Cellular Respiration�Harvesting Chemical Energy�General Biology A

ATP

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Objective 1: Explain the process of Cellular Respiration

• -Cellular Respiration is the process of breaking down organic molecules into energy forms ATP (Energy) which we can use to build and maintain our bodies.
• 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^-

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2006-2007

What’s the�point?

The point�is to make�ATP!

ATP

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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

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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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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

The goal of fermentation is to recycle NADH to NAD+ so it can be used in glycolysis again.

→

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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Fate of Pyruvate

• 2 Pyruvates are produced in glycolysis
• 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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citrate

acetyl CoA

Count the electron carriers!

pyruvate

reduction�of electron�carriers

This happens twice for each glucose molecule

x2

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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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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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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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