Elements Pure substances that cannot be broken down chemically into simpler kinds of matter More than 100 elements (92 naturally occurring)

90% of the mass of an organism is composed of 4 elements (oxygen, carbon, hydrogen and nitrogen) Each element unique chemical symbol Consists of 1-2 letters

Atoms The simplest particle of an element that retains all the properties of that element Properties of atoms determine the structure and properties of the matter they compose Our understanding of the structure of atoms based on scientific models, not observation

The Nucleus Central core Consists of positive charged protons and neutral neutrons Positively charged Contains most of the mass of the atom

The Protons All atoms of a given element have the same number of protons Number of protons called the atomic number (z) Number of protons balanced by an equal number of negatively charged electrons

The Neutrons The number varies slightly among atoms of the same element Different number of neutrons produces isotopes of the same element

Atomic Mass (A) Protons & neutrons are found in the nucleus of an atom Protons and neutrons each have a mass of 1 amu (atomic mass unit) The atomic mass of an atom is found by adding the number of protons & neutrons in an atom

The Electrons Negatively charged high energy particles with little or no mass Travel at very high speeds at various distances (energy levels) around the nucleus.

Electrons in the same energy level are approximately the same distance from the nucleus Outer energy levels have more energy than inner levels Each level holds only a certain number of electrons

Energy Levels Atoms have 7 energy levels The levels are K (closest to the nucleus), L, M, N, O, P, Q (furthest from the nucleus) The K level can only hold 2 electrons Levels L – Q can hold 8 electrons (octet rule)

Compounds Most elements do not exist by themselves Readily combine with other elements in a predictable fashion

A compound is a pure substance made up of atoms of two or more elements The proportion of atoms are always fixed Chemical formula shows the kind and proportion of atoms of each element that occurs in a particular compound

Molecules are the simplest part of a substance that retains all of the properties of the substance and exists in a free state Some molecules are large and complex

The tendency of elements to combine and form compounds depends on the number and arrangement of electrons in their outermost energy level Atoms are most stable when their outer most energy level is filled

Most atoms are not stable in their natural state Tend to react (combine) with other atoms in order to become more stable (undergo chemical reactions) In chemical reactions bonds are broken; atoms rearranged and new chemical bonds are formed that store energy

Covalent Bonds Formed when two atoms share one or more pairs of electrons

Ionic Bonds Some atoms become stable by losing or gaining electrons Atoms that lose electrons are called positive ions

Atoms that gain electrons are called negative ions Because positive and negative electrical charges attract each other ionic bonds form

Energy Transfer Much of the energy organisms need is provided by sugar (food) The net release of free energy is called an exergonic (exothermic) reaction

Reactions that involve a net absorption of free energy are called endergonic (endothermic) reactions Photosynthesis is an example Most reactions in living organisms are endergonic; therefore living organisms require a constant source of energy

Most chemical reactions require energy to begin The amount of energy needed to start the reaction is called activation energy

Certain chemical substances (catalysts) reduce the amount of activation energy required Biological catalysts are called enzymes

Enzymes are an important class of catalysts in living organisms Mostly protein Thousands of different kinds Each specific for a different chemical reaction

Enzyme Structure Enzymes work on substances called substrates Substrates must fit into a place on an enzyme called the active site Enzymes are reusable!

Reduction-Oxidation Reactions Many of the chemical reactions that help transfer energy in living organisms involve the transfer of electrons (reduction-oxidation = redox reactions)

Oxidation reaction – reactant loses electron(s) becoming more positive

Reduction reaction – reactant gains electron(s) becoming more negative

Solutions

Solutions A solution is a mixture in which 2 or more substances are uniformly distributed in another substance

Solute is the substance dissolved in the solution Particles may be ions, atoms, or molecules Solvent is the substance in which the solute is dissolved Water is the universal solvent

Solutions can be composed of varying proportions of a given solute in a given solvent --- vary in concentration (measurement of the amount of solute) A saturated solution is one in which no more solute can be dissolved Aqueous solution (water) are universally important to living things

Dissociation of water Breaking apart of the water molecule into two ions of opposite charge (due to strong attraction of oxygen atom of one molecule for H atom of another water molecule) H 2 O  H + (hydrogen ion) + OH - (hydroxide ion) H + + H 2 O  H 3 O (hydronium ion)

Acids and Bases One of the most important aspects of a living system is the degree of acidity or alkalinity

Acids Number of hydronium ions in solutions is greater than the number of hydroxide ions HCl  H + + Cl -

Bases Number of hydroxide ions in solution is greater than the number of hydronium ions NaOH  Na + + OH -

pH Scale logarithmic scale for comparing the relative concentrations of hydronium ions and hydroxide ions in a solution ranges from 0 to 14  Each pH is 10X stronger than next  e.g. ph 1 is 10 times stronger than ph 2

the lower the pH the stronger the acid the higher the pH the stronger the base pH 7.0 is neutral

Buffers Control of pH is very important Most enzymes function only within a very narrow pH Control is accomplished with buffers made by the body Buffers keep a neutral pH (pH 7)

Buffers neutralize small amounts of either an acid or base added to a solution Complex buffering systems maintain the pH values of your body’s many fluids at normal and safe levels