Cellular Respiration�Harvesting Chemical Energy

ATP

AP Biology

AP Biology

2006-2007

What’s the�point?

The point�is to make�ATP!

ATP

AP Biology

AP Biology

Harvesting stored energy

• Energy is stored in organic molecules
• carbohydrates, fats, proteins
• Heterotrophs eat these organic molecules → food
• digest organic molecules to get…
• raw materials for synthesis
• fuels for energy
• controlled release of energy
• “burning” fuels in a series of �step-by-step enzyme-controlled reactions

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Harvesting stored energy

• Glucose is the model
• catabolism of glucose to produce ATP

CO₂ + H₂O + heat

RESPIRATION = making ATP (& some heat)�by burning fuels in many small steps

CO₂ + H₂O + ATP (+ heat)

enzymes

ATP

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How do we harvest energy from fuels?

• Digest large molecules into smaller ones
• break bonds & move electrons from one molecule to another
• as electrons move they “carry energy” with them
• that energy is stored in another bond, �released as heat or harvested to make ATP

e^-

+

–

loses e-

gains e-

oxidized

reduced

redox

e^-

AP Biology

Two Types of Catabolic Pathways

• Catabolic pathways occur when molecules are broken down and their energy is released. Two catabolic pathways to know:
• Fermentation: the partial degradation of sugars that occur without the use of oxygen
• Aerobic respiration: the most prevalent and efficient pathway in which oxygen is consumed as a reactant along with the organic fuel.
• Carbohydrates, fats and proteins can all be broken down to release energy in cellular respiration. However, glucose is the primary molecule that is used in cellular respiration. The standard way of representing the process of cellular respiration shows glucose being broken down in the following reaction:
• C6H12O6 + 6O2 🡪 6CO2 + 6H20+ Energy (686 kcal/mol of glucose).

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How do we move electrons in biology?

• Moving electrons in living systems
• electrons cannot move alone in cells
• electrons move as part of H atom
• move H = move electrons

oxidation

reduction

e^-

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Coupling oxidation & reduction

• REDOX reactions in respiration
• release energy as breakdown organic molecules
• break C-C bonds
• strip off electrons from C-H bonds by removing H atoms
• C₆H₁₂O₆ → CO₂ = the fuel has been oxidized
• electrons attracted to more electronegative atoms
• in biology, the most electronegative atom?
• O₂ → H₂O = oxygen has been reduced
• couple REDOX reactions & �use the released energy to synthesize ATP

O₂

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Oxidation & reduction

• Oxidation
• adding O
• removing H
• loss of electrons
• releases energy
• exergonic

• Reduction
• removing O
• adding H
• gain of electrons
• stores energy
• endergonic

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Moving electrons in respiration

• Electron carriers move electrons by �shuttling H atoms around
• NAD⁺ → NADH (reduced)
• FAD⁺² → FADH₂ (reduced)

NADH

carries electrons as a reduced molecule

reducing power!

H

like $$�in the bank

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Overview of cellular respiration

• 4 metabolic stages
• Anaerobic respiration

1. Glycolysis

• respiration without O₂
• in cytosol
• Aerobic respiration
• respiration using O₂
• in mitochondria

2. Pyruvate oxidation

3. Krebs cycle

4. Electron transport chain

(+ heat)

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2007-2008

Cellular Respiration� Stage 1:� Glycolysis

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Glycolysis

• Breaking down glucose
• “glyco – lysis” (splitting sugar)

​

​

• ancient pathway which harvests energy
• where energy transfer first evolved
• transfer energy from organic molecules to ATP
• still is starting point for ALL cellular respiration
• but it’s inefficient
• generate only 2 ATP for every 1 glucose
• occurs in cytosol

In the�cytosol?�Why does�that make�evolutionary�sense?

That’s not enough�ATP for me!

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Evolutionary perspective

• Prokaryotes
• first cells had no organelles
• Anaerobic atmosphere
• life on Earth first evolved without free oxygen (O₂) in atmosphere
• energy had to be captured from organic molecules in absence of O₂
• Prokaryotes that evolved glycolysis are ancestors of all modern life
• ALL cells still utilize glycolysis

You mean�we’re related?�Do I have to invite�them over for �the holidays?

