The Body in Motion

Solomon • Berg • Martin Biology, Seventh Edition

Chapter 2

Atoms and Molecules:

The Chemical Basis of

Life

Copyright © 2005 Brooks/Cole — Thomson Learning

Biology, Seventh Edition CHAPTER 2 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

• 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



Functions of elements



• 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



Periodic chart (including Bohr models)



• 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



Characteristics of protons, neutrons, and electrons



• 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



Carbon Isotopes



• 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



Atomic orbitals



• 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



Covalent bonds



Number of Covalent bonds



Covalent bonds can be nonpolar or polar



• 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



Ionic bonding



• 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


Biology, Seventh Edition

Hydrogen bonding

CHAPTER 2 Atoms and Molecules: The Chemical Basis of Life



• 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


Biology, Seventh Edition CHAPTER 2 Atoms and Molecules: The Chemical Basis of Life pH values of common solutions


The Body in Motion

Chapter 2

Atoms and Molecules:

The Chemical Basis of

Life

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The Body in Motion

Chapter 2

Atoms and Molecules:

The Chemical Basis of

Life

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