Nature’s chemical language Elements, Atoms, Molecules, Chemical Bonds Water’s life supporting properties Chemical reaction CHAPTER 2 The Chemical Basis of Life By Dr. Par Mohammadian

Living organisms are composed of matter. Matter: Anything that occupies space and has mass (air, water, etc.). Matter is found on the Earth in three physical states: Solid, Liquid, Gas Matters are composed of elements. Element: A substance that cannot be broken down to other substances by ordinary chemical means Life requires about 25 chemical elements

Chemical elements combine in fixed ratios to form compounds Elements can combine to form compounds The smallest particle of an element is an atom Different elements have different types of atoms An atom is made up of protons and neutrons located in a central nucleus The nucleus is surrounded by electrons (The electrons orbit the nucleus) –The number of protons, the atomic number, determines which element it is –An atom’s mass number is the sum of the number of protons and neutrons

Electrons are arranged in shells –The outermost shell determines the chemical properties of an atom –In most atoms, a full outer shell holds eight electrons Electron arrangement determines the chemical properties of an atom

Chemical reactions enable atoms to give up or acquire electrons in order to complete their outer shells Chemical Bonds –These interactions usually result in atoms staying close together –The atoms are held together by chemical bonds: Ionic Bonds Covalent Bonds Hydrogen Bonds

Ionic bonds: Sharing of electrons Covalent bonds, the sharing of electrons, join atoms into molecules

Atoms in a covalently bonded molecule may share electrons equally, creating a nonpolar molecule If electrons are shared unequally, a polar molecule is created Water is a polar molecule THE PROPERTIES OF WATER

The charged regions on water molecules are attracted to the oppositely charged regions on nearby molecules –This attraction forms weak bonds called hydrogen bonds Water’s polarity leads to hydrogen bonding and other unusual properties Hydrogen bond

Water molecules stick together as a result of hydrogen bonding The Cohesion of Water Hydrogen bonds make liquid water cohesive Water’s hydrogen bonds moderate temperature

Like no other common substance, water exists in nature in all three physical states: –as a solid –as a liquid –as a gas Ice is less dense than liquid water

A solution is a liquid consisting of two or more substances evenly mixed Water as the Solvent of Life –The dissolving agent is called the solvent –The dissolved substance is called the solute aqueous solutions

Acidity is measured on the pH scale: –0-7 is acidic –8-14 is basic –Pure water and solutions that are neither basic nor acidic are neutral, with a pH of 7 The chemistry of life is sensitive to acidic and basic conditions Acid –A chemical compound that donates H + ions to solutions Base –A compound that accepts H + ions and removes them from solution

Cells are kept close to pH 7 by buffers Buffers are substances that resist pH change –They accept H + ions when they are in excess –They donate H + ions when they are depleted

Some ecosystems are threatened by acid precipitation Acid precipitation is formed when air pollutants from burning fossil fuels combine with water vapor in the air to form sulfuric and nitric acids Acid precipitation threatens the environment

Cells constantly rearrange molecules by breaking existing chemical bonds and forming new ones Chemical Reactions –Such changes in the chemical composition of matter are called chemical reactions –On the left side of the equation are the reactants, the starting materials –On the right side of the equation are the products, the end materials