chemistry 3

- The Structure and Function of Macromolecules

Overview: The Molecules of Life Another level in the hierarchy of biological

organization is reached when small organic molecules are joined together

Macromolecules Are large molecules composed of smaller

molecules Are complex in their structures Include proteins, carboydrates, lipids, and nucleic

acids like DNA

Most macromolecules are polymers, built from monomers

Three of the classes of life’s organic molecules are polymers

Carbohydrates Proteins Nucleic acids

A polymer Is a long molecule consisting of many

similar building blocks called monomers

The Synthesis and Breakdown of Polymers

Monomers form larger molecules by condensation reactions also called dehydration reactions

(a) Dehydration reaction in the synthesis of a polymer

HO H1 2 3 HO

HO H1 2 3 4

OH

H2O

Short polymerUnlinked monomer

Longer polymer

Dehydration removes a watermolecule, forming a new bond

Polymers can disassemble by Hydrolysis (also called digestion)

(b) Hydrolysis of a polymer

HO 1 2 3 OH

HO H1 2 3 4

H2O

HHO

Hydrolysis adds a watermolecule, breaking a bond

The Diversity of Polymers Each class of polymer

Is formed from a specific set of monomers All living organisms are composed of the

same types of polymers made up of the same monomer types – proteins, carbohydrates and nucleic acids.

However, each organism is composed of many unique polymers (unique proteins, carbohydrates and nucleic acids) based on the arrangement of monomers

An immense variety of polymers can be built from a small set of monomers

Carbohydrates serve as fuel and building material

Carbohydrates Include both simple sugars and their polymers

Monosaccharides (simple sugars) Are the simplest sugars Can be used for fuel - glucose Can be converted into other organic molecules

Nucleotides include a 5 carbon sugar, ribose or deoxyribose

Can be combined into polymers

Examples of monosaccharides

Triose sugars(C3H6O3)

Pentose sugars(C5H10O5)

Hexose sugars(C6H12O6)

H C OH

H C OH

H C OH

H C OH

H C OH

H C OH

HO C H

H C OH

H C OH

H C OH

H C OH

HO C H

HO C H

H C OH

H C OH

H C OH

H C OH

H C OH

H C OH

H C OH

H C OH

H C OH

C OC O

H C OH

H C OH

H C OH

HO C H

H C OH

C O

H

H

H

H H H

H

H H H H

H

H H

C C C COOOO

Ald

os

es

Glyceraldehyde

RiboseGlucose Galactose

Dihydroxyacetone

Ribulose

Ke

tos

es

Fructose

Monosaccharides May be linear Can form rings

H

H C OH

HO C H

H C OH

H C OH

H C

OC

H

1

2

3

4

5

6

H

OH

4C

6CH2OH 6CH2OH

5C

HOH

C

H OH

H

2 C

1C

H

O

H

OH

4C

5C

3 C

H

HOH

OH

H

2C

1 C

OH

H

CH2OH

H

H

OHHO

H

OH

OH

H5

3 2

4

(a) Linear and ring forms. Chemical equilibrium between the linear and ring structures greatly favors the formation of rings. To form the glucose ring, carbon 1 bonds to the oxygen attached to carbon 5.

OH3

O H OO

6

1

Notice the carbons are numbered and this numbering system remains when they form a ring in water.

DisaccharidesConsist of two monosaccharides

Are joined by a glycosidic linkageDehydration reaction in the synthesis of maltose. The bonding of two glucose units forms maltose. The glycosidic link joins the number 1 carbon of one glucose to the number 4 carbon of the second glucose. Joining the glucose monomers in a different way would result in a different disaccharide.

Dehydration reaction in the synthesis of sucrose. Sucrose is a disaccharide formed from glucose and fructose.Notice that fructose,though a hexose like glucose, forms a five-sided ring.

