Unit 2 – The Chemical Basis of Life

I. Life requires about 25 Chemical ElementsI. Life requires about 25 Chemical ElementsA. ElementsA. Elements

1. Matter – anything that occupies 1. Matter – anything that occupies space and has massspace and has mass

2. The various forms of matter are 2. The various forms of matter are composed of one or more chemical composed of one or more chemical elementselements

a) Element- a pure substance a) Element- a pure substance that cannot be broken down that cannot be broken down into other substances by into other substances by chemical meanschemical means

3. About 25 elements are essential to life.3. About 25 elements are essential to life.

a) Oxygen (O), Carbon (C), a) Oxygen (O), Carbon (C), Hydrogen Hydrogen (H), and Nitrogen (N) make up about (H), and Nitrogen (N) make up about

96% of the living matter in your body. 96% of the living matter in your body.

b) Calcium (Ca), Phosphorus (P), b) Calcium (Ca), Phosphorus (P), Potassium (K), Sulfur (S), and a few other Potassium (K), Sulfur (S), and a few other elements account for most of the elements account for most of the remaining 4%.remaining 4%.

c) Trace elements are elements that make up c) Trace elements are elements that make up less than 0.01% of your body mass, but are less than 0.01% of your body mass, but are critical to your healthcritical to your health

1) Example: Iron (Fe); essential for 1) Example: Iron (Fe); essential for carrying oxygen in your bloodcarrying oxygen in your blood

B. CompoundsB. Compounds1. A compound is a substance 1. A compound is a substance

containing two or more elements containing two or more elements that are that are chemically combined in a chemically combined in a fixed ratio.fixed ratio.

a) Example: water (Ha) Example: water (H22O)O)

2. A compound’s properties may 2. A compound’s properties may differ greatly from those of its differ greatly from those of its component component elements.elements.

a) Example:a) Example: Salt (NaCl)Salt (NaCl)

II. Chemical properties are based on the II. Chemical properties are based on the structure of atomsstructure of atoms

A. AtomsA. Atoms

1. An atom is the smallest 1. An atom is the smallest possible particle of an elementpossible particle of an element

a) comes from the Greek a) comes from the Greek word atomos meaning word atomos meaning “indivisible”“indivisible”

2. Structure of the atom2. Structure of the atoma) Proton- a subatomic particle with a a) Proton- a subatomic particle with a positive charge (+); found in the nucleuspositive charge (+); found in the nucleus

b) Electron- a subatomic particle with a b) Electron- a subatomic particle with a negative charge (-); found outside the negative charge (-); found outside the nucleus in energy levelsnucleus in energy levels

c) Neutron- a subatomic particle that is c) Neutron- a subatomic particle that is neutral (no charge); found in the nucleusneutral (no charge); found in the nucleus

d) Nucleus- the center of the atom d) Nucleus- the center of the atom containing protons and neutronscontaining protons and neutrons

E-E-

E-E-

E-E-

E-E-E-E-

P+P+

P+P+NN

NNNN

NNP+P+

P+P+

NNP+P+

NNNucleusNucleus

Electrons (E-) Electrons (E-)

Color RedColor Red

Energy Level – Color BlueEnergy Level – Color Blue

Protons + Protons +

Color GreenColor Green

Neutrons Neutrons

Color PurpleColor Purple

3. Atomic Number- the number of 3. Atomic Number- the number of protons in an atom’s nucleus; also the protons in an atom’s nucleus; also the number of electronsnumber of electrons

a) Left alone, an atom tends to a) Left alone, an atom tends to hold as hold as many electrons as protonsmany electrons as protons

b) The number of electrons is NOT b) The number of electrons is NOT constant like the number of constant like the number of

protonsprotons 1) The number of 1) The number of electrons electrons determines determines how the atom interacts how the atom interacts with with other atomsother atoms

2) The number of protons 2) The number of protons determines the atom’s determines the atom’s

propertiesproperties4. Atomic Mass- the number of 4. Atomic Mass- the number of protons plus the number of neutronsprotons plus the number of neutrons

B. IsotopesB. Isotopes1. Isotopes of an element have the 1. Isotopes of an element have the

same number of protons in same number of protons in their atoms their atoms but different numbers but different numbers of neutrons.of neutrons.

a) Example:a) Example:

C. Electrons and ReactivityC. Electrons and Reactivity

1. An atom’s electrons 1. An atom’s electrons determine determine how it reacts with how it reacts with other atoms.other atoms.

a) Electrons differ in the a) Electrons differ in the amount amount of energy they have of energy they have and how and how tightly they are held tightly they are held by the by the protons in the protons in the nucleus.nucleus.

b) Electrons in the highest energy b) Electrons in the highest energy level of an atom determine how level of an atom determine how that atom reacts.that atom reacts.

