Upload
doantu
View
213
Download
1
Embed Size (px)
Citation preview
Maria Immaculata iwo,SF ITB
BAB 2BAB 2CHEMISTRY OF LIFECHEMISTRY OF LIFE
2
2.1 Basic Chemistry
Matter is anything that takes up space and has weight; it can be a solid, a liquid, or a gas.
Therefore, not only are we humans matter, but so are the water we drink and the air we breathe.
Maria Immaculata iwo,SF ITB
3
Elements and Atoms• It is even more surprising that over 90% of the human
body is composed of just four elements: carbon, nitrogen,
oxygen, and hydrogen.
• Every element has a name and a symbol;
for example,
- carbon has been assigned the atomic symbol C (Fig. 2.1a).
Some of the symbols we use for elements are derived from Latin. For example,
the symbol for sodium is Na because natrium in Latin means sodium.
• Elements are composed of tiny particles called atoms.
Maria Immaculata iwo,SF ITB
4
Figure 2.1 Elements and atoms. a. The atomic symbol, number, and weight are given for common elements in the body.b. The structure of carbon shows that an atom contains the subatomic particles called protons (p) and neutrons (n) in the nucleus (colored pink) and electrons (colored blue) in shells about the nucleus.
Maria Immaculata iwo,SF ITB
5
Molecules and Compounds
Atoms often bond with each other to form a chemical unit called a molecule.
A molecule can contain atoms of the same kind, as when an oxygen atom joins with another oxygen atom to form oxygen gas.
Or the atoms can be different, as when an oxygen atom joins with two hydrogen atoms to form water.
When the atoms are different, a compound results.
Maria Immaculata iwo,SF ITB
6
Two types of bonds join atoms:
- the ionic bond
can be associated with inorganic molecules, which constitute nonliving matter,
- the covalent bond.
can be associated with organic molecules, which are unique to living things.
Double and Triple Bonds
Besides a single bond, in which atoms share only a pair of electrons, a double or a triple bond can form.
In a double bond, atoms share two pairs of electrons, and in a triple bond, atoms share three pairs of electrons between them. Structural formula: O——C——O
Molecular formula: CO2
Maria Immaculata iwo,SF ITB
7
Figure 2.4 Ionic reaction. a. During the formation of sodium chloride, an electron is transferred from the sodium atom to the chlorine atom. At the completion of the reaction, each atom has eight electrons in the outer shell, but each also carries a charge as shown.
b. In a sodium chloride crystal, bonding between ions creates a three-dimensional lattice in which each Na ion is surrounded by six Cl ions, and each Cl is surrounded by six Na.
Maria immaculata iwo, sf itbMaria Immaculata iwo,SF ITB
8
Fig. 2.5 Covalent reactions.After a covalent reaction, each atom will have filled its outer shell by sharing electrons. To determine this, it is necessary to count the shared electrons as belonging to both bonded atoms. Oxygen and nitrogen are most stable with eight electrons in the outer shell. Hydrogen is most stable with two electrons in the outer shell.
Maria Immaculata iwo,SF ITB
9
Water, Acids, and Bases
• Water is the most abundant molecule in living organisms, usually making up about 60–70% of the total body weight.
• Even so, water is an inorganic molecule because it does not contain carbon atoms. Carbon atoms are common to organic molecules.
• Water is a polar molecule with negative and positive ends:
Maria immaculata iwo, sf itbMaria Immaculata iwo,SF ITB
10
Hydrogen BondsA hydrogen bond occurs whenever a covalently bonded hydrogen is positive and attracted to a negatively charged atom nearby.
A hydrogen bond is represented
by a dotted line because it is
relatively weak and can be broken
rather easily.
Figure 2.6 Hydrogen bonding between water molecules. The polarity of the water molecules causes hydrogen bonds (dotted lines) to form between the molecules.
Maria immaculata iwo, sf itbMaria Immaculata iwo,SF ITB
11
Properties of Water
Polarity and hydrogen bonding cause water to have many properties beneficial to life, including the three to be mentioned here.
1. Water is a solvent for polar (charged) molecules and thereby facilitates chemical reactions both outside and within our bodies.
– water is a solvent that facilitates chemical reactions.– Ions and molecules that interact with water are said to be
hydrophilic. – Nonionized and nonpolar molecules that do not interact
with water are said to be hydrophobic.