Enzymes�of glycolysis are�“well-conserved”

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Overview

10 reactions

• convert �glucose (6C) to �2 pyruvate (3C)
• produces: �4 ATP & 2 NADH
• consumes:�2 ATP
• net yield: �2 ATP & 2 NADH

glucose

C-C-C-C-C-C

fructose-1,6bP

P-C-C-C-C-C-C-P

DHAP

P-C-C-C

G3P

C-C-C-P

pyruvate

C-C-C

DHAP = dihydroxyacetone phosphate

G3P = glyceraldehyde-3-phosphate

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Glycolysis summary

endergonic

invest some ATP

exergonic

harvest a little �ATP & a little NADH

net yield

• 2 ATP
• 2 NADH

4 ATP

ENERGY INVESTMENT

ENERGY PAYOFF

G3P

C-C-C-P

NET YIELD

like $$�in the �bank

-2 ATP

AP Biology

1st half of glycolysis (5 reactions)

P_i

3

6

4,5

ADP

NAD⁺

Glucose

hexokinase

phosphoglucose

isomerase

phosphofructokinase

Glyceraldehyde 3

-phosphate (G3P)

Dihydroxyacetone

phosphate

Glucose 6-phosphate

Fructose 6-phosphate

Fructose 1,6-bisphosphate

isomerase

glyceraldehyde

3-phosphate

dehydrogenase

aldolase

1,3-Bisphosphoglycerate

(BPG)

1,3-Bisphosphoglycerate

(BPG)

1

2

ATP

ADP

ATP

NADH

NAD⁺

NADH

P_i

CH₂

C

O

CH₂OH

P

O

CH₂

O

P

O

CHOH

C

CH₂

O

P

O

CHOH

CH₂

O

P

O

CH₂

O

P

O

P

O

CH₂

H

CH₂OH

O

CH₂

P

O

O

CH₂OH

P

O

Glucose “priming”

• get glucose ready to split
• phosphorylate glucose
• molecular rearrangement
• split destabilized glucose

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2nd half of glycolysis (5 reactions)

​

• NADH production
• G3P donates H
• oxidizes the sugar
• reduces NAD⁺
• NAD⁺ → NADH
• ATP production
• G3P → → → pyruvate
• PEP sugar donates P
• “substrate level phosphorylation”
• ADP → ATP

Payola!�Finally some �ATP!

7

8

H₂O

9

10

ADP

ATP

3-Phosphoglycerate

(3PG)

3-Phosphoglycerate

(3PG)

2-Phosphoglycerate

(2PG)

2-Phosphoglycerate

(2PG)

Phosphoenolpyruvate

(PEP)

Phosphoenolpyruvate

(PEP)

Pyruvate

Pyruvate

phosphoglycerate

kinase

phosphoglycero-�mutase

enolase

pyruvate kinase

ADP

ATP

ADP

ATP

ADP

ATP

H₂O

CH₂OH

CH₃

CH₂

O^-

O

C

P

H

CHOH

O^-

O^-

O^-

C

C

C

C

C

C

P

P

O

O

O

O

O

O

CH₂

NAD⁺

NADH

NAD⁺

NADH

Energy Harvest

G3P

C-C-C-P

P_i

P_i

6

DHAP

P-C-C-C

AP Biology

Substrate-level Phosphorylation

• In the last steps of glycolysis, where did the P come from to make ATP?
• the sugar substrate (PEP)

P is transferred from PEP to ADP

• kinase enzyme
• ADP → ATP

I get it!

The P_i came�directly from�the substrate!

ATP

AP Biology

Energy accounting of glycolysis

• Net gain = 2 ATP + 2 NADH
• some energy investment (-2 ATP)
• small energy return (4 ATP + 2 NADH)
• 1 6C sugar → 2 3C sugars

glucose → → → → → pyruvate

2x

6C

3C

All that work! �And that’s all �I get?

But�glucose has�so much more�to give!

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Is that all there is?

• Not a lot of energy…
• for 1 billon years⁺ this is how life on Earth survived
• no O₂ = slow growth, slow reproduction
• only harvest 3.5% of energy stored in glucose
• more carbons to strip off = more energy to harvest

Hard way�to make�a living!

glucose → → → → pyruvate

6C

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But can’t stop there!

• Going to run out of NAD⁺
• without regenerating NAD⁺, �energy production would stop!
• another molecule must accept H �from NADH
• so NAD⁺ is freed up for another round

Glycolysis

​

glucose + 2ADP + 2P_i + 2 NAD⁺ → 2 pyruvate + 2ATP + 2NADH

raw materials → products

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How is NADH recycled to NAD⁺?

Another molecule must accept H from NADH

NADH

pyruvate

acetyl-CoA

lactate

ethanol

NAD⁺

NAD⁺

NADH

NAD⁺

NADH

CO²

acetaldehyde

H₂O

Krebs

cycle

O₂

lactic acid�fermentation

with oxygen

aerobic respiration

without oxygen

anaerobic respiration

“fermentation”

which path you use depends on who you are…

alcohol�fermentation

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Fermentation (anaerobic)

• Bacteria, yeast

• Animals, some fungi

• beer, wine, bread

• cheese, anaerobic exercise (no O₂)

back to glycolysis→→

back to glycolysis→→

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Alcohol Fermentation

Count the�carbons!

