Chapter 4 Cellular Metabolism. 2 Introduction A.A living cell is the site of enzyme-catalyzed...

Chapter 4

Cellular Metabolism

2

Introduction A. A living cell is the site of enzyme-

catalyzed metabolic reactions that maintain life.

3

Metabolic ReactionsA. Metabolic reactions are of two types:

1. anabolic reactions, larger molecules are constructed from smaller ones, a process requiring energy.2. catabolic reactions, larger molecules are broken down, releasing energy. The reactions of metabolism are often reversible.

4

B. Anabolism 1. Anabolism provides the

substances needed for growth and repair.

2. These reactions occur by dehydration synthesis,

removinga molecule of water to join two

smaller molecules.

5

3. Polysaccharides, lipids, and proteins are constructed via dehydration synthesis.

a. To form fats, glycerol and fatty acids bond.

b. The bond between two amino acids is a peptide bond; two bound amino acids form a dipeptide, while many joined form a polypeptide.

6

Fig04.01

CH2OH

H H

OH

O

H OH

Monosaccharide

HHO

H

OH

CH2OH

H H

OH

O

H OH

Monosaccharide

HHO

H

OH

CH2OH

H H

OH

O

H OH

Disaccharide

H2O

Water

HHO

H

CH2OH

H H

OH

O

H OH

HO

H

OH

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7

Fig04.02

H C

H

Glycerol 3 fatty acid molecules

OH HO

H C OH HO

H C

C

C

C

(CH2)14 CH3

(CH2)14 CH3

(CH2)14 CH3OH HO

OH

O

O

C

C

C (CH2)14 CH3

O

O

O

H C

H

Fat molecule (triglyceride) 3 water molecules

H C

H C O

O

O

H

H2O

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(CH2)14 CH3

(CH2)14 CH3

H2O

H2O

8

C. Catabolism 1. Catabolism breaks apart larger

molecules into their building blocks.

2. These reactions occur by hydrolysis, wherein a molecule of water is inserted into a polymer which is

split into two smaller molecules.

9

Fig04.03

Amino acid

N

H

H

C C

H

R

Dipeptide molecule

Peptidebond

Amino acid

N

H

H

C C

H H H

R H

O

N

H

H

C C

H

R H

O

N

H

C C OH

R

H

OO

N

H

H

C C

H

R

N

H

C C OH

R

H

OO

H2O

Water

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10

Control of Metabolic Reactions:A. Enzymes control the rates of all the

metabolic reactions of the cell.B. Enzyme Action

1. Enzymes are complex proteins that function to lower the activation energy of a reaction so it may begin and proceed more rapidly. Enzymes are called catalysts.

11

2. Enzymes work in small quantities and are recycled by the cell.

3. Each enzyme is specific, acting on only one kind of substrate.

4. Active sites on the enzyme combine with the substrate and a reaction occurs.

5. The speed of enzymatic reactions depends on the number of

enzyme and substrate molecules available.

12

C. Factors That Alter Enzymes 1. Enzymes (proteins) can be denatured

by heat, pH extremes, chemicals, electricity, radiation, and by

other causes. Enzymes that only become active

when they combine with a nonprotein component are cofactors. Small

organic cofactors are called coenzymes.

Fig04.04

Product molecule

Unaltered enzyme molecule

Enzyme-substrate complex

Active site

(a) (b) (c)

Substrate molecules

Enzyme molecule

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14

Energy for Metabolic Reactions: A. Energy is the capacity to do work.B. Common forms of energy include

heat, light, sound, electrical energy, mechanical energy, and chemical energy.

C. Release of Chemical Energy - 1. Release of chemical energy in

the cell often occurs through the oxidation of glucose.

D. Cellular Respiration consists of 3 reactions: glycolysis, citric acid cycle & electron transport chain

1. ATPa. ATP molecules contain

three phosphates in a chain.b. A cell uses ATP for many

functions including active transport and

synthesis of various compounds.

16

½ O2

Fig04.05

1

3

4

2

Glycolysis 2 ATP

Glucose

High-energy electrons (e–)

High-energy electrons (e–)

High-energy electrons (e–)

2e– and 2H+

2 ATP

H2O

Electrontransport

chainATP32–34

CO2

2 Pyruvic acids (Each enters separately)

2 CO2

Acetyl CoA

Citric acid

Citric acid cycle

Oxaloacetic acid

Glycolysis The 6-carbon sugar glucose is broken down in the cytosolinto two 3-carbon pyruvic acid molecules with a net gainof 2 ATP and the release of high-energy electrons.