(a)

(b)

H

HO

H

HOH H

OH

O H

OH

CH2OH

H

HO

H

HOH H

OH

O H

OH

CH2OH

H

O

H

HOH H

OH

O H

OH

CH2OH

H

H2O

H2O

H

H

O

H

HOH

OH

O H

CH2OH

CH2OH HO

OHH

CH2OH

HOH H

H

HO

OHH

CH2OH

HOH H

O

O H

OHH

CH2OH

HOH H

O

HOH

CH2OH

H HO

O

CH2OH

H

H

OH

O

O

1 2

1 41– 4

glycosidiclinkage

1–2glycosidic

linkage

Glucose

Glucose Glucose

Fructose

Maltose

Sucrose

OH

H

H

In living systems, these reactions are always done by enzymes. Cellular enzymes are controlled

Notice that the chemical reactions take place at the functional groups

Polysaccharides

Polysaccharides Are polymers of sugars Serve many roles in organisms

Storage Starch is a polymer of glucose only Glycogen is also a polymer of glucose

Cell wall - structure Cellulose is a polymer of glucose Chitin

StarchIs a polymer consisting entirely of

glucose monomers

Is the major storage form of glucose in plants

Chloroplast Starch

Amylose Amylopectin

1 m

(a) Starch: a plant polysaccharide

Glycogen Consists of glucose monomers Is the major storage form of glucose in animals

Mitochondria Glycogen granules

0.5 m

(b) Glycogen: an animal polysaccharide

Glycogen

Variety from monomers and the covalent bond type

(c) Cellulose: 1– 4 linkage of glucose monomers

H O

O

CH2OH

HOH H

H

OH

OHH

H

HO

4

C

C

C

C

C

C

H

H

H

HO

OH

H

OH

OH

OH

H

O

CH2OH

HH

H

OH

OHH

H

HO

4 OH

CH2OH

O

OH

OH

HO41

O

CH2OH

O

OH

OH

O

CH2OH

O

OH

OH

CH2OH

O

OH

OH

O O

CH2OH

O

OH

OH

HO4

O1

OH

O

OH OHO

CH2OH

O

OH

O OH

O

OH

OH

(a) and glucose ring structures

(b) Starch: 1– 4 linkage of glucose monomers

1

glucose glucose

CH2OH

CH2OH

1 4 41 1

Which type of bond

depends on the enzyme

which is controlled by the cell.

Cellulose Is a major component of the tough walls that

enclose plant cells

Plant cells

0.5 m

Cell walls

Cellulose microfibrils in a plant cell wall

Microfibril

CH2OH

CH2OH

OH

OH

OO

OHO

CH2OHO

OOH

OCH2OH OH

OH OHO

O

CH2OH

OO

OH

CH2OH

OO

OH

O

O

CH2OHOH

CH2OHOHOOH OH OH OH

O

OH OH

CH2OH

CH2OH

OHO

OH CH2OH

OO

OH CH2OH

OH

b Glucos

e monomer

O

O

O

O

O

O

Parallel cellulose molecules areheld together by hydrogenbonds between hydroxyl

groups attached to carbonatoms 3 and 6.

About 80 cellulosemolecules associate

to form a microfibril, themain architectural unitof the plant cell wall.

A cellulose moleculeis an unbranched glucose polymer.

OH

OH

O

OOH

Cellulosemolecules

Figure 5.8

Cellulose is difficult to digest Cows have microbes in their stomachs to

facilitate this process

What do these microbes have that will allow them to break down cellulose?

Chitin, another important structural polysaccharide Is found in the exoskeleton of arthropods Can be used as surgical thread

(a) The structure of the chitin monomer.

O

CH2OH

OHH

H OH

H

NH

CCH3

O

H

H

(b) Chitin forms the exoskeleton of arthropods. This cicada is molting, shedding its old exoskeleton and emergingin adult form.

(c) Chitin is used to make a strong and flexible surgical

thread that decomposes after the wound or incision heals.

OH

Lipids are a diverse group of hydrophobic molecules

Lipids Are the one class of large biological molecules

that do not consist of polymers Share the common trait of being hydrophobic Include

Fats Phospholipids steroids

(b) Fat molecule (triacylglycerol)

H

H O HC

C

C

H

H OH

OH

H

HH H

HH

HH

H

HHH

H

HH

H

HH

HH

HH

H

HH

HH

H

HH

H

HC

CCC

CC

CC

CC

CC

CC

CC

Glycerol

Fatty acid(palmitic acid)

H

H

H

H

HH

HH

HH

HH

HH

HH

HH

HH

HH

HH

HHHH

HHH

HH

HH

H

H

HH

HH

HH

HH

HH

HH

HH

HH

HH

HH

HH

HH

HH

HH

HH

HH H

HH

HH

HH

HH

HH

HH

H

HH

HH

HH

HH

HH

HHH

HH

HO

O

O

O

O

OC

C

C C CC

CC

CC

CC

CC

CC

CC

C

C

CC

CCCC

CC

CC

CC

CC

CC

C CC

CC

CC

CC

CC

CC

CC

O

O

(a) Dehydration reaction in the synthesis of a fat

Ester linkage

The synthesis and structure of a fat, or triglycerol

Again, notice where the chemical reaction takes

place.