• Dead end process
• at ~12% ethanol, kills yeast
• can’t reverse the reaction

bacteria yeast

back to glycolysis→→

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Lactic Acid Fermentation

• Reversible process
• once O₂ is available, lactate is converted back to pyruvate by the liver

→

Count the�carbons!

animals�some fungi

back to glycolysis→→

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Pyruvate is a branching point

Pyruvate

mitochondria

Krebs cycle

aerobic respiration

fermentation

anaerobic�respiration

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Cellular Respiration� Stage 2 & 3:� Oxidation of Pyruvate� Krebs Cycle

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Glycolysis is only the start

• Glycolysis

​

​

​

• Pyruvate has more energy to yield
• 3 more C to strip off (to oxidize)
• if O₂ is available, pyruvate enters mitochondria
• enzymes of Krebs cycle complete the full oxidation of sugar to CO₂

3C

1C

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Cellular respiration

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Mitochondria — Structure

• Double membrane energy harvesting organelle
• smooth outer membrane
• highly folded inner membrane
• cristae
• intermembrane space
• fluid-filled space between membranes
• matrix
• inner fluid-filled space
• DNA, ribosomes
• enzymes
• free in matrix & membrane-bound

What cells would have a lot of mitochondria?

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Mitochondria – Function

What does this tell us about the evolution of eukaryotes?

Endosymbiosis!

Dividing mitochondria

Who else divides like that?

Advantage of highly folded inner membrane?

More surface area for membrane-bound enzymes & permeases

Membrane-bound proteins

Enzymes & permeases

Oooooh!�Form fits �function!

bacteria!

Permeases are membrane transport proteins

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Oxidation of pyruvate

• Pyruvate enters mitochondrial matrix

​

​

​

​

• 3 step oxidation process
• releases 2 CO₂ (count the carbons!)
• reduces 2 NAD → 2 NADH (moves e^-)
• produces 2 acetyl CoA
• Acetyl CoA enters Krebs cycle

3C

2C

1C

Where�does the�CO₂ go?

Exhale!

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Pyruvate oxidized to Acetyl CoA

Yield = 2C sugar + NADH + CO₂

reduction

oxidation

Coenzyme A

Pyruvate

Acetyl CoA

C-C-C

C-C

CO₂

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Krebs cycle

• aka Citric Acid Cycle
• in mitochondrial matrix
• 8 step pathway
• each catalyzed by specific enzyme
• step-wise catabolism of 6C citrate molecule
• Evolved later than glycolysis
• does that make evolutionary sense?
• bacteria →3.5 billion years ago (glycolysis)
• free O₂ →2.7 billion years ago (photosynthesis)
• eukaryotes →1.5 billion years ago (aerobic respiration = organelles → mitochondria)

1937 | 1953

Hans Krebs

1900-1981

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citrate

acetyl CoA

Count the carbons!

pyruvate

x2

oxidation�of sugars

This happens twice for each glucose molecule

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citrate

acetyl CoA

Count the electron carriers!

pyruvate

reduction�of electron�carriers

This happens twice for each glucose molecule

x2

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So we fully oxidized glucose

C₆H₁₂O₆

↓

CO₂

& ended up �with 4 ATP!

Whassup?

What’s the �point?

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• Krebs cycle produces large quantities of electron carriers
• NADH
• FADH₂
• go to Electron Transport Chain!

Electron Carriers = Hydrogen Carriers

What’s so �important about �electron carriers?

ATP

ADP�+ P_i

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Energy accounting of Krebs cycle

Net gain = 2 ATP

= 8 NADH + 2 FADH₂

ATP

pyruvate → → → → → → → → → CO₂

3C

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Value of Krebs cycle?

• If the yield is only 2 ATP then how was the Krebs cycle an adaptation?
• value of NADH & FADH₂
• electron carriers & H carriers
• reduced molecules move electrons
• reduced molecules move H⁺ ions
• to be used in the Electron Transport Chain

like $$�in the �bank

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Cellular respiration

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There is a better way!

• Electron Transport Chain
• series of proteins built into �inner mitochondrial membrane
• along cristae
• transport proteins & enzymes
• transport of electrons down ETC linked to pumping of H⁺ to create H⁺ gradient
• yields ~28 ATP from 1 glucose!
• only in presence of O₂ (aerobic respiration)

That�sounds more�like it!

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Electron Transport Chain

Intermembrane space

Mitochondrial matrix

Q

C

NADH �dehydrogenase

cytochrome�bc complex

cytochrome c�oxidase complex

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Remember the Electron Carriers?

G3P

Glycolysis

Krebs cycle

8 NADH

2 FADH₂

2 NADH

Time to�break open�the piggybank!

glucose

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Electron Transport Chain

intermembrane�space

mitochondrial�matrix

inner�mitochondrial�membrane

NAD⁺

Q

C

NADH

H₂O

H⁺

e^–

2H⁺ +

O₂

H⁺

H⁺

e^–

FADH₂

NADH �dehydrogenase

cytochrome�bc complex

cytochrome c�oxidase complex

FAD

e^–

H

H → e- + H⁺

NADH → NAD⁺ + H

H

Building proton gradient!