Maria immaculata iwo, sf itbMaria Immaculata iwo,SF ITB
12
Properties of Water
2. Water molecules are cohesive, and therefore liquids fill vessels, such as blood vessels.
Water molecules cling together because of hydrogen bonding, and yet water flows freely. This property allows dissolved and suspended molecules to be evenly distributed throughout a system.
Therefore, water is an excellent transport medium. Within our bodies, the blood that fills our arteries and veins is 92% water. Blood transports oxygen and nutrients to the cells and removes wastes such as carbon dioxide from the cells.
Maria immaculata iwo, sf itbMaria Immaculata iwo,SF ITB
13
Properties of Water
3. Water has a high heat of vaporization. Therefore, it absorbs much heat as it slowly rises, and gives off this heat as it slowly cools. It takes a large amount of heat to change water to steam. (Converting one gram of the hottest water to steam requires an input of 540 calories of heat energy.)
Water has a high heat of vaporization because hydrogen bonds must be broken before boiling occurs and water molecules vaporize—that is, evaporate into the environment. This property of water helps keep body temperature within normal limits.
Also, in a hot environment, we sweat; then the body cools as body heat is used to evaporate the sweat, which is mostly liquid water.
Maria Immaculata iwo,SF ITB
14
Acids and Bases
When water molecules dissociate (break up), they release an equal number of hydrogen ions (H) and hydroxide ions (OH):
Only a few water molecules at a time dissociate, and the actual number of H and OH is very small
(1x 107 moles/liter).Maria Immaculata iwo,SF ITB
15
Acids and Bases
Acids are substances that dissociate in water, releasing hydrogen ions (H). For example,
an important inorganic acid is hydrochloric acid (HCl), which dissociates in this manner:
Dissociation is almost complete; therefore, HCl is called a strong acid.
If hydrochloric acid is added to a beaker of water, the number of hydrogen ions (H) increases greatly.
Lemon juice, vinegar, tomatoes, and coffee are all acidic solutions. Maria Immaculata iwo,SF ITB
16
Acids and Bases
Bases are substances that either take up hydrogen ions (H) or release hydroxide ions (OH).
For example, an important inorganic base is sodium hydroxide (NaOH), which dissociates in this manner:
Dissociation is almost complete; therefore, sodium hydroxide is called a strong base.
If sodium hydroxide is added to a beaker of water, the number of hydroxide ions increases.
Milk of magnesia and ammonia are common basic solutions.Maria Immaculata iwo,SF ITB
17
pH Scale The pH scale, which ranges from 0 to 14, is used to indicate the acidity and basicity (alkalinity) of a solution.
pH 7, which is the pH of water, is neutral pH because water releases an equal number of hydrogen ions (H) and hydroxide ions (OH).
Any pH above 7 is a base, with more hydroxide ions than hydrogen ions.
Any pH below 7 is an acid, with more hydrogen ions than hydroxide ions. q As we move toward a higher pH, each unit has 10 times the basicity of the
previous unit, and as we move toward a lower pH, each unit has 10 times the acidity of the previous unit.
This means that even a small change in pH represents a large change in the proportional number of hydrogen and hydroxide ions in the body.
Maria Immaculata iwo,SF ITB
18
Figure 2.7 The pH scale. The proportionate amount of hydrogen ions to hydroxide ions is indicated by the diagonal line. Any solution with a pH above 7 is basic, while any solution with a pH below 7 is acidic.Maria Immaculata iwo,SF ITB
19
The pH of body fluids needs to be maintained within a narrow range, or else health suffers.
The pH of our blood when we are healthy is always about 7.4 — that is, just slightly basic (alkaline).
If the pH value drops below 7.35, the person is said to have acidosis; if it rises above 7.45, the condition is called alkalosis.
The pH stability is normally possible because the body has built-in mechanisms to prevent pH changes. Buffers are the most important of these mechanisms.
Buffers help keep the pH within normal limits because they are chemicals or combinations of chemicals that take up excess hydrogen ions (H) or hydroxide ions (OH).
For example,
the combination of carbonic acid (H2CO3) and the bicarbonate ion [HCO3-] helps keep the pH of the blood relatively constant because carbonic acid can dissociate to release hydrogen ions, while the bicarbonate ion can take them up!
Maria Immaculata iwo,SF ITB
20
Electrolytes
As we have seen, salts, acids, and bases are molecules that dissociate; that is, they ionize in water.
For example,
when a salt such as sodium chloride is put in water, the Na+ ion separates from the Cl- ion.
Substances that release ions when put into water are called electrolytes, because the ions can conduct an electrical current.