Fatty acidsVary in the length and number and locations of double bonds they contain

Saturated fatty acids Have the maximum number of hydrogen atoms possible (saturated with hydrogen) Have no double bonds

(a) Saturated fat and fatty acid

Stearic acid

(b) Unsaturated fat and fatty acidcis double bondcauses bending

Oleic acid

• Unsaturated fatty acids --Have one or more

double bonds

A single bond allows rotation, is longer and not a strong as a double bond

A double bond is stronger, shorter, and more rigid.

Bonds help to determine the 3-D shape of a molecule.

Phospholipids have only two fatty acids plus a phosphate group instead of a third fatty acid

Consists of a hydrophilic “head” and hydrophobic “tails”

CH2

O

PO O

O

CH2CHCH2

OO

C O C O

Phosphate

Glycerol

(a) Structural formula (b) Space-filling model

Fatty acids

(c) Phospholipid symbol

Hy

dro

ph

ob

ic t

ail

s

Hydrophilichead

Hydrophobictails

–

Hy

dro

ph

ilic

he

ad CH2 Choline

+N(CH3)3

The structure of phospholipids Results in a bilayer arrangement found in cell

membranes

Hydrophilichead

WATER

WATER

Hydrophobictail

Steroids - Are lipids characterized by a carbon skeleton consisting of four fused rings

One steroid, cholesterol Is found in cell membranes Is a precursor for some hormones

HO

CH3

CH3

H3C CH3

CH3

Is this molecule polar or nonpolar?

When written as a ring, all points are carbon unless written in otherwise.

Cholesterol fills in the spaces left by the kinks; stiffens the bilayer and makes it less fluid and less permeable.

How do you think bacteria, which do not use cholesterol, adjust the fluidity of their cell membrane?

Do concept check 5.3

Animal fats found in meat, butter, and cream are usually saturated, and solid at room temperature.

Plant oils like corn oil contain more unsaturated fatty acids.

Peanut and olive oil contain monounsaturated fatty acids.

Both saturated and trans fats correlate with heart problems and high levels or blood cholesterol. Atherosclerosis

Proteins have many structures, resulting in a wide range of functions

Enzymes Are often a type of protein that acts as a

catalyst, speeding up chemical reactions

Substrate(sucrose)

Enzyme (sucrase)

Glucose

OH

H O

H2O

Fructose

3 Substrate is convertedto products.

Substrate binds toenzyme.

1 Active site is available for a molecule of substrate, the

reactant on which the enzyme acts.

22

4 Products are released.

Is this part of the protein polar or nonpolar?

Enzyme remains unchanged, ready to work again.

Polypeptides Polypeptides

Are polymers of amino acids A protein

Can consist of only one large polypeptide Can consists of more than one polypeptides

(subunits) bound together by non-covalent interactions Hemoglobin

Some very small polypeptides are referred to as peptides

Amino acids Are organic molecules possessing both carboxyl and

amino groups Differ in their properties due to differing side chains,

called R groups

Proteins are composed of amino acid building blocks and are diverse in structure (shape) and function.

Amino acids have an amino group and an acid group bound to a central carbon.

This central carbon forms 4 single bonds. One with the amino group, one with the carboxylic acid, one with hydrogen, and the last with a variety of different chemical groups (R group).

Amino group

Acid group

20 different amino acids make up proteins

O

O–

H

H3N+ C C

O

O–

H

CH3

H3N+ C

H

C

O

O–

CH3 CH3

CH3

C C

O

O–

H

H3N+

CH

CH3

CH2

C

H

H3N+

CH3

CH3

CH2

CH

C

H

H3N+ C

CH3

CH2

CH2

CH3N+

H

C

O

O–

CH2

CH3N+

H

C

O

O–

CH2

NH

H

C

O

O–

H3N+ C

CH2

H2C

H2N C

CH2

H

C

Nonpolar

Glycine (Gly) Alanine (Ala) Valine (Val) Leucine (Leu) Isoleucine (Ile)

Methionine (Met) Phenylalanine (Phe)

C

O

O–

Tryptophan (Trp) Proline (Pro)

H3C

S

O

O–

Know the structure of an amino acid, not all the R groups.

