Lecture 9 – Cellular Respirationfacweb.northseattle.edu/lizthomas/Lecture 9.pdf · In this lecture •Cellular respiration •Redox reactions •Glycolysis –Pyruvate oxidation

Lecture 9 – Cellular Respiration

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First…watch this video. Seriously.

http://www.khanacademy.org/video/introduction-

to-cellular-respiration?playlist=Biology

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http://www.khanacademy.org/video/introduction-to-cellular-respiration?playlist=Biology







In this lecture

• Cellular respiration

• Redox reactions

• Glycolysis

– Pyruvate oxidation

• Krebs Cycle

• Electron Transport Chain

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Why do we do respiration?

• Cellular respiration provides most of our ATP

• The components of our diet provides the

reactants for cellular respiration

– Glucose is what we’ll study today

– Lipids and protein breakdown will be briefly

covered, and studied more in depth in another

course

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Biochemical pathways are:

Exergonic and endergonic reactions

Oxidation and reduction reactions

Enzymatic reactions

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The Big Picture

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From food to ATP • Amylase in saliva starts to break down starches

to disaccharides

• Stomach acid breaks apart large structures such

as cells and intercellular structures

• Amylase in the small intestine completes the

breakdown of all carbohydrates to disaccharides

• Maltases, lactases, and sucrases break down

disaccharides into monosaccharides

• Glucose is brought to all the cells in the body

through the circulatory system

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Energy production sites in the cell

• Glucose is brought inside the cell by

cotransport with sodium

• The mitochondria are where ATP is produced

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Electron energy levels

• An electron loses potential energy when it

shifts to a more electronegative atom

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C6H12O6 + 6O2 6CO2 + 6H2O + energy

Here, electrons are transferred from carbon and hydrogen to

oxygen

The electrons in this reaction lose a LOT of potential energy

Carbohydrates and fats are high-energy foods because they

have a lot of electrons associated with hydrogen

Figure 9.5

(a) Uncontrolled reaction (b) Cellular respiration

Explosive release of

heat and light energy

Controlled release of energy for

synthesis of ATP

Fre

e e

nerg

y, G

Fre

e e

nerg

y, G

H2 1/2 O2 2 H 1/2 O2

1/2 O2

H2O H2O

2 H+ 2 e

2 e

2 H+

ATP

ATP

ATP

(from food via NADH)

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The chemical reactions in respiration

Oxidation and reduction OIL RIG

Oxidation is “losing” Reduction is “gaining”

What is being lost and gained? Electrons!

Electrons are usually lost to oxygen

Oxygen is

highly

electro-

negative

The chemistry definition:

Oxidation and reduction reactions often occur in a pair,

and together are called redox reactions

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Oxidation and reduction

Losing a hydrogen atom Gaining a hydrogen atom

In biochemical reactions, hydrogen is what usually gets

swapped around

Hydrogen almost always bonds to an atom that is more

electronegative (C, O, N, P), and so loses its electron

The biology definition:

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Redox Reactions

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• Redox reactions

• Phosphorylation/dephosphorylation

– Carried out by kinases and phosphatases

– Phosphorylation increases chemical potential

energy and “primes” the molecule for work

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New players in the enzyme game

• NAD+ and NADH

• FAD and FADH2

NAD+ is derived

from niacin

NAD+ shuttles electrons through

the various stages of cellular

respiration

NAD+ is a coenzyme

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FAD is derived from riboflavin

NAD+ and FAD

• NAD+ accepts electrons and becomes NADH

• NAD+ is reduced into NADH

• NADH is a reducing agent, and is recycled back to

NAD+ through oxidation

Each NADH (the reduced form of NAD+) represents

stored energy that is tapped to synthesize ATP.

Each NADH produces 3 ATPs in the e- transport chain

NAD+ NADH

reduction

oxidation Oxidized

form

Reduced

form

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What is cellular respiration?

Glucose Energy

C6H12O6 + 6O2 6CO2 + 6H2O + energy Cellular respiration

38 ATP Heat

1 molecule of glucose produces 28 ATPs

Cellular respiration is the step-by-step

release and harness of the chemical

potential energy in glucose 17 NSCC BIOL211

- 686kcal/mol

Redox Reactions in Cellular Respiration

During cellular respiration, the fuel (such as glucose) is

oxidized, and O2 is reduced

becomes oxidized

becomes reduced

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Two types of cellular respiration:

• Aerobic respiration – takes place in the

presence of oxygen

• Anaerobic respiration – takes place in the

absence of oxygen

– Fermentation is a type of anaerobic respiration

where sugars are partially degraded

– Consumes compounds other than oxygen

The breakdown of organic

molecules is always exergonic

Cellular respiration includes both aerobic and anaerobic respiration but is

often used to refer to aerobic respiration

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The stages of aerobic respiration

