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Chapter 2

The Chemical Basis of LifeThe Chemical Basis of Life

-Part One-

The Chemical Basis of Life

• Key to understanding normal structure and function and understanding disease processes

• The information in this chapter will help in the understanding of how different organ systems function

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Basic Chemistry• Matter, Mass, and Weight

– Matter: anything that occupies space and has mass– Mass: the amount of matter in an object– Weight: the gravitational force acting on an object of

a given mass

• Elements and Atoms– Element: the simplest type of matter with unique

chemical properties; composed of atoms of only one kind

– Atom: smallest particle of an element that has chemical characteristics of that element

– Compounds: combination of two or more elements

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Periodic table of the elements

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Atomic Structure• Atoms: composed of

subatomic particles– Neutrons: no electrical

charge– Protons: one positive

charge– Electrons: one negative

charge

• Nucleus: formed by protons and neutrons

• Most of the volume of an atom occupied by electrons

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Atomic Number and Atomic Mass

• Atomic Number: equal to number of protons in each atom. Has a neutral charge. Thus, has an equal number of electrons

• Atomic Mass: number of protons plus number of neutrons

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Isotopes and Atomic Weight

• Isotopes: two or more forms of same element with same number of protons and electrons but different neutron number– For example; there are three types of hydrogen– Denoted by using symbol of element preceded by mass number as 1H,

2H, 3H

• Atomic Weight: averageaverage masses of naturally occurring isotopes

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Radioactive Isotopes

• Forms of atoms that emit radioactivity such as gamma rays, which can then be measured

• Unstable isotopes which undergo nuclear decay

• Used clinically and in research• Examples of uses

– Tracking hormone uptake– Treating cancer– Sterilization of materials to be used in surgery

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Electrons and Bonding

• Electrons occupy energy levels called electron shells

• Electrons closest to the nucleus are most strongly attracted

• Each shell has distinct properties– The number of electrons has an upper limit– Shells closest to the nucleus fill first

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Electrons and BondingElectrons and Bonding

• Bonding involves interactions between electrons in the outer shell (valence shell) These valence electrons of elements interact with each other to form chemical reactions

• Full valence shells do not form bonds

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Inert Elements

• Atoms are stable (inert) when the outermost shell is complete

• These are the element of the periodic table on the furthest right.

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Inert Elements

• Atoms will gain, lose, or share electrons to complete their outermost orbitals and reach a stable state

• Octet Rule– Atoms are considered stable when their

outermost orbital has 8 electrons– The exception to this rule is Shell 1, which

can only hold 2 electrons

Which are stable?

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Types of BONDS

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Electrons and Chemical Bonding• Intramolecular bonding occurs when

outermost electrons are either shared with or transferred to another atom– Ionic Bonding: atoms exchange

electrons – Covalent Bonding: two or more

atoms share electron pairs• Ion: an atom loses or gains

electrons and becomes charged– Cation: positively charged ion– Anion: negatively charged ion

• In an ionic bond, cations and anions are attracted to each other and remain close to each other

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Covalent BondingAtoms share one or more

pairs of electrons– Single covalent: two

atoms share one pair of electrons

– Double covalent: Two atoms share 4 electrons

– Nonpolar covalent: Electrons shared equally because nuclei attract the electrons equally

– Polar covalent: Electrons not shared equally because one nucleus attracts the electrons more than the other does

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Molecules and Compounds

• Molecules: two or more atoms chemically combine to form an independent unit

– Example: a hydrogen molecule (H2)

• Compounds: a substance composed of two or more different types of atoms chemically combined

– Example: water (H2O)

• Molecular Mass: determined by adding up atomic weights of its atoms or ions– Example: NaCl (22.99 + 35.45)

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• What is the difference between a compound and a molecule?• A molecule is formed when two or more atoms join together

chemically. A compound is a molecule that contains at least two different elements. All compounds are molecules but not all molecules are compounds.

• Molecular hydrogen (H2), molecular oxygen (O2) and molecular nitrogen (N2) are not compounds because each is composed of a single element. Water (H2O), carbon dioxide (CO2) and methane (CH4) are compounds because each is made from more than one element. The smallest bit of each of these substances would be referred to as a molecule. For example, a single molecule of molecular hydrogen is made from two atoms of hydrogen while a single molecule of water is made from two atoms of hydrogen and one atom of oxygen.

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Intermolecular Forces

• Forces between molecules

• Result from weak electrostatic attractions between oppositely charged parts or molecules, or between ions and molecules

• Weaker than forces producing chemical bonding

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Intermolecular Forces: Hydrogen Bonds

• Occur when the positively charged H of one molecule is attracted to the negatively charged O, N or F of another molecule – For example, in water the

positively charged hydrogen atoms of one water molecule bond with the negatively charged oxygen atoms of other water molecules

– Hydrogen bonds play an important role in determining the shape of complex molecules

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Polar vs Non polarThis is a very important concept that is used in

biology every day!• Polar molecules– Have a charge

because the electrons are NOT equally shared

– Dissolve in water– Ionic bond, polar

covalent bond and hydrogen bonding

– Example is Water

– Hydrophilic

• Non-polar– Do not have a charge– Nonpolar covalent

bonding where the charges are distributed evenly

– Do NOT dissolve in water

– Example is Oil

– Hydrophobic

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Intermolecular Forces: Solubility and Dissociation

• Solubility: ability of one substance to dissolve in another– For example, sugar or salt

dissolves in water• Dissociation or Separation: in

ionic compounds, cations are attracted to negative end and anions attracted to positive end of water molecules; the ions separate and each becomes surrounded by water molecules

• Electrolyte: dissociation of an ionic compound in water

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Electrolytes and Nonelectrolytes

• Electrolytes: solutions made by the dissociation of cations (+) and anions (-) in water– Have the capacity to conduct an electric current– Currents can be detected by electrodes

• Nonelectrolytes: solutions made by molecules that dissolve in water, but do not dissociate; do not conduct electricity

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Chemical Reactions

• Atoms, ions, molecules or compounds interact to form or break chemical bonds– Reactants: substances that enter into a chemical

reaction.– Products: substances that result from the reaction

• Chemical bonds are made (synthesis; anabolism) and broken (decomposition; catabolism) during chemical reactions

• Metabolism: collective term used for the sum of all of the anabolic and catabolic reactions in the body

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Synthetic Reactions• Two or more reactants chemically combine to form a

new and larger product. Anabolism.– Chemical bonds made; energy stored in the bonds. – Responsible for growth, maintenance and repair– Dehydration: synthetic reaction where water is a product– Produce chemicals characteristic of life: carbohydrates,

proteins, lipids, and nucleic acids

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Decomposition Reactions• A large reactant is broken down to form smaller products.

Catabolism.– Chemical bonds broken; energy released. – Hydrolysis: water is split into two parts that contribute to the

formation of the products– Example: the breakdown of ATP to form ADP and inorganic

phosphate with a concomitant release of free energy

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Reversible Reactions

• Chemical reactions in which the reaction can proceed either from reactants to products or from products to reactants.

• Equilibrium: rate of product formation is equal to rate of reactant formation

• Example: CO2 and H+ formation in plasma