O–

OH

CH2

C C

H

H3N+

O

O–

H3N+

OH CH3

CH

C C

HO–

O

SH

CH2

C

H

H3N+ C

O

O–

H3N+ C C

CH2

OH

H H H

H3N+

NH2

CH2

OC

C C

O

O–

NH2 O

C

CH2

CH2

C CH3N+

O

O–

O

Polar

Electricallycharged

–O O

C

CH2

C CH3N+

H

O

O–

O– O

C

CH2

C CH3N+

H

O

O–

CH2

CH2

CH2

CH2

NH3+

CH2

C CH3N+

H

O

O–

NH2

C NH2+

CH2

CH2

CH2

C CH3N+

H

O

O–

CH2

NH+

NHCH2

C CH3N+

H

O

O–

Serine (Ser) Threonine (Thr)Cysteine

(Cys)Tyrosine

(Tyr)Asparagine

(Asn)Glutamine

(Gln)

Acidic Basic

Aspartic acid (Asp)

Glutamic acid (Glu)

Lysine (Lys) Arginine (Arg) Histidine (His)

Know both the name and abbreviation of all amino acids along with their chemical nature – polar, nonpolar, charged, acidic, . . .

A peptide bond forms between the amino group on one amino acid and the carboxyl group on another

amino acid.

The formation of a peptide bonds in a tetrapeptide

Amino acids Are linked by peptide bonds between the amino

group of one amino acid and the acid group of the other amino acid

OH

DESMOSOMES

DESMOSOMESDESMOSOMES

OH

CH2

C

N

H

C

H O

H OH OH

Peptidebond

OH

OH

OH

H H

HH

H

H

H

H

H

H H

H

N

N N

N N

SHSide

chains

SH

OO

O O O

H2O

CH2 CH2

CH2 CH2CH2

C C C C C C

C CC C

Peptidebond

Amino end(N-terminus)

Backbone

(a)

(b) Carboxyl end(C-terminus)

The chemical reaction again takes place at the functional groups!

Each peptide bond is in a plane. This contributes to the shape of the protein.

Protein Conformation and Function

Two models of protein conformation

(a) A ribbon model

(b) A space-filling model

Groove

Groove

A protein’s specific conformation (shape and chemical nature)

determines how it functions.

Four Levels of Protein Structure

Primary structure Is the unique sequence of amino acids in a

polypeptide

–

Amino acid subunits

+H3NAmino

end

oCarboxyl end

oc

GlyProThrGlyThr

Gly

GluSeuLysCysProLeu

MetVal

Lys

ValLeu

AspAlaVal ArgGly

SerPro

Ala

Gly

lle

SerProPheHisGluHis

Ala

GluVal

ValPheThrAlaAsn

AspSer

GlyProArg

ArgTyrThr

lleAla

Ala

Leu

LeuSer

ProTyrSerTyrSerThr

Thr

Ala

ValVal

ThrAsnProLysGlu

ThrLys

SerTyrTrpLysAlaLeu

GluLle Asp

Covalent bonds

Peptide backbone imposes some restrictions on the folding of a protein. Why?

O C

helix

pleated sheetAmino acid

subunits NCH

C

O

C N

H

CO H

R

C NH

C

O H

C

R

N

HH

R C

O

R

C

H

NH

C

O H

NCO

R

C

H

NH

H

C

R

C

O

C

O

C

NH

H

R

C

C

O

N

HH

C

R

C

O

NH

R

C

H C

ON

HH

C

R

C

O

NH

R

C

H C

ON

HH

C

R

C

O

N H

H C R

N HO

O C N

C

RC

H O

CHR

N H

O C

RC

H

N H

O CH C R

N H

CC

N

R

H

O C

H C R

N H

O C

RC

H

H

C

RN

H

CO

C

NH

R

C

H C

O

N

H

C

Secondary structure Is the folding or coiling of the polypeptide into a

repeating configuration Includes the helix and the pleated sheet

H H

All based on hydrogen bonds between the peptide bonds of different amino acids

Tertiary structure Is the overall three-dimensional shape

of a polypeptide after it “folds” into a stable form.