What powers the proton (H⁺) pumps?…

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Stripping H from Electron Carriers

• Electron carriers pass electrons & H⁺ to ETC
• H cleaved off NADH & FADH₂
• electrons stripped from H atoms → H⁺ (protons)
• electrons passed from one electron carrier to next in mitochondrial membrane (ETC)
• flowing electrons = energy to do work
• transport proteins in membrane pump H⁺ (protons) across inner membrane to intermembrane space

ATP

ADP�+ P_i

TA-DA!!

Moving electrons�do the work!

H⁺

H⁺

H⁺

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But what “pulls” the �electrons down the ETC?

electrons�flow downhill �to O₂

oxidative phosphorylation

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Electrons flow downhill

• Electrons move in steps from �carrier to carrier downhill to oxygen
• each carrier more electronegative
• controlled oxidation
• controlled release of energy

make ATP�instead of�fire!

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We did it!

• Set up a H⁺ gradient
• Allow the protons �to flow through ATP synthase
• Synthesizes ATP

ADP + P_i → ATP

Each NADH can produce up to

2.5 ATP

Each FADH₂ can produce up to

1.5 ATP.

ATP

Are we�there yet?

“proton-motive” force

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Chemiosmosis

• The diffusion of ions across a membrane
• build up of proton gradient just so H+ could flow through ATP synthase enzyme to build ATP

Chemiosmosis links the Electron Transport Chain to ATP synthesis

So that’s�the point!

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Peter Mitchell

• Proposed chemiosmotic hypothesis
• revolutionary idea at the time

1961 | 1978

1920-1992

proton motive force

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H⁺

H⁺

O₂

+

Q

C

ATP

Pyruvate from

cytoplasm

Electron

transport

system

ATP

synthase

H₂O

CO₂

Krebs

cycle

Intermembrane

space

Inner

mitochondrial

membrane

1. Electrons are harvested and carried to the transport system.

2. Electrons provide energy �to pump protons across the membrane.

3. Oxygen joins with protons to form water.

2H⁺

NADH

NADH

Acetyl-CoA

FADH₂

ATP

4. Protons diffuse back in�down their concentration�gradient, driving the �synthesis of ATP.

Mitochondrial

matrix

2

1

H⁺

H⁺

O₂

H⁺

e^-

e^-

e^-

e^-

ATP

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Cellular respiration

2 ATP

2 ATP

~28 ATP

+

+

~32 ATP

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Summary of cellular respiration

• Where did the glucose come from?
• Where did the O₂ come from?
• Where did the CO₂ come from?
• Where did the CO₂ go?
• Where did the H₂O come from?
• Where did the ATP come from?
• What else is produced that is not listed �in this equation?
• Why do we breathe?

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Taking it beyond…

• What is the final electron acceptor in Electron Transport Chain?

• ETC backs up
• nothing to pull electrons down chain
• NADH & FADH₂ can’t unload H
• ATP production ceases
• cells run out of energy
• and you die!

O₂

• So what happens if O₂ unavailable?

AP Biology

2006-2007

Cellular Respiration� Other Metabolites & � Control of Respiration

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Cellular respiration

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Beyond glucose: Other carbohydrates

• Glycolysis accepts a wide range of carbohydrates fuels

• ex. starch, glycogen

• ex. galactose, fructose

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Beyond glucose: Proteins

amino group =

waste product excreted as ammonia, urea, or uric acid

2C sugar =

carbon skeleton =

enters glycolysis or Krebs cycle at �different stages

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Beyond glucose: Fats

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Carbohydrates vs. Fats

• Fat generates 2x ATP vs. carbohydrate
• more C in gram of fat
• more energy releasing bonds
• more O in gram of carbohydrate
• so it’s already partly oxidized
• less energy to release

That’s why�it takes so much�to lose a �pound a fat!

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Metabolism

• Coordination of chemical processes across whole organism
• digestion
• catabolism when organism needs energy or needs raw materials
• synthesis
• anabolism when organism has enough energy & a supply of raw materials
• by regulating enzymes
• feedback mechanisms
• raw materials stimulate production
• products inhibit further production

CO₂

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Metabolism

• Digestion
• digestion of carbohydrates, fats & proteins
• all catabolized through same pathways
• enter at different points
• cell extracts energy from every source

CO₂

Cells are�versatile & �selfish!

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Metabolism

• Synthesis
• enough energy?

build stuff!

• cell uses points in glycolysis & Krebs cycle as links to pathways for synthesis
• run pathways “backwards”
• have extra fuel, build fat!

Cells are�versatile & �thrifty!

AP Biology