The electrolyte balance in the blood and body tissues is important for good health because it affects the functioning of vital organs such as the heart and the brain.
Maria Immaculata iwo,SF ITB
21
Molecules of Life
Four categories of molecules, called carbohydrates, lipids, proteins, and nucleic acids, are unique to cells.
They are called macromolecules because each is composed of many subunits:
Maria Immaculata iwo,SF ITB
22
• During synthesis of macromolecules, the cell uses a dehydration reaction, so called because an —OH (hydroxyl group) and an —H (hydrogen atom)—the equivalent of a water molecule—are removed as the molecule forms (Fig. 2.8a).
• To break up macromolecules, the cell uses a hydrolysis reaction, in which the components of water are added (Fig. 2.8b).
Maria Immaculata iwo,SF ITB
23
Figure 2.8 Synthesis and degradation of macromolecules.
a. In cells, synthesis often occurs when subunits bond following a dehydration reaction (removal of H2O).
b. Degradation occurs when the subunits in a macromolecule separate after a hydrolysis reaction (addition of H2O).
Maria Immaculata iwo,SF ITB
24
Carbohydrates
Carbohydrates, like all organic molecules, always contain carbon (C) and hydrogen (H) atoms.
Carbohydrate molecules are characterized by the presence of the atomic grouping H—C—OH, in which the ratio of hydrogen atoms (H) to oxygen atoms (O) is approximately 2:1.
Because this ratio is the same as the ratio in water, the name “hydrates of carbon” seems appropriate.
Carbohydrates first and foremost function for quick, short-term energy storage in all organisms, including humans.
Figure 2.9 shows some foods that are rich in carbohydrates.
Maria Immaculata iwo,SF ITB
25
Figure 2.9
Common foods.
Carbohydrates such as bread and pasta are digested to sugars;
Lipids such as oils are digested to glycerol and fatty acids
Proteins such as meat are digested to amino acids.
Cells use these subunit molecules to build their own macromolecules.
Maria Immaculata iwo,SF ITB
26
Simple Carbohydrates• If the number of carbon atoms in a carbohydrate is low
(between three and seven), it is called a simple sugar, or monosaccharide.
• The designation pentose means a 5-carbon sugar, and the designation hexose means a 6-carbon sugar.
• Glucose, the hexose our bodies use as an immediate source of energy, can be written in any one of these ways:
Maria Immaculata iwo,SF ITB
27
Other common hexoses are fructose, found in fruits, and galactose, a constituent of milk.
A disaccharide (di, two; saccharide, sugar) is made by joining only two monosaccharides together by a dehydration reaction (see Fig. 2.8a).
Maltose is a disaccharide that contains
two glucose molecules:
When glucose and fructose join, the disaccharide sucrose forms. • Sucrose, which is ordinarily derived from sugarcane and sugar
beets, is commonly known as table sugar.
Complex Carbohydrates (Polysaccharides)• Macromolecules such as starch, glycogen, and cellulose are
polysaccharides that contain many glucose units.
Maria Immaculata iwo,SF ITB
28
Starch and GlycogenStarch and glycogen are storage forms of glucose in plants and animals.
Starch has fewer side branches, or chains of glucose that branch off from the main chain, than does glycogen, as shown in Fig. 2.10 and 2.11.
Flour, usually acquired by grinding wheat and used for baking, is high in starch, and so are potatoes.
After we eat starchy foods such as potatoes, bread, and cake, glucose enters the bloodstream, and the liver stores glucose as glycogen.
In between eating, the liver releases glucose so that the blood glucose concentration is always about 0.1%.
If blood contains more glucose, it spills over into the urine, signaling that the condition diabetes mellitus exists.
Maria Immaculata iwo,SF ITB
29
Figure 2.10
Starch structure and function.
Starch has straight chains of glucose molecules.
Some chains are also branched, as indicated.
The electron micrograph shows starch granules in potato cells.
Starch is the storage form of glucose in plants. Maria Immaculata iwo,SF ITB
30
CelluloseThe polysaccharide cellulose is found in plant cell walls.
In cellulose, the glucose units are joined by a slightly different type of linkage from that in starch or glycogen.
Humans are unable to digest foods containing this type of linkage; therefore, cellulose largely passes through our digestive tract as fiber, or roughage.
It is believed that fiber in the diet is necessary to good health, and it may even help prevent colon cancer.
Maria Immaculata iwo,SF ITB
31
Figure 2.11 Glycogen structure and function.
