400 likes | 406 Views
Atoms and Molecules: The Chemical Basis of Life. A knowledge of chemistry is essential for understanding organisms Important to biology are inorganic compounds, including water, simple acids and bases, and simple salts. Elements
E N D
A knowledge of chemistry is essential for understanding organisms • Important to biology are inorganic compounds, including water, simple acids and bases, and simple salts
Elements • Substances that cannot be broken down into simpler substances by ordinary chemical reactions • Each has a chemical symbol
Four elements comprise the mass of most organisms • Oxygen, carbon, hydrogen, and nitrogen • In addition, other elements, such as calcium, and trace elements are present
Atom • The smallest portion of an element that retains its chemical properties • Subatomic particles include • Electron—carries a negative charge • Proton—carries a positive charge • Neutron—uncharged particle
Every element has • A fixed number of protons in the atomic nucleus, known as the atomic number • The periodic table is a chart of the elements arranged by atomic number
The atomic mass of an atom • Is a number that indicates how much matter it contains • Is expressed by the atomic mass unit (amu), also known as the dalton • Is indicated by a superscript to the left of the chemical symbol
Isotopes • Are two or more forms of atoms of the same element • Contain the same number of protons and electrons, but the number of neutrons varies • Radioisotopes break down and emit radiation
Electrons move through orbitals • Electrons at the same principal energy level make up an electron shell
Electrons in a shell distant from the nucleus have greater energy • Valence electrons occupy the valence shell • Changes in electron energy levels are important in energy conversions in organisms
The chemical behavior of an atom is determined by the number and arrangement of its valence electrons • When the valence shell is not full, the atom tends to lose, gain, or share electrons
A chemical compound consists of atoms of two or more elements • Atoms combine in a fixed ratio • Atoms may join to form a molecule
A chemical formula describes the chemical composition of a substance • Simplest formula • Molecular formula • Structural formula
Molecular mass • Sum of the atomic masses of the component atoms of a single molecule • One mole is the amount of an element or compound whose mass in grams is equivalent to its atomic or molecular mass • The mole allows for comparison of atoms and molecules of very different mass
Chemical reactions in an organism • Described by chemical equations • Reactants are written on the left • Products are written on the right • Reactions can proceed simultaneously in both directions • At dynamic equilibrium, forward and reverse rates of reaction are equal
Chemical bonds • Forces of attraction that hold atoms of a compound together • The two principal types are • Covalent bonds • Ionic bonds • Bond energy • Energy necessary to break a chemical bond
Covalent bonds • Share electrons between atoms • Each atom has a filled valence shell • Covalent compound • Compound consisting mainly of covalent bonds • Example is hydrogen gas molecule • Bond can be single, double, or triple
Ion • Particle with one or more units of electrical charge • Results when an atom gains or loses electrons • Cations—positively charged ions • Anions—negatively charged ions • Cations and anions are involved in biological processes, such as muscle contraction
Sodium, potassium, and chloride ions are essential for this nerve cell to stimulate these muscle fibers
Ion bonds • Formed due to attraction between a cation and an anion • An ionic compound is a substance consisting of cations and anions bonded together • An example of ionic bond is the attraction between sodium ions and chloride ions
Hydrogen bonds • Tend to form between an atom with partial negative charge and a hydrogen atom covalently bonded to oxygen or nitrogen • Readily formed and broken • While individually weak, hydrogen bonds are strong when present in large numbers
Many energy conversions in a cell involve an electron transfer from one substance to another • Known as oxidation-reduction, or redox reaction
Large part of the mass of most organisms is water • Water is important as internal constituent and environmental factor
Water facilitates chemical reactions • Hydrophilic substances—interact readily with water, such as table salt • Hydrophobic substances—not disrupted or dissolved by water, such as fats
Water exists as gas, liquid, or solid • Hydrogen bonds are formed or broken as water changes state
Acid • Substance that dissociates in solution to yield hydrogen ions and an anion • Base • Substance that dissociates to yield a hydroxide ion and a cation when dissolved in water
The degree of a solution’s acidity is expressed in pH • Definition of pH • The negative logarithm of the hydrogen ion concentration • Expressed in moles per liter
Neutral solution • pH of 7 • Acidic solution • pH value of less than 7 • Base solution • pH greater than 7 • An acid and a base react to form a salt plus water