The Chemistry of Life Chapters 2

Think about it… Why must we understand chemistry first in order to understand biology? Write down 3 reasons. Try and be specific!

Chapter 2: The Chemical Basis of Life

Elements, Atoms, and Molecules Living organisms are composed of matter Matter is composed of elements Element - substance that cannot be broken down 92 elements in nature—only a few exist in a pure state Life requires 25 essential elements; some are called trace elements

Elements, Atoms, and Molecules Compound—substance of 2+ different elements combined in a fixed ratio many compounds that consist of only 2 elements Table salt + Sodium Chlorine Sodium Chloride

Elements, Atoms, and Molecules An atom is the smallest unit of matter made of 100+ subatomic particles Proton—positive electrical charge Electron—negative electrical charge Neutron—electrically neutral Elements differ in # of protons, neutrons, and electrons Protons and neutrons - in the nucleus electrons orbit around the nucleus How do electrons stay orbiting around the nucleus?

Electron cloud Nucleus 2e– Figure 2.4A Two models of a helium atom 2 Protons Mass number = 4 2 Neutrons 2 Electrons

Elements, Atoms, and Molecules The number of protons = atomic number Electron cloud 6e– Nucleus 6 Protons Mass number = 12 6 Neutrons 6 Electrons

Elements, Atoms, and Molecules all atoms of an element have the same atomic number, some differ in mass number Isotopes = same numbers of protons and electrons; different numbers of neutrons

Radioactive Isotopes Read page 21 on radioactive isotopes and do the following: How are radioactive isotopes used in research? How are they used in diagnoses of medical diseases? What are the dangers of using tracers in medicine

Elements, Atoms, and Molecules Only electrons are involved in chemical activity Electrons occur in energy levels called electron shells Hydrogen Helium First shell Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon Second shell Sodium Magnesium Aluminum Silicon Phosphorus Sulfur Chlorine Argon Third shell

Concept Check 1. The reactive properties or chemical behavior of an atom mostly depend on the number of a. the electrons in each electron shell of the atom. b. the neutrons found in the nucleus. c. the filled electron shells. d. the electrons in the outer electron shell of the atom. Answer: 4

Elements, Atoms, and Molecules Atoms want to fill their outer electron shells atoms share, donate, or receive electrons This results in attractions between atoms called chemical bonds Ionic bonds vs. covalent bonds

Elements, Atoms, and Molecules ion – atom/molecule with a +/- charge When an electron is lost, a + charge results; when one is gained, a - charge results Two ions with opposite charges attract each other When the attraction holds ions together, it is called an ionic bond

Sodium chloride (NaCl) Transfer of electron + – Na Sodium atom Cl Chlorine atom Na+ Sodium ion Cl– Chloride ion Figure 2.7A Formation of an ionic bond, producing sodium chloride. Sodium chloride (NaCl)

Elements, Atoms, and Molecules covalent bond - when atoms share outer-shell electrons A molecule is formed when atoms are held together by covalent bonds

Elements, Atoms, and Molecules Atoms in a covalently bonded molecule compete for shared electrons attraction (pull) for shared electrons is called electronegativity More electronegative = pull harder In molecules of only 1 element, the pull toward each atom is =, because each atom has the same electronegativity The bonds formed are called nonpolar covalent bonds

Elements, Atoms, and Molecules Water has atoms w/ different electronegativities Oxygen attracts the shared electrons more strongly than hydrogen So, the shared electrons spend more time near oxygen The result is a polar covalent bond (–) (–) O H H (+) (+)

Elements, Atoms, and Molecules Some chemical bonds are weak Water molecules are electrically attracted to oppositely charged regions on neighboring molecules Because the positively charged region is always a hydrogen atom, the bond is called a hydrogen bond Hydrogen bond

Water Hydrogen bonding causes molecules to stick together, called cohesion help transport water and nutrients up the plant Cohesion is related to surface tension—a measure of how difficult it is to break the surface of a liquid

Adhesion Water-conducting cells Cohesion 150 µm Direction of water movement Cohesion Campbell, Neil, and Jane Reece, Biology, 8th ed., Figure 3.3 Water Transport in Plants. 150 µm

What will break hydrogen bonds? Water Adhesion – when water molecules stick to other substances. What will break hydrogen bonds?

Water solution - a liquid consisting of a uniform mixture of two or more substances dissolving agent = solvent substance dissolved = solute Suspension - a substance that doesn’t dissolve but separates into tiny pieces Water keeps pieces suspended so they don’t settle out

Ion in Salt solution crystal Figure 2.14 A crystal of salt (NaCl) dissolving in water. Ion in solution Salt crystal

Water water molecules can break apart into ions Some are hydrogen ions (H+) Some are hydroxide ions (OH–) Both are reactive Chemicals other than water can contribute H+ to a solution = acids An acidic solution has a higher concentration of H+ than OH–

Water Some chemicals accept hydrogen ions and remove them from solution = bases For example, sodium hydroxide (NaOH) provides OH– that combines with H+ to produce H2O (water) A pH scale - used to describe whether a solution is acidic or basic pH ranges from 0 (most acidic) to 14 (most basic) A solution that is neither acidic or basic is neutral (pH = 7)

(Higher concentration of H+) (Lower concentration of H+) pH scale 1 Battery acid 2 Lemon juice, gastric juice 3 Grapefruit juice, soft drink, vinegar, beer (Higher concentration of H+) Increasingly ACIDIC Acidic solution 4 Tomato juice 5 Rain water 6 Human urine Saliva NEUTRAL [H+]=OH–] 7 Pure water Human blood, tears 8 Seawater Neutral solution 9 Figure 2.15 The pH scale represents the relative concentration of H+ and OH–. 10 (Lower concentration of H+) Increasingly BASIC Milk of magnesia 11 Household ammonia 12 Household bleach 13 Oven cleaner Basic solution 14

Acidic solution Basic solution Neutral solution Figure 2.15 The pH scale represents the relative concentration of H+ and OH–. Acidic solution Neutral solution Basic solution

Reactions The formation of water from hydrogen and oxygen is an example of a chemical reaction The reactants (H2 and O) are converted to H2O, the product What are a few examples of reactions that living things (plants/animals) must go through in order to survive? Find the reaction equation for them. What are the products/reactants?

Interpreting Data 3. This is the general equation for photosynthesis—the process of capturing sunlight energy and converting it to chemical energy. Which of the following are the reactants of this reaction? a. C6H12O6 and O2. b. CO2 and H2O. Answer: 2