The Chemical Basis of Life Chapter 2 The Chemical Basis of Life

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

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

2.2 Trace elements are common additives to food and water 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)

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 Sodium Chlorine Sodium Chloride

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

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 Isotopes Isotopes are atoms with the same atomic number but different atomic masses (different number of neutrons)

2.5 Radioactive isotopes can help or harm us 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

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

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

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

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 Transfer of electron Na Sodium atom Cl Chlorine atom

LE 2-7b Na+ Cl-

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

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

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

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

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

Water also moderates temperature by evaporative cooling The surface cools as the hottest molecules leave

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

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

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

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

2.16 Acid precipitation threatens the environment 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

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 + 2 H2 O2 2 H2O

Living cells carry out thousands of chemical reactions that rearrange matter in significant ways Beta-carotene Vitamin A (2 molecules)