1) The first or lowest 1) The first or lowest energy energy level (nearest level (nearest the nucleus) can the nucleus) can hold two hold two electrons.electrons.

2) Every energy level after 2) Every energy level after the the first can hold up to 8 first can hold up to 8 electrons.electrons.

III. Chemical bonds join atoms to one III. Chemical bonds join atoms to one anotheranother

A. Ionic BondsA. Ionic Bonds1. An ionic bond occurs when 1. An ionic bond occurs when

an an atom transfers an electron to atom transfers an electron to another atomanother atoma) Example:a) Example:

2. Ions- atoms (or groups of 2. Ions- atoms (or groups of atoms) that have become atoms) that have become electrically charged as a result electrically charged as a result of gaining or losing electronsof gaining or losing electrons

B. Covalent BondsB. Covalent Bonds1. A covalent bond forms when 1. A covalent bond forms when

two two atoms share electronsatoms share electrons2. The number of bonds an atom 2. The number of bonds an atom

can can form usually equals the form usually equals the number of number of additional electrons that additional electrons that will fill its will fill its highest energy levelhighest energy level

a) Example:a) Example:

3. Molecules- two or more atoms held 3. Molecules- two or more atoms held together by covalent bondstogether by covalent bonds

a) chemical formula- tells you a) chemical formula- tells you the the number and types of atoms in number and types of atoms in a a moleculemolecule

b) structural formula- b) structural formula- indicates how indicates how atoms in a atoms in a molecule are linked by molecule are linked by bondsbonds

c) space-filling model- a c) space-filling model- a drawing drawing that depicts a 3-D that depicts a 3-D model of a model of a moleculemolecule

C. Chemical ReactionsC. Chemical Reactions

1. A chemical reaction is the 1. A chemical reaction is the breaking breaking of old bonds and the of old bonds and the formation of new formation of new bonds that result bonds that result in new substancesin new substances

a) Example: a) Example:

b) Reactants- the starting b) Reactants- the starting materials for a materials for a chemical reactionchemical reaction

c) Products- the ending materials c) Products- the ending materials in a in a chemical reactionchemical reaction

FYI…FYI…2. Exothermic reactions- chemical 2. Exothermic reactions- chemical reactions that release energyreactions that release energy

3. Endothermic reactions- chemical 3. Endothermic reactions- chemical reactions that absorb energyreactions that absorb energy

IV. Life depends on the unique IV. Life depends on the unique properties of waterproperties of water

A. The structure of waterA. The structure of water1. Water is made up of 2 1. Water is made up of 2

hydrogen hydrogen atoms and one oxygen atoms and one oxygen atomatom

a) Oxygen pulls electrons a) Oxygen pulls electrons much more strongly much more strongly

than does than does hydrogen. This hydrogen. This unequal pull unequal pull results in the results in the shared electrons shared electrons spending spending more of their time more of their time with the with the oxygen atom (creates oxygen atom (creates a V a V shape). shape).

b) The oxygen end has a slight b) The oxygen end has a slight negative charge, while the negative charge, while the hydrogen atoms are slightly hydrogen atoms are slightly positive. This is called a positive. This is called a polarpolar molecule.molecule.

2. Hydrogen bond- a bond 2. Hydrogen bond- a bond created by the weak attraction created by the weak attraction of a slightly positive hydrogen of a slightly positive hydrogen atom to a slightly negative atom to a slightly negative portion of another molecule.portion of another molecule.

B. Water’s Life-Supporting PropertiesB. Water’s Life-Supporting Properties

1. Cohesion and Adhesion1. Cohesion and Adhesion

a) Cohesion – the tendency of a) Cohesion – the tendency of molecules of the same molecules of the same

kind to stick kind to stick to one anotherto one another

1) Example: beads of 1) Example: beads of water on water on the outside of a the outside of a glassglass

b) Adhesion- the attraction between b) Adhesion- the attraction between unlike moleculesunlike molecules

1) Example:1) Example:

Figure 4-13Cohesion and adhesion contribute to the rise of water molecules within a tree's water transport system. The dotted lines in the diagram indicate hydrogen bonds.

