Organisms must take in energy from outside sources.

Energy is incorporated into organic molecules such as glucose in the process of photosynthesis.

Glucose is then broken down in cellular respiration. The energy is stored in ATP.

Fig. 9-2

Lightenergy

ECOSYSTEM

Photosynthesis in chloroplastsCO2 +

H2O Cellular respiration

in mitochondria

Organicmolecule

s+ O2

ATP powers most cellular work

Heatenergy

ATP

The Flow of Energy from Sunlight to ATP

Energy in food is stored as carbohydrates (such as glucose), proteins & fats. Before that energy can be used by cells, it must be released and transferred to ATP.

Aerobic Cellular Respiration: the process that releases energy by breaking down food (glucose) molecules in the presence of oxygen. Formula: C6H12O6 + 6O2 → 6CO2 + 6H2O +~ 36 ATP

Fermentation: the partial breakdown of glucose without oxygen. It only releases a small amount of ATP.

Glycolysis: the first step of breaking down glucose—it splits glucose (6C) into 2 pyruvic acid molecules (3C each)

The transfer of electrons during chemical reactions releases energy stored in organic compounds such as glucose.

Oxidation-reduction reactions are those that involve the transfer of an electron from one substance to another.

Chemical reactions that transfer electrons between reactants are called oxidation-reduction reactions, or redox reactions

• In oxidation, one substance loses electrons, or is oxidized

In reduction, a substance gains electrons, or is reduced (the amount of positive charge is reduced)

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Na will easily lose its outer electron to Cl . Why?In this reaction, which atom is oxidized?Which is reduced?

becomes oxidized

becomes reduced

In cellular respiration, glucose is broken down and loses its electrons in the process. The glucose becomes oxidized and the Oxygen is reduced.

Redox Reactions of Cellular Respiration

In cellular respiration, glucose is broken down in a series of steps.

As it is broken down, electrons from glucose are transferred to NAD+, a coenzyme

When it receives the electrons, it is converted to NADH. NADH

represents stored energy that can be used to make ATP

NADH passes the electrons to the electron transport chain, a series of proteins embedded in the inner membrane of the mitochondria.

The electrons (and the energy they carry) are transferred from one protein to the next in a series of steps.

Free

ene

rgy,

G

Free

ene

rgy,

G

(a) Uncontrolled reaction

H2O

H2 + 1/2 O2

Explosiverelease of

heat and lightenergy

(b) Cellular respiration

Controlledrelease ofenergy for

synthesis ofATP

2 H+ + 2 e–

2 H + 1/2 O2

(from food via NADH)

ATP

ATP

ATP

1/2 O22 H+

2 e–Electron transport

chain

H2O

Energy is released a little at a time, rather than one big explosive reaction:

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

molecules of pyruvate) The Citric Acid cycle/Kreb’s Cycle (completes

the breakdown of glucose) Electron Transport Chain and Oxidative

phosphorylation (accounts for most of the ATP synthesis)

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Substrate-levelphosphorylation

ATP

Cytosol

Glucose Pyruvate

Glycolysis

Electronscarried

via NADH

An Overview of Cellular Respiration–Part 1

Mitochondrion

Substrate-levelphosphorylation

ATP

Cytosol

Glucose Pyruvate

Glycolysis

Electronscarried

via NADH

Substrate-levelphosphorylation

ATP

Electrons carriedvia NADH and

FADH2

Citricacidcycle

An Overview of Cellular Respiration—Part 2

Mitochondrion

Substrate-levelphosphorylation

ATP

Cytosol

Glucose Pyruvate

Glycolysis

Electronscarried

via NADH

Substrate-levelphosphorylation

ATP

Electrons carriedvia NADH and

FADH2

Oxidativephosphorylation

ATP

Citricacidcycle

Oxidativephosphorylation:electron transport

andchemiosmosis

An Overview of Cellular Respiration—Part 3

Substrate-level phosphorylation:

Phosphate is added to ADP to make ATP by using an enzyme:

Oxidative phosphorylation:

Phosphate is added to ADP to make ATP by ATP Synthase—a protein embedded in the mitochondria membrane (requires O2)WAY MORE

EFFICIENT!! PRODUCES LOTS MORE ATP!

“Glyco”=sugar; “lysis”=to split In this first series of reactions, glucose

(C6) is split into two molecules of pyruvic acid (C3).

This occurs in the cytoplasm of cells and does not require oxygen.

This releases only 2 ATP molecules, not enough for most living organisms.

