Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings PowerPoint Lectures for Biology: Concepts and Connections, Fifth Edition – Campbell, Reece, Taylor, and Simon Lectures by Chris Romero Chapter 2 The Chemical Basis of Life

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Nature's Chemical Language Chemicals play an important role in all organisms The rattlebox moth provides a good example of chemicals used in mating and defense

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings ELEMENTS, ATOMS, AND MOLECULES 2.1 Living organisms are composed of about 25 chemical elements Elements are the basic chemical units that cannot be broken apart by typical chemical processes There are 92 naturally occurring elements – 25 are required by living organisms – 4 make up 96.3 of the human body

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings CONNECTION 2.2 Trace elements are common additives to food and water Trace elements are essential in minute quantities for proper biological functioning – Example: iodine is a trace element that prevents goiter – Many foods are fortified with trace elements and vitamins (which consist of two or more elements)

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 2.3 Elements can combine to form compounds Compounds contain two or more elements in a fixed ratio Different arrangements of the atoms of elements determine the unique properties of each compound The smallest unit of an element is an atom

LE 2-3 SodiumChlorine Sodium Chloride

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 2.4 Atoms consist of protons, neutrons, and electrons Subatomic particles – Protons and neutrons occupy the central region (nucleus) of an atom A proton has a single positive charge A neutron is electrically neutral – Electrons surround the nucleus An electron has a single negative charge

LE 2-4a Protons Neutrons Electrons Helium atom Mass number = Protons Neutrons Electrons Carbon atom Mass number = 12 Electron cloud Nucleus 2e – 6e –

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Differences in Elements – All the atoms of a particular element have the same number of protons – The number of protons-the atomic number- defines the element's unique properties – An atom's mass number (atomic mass) is the sum of its protons and neutrons

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Isotopes – Isotopes are atoms with the same atomic number but different atomic masses (different number of neutrons)

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings CONNECTION 2.5 Radioactive isotopes can help or harm us Radioactive isotopes are useful as tracers to study the fate of elements and molecules in living systems Radioactive tracer isotopes are often used in combination with sophisticated imaging instruments for medical diagnosis Uncontrolled exposure to radioactive material can harm living organisms

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 2.6 Electron arrangement determines the chemical properties of an atom Electrons in an atom are arranged in electron shells, which may contain different numbers of electrons

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The chemical reactivity of an atom depends on the number of electrons in the outer shell – Atoms whose outer shells are not full share or transfer electrons to other atoms, forming molecules – Two major types of chemical bonds between atoms form compounds Ionic bonds Covalent bonds

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 2.7 Ionic bonds are attractions between ions of opposite charge An ion is a charged atom that has lost or gained electrons in its outer shell – A positively charged ion (cation) is an atom that has lost an electron – A negatively charged ion (anion) is an atom that has gained an electron

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings An electrical attraction between ions with opposite charges results in an ionic bond Example: sodium chloride (table salt) results from an ionic bond between sodium and chlorine Animation: Ionic Bonds Animation: Ionic Bonds

LE 2-7a-1 Transfer of electron Na Sodium atom Cl Chlorine atom

LE 2-7a-2 Na  Sodium ion Cl  Chloride ion Sodium chloride (NaCl)

LE 2-7b Na  Cl 

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 2.8 Covalent bonds join atoms into molecules through electron sharing Covalently bonded atoms share one or more pairs of outer shell electrons, forming a molecule In a double bond, two pairs of electrons are shared Covalent bonds can be represented in various ways Animation: Covalent Bonds Animation: Covalent Bonds

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 2.9 Unequal electron sharing creates polar molecules A molecule whose covalently bonded atoms share electrons equally is nonpolar A molecule whose covalently bonded atoms share electrons unequally is polar – One part of the molecule is slightly positive, and one part is slightly negative

LE 2-9 A water molecule

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 2.10 Hydrogen bonds are weak bonds important in the chemistry of life The attraction between slightly positive regions and slightly negative regions creates hydrogen bonds Hydrogen bonding occurs in many biologically important compounds – Water – DNA – Proteins

LE 2-10 Hydrogen bond

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Animation: Water Structure Animation: Water Structure

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings WATER'S LIFE-SUPPORTING PROPERTIES 2.11 Hydrogen bonds make liquid water cohesive Cohesion is the tendency of molecules to stick together Surface tension results from the cohesion of water molecules Animation: Water Transport Animation: Water Transport

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 2.12 Water's hydrogen bonds moderate temperature When water is heated, the heat energy is absorbed, disrupting hydrogen bonds – The water stores a large amount of heat while warming only a few degrees When water is cooled, heat energy is released as hydrogen bonds are formed – The temperature of the water is lowered slowly

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Water also moderates temperature by evaporative cooling – The surface cools as the hottest molecules leave

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 2.13 Ice is less dense than liquid water Hydrogen bonds in ice create a stable, three- dimensional structure Ice is less dense than water, because it has fewer molecules in the same volume

LE 2-13 Hydrogen bond Ice Hydrogen bonds are stable Liquid water Hydrogen bonds constantly break and re-form

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 2.14 Water is the solvent of life A solution is a homogeneous mixture of a liquid solvent and one or more dissolved solutes Because water is a polar molecule, it readily forms solutions with many other polar and ionic compounds A solution in which water is the solvent is an aqueous solution

LE 2-14 Ion in solution Salt crystal

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 2.15 The chemistry of life is sensitive to acidic and basic conditions A compound that releases H+ ions in solution is an acid A compound that accepts H+ ions in solution is a base Acidity is measured on the pH scale from 0 (most acidic) to 14 (most basic) The pH of most cells is kept close to 7 (neutral) by buffers that resist pH change

LE 2-15 Acidic solution OH  HH HH HH HH HH HH HH HH HH HH HH HH HH Increasingly ACIDIC (Higher concentration of H  ) Neutral solution OH  HH HH Basic solution NEUTRAL  H     OH   pH scale Lemon juice, gastric juice Grapefruit juice, soft drink Tomato juice Human urine Pure water Human blood Seawater Milk of magnesia Household ammonia Household bleach Oven cleaner Increasingly BASIC (Lower concentration of H  )

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings CONNECTION 2.16 Acid precipitation threatens the environment Acid precipitation is formed when air pollutants from burning fossil fuels combine with water vapor in the air to form sulfuric and nitric acids Some ecosystems and structures are threatened by acid precipitation

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings CHEMICAL REACTIONS 2.17 Chemical reactions change the composition of matter In a chemical reaction, reactants interact, leading to products Atoms are rearranged, but the number of atoms stays constant on both sides of the equation

LE 2-17a  2 H 2 O2O2 2 H 2 O 

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Living cells carry out thousands of chemical reactions that rearrange matter in significant ways

LE 2-17b Beta-caroteneVitamin A (2 molecules)