3. Low Density of Ice3. Low Density of Icea) Density is the amount of matter a) Density is the amount of matter

in in a a given volume.given volume.

b) The solid state is more b) The solid state is more dense than the liquid state.dense than the liquid state.

c) This is important to living c) This is important to living things things because if ice didn’t float, because if ice didn’t float, bodies of bodies of water would freeze from water would freeze from bottom to top, bottom to top, trapping the fish and trapping the fish and other organisms in other organisms in a shrinking layer a shrinking layer of water without access of water without access to to nutrients.nutrients.

Figure 4-15Figure 4-15Ice floats because its molecules are less densely Ice floats because its molecules are less densely packed than those in liquid water.packed than those in liquid water.

C. Water’s ability to Dissolve C. Water’s ability to Dissolve other substancesother substances

a) Solution- a uniform a) Solution- a uniform mixture of mixture of two or more two or more substancessubstances

1) Solvent- the 1) Solvent- the substance substance that that dissolves the other dissolves the other substancesubstance

2) Solute- the 2) Solute- the substance substance that is that is dissolveddissolved

b) Aqueous solution- a solution in b) Aqueous solution- a solution in which water is the solventwhich water is the solvent

1) Water is the main solvent in 1) Water is the main solvent in all all cells, blood, and plant sapcells, blood, and plant sap

Figure 4-16Figure 4-16Sodium chloride dissolves as Na+ and Cl- ions Sodium chloride dissolves as Na+ and Cl- ions become attracted to water molecules and break become attracted to water molecules and break away from the surface of the solid.away from the surface of the solid.

D. Acids, Bases, and pHD. Acids, Bases, and pH

1. Acid- A compound that 1. Acid- A compound that donates donates H+ ions to a solution H+ ions to a solution

2. Base- A compound that 2. Base- A compound that removes removes H+ ions from an H+ ions from an aqueous solutionaqueous solution

3. pH scale-describes how 3. pH scale-describes how acidic or acidic or basic a solution isbasic a solution is

a) pH scale ranges from a) pH scale ranges from 0 to 0 to 14, 14, with 0 being with 0 being the most the most acidic and 14 acidic and 14 being the being the most basic.most basic.

b) Pure water and b) Pure water and aqueous aqueous solutions that solutions that have equal have equal amounts of amounts of H+ and OH- ions H+ and OH- ions are said are said to be neutral (pH of to be neutral (pH of 7).7).

4. Buffers- substances that 4. Buffers- substances that cause a solution to resist cause a solution to resist changes in pH; works by changes in pH; works by accepting H+ ions when their accepting H+ ions when their levels rise and donating H+ levels rise and donating H+ ions when their levels fallions when their levels fall

V. Carbon is the main ingredient of V. Carbon is the main ingredient of Organic moleculesOrganic molecules

A. Carbon Skeletons and A. Carbon Skeletons and Functional Functional GroupsGroups

1. Carbon has 4 electrons in 1. Carbon has 4 electrons in its its highest energy level, highest energy level, which means which means they can form up they can form up to 4 bonds with to 4 bonds with other atoms.other atoms.

2. Most carbon-based 2. Most carbon-based molecules are organic molecules are organic molecules; non-carbon-based molecules; non-carbon-based molecules are classified as molecules are classified as inorganic molecules.inorganic molecules.

3. Carbon can also bond with 3. Carbon can also bond with atoms of other elementsatoms of other elements

a) Hydrocarbons- organic a) Hydrocarbons- organic molecules that are composed of molecules that are composed of

only carbon and hydrogenonly carbon and hydrogen

4. Functional group- a group of 4. Functional group- a group of atoms within a molecule that atoms within a molecule that interacts in predictable ways with interacts in predictable ways with other molecules other molecules

a) Hydrophilic- molecules a) Hydrophilic- molecules that that attract water molecules attract water molecules

(hydroxyl groups)(hydroxyl groups)

B. Monomers and PolymersB. Monomers and Polymers

1. Monomers – small molecular 1. Monomers – small molecular unit that unit that is the building block of a is the building block of a larger larger moleculemolecule

2. Polymers- long chains of small 2. Polymers- long chains of small molecular units (monomers)molecular units (monomers)

a) Every living cell has a) Every living cell has thousands of thousands of different kinds of different kinds of polymers and yet polymers and yet all of these all of these polymers are built from polymers are built from a a collection of fewer than 50 kinds collection of fewer than 50 kinds

of monomers.of monomers.