Citric Acid CycleThe 3-carbon pyruvic acids generated by glycolysis enterthe mitochondria separately. Each loses a carbon (generating CO2) and is combined with a coenzyme to form a 2-carbon acetyl coenzyme A (acetyl CoA). More high-energy electrons are released.

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Cyt

oso

lM

itoch

ond

rio

n

Electron Transport ChainThe high-energy electrons still contain most of thechemical energy of the original glucose molecule.Special carrier molecules bring the high-energy electronsto a series of enzymes that store much of the remainingenergy in more ATP molecules. The other products areheat and water. The function of oxygen as the finalelectron acceptor in this last step is why the overallprocess is called aerobic respiration.

Each acetyl CoA combines with a 4-carbon oxaloaceticacid to form the 6-carbon citric acid, for which the cycleis named. For each citric acid, a series of reactionsremoves 2 carbons (generating 2 CO2’s), synthesizes1 ATP, and releases more high-energy electrons.The figure shows 2 ATP, resulting directly from 2turns of the cycle per glucose molecule that entersglycolysis.

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c. Energy is stored in the last phosphate bond of ATP.d. Energy is stored while converting ADP to ATP; when energy is released, ATP becomes ADP, ready to be regenerated into ATP.

Fig04.06

ATP

P P P

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Fig04.07

P P

ADP

ATPEnergy transferredfrom cellularrespiration usedto reattachphosphate

Energy transferredand utilized bymetabolicreactions whenphosphate bondis broken

P P P

PP

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20

2. Glycolysisa. The first part of cellular respiration is the splitting of 6-C glucose that occurs through a series of enzyme-

catalyzed steps called glycolysis.b. Glycolysis occurs in the cytosol and

does not require oxygen (is anaerobic).

c. Energy from ATP is used to start the process but there is a net gain of energy as a result.

21

3. Aerobic Respiration 1. Oxygen is needed for aerobic

respiration, which occurs within the mitochondria.

2. There is a much greater gain of ATP molecules from aerobic respiration.

3. The final products of glucose oxidation are carbon

dioxide, water, and energy.

22

Fig04.09

High-energyelectrons

Complete oxidationof acetyl coenzyme Ato H2O and CO2 produceshigh-energy electrons,which yield much ATPvia the electron transportchain

Breakdown of simplemolecules to acetylcoenzyme Aaccompanied byproduction of limitedATP and high-energyelectrons

H2O

2e– and 2H+

Waste products

½ O2–NH2

CO2

CO2

Citricacidcycle

Electrontransport

chain

Amino acids

Acetyl coenzyme A

Simple sugars(glucose)

Glycerol Fatty acids

Proteins (egg white)

Carbohydrates (toast, hash browns)

Food

Fats (butter)

Pyruvic acid

ATP

ATP

ATP

Breakdown of largemolecules tosimple molecules

Glycolysis

1

2

3

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23

Metabolic Pathways:A. The enzymes controlling either an

anabolic or catabolic sequence of reactions must act in a specific order.

B. A sequence of enzyme-controlled reactions is called a metabolic

pathway.

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C. Regulation of Metabolic Pathways1. The rate of a metabolic pathway is

determined by a regulatory enzyme responsible for one of its steps.

2. A rate-limiting enzyme is the first step in a series.

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DNA (Deoxyribonucleic acid):A. Deoxyribonucleic acid (DNA) contains

the genetic code needed for the synthesis of each protein (including enzymes) required by the cell.

B. Genetic Information 1. A gene is a portion of a DNA

molecule that contains the genetic information for making a single

protein. The complete set of instructions is the genome.

26

C. DNA Molecules 1. The nucleotides of DNA form a

sugar-phosphate backbone with bases extending into the interior of the DNA molecule.

2. The nucleotides of one DNA strand are compatible to those in the other

strand (adenine pairs with thymine; cytosine with guanine) and so exhibit complementary base pairing.

27

3. The DNA molecule twists to form a double helix and may be millions of base pairs long.

28AT

T A

T A

A

Fig04.10

C

C

A

C

C G

G

C

G

C G

C G

G

G

T

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T A

T

29

D. DNA Replication1. Each new cell must be provided with

an exact replica of the parent cell's DNA.

2. DNA replication occurs during interphase. a. The DNA molecule splits.b. Nucleotides form

complementary pairs with the original strands.

3. Each new DNA molecule consists of one parental strand and one

newly- synthesized strand of DNA. 

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T A

A T

T A

T A

AT

T A

AT

Fig04.11

C

C

A T

C

C G

G

C

C

G

CG

A

A

T

T

C G

C

A T

Newly formed DNA molecules

Region of replication

Original DNA molecule

C

G

G

G

G

G

G

GG

G

C C

C

C

C G

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T A

T A

T A

A

31

Protein Synthesis:A. The Genetic Code- Instructions for

making proteins1. The genetic code is the

correspondence of gene and protein building block sequences.

