electron transport chain

NAD+ reused in glycolysis (way to keep generating ATP without O2)

Alcohol fermentation

• Pyruvate + NADH ethanol + NAD+ + CO2

• Bacteria

• Yeast by humans for:

2 ADP + 2 P i 2 ATP

Glucose Glycolysis

2 Pyruvate

2 NADH 2 NAD+

+ 2 H+ CO2

2 Acetaldehyde 2 Ethanol

(a) Alcohol fermentation

2

Lactic acid fermentation

Pyruvate + NADH lactate + NAD+

• Bacteria, fungi in cheese making

• Human muscle cells use lactic acid fermentation to generate Pyruvate + NADH lactate + NAD+

• ATP when O2 is low.

Glucose

2 ADP + 2 P i 2 ATP

Glycolysis

2 NAD+ 2 NADH

+ 2 H+ 2 Pyruvate

2 Lactate

(b) Lactic acid fermentation

Fermentation (no O2) vs. Aerobic Respiration

• Both use glycolysis to oxidize glucose (and other organic fuels ) to pyruvate

• ATP

– Cellular respiration 38 ATP per glucose

– Fermentation 2 ATP per glucose

• Obligate anaerobes – fermentation – cannot survive in the presence of O2

– Ex. clostridium botulinum

• Facultative anaerobes

– Yeast and many bacteria – can survive using either fermentation or cellular

respiration (pyruvate can be used either way) – Ex. E. coli, Streptococcus

Glucose

Glycolysis

Pyruvate

CYTOSOL

No O2 present:

Fermentation

O2 present:

Aerobic cellular

respiration

MITOCHONDRION

Acetyl CoA Ethanol or

lactate Citric acid cycle

Facultative anaerobe

The Evolutionary Significance of Glycolysis

• Glycolysis occurs in nearly all organisms

• Glycolysis probably evolved in ancient prokaryotes before O2 on planet

Glycolysis and the citric acid cycle connect to other metabolic pathways

The Versatility of Catabolism

• Glycolysis and fuel

– Carbohydrates – many accepted

– Proteins amino acids; glycolysis or the citric acid cycle

– Fats glycerol glycolysis

– Fatty acids acetyl CoA

– An oxidized gram of fat produces >2X ATP as oxidized gram of carbohydrate

Proteins Carbohydrates

Amino acids

Sugars

Fats

Glycerol Fatty acids

Glycolysis

Glucose

Glyceraldehyde-3-

Pyruvate

P

NH3

Acetyl CoA

Citric acid cycle

Oxidative phosphorylation

Fermentation and anaerobic respiration enable cells to produce ATP

without the use of oxygen

• Most 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 or anaerobic respiration to produce ATP

Regulation of Cellular Respiration via Feedback Mechanisms

• Feedback inhibition is the most common mechanism for control

• If ATP concentration begins to drop, respiration speeds up; when there is plenty of ATP, respiration slows down

• Control of catabolism is based mainly on regulating the activity of enzymes at strategic points in the catabolic pathway

Biosynthesis (Anabolic Pathways)

• The body uses small molecules to build other substances

• These small molecules may come directly from food, from glycolysis, or from the citric acid cycle