C. Building and Breaking PolymersC. Building and Breaking Polymers

1. Each time a monomer is 1. Each time a monomer is added added to a chain, a water to a chain, a water molecule is molecule is released; this is released; this is called dehydration called dehydration synthesis.synthesis.

2. Cells break bonds between 2. Cells break bonds between monomers by adding water to monomers by adding water to them; this is called hydrolysis them; this is called hydrolysis reaction.reaction.

Figure 5-4Figure 5-4In the dehydration reaction, two monomers bond to each In the dehydration reaction, two monomers bond to each other, making a polymer chain longer. The hydroxyl group of other, making a polymer chain longer. The hydroxyl group of one monomer reacts with a hydrogen atom from the other one monomer reacts with a hydrogen atom from the other monomer. The reactions involved ultimately release a water monomer. The reactions involved ultimately release a water molecule.molecule.

Figure 5-5Figure 5-5In the hydrolysis reaction, the addition of a water In the hydrolysis reaction, the addition of a water molecule breaks the polymer chain. molecule breaks the polymer chain.

VI. Carbohydrates provide fuel and VI. Carbohydrates provide fuel and building materialbuilding material

A. SugarsA. Sugars1. Carbohydrate- an organic 1. Carbohydrate- an organic

compound made up of compound made up of sugar sugar moleculesmolecules

a) contains the elements a) contains the elements carbon, hydrogen, and carbon, hydrogen, and

oxygen oxygen in the ratio of 1 in the ratio of 1 carbon: 2 carbon: 2 hydrogen: 1 hydrogen: 1 oxygenoxygen

b) the carbon skeletons of b) the carbon skeletons of carbohydrates have a carbohydrates have a

ring ring shapeshape

2. Monosaccharides- simple 2. Monosaccharides- simple sugars that contain just one sugar sugars that contain just one sugar unitunit

a) Examples: glucose, a) Examples: glucose, fructose, and fructose, and galactosegalactose

b) Sugar molecules, b) Sugar molecules, particularly particularly glucose, are the glucose, are the main fuel supply main fuel supply for cellular workfor cellular work

Figure 5-6Figure 5-6The complete structural diagram of the The complete structural diagram of the monosaccharide glucose (left) shows all its atoms. monosaccharide glucose (left) shows all its atoms. The simplified representation (right) shows just the The simplified representation (right) shows just the core ring formed by some of the carbon and oxygen core ring formed by some of the carbon and oxygen atoms. Ring shapes are common in sugar molecules atoms. Ring shapes are common in sugar molecules found in nature.found in nature.

3. Disaccharides- consists of 2 3. Disaccharides- consists of 2 monosaccharides (double sugar)monosaccharides (double sugar)

a) the most common a) the most common disaccharide disaccharide is sucrose; consists is sucrose; consists of a glucose of a glucose molecule linked to a molecule linked to a fructose fructose moleculemolecule

1) major carbohydrate 1) major carbohydrate in in plant sapplant sap

Figure 5-7Figure 5-7Sucrose is a disaccharide (double sugar) consisting Sucrose is a disaccharide (double sugar) consisting of two monosaccharides linked together.of two monosaccharides linked together.

B. PolysaccharidesB. Polysaccharides1. Polysaccharides are long 1. Polysaccharides are long

polymer polymer chains made up of simple chains made up of simple sugar sugar monomersmonomers

a) Starch is a polysaccharide a) Starch is a polysaccharide found found in plant cellsin plant cells

1) Examples: potatoes, 1) Examples: potatoes, rice, rice, and corn are all rich and corn are all rich in starchin starch

b) Animal cells store excess sugar b) Animal cells store excess sugar in the in the form of glycogen; this is form of glycogen; this is stored as stored as granules in liver and granules in liver and muscle cells.muscle cells.

c) Cellulose serves as a building c) Cellulose serves as a building material in plants; they material in plants; they

protect cells protect cells and stiffen the and stiffen the plant.plant.

