Bài giảng 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
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
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)
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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 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 2 2 2 Protons Neutrons Electrons Helium atom Mass number = 4 6 6 6 Protons Neutrons Electrons Carbon atom Mass number = 12 Electron cloud Nucleus 2e – 6e – 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) 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 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 - 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 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 Animation: Water Structure 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 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 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 LE 2-15 Acidic solution OH H + H + H + H + OH H + H + H + OH OH OH OH OH H + H + H + H + H + H + Increasingly ACIDIC (Higher concentration of H + ) Neutral solution OH OH OH OH OH OH 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 + ) 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 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 O 2 2 H 2 O + Living cells carry out thousands of chemical reactions that rearrange matter in significant ways LE 2-17b Beta-carotene Vitamin A (2 molecules)
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