Cellular Metabolism - ddamm.weebly.com · Chapter 4 Cellular Metabolism. 2 Introduction A. A living...

Chapter 4

Cellular Metabolism

2

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

metabolic reactions that maintain life.

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3

Metabolic Reactions A. 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.


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B. Anabolism 1. Anabolism provides the substances

needed for growth and repair. 2. These reactions occur by

dehydration synthesis, removing a molecule of water to join two

smaller molecules.


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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.


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Fig04.01

CH2OH H H

OH

O

H OH

Monosaccharide +

H HO

H

OH

CH2OH H H

OH

O

H OH

Monosaccharide

H HO

H

OH

CH2OH H H

OH

O

H OH

Disaccharide

H2O

Water +

H HO

H CH2OH

H H OH

O

H OH

H O H

OH

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

7

Fig04.02

H C H

Glycerol 3 fatty acid molecules +

OH HO

H C OH HO

H C

C

C

C

(CH 2 ) 14 CH 3

(CH 2 ) 14 CH 3

(CH 2 ) 14 CH 3 OH HO O H

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


(CH2)14 CH3

(CH2)14 CH3

H2O H2O

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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.


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Fig04.03

Amino acid

N H

H C C H

R Dipeptide molecule + +

Peptide!bond!

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

O O N

H

H C C H

R N

H C C OH R

H

O O H2O

Water


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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.


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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.


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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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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. ATP a. ATP molecules contain three

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

functions including active transport and synthesis of various compounds.

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½ O2!

Fig04.05

1

3

4

2

Glycolysis 2 ATP

Glucose

High-energy electrons (e–)



2e– and 2H+

2 ATP

H2O

Electron!transport !

chain!ATP 32–34!

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 cytosol!into two 3-carbon pyruvic acid molecules with a net gain!of 2 ATP and the release of high-energy electrons.!

Citric Acid Cycle!The 3-carbon pyruvic acids generated by glycolysis enter!the 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.!


Cyt

osol

Mito

chon

drio

n

Electron Transport Chain!The high-energy electrons still contain most of the!chemical energy of the original glucose molecule.!Special carrier molecules bring the high-energy electrons!to a series of enzymes that store much of the remaining!energy in more ATP molecules. The other products are!heat and water. The function of oxygen as the final!electron acceptor in this last step is why the overall!process is called aerobic respiration.

Each acetyl CoA combines with a 4-carbon oxaloacetic!acid to form the 6-carbon citric acid, for which the cycle!is named. For each citric acid, a series of reactions!removes 2 carbons (generating 2 CO2’s), synthesizes!1 ATP, and releases more high-energy electrons.!The figure shows 2 ATP, resulting directly from 2!turns of the cycle per glucose molecule that enters!glycolysis.

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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


Fig04.07

P P

ADP

ATP Energy transferred!from cellular!respiration used!to reattach!phosphate

Energy transferred!and utilized by!metabolic!reactions when!phosphate bond!is broken

P P P

P P


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2. Glycolysis a. 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.


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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.


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Fig04.09

High-energy!electrons!

Complete oxidation!of acetyl coenzyme A!to H2O and CO2 produces!high-energy electrons,!which yield much ATP!via the electron transport!chain

Breakdown of simple!molecules to acetyl!coenzyme A!accompanied by!production of limited!ATP and high-energy!electrons

H2O 2e– and 2H+

Waste products ½ O2 –NH2

CO2

CO2

Citric!acid!

cycle

Electron!transport!

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 large!molecules to!simple molecules

Glycolysis

1

2

3

© Royalty-Free/Corbis


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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 Pathways 1. 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.


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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.


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

28 A T

T A

T A

A

Fig04.10

C

C

A

C

C G

G

C

G

C G

C G

G

G

T


T A

T

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D. DNA Replication

1. 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

A T

T A

A T

Fig04.11

C

C A T

C C G

G

C

C

G C G

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

G G

G

C C

C C G


T A

T A

T A

A

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Protein Synthesis: A. The Genetic Code- Instructions for making

proteins 1. 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. Translation a. 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.


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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.


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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.


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Fig04.13

Messenger!RNA

1 DNA!information!is copied, or!transcribed,!into mRNA!following!complementary!base pairing

2 mRNA leaves!the nucleus!and attaches!to a ribosome

3 Translation begins as tRNA anticodons!recognize complementary mRNA codons,!thus bringing the correct amino acids into!position on the growing polypeptide chain

4 As the ribosome!moves along the!mRNA, more amino!acids are added

5 At the end of the mRNA,!the ribosome releases!the new protein

6 tRNA molecules!can pick up another!molecule of the!same amino acid!and be reused

Amino acids!attached to tRNA

Polypeptide !chain!

Cytoplasm DNA!double!helix

DNA!strands!pulled!apart

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

“rea

ding

”

DNA !strand!

Messenger!RNA

A T

A A

T T

T

A T A T

A T

A A T

U A

U A

U A

G C

C

G C G C

G C

G C G C

G C G G

C C

G C

C G U A C G C

G

G

G G

G

G

G G

G

G

C C

C 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


C G U A C G G C G C A T C G G C

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Fig04.14

Messenger!RNA

Transfer!RNA

Next amino acid!

Anticodon

Codons

Growing!polypeptide!chain

1

1

2

2

3

3

4

4

5

5

6

6

7

C U G G

Messenger!RNA

Transfer!RNA

Next!amino acid

Ribosome 1

1

2

2

3

3 7

4

4

5

5

6 7

C C C G U

C U G C G U

Messenger!RNA

Transfer!RNA

Next amino acid Anticodon

Codons

Growing!polypeptide!chain

1

1

2

2

3

3

4

4

5

5

6

6

7

Peptide bond!

C U G C G U

C C G C G U

6

Messenger!RNA

Transfer!RNA Next!amino acid

1

1

2

2

3

3

4

4

5

5

6 7

6 7

U C G G A A A A A A G G G G G G G C C C C C C C U U

U C G G A A A A A A G G G G G G G G C C C C C C C U U



The transfer RNA molecule!for the last amino acid added!holds the growing polypeptide!chain and is attached to its!complementary codon on mRNA.

A second tRNA binds!complementarily to the!next codon, and in doing!so brings the next amino!acid into position on the ribosome.!A peptide bond forms, linking!the new amino acid to the!growing polypeptide chain.

The tRNA molecule that!brought the last amino acid!to the ribosome is released!to the cytoplasm, and will be!used again. The ribosome!moves to a new position at!the next codon on mRNA.

A new tRNA complementary to!the next codon on mRNA brings!the next amino acid to be added!to the growing polypeptide chain.

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1

3

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