1) Most animals cannot 1) Most animals cannot digest digest cellulose because they cellulose because they lack lack the molecule the molecule necessary to necessary to break the break the bonds between the bonds between the glucose glucose monomers in monomers in cellulose.cellulose.

2) Cellulose is referred to as fiber 2) Cellulose is referred to as fiber and serves to keep the digestive and serves to keep the digestive system healthy.system healthy.

d) Almost all d) Almost all carbohydrates are carbohydrates are hydrophilic; this is due to the hydrophilic; this is due to the

many hydroxyl groups in their many hydroxyl groups in their sugar units. Therefore, sugar units. Therefore,

monosaccharides and monosaccharides and disaccharides disaccharides

dissolve readily dissolve readily in water.in water.

Figure 5-8Glycogen, cellulose, and starch are three types of polysaccharides found in food. Though all three polymers are composed of the same monomer, glucose, the way the glucose monomers link together is different for each.

VII. Lipids include Fats and SteroidsVII. Lipids include Fats and SteroidsA. Characteristics of LipidsA. Characteristics of Lipids

1. One of a class of water-1. One of a class of water-avoiding avoiding compoundscompounds

a) Water-avoiding a) Water-avoiding molecules molecules are said to be are said to be hydrophobichydrophobic

B. FatsB. Fats

1. Consists of a three-carbon 1. Consists of a three-carbon backbone called glycerol backbone called glycerol

attached attached to three fatty acidsto three fatty acids

a) Saturated fat- a fat in which all 3 a) Saturated fat- a fat in which all 3 fatty acid chains contain the maximum fatty acid chains contain the maximum possible number of hydrogen atomspossible number of hydrogen atoms

1) Diets rich in this type of fat are 1) Diets rich in this type of fat are said to be unhealthy because said to be unhealthy because

they they promote the buildup of promote the buildup of lipid-containing lipid-containing deposits called deposits called plaques, within the walls plaques, within the walls of blood of blood vessels. vessels.

2) They are solid at room 2) They are solid at room temperaturetemperature

3) Examples: lard, butter3) Examples: lard, butter

b) Unsaturated fat- a fat that b) Unsaturated fat- a fat that contains less than the contains less than the maximum number of hydrogen maximum number of hydrogen atoms in one or more of its atoms in one or more of its fatty acid chains because fatty acid chains because some of its carbon atoms are some of its carbon atoms are double-bonded to each otherdouble-bonded to each other

1) Examples: Vegetable oil, 1) Examples: Vegetable oil, corn corn oil, olive oil; fats in oil, olive oil; fats in fruits, fruits, vegetables, and fishvegetables, and fish

C. SteroidsC. Steroids

1. Classified as lipids because 1. Classified as lipids because they are they are hydrophobic, but they are hydrophobic, but they are different from different from fats in structure and fats in structure and function.function.

a) Example: estrogen, a) Example: estrogen, testosterone, testosterone, cholesterolcholesterol

2. The best-known steroid is 2. The best-known steroid is cholesterol. cholesterol. This is an essential This is an essential molecule found in the molecule found in the membranes membranes that surround your cells. that surround your cells.

Figure 5-10Figure 5-10The only difference in these two steroid The only difference in these two steroid hormones is the location of their functional hormones is the location of their functional groups. Yet, these two molecules contribute groups. Yet, these two molecules contribute to major differences in the appearance and to major differences in the appearance and behavior of male and female mammals.behavior of male and female mammals.

VIII. Proteins perform most functions VIII. Proteins perform most functions in cellsin cells

A. The Functions of ProteinsA. The Functions of Proteins1. A protein is a polymer 1. A protein is a polymer

constructed from a set of constructed from a set of just 20 just 20 kinds of monomers kinds of monomers called amino called amino acids.acids.

a) responsible for almost a) responsible for almost all of all of the day-to-day the day-to-day functioning of functioning of organismsorganisms

1) example: hair, fur, 1) example: hair, fur, make up muscles, make up muscles,

and and provide long-term provide long-term nutrient nutrient storagestorage

B. Amino AcidsB. Amino Acids

1. An amino acid monomer 1. An amino acid monomer consists of consists of central carbon atom central carbon atom bonded to four bonded to four partners. One partner partners. One partner is a hydrogen is a hydrogen atom. Two others are atom. Two others are a carboxyl group a carboxyl group and an amino and an amino group.group.

