2.1. KEY CONCEPT All living things are based on atoms and their interactions. 34 Reinforcement Unit 1 Resource Book

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1 2.1 ATOMS, IONS, AND MOLECULES KEY CONCEPT All living things are based on atoms and their interactions. All matter, whether living or nonliving, is made of the same tiny building blocks, called atoms. An atom is the smallest basic unit of matter. All atoms have the same basic structure, composed of three smaller particles. Protons: A proton is a positively charged particle in an atom s nucleus. The nucleus is the dense center of an atom. Neutrons: A neutron has no electrical charge, has about the same mass as a proton, andisalsofoundinanatom snucleus. Electrons: An electron is a negatively charged particle found outside the nucleus. Electrons are much smaller than either protons or neutrons. Different types of atoms are called elements, whichcannotbebrokendownbyordinary chemical means. Which element an atom is depends on the number of protons in the atom s nucleus. For example, all hydrogen atoms have one proton, and all oxygen atoms have 16 protons. Only about 25 different elements are found in organisms. Atoms of different elements can link, or bond, together to form compounds. Atoms form bonds in two ways. Ionic bonds: An ion is an atom that has gained or lost one or more electrons. Some atoms form positive ions, which happens when an atom loses electrons. Other atoms form negative ions, which happens when an atom gains electrons. An ionic bond forms through the electrical force between oppositely charged ions. Covalent bonds: A covalent bond forms when atoms share one or more pairs of electrons. A molecule is two or more atoms that are held together by covalent bonds. 1. What are the parts of an atom? 2. What makes atoms of one element different from the atoms of another element? 3. How are ionic bonds and covalent bonds different? 34 Unit 1 Resource Book

2 2.2 PROPERTIES OF WATER KEY CONCEPT Water s unique properties allow life to exist on Earth. The structure of the water molecule gives water unique properties. Water is a polar molecule, which means that it has a region with a slight negative charge (the oxygen atom), and a region with a slight positive charge (the hydrogen atoms). The oppositely charged regions of water molecules interact to form hydrogen bonds. A hydrogen bond is an attraction between a slightly positive hydrogen atom and a slightly negative atom. Hydrogen bonds are responsible for several important properties of water. High specific heat: Water resists changes in temperature; it must absorb a large amount of heat energy to increase in temperature. Cohesion: The attraction among molecules of a substance is called cohesion. Cohesion due to hydrogen bonds makes water molecules stick together. Adhesion: The attraction among molecules of different substances is called adhesion. Water molecules stick to many other materials because of hydrogen bonds. Many compounds that are important for life dissolve in water. Water is the largest component of cells interiors, and chemical reactions in the cell take place in this water. When one substance dissolves in another, a solution is formed. The substance present in the greatest amount is called the solvent. Substances that are present in lower amounts and dissolve in the solvent are called solutes. Polar solvents, such as water, dissolve polar molecules and ions. When some substances dissolve in water they break up into ions. A compound that releases a hydrogen ion (a proton) when it dissolves in water is an acid. Bases are compounds that remove, or accept, hydrogen ions. A solution s acidity, or its hydrogen ion concentration, is measured on the ph scale. An acid has a low ph (ph below 7) and a high hydrogen ion concentration. A base has a high ph (ph above 7) and a low hydrogen ion concentration. Organisms must maintain a stable ph. Even a small change in ph can disrupt many biological processes. 1. How do hydrogen bonds form? 2. What are the two parts of a solution? 3. What does ph measure? 38 Unit 1 Resource Book

3 2.3 CARBON-BASED MOLECULES KEY CONCEPT Carbon-based molecules are the foundation of life. Carbon atoms are the basis of most molecules that make up living things. Many carbon-based molecules are large molecules called polymers that are made of many smaller, repeating molecules called monomers. There are four main types of carbon-based molecules in living things. Carbohydrates includesugarsandstarches,andareoftenbrokendownasasource of chemical energy for cells. Some carbohydrates are part of cell structure, such as cellulose, which makes up plant cell walls. Lipids include fats and oils and, like carbohydrates, are often broken down as a source of chemical energy for cells. One type of lipid, called a phospholipid, makes up most of all cell membranes. Proteins have a large number of structures and functions. Some proteins are needed for muscle movement; another protein, called hemoglobin, transports oxygen in blood. Another type of proteins, called enzymes, speed up chemical reactions in cells. Nucleic acids are molecules that store genetic information and build proteins. DNA stores genetic information in cells, and RNA helps to build the proteins for which DNA codes. Type of Molecule Functions Example Carbohydrate Lipid Protein Nucleic acid Unit 1 Resource Book

4 2.4 CHEMICAL REACTIONS KEY CONCEPT Life depends on chemical reactions. At the most fundamental level, every process that takes place in an organism depends on chemical reactions. In a chemical reaction, substances are changed into different substances by the breaking and forming of chemical bonds. The substances that are present at the start of a chemical reaction, and are changed by the reaction, are called reactants. The substances that are formed by a chemical reaction are the products. Chemical bonds must be broken in the reactants and new ones must be formed in the products. Energy must be added to break chemical bonds. In contrast, energy is always released when new bonds form. The amount of energy needed to break a bond, or the amount of energy released when a bond forms, is called bond energy. All chemical reactions require the input of at least a small amount of energy in order for bonds to break in the reactants and for the reaction to start. The energy needed to start a chemical reaction is the activation energy. In general, there are two types of energy changes that can occur during a chemical reaction. Exothermic reaction: An exothermic chemical reaction releases more energy than it absorbs. The bonds that are broken in the reactants of an exothermic reaction have a higher bond energy than the new bonds that form in the products. Energy is usually released as heat or light. Endothermic reaction: An endothermic chemical reaction absorbs more energy than it releases. The bonds that are broken in the reactants of an endothermic reaction have a lower bond energy than the new bonds that form in the products. The energy that is absorbed makes up for the difference. 1. What are the two parts of a chemical reaction? 2. What is activation energy? 3. How are exothermic reactions different from endothermic reactions? 46 Unit 1 Resource Book

5 2.5 ENZYMES KEY CONCEPT Enzymes are catalysts for chemical reactions in living things. Chemical reactions require the addition of energy, called activation energy, to take place. Evenifachemicalreactionstarts,itmaynothappenveryquickly. However,boththe activation energy and the rate of a chemical reaction can be changed by a chemical catalyst. A catalyst is a substance that decreases the activation energy for a reaction and increases the rate of the reaction. In living things, enzymes are catalysts for chemical reactions. Almost all enzymes are proteins, and almost every process in living things needs enzymes. The function of each enzyme depends on its structure. A change in biological conditions within an organism can affect the shape of an enzyme, which can decrease or prevent an enzyme from working properly. For example, enzymes function best in a small range around an organism s normal temperature and ph. The shape of an enzyme is important because it allows only certain molecules to bind to the enzyme. The specific molecules that an enzyme acts on are called substrates. One waytothinkofenzymefunctioniscalledthelock-and-keymodel.substratesbindtoan enzyme like the way in which a key fits into a lock. If an enzyme s structure changes, the substrates cannot bind to the enzyme. Substrates bind to an enzyme. The enzyme brings molecules close together so that they can react with one another. The bonds inside the substrates are stretched slightly out of position, which weakens the bonds. Less energy is needed to break weaker bonds. The reaction takes place and the product is released from the enzyme. The enzyme can then bind to more of the substrate molecules. 1. What is a catalyst? 2. Where are enzymes found? 3. How does an enzyme work? 50 Unit 1 Resource Book