Results from interactions between amino acids and R groups

CH2CH

OH

O

CHO

CH2

CH2 NH3+ C-O CH2

O

CH2SSCH2

CH

CH3

CH3

H3C

H3C

Hydrophobic interactions and van der Waalsinteractions

Polypeptidebackbone

Hydrogenbond

Ionic bond

CH2

Disulfide bridge

What are these?

The distribution of polar and nonpolar amino acids is important in how a protein folds. The nonpolar side chains tend to cluster in the interior of a molecule, avoiding contact with water, while the polar side chains arrange themselves near the outside.

Hydrophobic areas also tend to be found spanning the lipid bilayer of membranes like the plasma membrane.

Transmembrane proteins often cross the membrane in an alpha helix because the peptide bond itself is hydrophic unless all partial charges are equalized in an alpha helix or beta sheet.

Quaternary structure

Is the overall protein structure that results from the aggregation of two or more polypeptide subunits

Hemoglobin contains

two alpha globin subunits and

two beta globin subunits.

Heme is the site where oxygen is carried

There are many large multi-subunit proteins in cells.

Larger protein molecules may contain more than one polypeptide chain or subunit. The region that interacts with another molecule through noncovalent bonds is the binding site.

Sickle-cell disease results from a single amino acid substitution in the protein hemoglobin

Fibers of abnormalhemoglobin deform cell into sickle shape.

Primary structure

Secondaryand tertiarystructures


Function

Red bloodcell shape

Hemoglobin A

Molecules donot associatewith oneanother, eachcarries oxygen.

Normal cells arefull of individualhemoglobinmolecules, eachcarrying oxygen

10 m 10 m

Primary structure

Secondaryand tertiarystructures


Function

Red bloodcell shape

Hemoglobin S

Molecules interact with one another tocrystallize into a fiber, capacity to carry oxygen is greatly reduced.

subunit subunit

1 2 3 4 5 6 7 3 4 5 6 721

Normal β hemoglobin

Sickle-cell β hemoglobin . . .. . .

Figure 5.21

Exposed hydrophobic

region

Val ThrHis Leu Pro Glul Glu Val His Leu Thr Pro Val Glu

The sickle-cell hemoglobin does not fold into the proper shape because the amino acid sequence (Primary structure) is incorrect.

20 different amino acids make up proteins

O

O–

H

H3N+ C C

O

O–

H

CH3

H3N+ C

H

C

O

O–

CH3 CH3

CH3

C C

O

O–

H

H3N+

CH

CH3

CH2

C

H

H3N+

CH3

CH3

CH2

CH

C

H

H3N+ C

CH3

CH2

CH2

CH3N+

H

C

O

O–

CH2

CH3N+

H

C

O

O–

CH2

NH

H

C

O

O–

H3N+ C

CH2

H2C

H2N C

CH2

H

C

Nonpolar

Glycine (Gly) Alanine (Ala) Valine (Val) Leucine (Leu) Isoleucine (Ile)

Methionine (Met) Phenylalanine (Phe)

C

O

O–

Tryptophan (Trp) Proline (Pro)

H3C

S

O

O–

Know the structure of an amino acid, not all the R groups.

O–

OH

CH2

C C

H

H3N+

O

O–

H3N+

OH CH3

CH

C C

HO–

O

SH

CH2

C

H

H3N+ C

O

O–

H3N+ C C

CH2

OH

H H H

H3N+

NH2

CH2

OC

C C

O

O–

NH2 O

C

CH2

CH2

C CH3N+

O

O–

O

Polar

Electricallycharged

–O O

C

CH2

C CH3N+

H

O

O–

O– O

C

CH2

C CH3N+

H

O

O–

CH2

CH2

CH2

CH2

NH3+

CH2

C CH3N+

H

O

O–

NH2

C NH2+

CH2

CH2

CH2

C CH3N+

H

O

O–

CH2

NH+

NHCH2

C CH3N+

H

O

O–

Serine (Ser) Threonine (Thr)Cysteine

(Cys)Tyrosine

(Tyr)Asparagine

(Asn)Glutamine

(Gln)

Acidic Basic

Aspartic acid (Asp)

Glutamic acid (Glu)

Lysine (Lys) Arginine (Arg) Histidine (His)

Know both the name and abbreviation of all amino acids along with their chemical nature – polar, nonpolar, charged, acidic, . . .