Glycolysis Breaks glucose in

half

Citric Acid Cycle*

Electron Transport

Chain

Consumes two ATP

Generates 4 ATP

Net 2 ATP

Generates 2 ATP Rearranges the half-

glucose molecule

Generates 34 ATP Electrons from

glucose are passed

around

*AKA the Krebs cycle and the TCA cycle 21 NSCC BIOL211

The Big Picture

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Figure 9.6-1

Electrons

carried

via NADH

Glycolysis

Glucose Pyruvate

CYTOSOL MITOCHONDRION

ATP

Substrate-level

phosphorylation

Glycolysis

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• The players:

– Glucose

– Pyruvate

– ADP/ATP

– Enzymes

• The processes:

– Substrate-level phosphorylation

• The locations

– Cytoplasm

Glycolysis

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The production of

ATP from ADP by

direct transfer of a

phosphate group

from a

phosphorylated

protein

Glycolysis

Broken down into two stages:

• The “investment” phase (uses 2 ATP)

• The “payoff” phase (produces 2 ATP)

Beginning structure: End structures:

- Breaking down of glucose

- Can be done with or without oxygen “glyco” = glucose

“lysis” = breaking

apart

“You have to spend money to make money” 25 NSCC BIOL211

Glycolysis

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Glycolysis

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Glycolysis

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Glycolysis

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Glycolysis

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Glycolysis

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Glycolysis

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Glycolysis

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Glycolysis

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Glycolysis

• Begin:

– Glucose

– NAD+

– ADP

• End:

– 2 pyruvate

– 2 NADH

– 2 ATP

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Glycolysis

Most of glycose’s original energy

is still present in pyruvate!

• In the presence of O2, pyruvate enters the

mitochondrion where the oxidation of glucose

is completed during TCA cycle

• Without O2, pyruvate undergoes fermentation

into either ethanol or lactic acid

Glycolysis

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Glycolysis

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Pyruvate Oxidation

Glycolysis feeds into TCA cycle ONLY when oxygen is present!!

• Before pyruvate can be fed into TCA cycle, it

must become acetyl-CoA (acetyl-coenzyme A)

• It does this through pyruvate oxidation

– Produces one NADH from NAD+

– Three-carbon pyruvate is converted into two-

carbons + acetyl-CoA

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Pyruvate Oxidation

Think of acetyl-

CoA as a

transporter for

the carbon

atoms from

pyruvate

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Pyruvate Oxidation

• Acetyl-CoA couples with oxaloacetate, the

first molecule in TCA cycle

• Acetyl-CoA + oxaloacetate = citrate

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Pyruvate Oxidation

• The players: – Acetyl-CoA

– Oxaloacetate

– Plus many more carbon skeleton intermediates

– Enzymes

• The processes: – Hydrolysis

– Redox reactions

• The locations: – Mitochondrial matrix

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The Citric Acid Cycle

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Begin:

Oxaloacetate

End:

Oxaloacetate

The citric acid cycle, also called the Krebs cycle,

completes the break down of pyruvate to CO2

1 ADP

3 NAD+

1 FAD

1 ATP

3 NADH

1 FADH2

The citric acid cycle has eight steps, each catalyzed by a specific enzyme

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The Citric Acid Cycle Step 1 and 2: Overview

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CoA is recycled

here to go back

to pyruvate

oxidation

Step 1 and 2: In detail

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Step 3 and 4: In detail and overview

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The Citric Acid Cycle Step 5 and 6: Overview

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Step 5 and 6: In detail

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Step 7 and 8: Overview

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Step 7 and 8:

In detail

• The citric acid cycle is the entry point for other

catabolic pathways

• Acetyl-CoA can be derived from carbohydrates,

proteins, and fats

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• Begin:

– Acetyl-CoA

– Oxaloacetate

– 3NAD+

– 2 FAD

– 1ADP

• End:

– 3 CO2s

– Oxaloacetate

– 3 NADH

– 2 FADH2

– 1 ATP

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The whole point of

TCA cycle is to

produce NADH

and FADH2

This is for one pyruvate. Remember, one glucose molecule produces two pyruvates!

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The electron transport chain

• The whole cellular process is about producing

ATP. Why do we care about NADH and

FADH2?

– These molecules then get oxidized in the electron

transport chain

– Every NADH will produce 3 ATP

– Every FADH2 will produce 2 ATP

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End:

ATP

10 NAD+

2 FAD

Begin:

ADP

10 NADH

2 FADH2


The electron transport chain breaks the large free-energy drop from food to O2

into smaller steps that release energy in manageable amounts

Electrons are passed along at lower and lower energy levels to release their energy

• The players:

– FADH2, NADH, ADP

– ATP Synthase

– Cytochromes and membrane proteins

• The processes:

– Chemiosmosis

• The location:

– Intermembrane space of the mitochondria

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• The electron transport chain is a series of

proteins that pass along electrons

– Electrons come from NADH and FADH2

– Proteins are embedded in the matrix membrane

– Each time an electron is passed, it releases energy

– That energy is used to drive protons across the

membrane into the intermembrane space

– This creates an electrochemical gradient

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NADH NAD+ + H+ + 2e-

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CoQ

CytC

CytB

O2

Energ

y in

creas

es

Release of energy

Release of energy

Release of energy

CoQ, CytC, and

CytB are all

membrane proteins

on the inner matrix

membrane

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FADH2’s electrons are

lower energy than

NADH, and so enter

the electron transport

chain at a protein

further along in the

chain

The transport proteins alternate

reduced and oxidized states as they

accept and donate electrons

Oxygen accepts those now

very-low energy electrons.