Glycogen is more branched than starch.
The electron micrograph shows glycogen granules in liver cells.
Glycogen is the storage form of glucose in humans.
Maria Immaculata iwo,SF ITB
32
LipidsLipids contain more energy per gram than other biological molecules, and some function as long-term energy storage molecules in organisms.
Others form a membrane that separates a cell from its environment and has inner compartments as well.
Steroids are a large class of lipids that includes, among other molecules, the sex hormones.
Lipids are diverse in structure and function, but they have a common characteristic: They do not dissolve in water.
Their low solubility in water is due to an absence of polar groups.
They contain little oxygen and consist mostly of carbon and
hydrogen atoms.Maria Immaculata iwo,SF ITB
33
Fats and Oils
The most familiar lipids are those found in fats and oils.
Fats, which are usually of animal origin (e.g., lard and butter), are solid at room temperature.
Oils, which are usually of plant origin (e.g., corn oil and soybean oil), are liquid at room temperature.
Fat has several functions in the body:• It is used for long-term energy storage, it insulates against heat
loss, and it forms a protective cushion around major organs.• Fats and oils form when one glycerol molecule reacts with three
fatty acid molecules (Fig. 2.12). • A fat is sometimes called a triglyceride, because of its three-part
structure, or a neutral fat, because the molecule is nonpolar and carries no charge.
LDL HDL
Maria Immaculata iwo,SF ITB
34
Emulsification
Emulsifiers can cause fats to mix with water. They contain molecules with a nonpolar end and a polar end. The molecules position themselves about an oil droplet so that their nonpolar ends project. Now the droplet disperses in water, which means that emulsification has occurred.
• Emulsification takes place when dirty clothes are washed with soaps or detergents. Also, prior to the digestion of fatty foods, fats are emulsified by bile.
• The gallbladder stores bile for emulsifying fats prior to the digestive process.
Maria Immaculata iwo,SF ITB
35
Saturated and Unsaturated Fatty AcidsA fatty acid is a carbon–hydrogen chain that ends with the acidic group —COOH (Fig. 2.12).
Most of the fatty acids in cells contain 16 or 18 carbon atoms per molecule, although smaller ones with fewer carbons are also known.
Fatty acids are either saturated or unsaturated.
Saturated fatty acids have only single covalent bonds because the carbon chain is saturated, so to speak, with all the hydrogens it can hold.
Saturated fatty acids account for the solid nature at room temperature of fats such as lard and butter.
Maria Immaculata iwo,SF ITB
36
Unsaturated fatty acids have double bonds between carbon atoms wherever fewer than two hydrogens are bonded to a carbon atom.
Unsaturated fatty acids account for the liquid nature of vegetable oils at room temperature.
Hydrogenation of vegetable oils can convert them to margarine and other products.
Maria Immaculata iwo,SF ITB
37
Figure 2.12 Synthesis and degradation of a fat molecule.
Fatty acids can be saturated (no double bonds between carbon atoms) or unsaturated (have double bonds, colored yellow, between carbon atoms). When a fat molecule forms, three fatty acids combine with glycerol, and three water molecules are produced. Maria Immaculata iwo,SF ITB
38
PhospholipidsPhospholipids, as their name implies, contain a phosphate group (Fig. 2.13).
Essentially, they are constructed like fats, except that in place of the third fatty acid, there is a phosphate group or a grouping that contains both phosphate and nitrogen.
Phospholipid molecules are not electrically neutral, as are fats, because the phosphate and nitrogen containing groups are ionized.
They form the so-called hydrophilic head of the molecule, while the rest of the molecule becomes the hydrophobic tails.
Phospholipids are the backbone of cellular membranes; they spontaneously form a bilayer in which the hydrophilic heads face outward toward watery solutions and the tails form the hydrophobic interior.
Maria Immaculata iwo,SF ITB
39
Figure 2.13 Phospholipid structure and function.
a. Phospholipids are structured like fats, but one fatty acid is replaced by a polar phosphate group.
b. Therefore, the head is polar while the tails are nonpolar.
c. This causes the molecule to arrange itself as shown when exposed to water.
Maria Immaculata iwo,SF ITB
40
SteroidsSteroids are lipids that have an entirely different structure from those of fats.
Steroid molecules have a backbone of four fused carbon rings. Each one differs primarily by the functional groups attached to the rings.
Cholesterol is a component of an animal cell’s outer membrane and is the precursor of several other steroids, such as the sex hormones estrogen and testosterone.