2. What is different about each 2. What is different about each amino amino acid is the “side group” called acid is the “side group” called the R-the R- group.group.

Figure 5-12Figure 5-12All amino acids consist of a central carbon bonded All amino acids consist of a central carbon bonded to an amino group, a carboxyl group, and a to an amino group, a carboxyl group, and a hydrogen atom. The fourth bond is with a unique hydrogen atom. The fourth bond is with a unique side group. The differences in side groups convey side group. The differences in side groups convey different properties to each amino acid.different properties to each amino acid.

C. Building a ProteinC. Building a Protein1. Polypeptide- a chain of linked 1. Polypeptide- a chain of linked

amino amino acidsacids

2. Each link is created by a 2. Each link is created by a dehydration dehydration reaction between reaction between the amino group of the amino group of one amino acid one amino acid and the carboxyl group and the carboxyl group of another of another amino acid.amino acid.

3. The protein “alphabet” consists 3. The protein “alphabet” consists of 20 of 20 “letters” or amino acids.“letters” or amino acids.

a) each protein has its own a) each protein has its own unique unique sentence of amino acidssentence of amino acids

Figure 5-13Figure 5-13The order of amino acids makes each polypeptide The order of amino acids makes each polypeptide unique. There are 129 amino acids in this protein, unique. There are 129 amino acids in this protein, called lysozyme. The three-letter symbols are called lysozyme. The three-letter symbols are abbreviations for the amino acid names.abbreviations for the amino acid names.

D. Protein ShapeD. Protein Shape1. The shape of a protein is 1. The shape of a protein is

influenced influenced by the following:by the following:

a) temperaturea) temperatureb) pH b) pH

c) other qualities of its c) other qualities of its environmentenvironment

1) denaturation- loss of 1) denaturation- loss of normal normal shape of a protein due shape of a protein due to heat to heat or other factoror other factor

IX. Enzymes are proteins that speed IX. Enzymes are proteins that speed up specific reactions in cellsup specific reactions in cells

A. Enzymes and Activation EnergyA. Enzymes and Activation Energy

1. To start a chemical 1. To start a chemical reaction, it is reaction, it is first necessary to first necessary to weaken chemical weaken chemical bonds in the bonds in the reactant molecules. reactant molecules. This This activation process requires that activation process requires that

the molecules absorb energy.the molecules absorb energy.

a) activation energy- the a) activation energy- the minimum amount of minimum amount of

energy energy required to trigger a required to trigger a chemical chemical reactionreaction

2. Catalysts- compounds that speed 2. Catalysts- compounds that speed up chemical reactionsup chemical reactions

a) The main catalysts of chemical a) The main catalysts of chemical reactions in organisms are reactions in organisms are

specialized specialized proteins called enzymes.proteins called enzymes.1) An enzyme doesn’t supply 1) An enzyme doesn’t supply

activation energy, but activation energy, but instead instead lowers the energy lowers the energy requirement requirement barrier so that barrier so that the reaction can the reaction can proceed at proceed at normal cell normal cell temperaturestemperatures

2) Each enzyme catalyzes a a 2) Each enzyme catalyzes a a specific kind of chemical specific kind of chemical

reactionreaction

Figure 5-15Figure 5-15The activation energy barrier is like a wall between The activation energy barrier is like a wall between two parts of a pond. If an enzyme lowers the wall, two parts of a pond. If an enzyme lowers the wall, more frogs have enough energy to reach the other more frogs have enough energy to reach the other side.side.

B. How enzymes workB. How enzymes work1. The substrate of each enzyme fits 1. The substrate of each enzyme fits the active site (like a key in a lock)the active site (like a key in a lock)

a) substrate- specific reactant a) substrate- specific reactant acted on by an enzymeacted on by an enzyme

b) active site- region of an b) active site- region of an enzyme into which a particular enzyme into which a particular substrate fitssubstrate fits

Figure 5-16Figure 5-16A substrate binds to an enzyme at an active site. The enzyme-A substrate binds to an enzyme at an active site. The enzyme-substrate interaction lowers the activation energy required substrate interaction lowers the activation energy required for the reaction to proceed. In this example, water is added to for the reaction to proceed. In this example, water is added to the weakened bond in sucrose, breaking sucrose into the weakened bond in sucrose, breaking sucrose into glucose and fructose.glucose and fructose.