Protein conformation Depends on

the sequence of amino acid side chains (with R groups) and the physical and chemical conditions of the protein’s environment

Denaturation is when a protein unravels and loses its native conformation

Denaturation

Renaturation

Denatured proteinNormal protein

What kinds of bonds are broken here?

Increased temperature

Change in pH

Organic solvent (hydrophobic)

What kinds of bonds are not broken here?

The Protein-Folding Problem

Most proteins Probably go through several intermediate states

on their way to a stable conformation. Many proteins are being made in the cell all of

the time. How do the fold correctly, how do they interact with their subunits correctly?

Chaperonins Are protein molecules that assist in the proper

folding of other proteins

Hollowcylinder

Cap

Chaperonin(fully assembled)

Steps of ChaperoninAction: An unfolded polypeptide enters the cylinder from one end.

The cap attaches, causing the cylinder to change shape insuch a way that it creates a hydrophilic environment for the folding of the polypeptide.

The cap comesoff, and the properlyfolded protein is released.

Correctlyfoldedprotein

Polypeptide

2

1

3

X-ray crystallography Is used to determine a protein’s three-

dimensional structureX-raydiffraction pattern

Photographic film

Diffracted X-rays

X-raysource

X-ray beam

Crystal Nucleic acid Protein

(a) X-ray diffraction pattern (b) 3D computer modelFigure 5.24


Nucleic acids store and transmit hereditary information

Genes Are the units of inheritance

Code for the amino acid sequence of polypeptides Are made of nucleic acids

There are two types of nucleic acids Deoxyribonucleic acid (DNA) Ribonucleic acid (RNA)

Deoxyribonucleic acid (DNA)

Stores information for the synthesis of specific proteins –DNA is the “ genetic material” inherited from parents

Directs RNA synthesis Directs protein synthesis

indirectly through messenger RNA

1

2

3

Synthesis of mRNA in the nucleus

Movement of mRNA into cytoplasm

via nuclear pore

Synthesisof protein

NUCLEUS

CYTOPLASM

DNA

mRNA

Ribosome

AminoacidsPolypeptide

mRNA

The Structure of Nucleic Acids Nucleic acids

Exist as polymers called polynucleotides Each polynucleotide Consists of monomers called

nucleotides

(a) Polynucleotide, or nucleic acid

3’C

5’ end

5’C

3’C

5’C

3’ endOH

O

O

O

O Nitrogenousbase

Nucleoside

O

O

O

O P CH2

5’C

3’CPhosphate

group Pentosesugar

(b) Nucleotide

O

Nucleotide Monomers Are made up of nucleosides and phosphate

groups

(c) Nucleoside componentsFigure 5.26

CHCH

Uracil (in RNA)U

Ribose (in RNA)

Nitrogenous bases Pyrimidines

CN

NC

OH

NH2

CHCH

OC

NH

CH

HNC

O

CCH3

N

HNC

C

HO

O

CytosineC

Thymine (in DNA)T

NHC

N C

CN

C

CH

N

NH2 O

NHC

NHH

CC

N

NH

C NH2

AdenineA

GuanineG

Purines

OHOCH2

H

H H

OH

H

OHOCH2

HH H

OH

H

Pentose sugars

Deoxyribose (in DNA) Ribose (in RNA)OHOH

CH

CH

Uracil (in RNA)U

4’

5”

3’OH H

2’

1’

5”

4’

3’ 2’

1’

Nitrogenousbase

Nucleoside

O

O

O

O P CH2

5’C

3’CPhosphate

group Pentosesugar

(b) Nucleotide

O

pyrimidines

Nucleotide polymers are made up of nucleotides linked by the–OH group on the 3´ carbon of one nucleotide and the phosphate on the 5´ carbon on the next So they “grow” at the 3’ end.

The sequence of bases along a nucleotide polymer Is unique for

each gene

The DNA Double Helix

Anti-parallel

complementary

Complementary base pairs


DNA and Proteins as Tape Measures of Evolution

Molecular comparisons Help biologists sort out the evolutionary

connections among species Ribosomal RNA gene sequence is conserved. Look for differences.

chemistry 3

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