Oxygen is the terminal

electron acceptor

• The release of energy is used to pump H+

across the matrix membrane into the

intermembrane space

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• H+ are pumped against their gradient using the

energy released from passing electrons to lower and

lower energy states

• This creates an electrochemical gradient

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We can then

couple the

potential energy

in the

electrochemical

gradient to

another

biochemical

reaction

• A bigger picture:

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What is that electrochemical gradient used for?

To create ATP!

How is that done?

Through a protein called ATP synthase

ATP synthase translates the potential energy in the electrochemical

gradient into the potential energy in the phosphate bonds of ATP

The flow of H+ with its electrochemical gradient is an exergonic reaction

ATP synthase couples an exergonic reaction with an endergonic reaction

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• ATP synthase is a turbine that

connects the flow of protons

to ADP ATP

phosphorylation

• This is called chemiosmosis

Electrochemical energy Kinetic energy Chemical energy

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As the turbine turns with

the “current” of protons

flowing past, it

phosphorylates ADP into

ATP

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ATP Synthase: another view

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All together:

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• 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

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Energy flows in this direction:

glucose NADH electron transport chain proton-motive force ATP

What happens without oxygen?

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Fermentation

• Anaerobic respiration uses an electron

transport chain with a final electron acceptor

other than O2, for example sulfate

• Produces much less energy than aerobic

respiration

– Only source of ATP is substrate-level

phosphorylation

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• Two common types of fermentation:

– Lactic acid fermentation • Lactic acid fermentation by some fungi and bacteria is used to

make cheese and yogurt

• Human muscle cells use lactic acid fermentation to generate ATP

when O2 is scarce

– Alcohol fermentation

• Alcohol fermentation by yeast is used in brewing,

winemaking, and baking

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Fermentation

• Pyruvate is converted

to ethanol in two steps

– NADH produced in

glycolysis is oxidized

to NAD+

– Glucose is not

conpletely digested

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Alcohol Fermentation

• Pyruvate is

converted to

lactate in one

step

– NADH produced

during glycolysis

is oxidized to

NAD+

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Lactic Acid Fermentation

Comparing Aerobic and Anaerobic

Respiration

Aerobic Respiration Anaerobic Respiration

Glycolysis Yes Yes

Krebs Cycle Yes No

Electron Transport

Chain

Yes No

ATP Production 32 per glucose 2 per glucose

NADH production Yes Yes

FADH2 production Yes No

Terminal electron

acceptor

O2 Pyruvate or acetaldehyde

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Who uses what pathway?

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• Obligate anaerobes carry out fermentation or anaerobic respiration and cannot survive in the presence of O2

• Yeast and many bacteria are facultative anaerobes, meaning that they can survive using either fermentation or cellular respiration

• We require oxygen to live, and are obligate aerobes

• In a facultative anaerobe, pyruvate is a fork in the metabolic road that leads to two alternative catabolic routes

Figure 9.18

Glucose

CYTOSOL Glycolysis

Pyruvate

No O2 present:

Fermentation

O2 present:

Aerobic cellular

respiration

Ethanol,

lactate, or

other products

Acetyl CoA

MITOCHONDRION

Citric

acid

cycle

Catabolism of other biomolecules

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• Proteins must be digested to amino acids; amino groups can feed glycolysis or the citric acid cycle

• Fats are digested to glycerol (used in glycolysis) and fatty acids (used in generating acetyl CoA)

• Fatty acids are broken down by beta oxidation and yield acetyl CoA

• An oxidized gram of fat produces more than twice as much ATP as an oxidized gram of carbohydrate

Controlling Respiration

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ATP and citrate inhibit

phosphofructokinase

AMP is a positive allosteric

regulator of phosphofructokinase

If you have a lot of ATP or citrate (that

means a lot of energy) glycolysis is shut

down

If you have a lot of AMP (very little

energy is present) glycolysis is

stimulated

If ATP concentration begins to drop,

respiration speeds up; when there is

plenty of ATP, respiration slows down

The Evolutionary Significance of

Glycolysis

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• Ancient prokaryotes are thought to have used

glycolysis long before there was oxygen in the

atmosphere

• Very little O2 was available in the atmosphere until

about 2.7 billion years ago, so early prokaryotes likely

used only glycolysis to generate ATP

• Glycolysis is a very ancient process

Vocabulary

• Glycolysis

• Krebs/TCA cycle

• Redox reactions

• Terminal electron acceptor

• Chemiosmosis

• Oxidative phosphorylation

• Proton-motive force

• Fermentation

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Lecture 9 – Cellular Respirationfacweb.northseattle.edu/lizthomas/Lecture 9.pdf · In this lecture •Cellular respiration •Redox reactions •Glycolysis –Pyruvate oxidation

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