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2. Transcription a. RNA molecules are single-

stranded and contain ribose rather than deoxyribose, and uracil rather than thymine.

b. Messenger RNA (mRNA) molecules are synthesized in the nucleus in a sequence

complementary to the DNA template in a process

called transcription.

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3. Translationa. Each amino acid corresponds to a triplet of DNA nucleotides; a triplet of nucleotides in

messenger RNA is called a codon.b. Messenger RNA can move out of

the nucleus and associate with ribosomes in the cytoplasm where the protein will be constructed in a process called translation.

34

c. In the cytoplasm, a second kind of RNA, called transfer

RNA, has a triplet of nucleotides called the anticodon, which is complementary to nucleotides of

the messenger RNA codon.d. The ribosome holds the

messenger RNA in position while the transfer RNA carries in

the correct amino acid in sequence, with anticodons matching up to codons.

35

e. The ribosome contains enzymes needed to

join the amino acids together.

f. As the amino acids are joined, the new protein molecule into its unique shape.

36

Fig04.13

MessengerRNA

1 DNAinformationis copied, ortranscribed,into mRNAfollowingcomplementarybase pairing

2 mRNA leavesthe nucleusand attachesto a ribosome

3 Translation begins as tRNA anticodonsrecognize complementary mRNA codons,thus bringing the correct amino acids intoposition on the growing polypeptide chain

4 As the ribosomemoves along themRNA, more aminoacids are added

5 At the end of the mRNA,the ribosome releasesthe new protein

6 tRNA moleculescan pick up anothermolecule of thesame amino acidand be reused

Amino acidsattached to tRNA

Polypeptide chain

Cytoplasm

DNAdoublehelix

DNAstrandspulledapart

Transcription Translation

Nucleus

Direction of “reading”

C

Codon 1

Codon 2

Codon 3

Codon 4

Codon 5

Codon 6

Codon 7

G

G

G

G

G

A

A

A

U

U

C

C

C

C

C

C

G

G

G

A

Methionine

Glycine

Amino acids represented

Serine

Alanine

Threonine

Alanine

Glycine

Dire

ctio

n of

“re

adin

g”

DNA strand

MessengerRNA

AT

A

A

T

T

T

A TA T

A T

A

A T

U A

U A

U A

G C

C

G CG C

G C

G C

G C

G C

G

G

C

C

G C

C GU AC GC

G

G

GG

G

G

GG

G

G

C

CC

C

C

C

C

C

C

C

A

A

A

A

A

T

T A

A T

A T

A T

A T

C G

G C

G C

G C

T A

T A

T A

C G

A T

G C

T A

C G

T A

C G

C G

G C

A T

T A

C G

G C

T

T

G

C G

C G

C G

C G

C G

C G

C G

G C

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C G

U A

C G

G C

G C

A T

C G

G C

37

Fig04.14

MessengerRNA

TransferRNA

Next amino acid

Anticodon

Codons

Growingpolypeptidechain

1

1

2

2

3

3

4

4

5

5

6

6

7

CU G G

MessengerRNA

TransferRNA

Nextamino acid

Ribosome

1

1

2

2

3

37

4

4

5

5

6 7

CC

C G U

CU G C G U

MessengerRNA

TransferRNA

Next amino acid

Anticodon

Codons

Growingpolypeptidechain

1

1

2

2

3

3

4

4

5

5

6

6

7

Peptide bond

CU G C G U

C CG C GU

6

MessengerRNA

TransferRNA

Nextamino acid

1

1

2

2

3

3

4

4

5

5

6 7

6 7

UCG GA AA A A AG G G G G G GC C C C C C CU U

UCG GA AA A A AG G G G G G G GC C C C C C CU U

UCG GA AA A A AG G G G G G G GC C C C C C CU U

UCG GA AA A A AG G G G G G G GC C C C C C CU U

The transfer RNA moleculefor the last amino acid addedholds the growing polypeptidechain and is attached to itscomplementary codon on mRNA.

A second tRNA bindscomplementarily to thenext codon, and in doingso brings the next aminoacid into position on the ribosome.A peptide bond forms, linkingthe new amino acid to thegrowing polypeptide chain.

The tRNA molecule thatbrought the last amino acidto the ribosome is releasedto the cytoplasm, and will beused again. The ribosomemoves to a new position atthe next codon on mRNA.

A new tRNA complementary tothe next codon on mRNA bringsthe next amino acid to be addedto the growing polypeptide chain.

2

1

3

4

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