The male sex hormone, testosterone, is formed primarily in the testes, and the female sex hormone, estrogen, is formed primarily in the ovaries.
Testosterone and estrogen differ only by the functional groups attached to the same carbon backbone, yet they have a profound effect on the body and on our sexuality (Fig. 2.14a,b).
Testosterone is a steroid
that causes males to have
greater muscle strength
than females.
Maria Immaculata iwo,SF ITB
41
ProteinsProteins perform a myriad of functions, including the following:
• Proteins such as collagen and keratin (which makes up hair and nails) are fibrous structural proteins that lend support to ligaments,
• Many hormones, which are messengers that influence cellular metabolism, are proteins.
• The proteins actin and myosin account for the movement of cells and the ability of our muscles to contract.
• Some proteins transport molecules in the blood; for example, hemoglobin is a complex protein in our blood that transports oxygen.
• Antibodies in blood and other body fluids are proteins that combine with pathogens or their toxins.
• Enzymes are globular proteins that speed chemical reactions.
Maria Immaculata iwo,SF ITB
42
Structure of Proteins
Proteins are macromolecules composed of amino acid subunits.
An amino acid has a central carbon atom bonded to a hydrogen atom and three groups.
The name of the molecule is appropriate because one of these groups is an amino group and another is an acidic group.
The third group is called an R group because it is the Remainder of the molecule (Fig. 2.15a).
Amino acids differ from one another by their R group; the R group varies from having a single carbon to being a complicated ring structure.
When two amino acids join, a dipeptide results; a polypeptide is a chain of amino acids (Fig. 2.15b).
Maria Immaculata iwo,SF ITB
43
Figure 2.15 Levels of polypeptide structure.
a. Amino acids are the subunits of polypeptides. Note that an amino acid contains nitrogen.
b. Polypeptides differ by the sequence of their amino acids, which are joined by peptide bonds.
c. A polypeptide often twists to become a coil due to hydrogen bonding between members of the peptide bonds.
d. The third level of polypeptide structure is due to various types of bonding between the R groups of the amino acids.
Maria Immaculata iwo,SF ITB
44
Enzymatic Reactions
Metabolism is the sum of all the chemical reactions that occur in a cell.
An enzyme is a protein molecule that functions as an organic catalyst to speed a particular metabolic reaction.
The energy that must be supplied is called the energy of activation.
In the body, enzymes lower the energy of activation by forming a complex with particular molecules.
In a cell, an enzyme brings together certain molecules and causes them to react with one another.• Enzymes are proteins necessary to metabolism.
Maria Immaculata iwo,SF ITB
45
Enzyme-Substrate ComplexIn any reaction, the molecules that interact are called reactants, while the substances that form as a result of the reaction are the products.
The reactants in an enzymatic reaction are its substrate(s).
Enzymes are often named for their substrate(s); for example, maltase is the enzyme that digests maltose.
Enzymes have a specific region, called an active site, where the reaction occurs.
An enzyme’s specificity is caused by the shape of the active site, where the enzyme and its substrate(s) fit together, much like pieces of a jigsaw puzzle (Fig. 2.16). • After a reaction is complete and the products are released, the enzyme is
ready to catalyze its reaction again:
E S → ES → E P
(where E = enzyme, S = substrate, ES = enzyme-substrate complex,
and P = product).Maria Immaculata iwo,SF ITB
46
Figure 2.16 Enzymatic action. An enzyme has an active site, where the substrates come together and react. The products are released, and the enzyme is free to act again.
a. In synthesis, the substrates join to produce a larger product.
b. In degradation, the substrate breaks down to smaller products.
Maria Immaculata iwo,SF ITB
47
Types of Reactions
• Certain types of chemical reactions are common to metabolism.
– Synthesis Reactions
– Degradation Reactions
– Replacement Reactions
Maria Immaculata iwo,SF ITB
48
Synthesis Reactions
During synthesis reactions, two or more reactants combine to form a larger and more complex product (Fig. 2.16a).
The dehydration synthesis reaction i.e., the joining of subunits to form a macromolecule, is an example of a synthesis reaction.
When glucose molecules join in the liver, forming glycogen, a synthesis reaction has occurred.
Notice
that synthesis reactions always involve bond formation and therefore an input of energy.
Maria Immaculata iwo,SF ITB
49
Degradation Reactions
During degradation reactions, a larger and more complex molecule breaks down into smaller, simpler products (Fig. 2.16b).
The hydrolysis reactions that break down macromolecules into their subunits are examples of degradation reactions, also called decomposition reactions.• When protein is digested to amino acids in the
stomach, a degradation reaction has occurred.
Maria Immaculata iwo,SF ITB
50
Replacement Reactions Replacement reactions involve both degradation and synthesis.
For example,
when ADP joins with inorganic phosphate, P, and ATP forms, the last hydrogen in ADP is replaced by a P (see Fig. 2.18).
The P loses a hydroxyl group.
The hydrogen and hydroxyl group join to become water.
Maria Immaculata iwo,SF ITB
51
Figure 2.18 ATP reaction. ATP, the universal energy currency of cells, is composed of adenosine and three phosphate groups (called a triphosphate).
When cells require energy, ATP undergoes hydrolysis, producing ADP, P , with the release of energy. (The P stands for inorganic phosphate.)
Later, ATP is rebuilt when energy is supplied and ADP joins with P .Maria Immaculata iwo,SF ITB
52
Nucleic Acids
• Nucleic acids are huge macromolecules composed of nucleotides.• Every nucleotide is a molecular complex of three types of subunit
molecules
a phosphate (phosphoric acid),
a pentose sugar,
a nitrogen-containing base:
Maria Immaculata iwo,SF ITB
53
Nucleic Acids
Nucleic acids store hereditary information that determines which proteins a cell will have.
Two classes of nucleic acids are in cells:
DNA (deoxyribonucleic acid)
RNA (ribonucleic acid)• DNA makes up the hereditary units called genes. • Genes pass on from generation to generation the instructions for
replicating DNA, making RNA, and joining amino acids to form the proteins of a cell.
• RNA is an intermediary in the process of protein synthesis, conveying information from
• DNA regarding the amino acid sequence in proteins.Maria Immaculata iwo,SF ITB
54
Nucleic Acids
The nucleotides in DNA contain the 5-carbon sugar deoxyribose;
the nucleotides in RNA contain the sugar ribose.
This difference accounts for their respective names.
there are four different types of bases in DNA: – A adenine, – T thymine, – G guanine, – C cytosine.
• The base can have two rings (adenine or guanine) or one ring (thymine or cytosine).
Maria Immaculata iwo,SF ITB
55
Nucleic Acids
In RNA,
the base uracil replaces the base thymine.• These structures are nitrogen-containing bases—that is,
a nitrogen atom is a part of the ring. • Like other bases, the presence of the nitrogen-
containing base in DNA and RNA raises the pH of a solution.
Maria Immaculata iwo,SF ITB
56
Figure 2.17 Overview of DNA structure.
a. Double helix. b. Complementary base pairing between strands.
c. Ladder configuration.
Notice that the uprights are composed of phosphate and sugar molecules and that the rungs are complementary paired bases.
Maria Immaculata iwo,SF ITB
57Maria Immaculata iwo,SF ITB
58
Nucleic Acids• Both DNA and RNA are polymers of nucleotides; only
DNA is double stranded.
• DNA makes up the genes, and along with RNA, specifies protein synthesis.
Maria Immaculata iwo,SF ITB
59
ATP (Adenosine Triphosphate)
Individual nucleotides can have metabolic functions in cells.• Some nucleotides are important in energy transfer. • When adenosine (adenine plus ribose) is modified by the
addition of three phosphate groups, it becomes ATP (adenosine triphosphate), the primary energy carrier in cells.
• Cells require a constant supply of ATP. – To obtain it, they break down glucose and convert the energy that is
released into ATP molecules.
• The amount of energy in ATP is just right for more chemical reactions in cells.
• ATP is sometimes called a high-energy molecule because the last two phosphate bonds are unstable and easily broken.
Maria Immaculata iwo,SF ITB
60
• Usually in cells, the terminal phosphate bond is hydrolyzed, leaving the molecule ADP (adenosine diphosphate) and a molecule of inorganic phosphate, P (Fig. 2.18).
• The breakdown of ATP releases energy because the products of hydrolysis (ADP and P ) are more stable than ATP.
• After ATP breaks down and the energy is used for a cellular purpose, ATP is rebuilt by the addition of P to ADP again; this can be seen by reading Figure 2.18 from right to left.
• There is enough energy in one glucose molecule to build 36 ATP molecules in this way.
• Homeostasis is only possible because cells continually produce and use ATP molecules.
• ATP is the energy “currency” of cells because its breakdown supplies energy for many cellular processes.
Maria Immaculata iwo,SF ITB