Energy investment phase

Glucose

2 ADP + 2 P 2 ATP used

formed4 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 H2OGlucoseNet

4 ATP formed – 2 ATP used 2 ATP

2 NAD+ + 4 e– + 4 H+ 2 NADH + 2 H+

Glycolysis

The Citric Acid Cycle (also called the Kreb’s Cycle) completes the breakdown of pyruvate and the release of energy from glucose.

It occurs in the matrix of the mitochondria.

In the presence of oxygen, pyruvate enters the mitochondria.

Before the pyruvate can enter the Citric Acid Cycle, however, it must be converted to Acetyl Co-A.

Some energy is released and NADH is formed.

CYTOSOL MITOCHONDRION

NAD+ NADH + H+

2

1 3

Pyruvate

Transport protein

CO2Coenzyme A

Acetyl CoA

Converting Pyruvate to Acetyl CoA:

The Acetyl Co-A enters the Citric Acid Cycle in the matrix of the mitochondria.

The Citric Acid cycle breaks down the Acetyl Co-A in a series of steps, releasing CO2

It produces 1 ATP, 3 NADH, and 1 FADH2 per turn.

• The Citric Acid cycle (also called the Krebs Cycle) has eight steps, each catalyzed by a specific enzyme

• The acetyl group of acetyl CoA joins the cycle by combining with oxaloacetate, forming citrate (Citric Acid).

• The next seven steps decompose the citrate (Citric Acid) back to oxaloacetate, making the process a cycle

The Citric Acid Cycle

Oxaloacetate + Acetyl CoA Citric Acid

Acetyl CoACoA—SH

Citrate

H2O

IsocitrateNAD+

NADH+ H+

CO2

-Keto-glutarate

CoA—SH

CO2NAD+

NADH+ H+Succinyl

CoA

CoA—SH

P iGTP GDP

ADP

ATP

SuccinateFAD

FADH2

Fumarate

CitricacidcycleH2O

Malate

Oxaloacetate

NADH+H+

NAD+

1

2

3

4

5

6

7

8

The Citric Acid Cycle:

• Each Citric Acid Cycle only produces 1 ATP molecule. The rest of the energy from pyruvate is in the NADH and FADH2.

• The NADH and FADH2 produced by the Citric Acid cycle relay electrons extracted from food to the electron transport chain.

The electron transport chain is in the cristae of the mitochondrion

Most of the chain’s components are proteins, which exist in multiprotein complexes

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

Electrons drop in free energy as they go down the chain and are finally passed to O2, forming H2O

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Oxygen is the final electron acceptor.

NADH

NAD+2FADH2

2 FADMultiproteincomplexesFAD

Fe•SFMN

Fe•SQ

Fe•S

Cyt b

Cyt c1

Cyt cCyt a

Cyt a3

IV

Free

ene

rgy

(G) r

ela t

ive

to O

2 (kc

a l/m

o l)

50

40

30

20

10 2(from NADHor FADH2)

0 2 H+ + 1/2 O2

H2O

e–

e–

e–

The Electron Transport Chain

Electrons are transferred from NADH or FADH2 to the electron transport chain

Electrons are passed through a number of proteins to O2

The chain’s function is to break the large free-energy drop from food to O2 into smaller steps that release energy in manageable amounts

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

INTERMEMBRANE SPACE

Rotor

H+

Stator

Internalrod

Cata-lyticknob

ADP+P ATP

iMITOCHONDRIAL MATRIX

ATPSynthase

Protein complexof electroncarriers

H+

H+H+

Cyt c

Q

V

FADH2 FAD

NAD+NADH(carrying electronsfrom food)

Electron transport chain

2 H+ + 1/2O2 H2O

ADP + P i

Chemiosmosis

Oxidative phosphorylation

H+

H+

ATP synthase

ATP

21

Chemiosmosis couples the electron transport chain to ATP synthesis

During cellular respiration, most energy flows in this sequence: glucose NADH electron transport chain proton-motive force ATP

About 40% of the energy in a glucose molecule is transferred to ATP during cellular respiration, making about 38 ATP

+ 6 O2 6CO2 + 6H2O + 38 ATP

Maximum per glucose: About36 or 38 ATP

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

Oxidativephosphorylation:electron transport

andchemiosmosis

Citricacidcycle

2AcetylCoA

Glycolysis

Glucose2

Pyruvate

2 NADH 2 NADH 6 NADH 2 FADH2

2 FADH2

2 NADHCYTOSOL Electron shuttles

span membraneor

MITOCHONDRION

ATP Yield per molecule of glucose at each stage of cellular respiration: