Chemistry Review AP Biology. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin...

Chemistry ReviewAP Biology

• Matter is anything that takes up space and has mass.

• An element is a substance that cannot be broken down into other substances by chemical reactions.

Matter consists of chemical elements in pure form and in combinations called compounds

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

A compound is a substance consisting of two or more elements in a fixed ratio.

• Table salt (sodium chloride or NaCl) is a compound with equal numbers of chlorine and sodium atoms.

• While pure sodium is a metal and chlorine is a gas, their combination forms an edible compound, an emergent property.


Fig. 2.2

About 25 of the 92 natural elements are known to be essential for life.

• Four elements - carbon (C), oxygen (O), hydrogen (H), and nitrogen (N) - make up 96% of living matter.

• Most of the remaining 4% of an organism’s weight consists of phosphorus (P), sulfur (S), calcium (Ca), and potassium (K).

Life requires about 25 chemicalelements


Trace elements are required by an organism but only in minute quantities.

• Some trace elements, like iron (Fe), are required by all organisms.


– Other trace elements are required only by some species.• For example, a daily intake

of 0.15 milligrams of iodine is required for normal activity of the human thyroid gland.

Fig. 2.4

Each element consists of unique atoms.

An atom is the smallest unit of matter that still retains the properties of an element.

• Atoms are composed of even smaller parts, called subatomic particles.

• Neutrons • Protons• Electrons

Atomic structure determines the behavior of an element


Atomic Structure

Atomic Number• Represents number of protons in the nucleus

• Tells us the number of protons and the number of electrons that are found in a neutral atom of a specific element

Unless otherwise indicated, atoms have equal numbers of protons and electrons, so no net charge

Mass Number

• The sum of the number of protons and neutrons in the nucleus of an atom

• atomic weight of an atom, a measure of its mass, can be approximated by the mass number

Isotopes

• Two atoms of the same element that differ in the number of neutrons

• Most isotopes are stable; they do not tend to lose particles

http://www.earth.northwestern.edu/people/seth/107/Time/carbon.jpg

• The nuclei of some isotopes are unstable and decay spontaneously, emitting particles and energy.

• 14C is a one of these unstable or radioactive isotopes.

• When 14C decays, a neutron is converted to a proton and an electron.

• This converts 14C to 14N, changing the identity of that atom.

Radioactive Isotopes

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• Radioactive decay rates can be used to date fossils.

• Radioactive isotopes can be used to trace atoms in metabolism.

Radioactive isotopes have many applications in biological research

Radioactive isotopes are also used to diagnose medical disorders.

• the rate of excretion in the urine can be measured after injection into the blood of known quantity of radioactive isotope.

• radioactive tracers can be used with imaging instruments to monitor chemical processes in the body.


Fig. 2.7

• Energy is the ability to do work.

• Potential energy is the energy that matter stores because of its position or location.

Electrons of an atom may vary in the amount of energy that they possess.


• The first electron shell can hold only 2 electrons.

• The second shell can hold up to 8 electrons.

• The chemical behavior of an atom depends mostly on the number of electrons in its outermost shell, the valence shell.

Chemical behavior of an atom is determined by its electron configuration


http://hyperphysics.phy-astr.gsu.edu/hbase/solids/imgsol/valen.gif

• Atoms with incomplete valence shells interact by either sharing or transferring valence electrons.

• These interactions typically result in the atoms remaining close together, held by an attractions called chemical bonds.

• The strongest chemical bonds are covalent bonds and ionic bonds.

Atoms combine by chemical bonding to form molecules


Chemical Bonds

Non-Polar Covalent

• Sharing of a pair of valence electrons by two atoms

• Bond between two atoms of the same element

• Between atoms that have similar electronegativities

Polar Covalent• Electrons are not shared equally by the two atoms

• When one atom has a much higher electronegativity than the other

Ionic• When 2 atoms are so

unequal in their attraction for valence electrons that one atom strips an electron completely from the other

• When one atom has a much higher electronegativity than the other

Hydrogen bonds form when a hydrogen atom that is already covalently bonded to a strongly electronegative atom is attracted to another strongly electronegative atom.

Chemical Bonds


• In chemical reactions chemical bonds are broken and reformed, leading to new arrangements of atoms.

• The starting molecules in the process are called reactants and the end molecules are called products.

• In a chemical reaction, all of the atoms in the reactants must be accounted for in the products.

• The reactions must be “balanced.”

Chemical reactions make and break chemical bonds


ReferencesIllustrations credited to Pearson Education have been borrowed from BIOLOGY 6th Edition, by Campbell and Reece, ©2002. These images have been scanned from the originals by permission of the publisher. These illustrations may not be reproduced in any format for any purpose without express written permission from the publisher.

Ch. 3WATER AND THE FITNESS OF

THE ENVIRONMENTAP Biology

• In a water molecule two hydrogen atoms form single polar covalent bonds with an oxygen atom.

• Because oxygen is more electronegative, the region around oxygen has a partial negative charge.

• The region near the two hydrogen atoms has a partial positive charge.

• A water molecule is a polar molecule with opposite ends of the molecule with opposite charges.

The polarity of water molecules results in hydrogen bonding


Water has a variety of unusual properties because of attractions between these polar molecules• Water molecules are cohesive and that surface

tension makes air-water boundaries distinctive microhabitats

• Pure water freezes at 0C but is most dense at 4C. That is, solid water (ice) is less dense than cold liquid water

• Water has a high heat capacity• Water is a versatile solvent

Cohesion among water molecules plays a key role in the transport of

water against gravity in plants.• Water that evaporates from a leaf is replaced by water from vessels in

the leaf.

• Hydrogen bonds cause water molecules leaving the veins to tug on

molecules further down.

• This upward pull is transmitted to the roots.

• Adhesion, clinging

of one substance to

another, contributes

too, as water adheres

to the wall of the

vessels.

Organisms depend on the cohesion of water molecules


Surface tension, a measure of the force necessary to stretch or break the surface of a liquid, is related to cohesion.

• Water has a greater surface tension than most other liquids because hydrogen bonds among surface water molecules resist stretching or breaking the surface.

• Water behaves as if covered by an invisible film.

• Some animals can stand, walk, or run on water without breaking the surface.

Organisms depend on the cohesion of water molecules


Water is unusual because it is less dense as a solid than as a liquid.

• Most materials contract as they solidify, but water expands.

• At temperatures above 4oC, water behaves like other liquids, expanding when it warms and contracting when it cools.

• Water begins to freeze when its molecules are no longer moving vigorously enough to break their hydrogen bonds.

Oceans and lakes don’t freeze solid because ice floats


Therefore, ice floats on the cool water below.

This oddity has important consequences for life.

• If ice sank, eventually all ponds, lakes, and even the ocean would freeze solid.

• During the summer, only the upper few inches of the ocean would thaw.

• Instead, the surface layer of ice insulates liquid water below, preventing it from freezing and allowing life to exist under the frozen surface.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin CummingsFig. 3.6

• Water stabilizes air temperatures by absorbing heat from warmer air and releasing heat to cooler air.

• Water can absorb or release relatively large amounts of heat with only a slight change in its own temperature.

Water moderates temperatures onEarth


• Water stabilizes temperature because it has a high specific heat.

• The specific heat = amount of heat that must be absorbed or lost for 1g of that substance to change its temperature by 1oC.

• Water resists changes in temperature because it takes a lot of energy to speed up its molecules.

• Heat must be absorbed to break hydrogen bonds and is released when hydrogen bonds form.



• The impact of water’s high specific heat ranges from the level of the whole environment of Earth to that of individual organisms.

• A large body of water can absorb a large amount of heat from the sun in daytime and during the summer, while warming only a few degrees.

• At night and during the winter, the warm water will warm cooler air.

• The water that dominates the composition of biological organisms moderates changes in temperature better than if composed of a liquid with a lower specific heat.



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• The transformation of a molecule from a liquid to a gas is called vaporization or evaporation.

• This occurs when the molecule moves fast enough that it can overcome the attraction of other molecules in the liquid.

• Even in a low temperature liquid (low average kinetic energy), some molecules are moving fast enough to evaporate.

• Heating a liquid increases the average kinetic energy and increases the rate of evaporation.

• Evaporative cooling

Water moderates temperatures


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A liquid that is a completely homogeneous mixture of two or more substances is called a solution.

• A sugar cube in a glass of water will eventually dissolve to form a uniform mixture of sugar and water.

The dissolving agent is the solvent and the substance that is dissolved is the solute.

• In our example, water is the solvent and sugar the solute.

In an aqueous solution, water is the solvent.

Water is not a universal solvent, but it is very versatile because of the polarity of water molecules.

Water is the solvent of life


Water is an effective solvent because it so readily forms hydrogen bonds with charged and polar covalent molecules.

• For example, when a crystal of salt (NaCl) is placed in water, the Na+ cations form hydrogen bonds with partial negative oxygen regions of water molecules.

• The Cl- anions form hydrogen bonds with the partial positive hydrogen regions of water molecules.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 3.7

Any substance that has an affinity for water is hydrophilic.

Substances that have no affinity for water are hydrophobic.


Occasionally, a hydrogen atom shared by two water molecules shifts from one molecule to the other.

• The hydrogen atom leaves its electron behind and is transferred as a single proton - a hydrogen ion (H+).

• The water molecule that lost a proton is now a hydroxide ion (OH-).

• The water molecule with the extra proton is a hydronium ion (H3O+).

Introduction to pH


Unnumbered Fig. 3.47

An acid is a substance that increases the hydrogen ion concentration in a solution.

• When hydrochloric acid is added to water, hydrogen ions dissociate from chloride ions:

• HCl -> H+ + Cl-

Addition of an acid makes a solution more acidic.

Organisms are sensitive to changes inpH


Any substance that reduces the hydrogen ion concentration in a solution is a base.

Some bases reduce H+ directly by accepting hydrogen ions.

• Ammonia (NH3) acts as a base when the nitrogen’s unshared electron pair attracts a hydrogen ion from the solution, creating an ammonium in (NH4

+).

•NH3 + H+ <=> NH4+

Solutions with more OH- than H+are basic solutions.


In any solution the product of their H+ and OH- concentrations is constant at 10-14.

• [H+] [OH-] = 10-14

• In a neutral solution, [H+] = 10-7 M and [OH-] = 10-7

MAdding acid to a solution shifts the balance between H+ and OH- toward H+ and leads to a decline in OH-.

• If [H+] = 10-5 M, then [OH-] = 10-9 M• Hydroxide concentrations decline because some of additional acid combines with hydroxide to form water.

Adding a base does the opposite, increasing OH- concentration and dropping H+ concentration.


• In a neutral solution [H+] = 10-7 M, and the pH = 7.

• Values for pH decline as [H+] increase.

• While the pH scale is based on [H+], values for [OH-] can be easily calculated from the product relationship.


Fig. 3.9

The pH of a neutral solution is 7.

Acidic solutions have pH values less than 7 and basic solutions have pH values more than 7.

Most biological fluids have pH values in the range of 6 to 8.

• However, pH values in the human stomach can reach 2.

Each pH unit represents a tenfold difference in H+ and OH- concentrations.

• A small change in pH actually indicates a substantial change in H+ and OH- concentrations.


The chemical processes in the cell can be disrupted by changes to the H+ and OH- concentrations away from their normal values near pH 7.

To maintain cellular pH values at a constant level, biological fluids have buffers.

Buffers resist changes to the pH of a solution when H+ or OH- is added to the solution.

• Buffers accept hydrogen ions from the solution when they are in excess and donate hydrogen ions when they have been depleted.


Buffers typically consist of a weak acid and its corresponding base.

• One important buffer in human blood and other biological solutions is carbonic acid.

• The chemical equilibrium between carbonic acid and bicarbonate acts at a pH regulator.

• The equilibrium shifts left or right as other metabolic processes add or remove H+ from the solution.


CARBON AND THE MOLECULAR DIVERSITY OF LIFE

AP Biology

Ch. 4

With a total of 6 electrons, a carbon atom has 2 in the first shell and 4 in the second shell.

• Carbon usually completes its valence shell by sharing electrons with other atoms in four covalent bonds.

• This tetravalence by carbon makes large, complex molecules possible.

Carbon atoms are the most versatile building blocks of molecules


The electron configuration of carbon gives it compatibility to form covalent bonds with many different elements.

The valences of carbon and its partners can be viewed as the building code that governs the architecture of organic molecules.


Fig. 4.3

Carbon chains form the skeletons of most organic molecules.

• The skeletons may vary in length and may be straight, branched, or arranged in closed rings.

• The carbon skeletons may also include double bonds.

Variation in carbon skeletons contributes to the diversity of organic

molecules


• The components of organic molecules that are most commonly involved in chemical reactions are known as functional groups.

• Each functional groups behaves consistently from one organic molecule to another.

• The number and arrangement of functional groups help give each molecule its unique properties.

Functional groups contribute to the molecular diversity of life


In a hydroxyl group (-OH), a hydrogen atom forms a

polar covalent bond with an oxygen atom, which forms a

polar covalent bond to the carbon skeleton.• Because of these polar covalent bonds hydroxyl groups

improve the solubility of organic molecules.

• Organic compounds with hydroxyl groups are alcohols and

their names typically end in -ol.


A carbonyl group (>CO) consists of an oxygen atom joined to the carbon skeleton by a double bond.

• If the carbonyl group is on the end of the skeleton, the compound is an aldelhyde.

• If not, then the compound is a ketone.• Isomers with aldehydes versus ketones have different properties.


A carboxyl group (-COOH) consists of a carbon atom with a double bond to an oxygen atom and a single bond to a hydroxyl group.

• Compounds with carboxyl groups are carboxylic acids.• A carboxyl group acts as an acid because the combined

electronegativities of the two adjacent oxygen atoms increase the dissociation of hydrogen as an ion (H+).


An amino group (-NH2) consists of a nitrogen atom attached to two hydrogen atoms and the carbon skeleton.

• Organic compounds with amino groups are amines.• The amino group acts as a base because ammonia can pick up a hydrogen ion (H+) from the solution.

• Amino acids, the building blocks of proteins, have amino and carboxyl groups.


A sulfhydryl group (-SH) consists of a sulfur atom

bonded to a hydrogen atom and to the backbone.• This group resembles a hydroxyl group in shape.

• Organic molecules with sulfhydryl groups are thiols.

• Sulfhydryl groups help stabilize the structure of proteins.


A phosphate group (-OPO32-) consists of phosphorus

bound to four oxygen atoms (three with single bonds and one with a double bond).

• A phosphate group connects to the carbon backbone via one of its oxygen atoms.

• Phosphate groups are anions with two negative charges as two protons have dissociated from the oxygen atoms.

• One function of phosphate groups is to transfer energy between organic molecules.


THE STRUCTURE AND FUNCTION OF

MACROMOLECULEShttp://www.reciprocalnet.org/common/graphs/pepsin.jpg

Cells join smaller organic molecules together to form larger molecules.

These larger molecules, macromolecules, may be composed of thousands of atoms and weigh over 100,000 daltons.

The four major classes of macromolecules are: carbohydrates, lipids, proteins, and nucleic acids.

Introduction


Three of the four classes of macromolecules form chainlike molecules called polymers.

• Polymers consist of many similar or identical building blocks linked by covalent bonds.

The repeated units are small molecules called monomers.

• Some monomers have other functions of their own.

Most macromolecules are polymers


Monomers are connected by covalent bonds via a condensation reaction or dehydration reaction.

• One monomer provides a hydroxyl group and the other provides a hydrogen and together these form water.

• This process requires energy and is aided by enzymes.


Fig. 5.2a

The covalent bonds connecting monomers in a polymer are disassembled by hydrolysis.

• In hydrolysis as the covalent bond is broken a hydrogen atom and hydroxyl group from a split water molecule attaches where the covalent bond used to be.

• Hydrolysis reactions dominate the digestive process, guided by specific enzymes.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin CummingsFig. 5.2b

Carbohydrates include both sugars and polymers.The simplest carbohydrates are monosaccharides or simple sugars.Disaccharides, double sugars, consist of two monosaccharides joined by a condensation reaction.Polysaccharides are polymers of monosaccharides.

Introduction


Monosaccharides generally have molecular formulas that are some multiple of CH2O.

• For example, glucose has the formula C6H12O6.

• Most names for sugars end in -ose.

Sugars, the smallest carbohydrates serve as a source of fuel and carbon sources


Two monosaccharides can join with a glycosidic linkage to form a dissaccharide via dehydration.

• Maltose, malt sugar, is formed by joining two glucose molecules.

• Sucrose, table sugar, is formed by joining glucose and fructose and is the major transport form of sugars in plants.


Fig. 5.5a

Polysaccharides are polymers of hundreds to thousands of monosaccharides joined by glycosidic linkages.One function of polysaccharides is as an energy storage macromolecule that is hydrolyzed as needed.Other polysaccharides serve as building materials for the cell or whole organism.

Polysaccharides, the polymers of sugars, have storage and structural roles


Starch is a storage polysaccharide composed entirely of glucose monomers.

• One unbranched form of starch, amylose, forms a helix.

• Branched forms, like amylopectin, are more complex.


Fig. 5.6a

Plants store starch within plastids, including chloroplasts.Plants can store surplus glucose in starch and withdraw it when needed for energy or carbon.Animals that feed on plants, especially parts rich in starch, can also access this starch to support their own metabolism.


Animals also store glucose in a polysaccharide called glycogen.

• Glycogen is highly branched, like amylopectin.• Humans and other vertebrates store glycogen in the liver and muscles but only have about a one day supply.


Insert Fig. 5.6b - glycogenFig. 5.6b

Structural polysaccharides form strong building materials.Cellulose is a major component of the tough wall of plant cells.

• Cellulose is also a polymer of glucose monomers, but using beta rings.


Fig. 5.7c

Another important structural polysaccharide is chitin, used in the exoskeletons of arthropods (including insects, spiders, and crustaceans).

• Chitin is similar to cellulose, except that it contains a nitrogen-containing appendage on each glucose.

• Pure chitin is leathery, but the addition of calcium carbonate hardens the chitin.

Chitin also forms the structural support for the cell walls of many fungi.


Fig. 5.9

Lipids are an exception among macromolecules because they do not have polymers.The unifying feature of lipids is that they all have little or no affinity for water.

• This is because their structures are dominated by nonpolar covalent bonds.

Lipids are highly diverse in form and function.

Lipids Introduction


Although fats are not strictly polymers, they are large molecules assembled from smaller molecules by dehydration reactions.A fat is constructed from two kinds of smaller molecules, glycerol and fatty acids.

Fats store large amounts of energy



Fig. 5.10a

• Glycerol consists of a three-carbon skeleton with a hydroxyl group attached to each.

• A fatty acid consists of a carboxyl group attached to a long carbon skeleton, often 16 to 18 carbons long.

The many nonpolar C-H bonds in the long hydrocarbon skeleton make fats hydrophobic.In a fat, three fatty acids are joined to glycerol by an ester linkage, creating a triacylglycerol.


Fig. 5.10b

The three fatty acids in a fat can be the same or different.Fatty acids may vary in length (number of carbons) and in the number and locations of double bonds.

• If there are no carbon-carbon double bonds, then the molecule is a saturated fatty acid - a hydrogen at every possible position.


Fig. 5.11a

• If there are one or more carbon-carbon double bonds, then the molecule is an unsaturated fatty acid - formed by the removal of hydrogen atoms from the carbon skeleton.

• Saturated fatty acids are straight chains, but unsaturated fatty acids have a kink wherever there is a double bond.


Fig. 5.11b

Fats with saturated fatty acids are saturated fats.

• Most animal fats are saturated.• Saturated fats are solid at room temperature.• A diet rich in saturated fats may contribute to cardiovascular disease (atherosclerosis) through plaque deposits.

Fats with unsaturated fatty acids are unsaturated fats.

• Plant and fish fats, known as oils, are liquid are room temperature.

•The kinks provided by the double bonds prevent the molecules from packing tightly together.


The major function of fats is energy storage.• A gram of fat stores more than twice as much energy as a gram of a polysaccharide.

• Plants use starch for energy storage when mobility is not a concern but use oils when dispersal and packing is important, as in seeds.

• Humans and other mammals store fats as long-term energy reserves in adipose cells.

Fat also functions to cushion vital organs.A layer of fats can also function as insulation.

• This subcutaneous layer is especially thick in whales, seals, and most other marine mammals


Phospholipids have two fatty acids attached to glycerol and a phosphate group at the third position.

• The phosphate group carries a negative charge.

• Additional smaller groups may be attached to the phosphate group.

Phospholipids are major components of cell membranes


At the surface of a cell phospholipids are arranged as a bilayer.

• Again, the hydrophilic heads are on the outside in contact with the aqueous solution and the hydrophobic tails from the core.

• The phospholipid bilayer forms a barrier between the cell and the external environment.

They are the major component of membranes.


Fig. 5.12b

Steroids are lipids with a carbon skeleton consisting of four fused carbon rings.

• Different steroids are created by varying functional groups attached to the rings.

Steroids include cholesterol and certain hormones


Fig. 5.14

Cholesterol, an important steroid, is a component in animal cell membranes.Cholesterol is also the precursor from which all other steroids are synthesized.

• Many of these other steroids are hormones, including the vertebrate sex hormones.

While cholesterol is clearly an essential molecule, high levels of cholesterol in the blood may contribute to cardiovascular disease.


Proteins are instrumental in about everything that an organism does.

• These functions include structural support, storage, transport of other substances, intercellular signaling, movement, and defense against foreign substances.

• Proteins are the overwhelming enzymes in a cell and regulate metabolism by selectively accelerating chemical reactions.

Humans have tens of thousands of different proteins, each with their own structure and function.

Protein Introduction


Proteins are the most structurally complex molecules known.

• Each type of protein has a complex three-dimensional shape or conformation.

All protein polymers are constructed from the same set of 20 monomers, called amino acids.Polymers of proteins are called polypeptides.A protein consists of one or more polypeptides folded and coiled into a specific conformation.


Amino acids consist of four components attached to a central carbon, the alpha carbon.These components include a hydrogen atom, a carboxyl group, an amino group, and a variable R group (or side chain).

• Differences in R groups produce the 20 different amino acids.

• R group determines chemical properties

A polypeptide is a polymer of amino acids connected in a specific sequence


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Amino acids are joined together when a dehydration reaction removes a hydroxyl group from the carboxyl end of one amino acid and a hydrogen from the amino group of another.

• The resulting covalent bond is called a peptide bond.


Fig. 5.16

A functional protein consists of one or more polypeptides that have been precisely twisted, folded, and coiled into a unique shape. It is the order of amino acids that determines what the three-dimensional conformation will be.

A protein’s function depends on its specific conformation


Fig. 5.17

Three levels of structure: primary, secondary, and tertiary structure, are used to organize the folding within a single polypeptide.Quarternary structure arises when two or more polypeptides join to form a protein.

Protein Structure


A protein’s conformation can change in response to the physical and chemical conditions.

Alterations in pH, salt concentration, temperature, or other factors can unravel or denature a protein.

• These forces disrupt the hydrogen bonds, ionic bonds, and disulfide bridges that maintain the protein’s shape.

Some proteins can return to their functional shape after denaturation, but others cannot, especially in the crowded environment of the cell.


The amino acid sequence of a polypeptide is programmed by a gene.A gene consists of regions of DNA, a polymer of nucleic acids.DNA (and their genes) is passed by the mechanisms of inheritance.

Nucleic Acid Introduction


There are two types of nucleic acids: ribonucleic acid (RNA) and deoxyribonucleic acid (DNA).DNA provides direction for its own replication.DNA also directs RNA synthesis and, through RNA, controls protein synthesis.

Nucleic acids store and transmit hereditary information


Organisms inherit DNA from their parents.

• Each DNA molecule is very long and usually consists of hundreds to thousands of genes.

• When a cell reproduces itself by dividing, its DNA is copied and passed to the next generation of cells.


While DNA has the information for all the cell’s activities, it is not directly involved in the day to day operations of the cell.

• Proteins are responsible for implementing the instructions contained in DNA.

Each gene along a DNA molecule directs the synthesis of a specific type of messenger RNA molecule (mRNA).The mRNA interacts with the protein-synthesizing machinery to direct the ordering of amino acids in a polypeptide.


The flow of genetic information is from DNA -> RNA -> protein.

• Protein synthesis occurs in cellular structurescalled ribosomes.

• In eukaryotes, DNA is located in the nucleus, but most ribosomes are in the cytoplasm with mRNA as an intermediary.


Fig. 5.28

Nucleic acids are polymers of monomers called nucleotides.Each nucleotide consists of three parts: a nitrogen base, a pentose sugar, and a phosphate group

A nucleic acid strand is a polymer of nucleotides.


The nitrogen bases, rings of carbon and nitrogen, come in two types: purines and pyrimidines.

• Pyrimidines have a single six-membered ring.• The three different pyrimidines, cytosine (C), thymine (T), and uracil (U) differ in atoms attached to the ring.

• Purine have a six-membered ring joined to a five-membered ring.

• The two purines are adenine (A) and guanine (G).


The pentose joined to the nitrogen base is ribose in nucleotides of RNA and deoxyribose in DNA.

• The only difference between the sugars is the lack of an oxygen atom on carbon two in deoxyribose.

• The combination of a pentose and nucleic acid is a nucleoside.

The addition of a phosphate group creates a nucleoside monophosphate or nucleotide.


Polynucleotides are synthesized by connecting the sugars of one nucleotide to the phosphate of the next with a phosphodiester link.This creates a repeating backbone of sugar-phosphate units with the nitrogen bases as appendages.


The sequence of nitrogen bases along a DNA or mRNA polymer is unique for each gene.Genes are normally hundreds to thousands of nucleotides long.The number of possible combinations of the four DNA bases is limitless.The linear order of bases in a gene specifies the order of amino acids - the primary structure of a protein.The primary structure in turn determines three-dimensional conformation and function.


An RNA molecule is a single polynucleotide chain.DNA molecules have two polynucleotide strands that spiral around an imaginary axis to form a double helix.

• The double helix was first proposed as the structure of DNA in 1953 by James Watson and Francis Crick.

Inheritance is based on replication of the DNA double helix


Chemistry Review AP Biology. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin...

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