Chapter 2

The Chemical Basis of LifeThe Chemical Basis of Life

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

– Elements: The simplest type of matter with unique chemical properties

• 112 known elements (92 naturally occurring)

– Atoms: Smallest particle of an element that has chemical characteristics of that element

Atomic Structure• Atoms: composed of subatomic

particles

– Neutrons: no electrical charge

– Protons: positive charge

– Electrons: negative charge

• Nucleus

– Formed by protons and neutrons

– Most of volume of atom occupied by electrons

• Relative size:

– gumball vs. stadium.

Atomic Structure• Atomic Symbol• Atomic Number:

Equal to the number of protons in an atom.

• Mass Number: Number of protons and neutrons in an atom.

Particle Electric Charge

Atomic mass

Location

Proton +1 1 Nucleus

Neutron 0 1 Nucleus

Electron -1 0 Electron shells

• Atomic number– Equal to the number of protons in each atom

• Mass number– Equal to the number of protons and neutrons in

each atom• Electrons determine the chemical properties of the

atom.• Electron shells - Electrons encircle the nucleus in

electron shells or energy levels.– Orbital - an electron can occupy any position in a

certain volume of space called an orbital• The innermost shell can hold up to two electrons. • The outermost shell, holding the valence electrons, can

have eight electrons.

Atomic Configuration• Electron arrangement is determined by total number of

electrons and electron shell they occupy. (Bohr Model)

1. Electrons with least amount of potential energy are located in K shell closest to nucleus; electrons having more potential energy are located in shells farther from the nucleus.

2. Inner shell contains up to two electrons; additional shells contain eight electrons.

3. Periodic table is arranged according to number of electrons in outer shell.

Atomic Structure of Isotopes

• Isotopes have the same atomic number but a different mass. – same number of

protons but differ in the number of neutrons

– e.g., a carbon atoms has six protons but may have more or less than usual six neutrons.

•Isotopes

–Two or more forms of the same element that have the same number of protons and electrons but have a different number of neutrons.

Atomic Mass• Individual atoms have very little mass.

– Hydrogen = 1.67 X 10 -24g.

• Unified atomic mass units (u) or Dalton (D)– Proton or neutron has an atomic mass of 1 .

• Atomic Mass = the average mass of its naturally occurring isotopes taking into account the relative abundance of each isotope. – Carbon has the isotopes 12C, 13C and 14C. – The atomic mass of Carbon is 12.01 D

Electrons and Chemical Bonding

• The chemical behavior of an atom is determined largely by its outermost electrons (valence electrons).

• Chemical bonding occurs when the outermost electrons are transferred or shared between atoms.

• Chemical bonding can be grouped into categories: Ionic, covalent, metallic and hydrogen bonds

Ionic Bonding• Ions are charged particles.

– cations - Positively charged ions ( have lost an electron)

– anions - Negatively charged ions (have gained an electron)

• Oppositely charged molecules are attracted to each other and tend to remain close to each other.

• Ionic bonding occurs when one or more valence electrons from one atom are completely transferred to a second atom.

• Common ions found in the body include:– Ca++, Na+, K+, H+, OH-, Cl-

• Ionic compounds readily form crystals.• Ionic bonds tend to dissociate in water.

Covalent Bonding• Covalent bonding: atoms share one or more

pairs of valence electrons to form stable valence shells.– Single covalent bond: One electron pair is

shared between two atoms.– Double covalent bond: Two atoms share 4

electrons– Polar and non polar covalent bonds– Nonpolar covalent bond: Electrons are shared

equally– Polar covalent bonds: Electrons are not shared

equally

Intermolecular Forces

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

• Weaker than forces producing chemical bonding

Hydrogen Bonding • Hydrogen bond: Weak attractive force between

slightly positive hydrogen atom of one molecule and slightly negative atom in another molecule

Many hydrogen bonds taken together are relatively strong.

Hydrogen bonds between complex molecules of cells help maintain structure and function.

Intermolecular Forces

• Solubility: Ability of one substance to dissolve in another

– Example: Sugar dissolves in water

• Dissociation or Separation

– Ionic compounds

– Cations are attracted to negative end and anions attracted to positive end of water molecules

Intermolecular Forces

• Electrolytes: Cations (+) and anions (-) that dissociate in water– Capacity to conduct an electric current– Currents can be detected by electrodes

• Nonelectrolytes: Molecules that do not dissociate form solutions that do not conduct electricity

Chemical Reactions• Chemical Reactions: Atoms, ions, molecules or

compounds interact to form or break chemical bonds– Metabolism: All anabolic and catabolic

reactions in the body• Catabolism: Decomposition reactions

– Hydrolysis: Reactions that use water

• Anabolism: Growth, maintenance, and repair of the body in synthesis reactions

– Produce molecules characteristic of life: ATP, proteins, carbohydrates, lipids, and nucleic acids

Synthesis and Decomposition Reactions• Synthesis Reactions

– Two or more reactants chemically combine to form a larger product

– Anabolism: All body’s synthesis reactions

• Decomposition Reactions

– Reverse of synthesis reactions

– Catabolism: Reactions of decomposition in body

Oxidation-Reduction Reactions

• Oxidation– Loss of an electron by an atom

• Reduction– Gain of an electron by an atom

• Oxidation-Reduction Reactions– The complete or partial loss of an electron by

one atom is accompanied by the gain of that electron by another atom

Rate of Chemical Reactions• Reactant type - differ in their ability to undergo

chemical reactions.• Concentration - greater concentrations of

reactants generally cause reactions to proceed faster.

• Temperature - reaction speed increases with higher temps.

• Catalysts - substance that increases the rate at which a chemical reaction proceeds. – Enzymes - are protein molecules that act as

catalysts in the body.

Energy

• Energy: The capacity to do work– Potential Energy: Stored energy– Kinetic Energy: Does work and moves matter– Mechanical Energy: Energy resulting from the

position or movement of objects– Chemical Energy: Form of potential energy in

the chemical bonds of a substance– Heat Energy: Energy that flows between objects

of different temperatures

Energy and Chemical Reactions

Speed of Chemical Reactions

• Activation Energy: Minimum energy reactants must have to start a chemical reaction

– Catalysts: Substances that increase the rate of chemical reactions without being permanently changed or depleted

– Enzymes: Increase the rate of chemical reactions by lowering the activation energy necessary for reaction to begin

Activation Energy and Enzymes

Properties of Water - functions in living organisms

• Stabilizing body temperature

– Water has a high specific heat and tends to resist large temperature fluctuations.• Evaporation (540 calories / gram of water).

• Protection

– Lubricant and Cushion

• Chemical reactions

– Water is an excellent solvent.

• Mixing medium– solutions: any liquid that contains dissolved

substances– suspensions: a liquid that contains non-

dissolved materials that settle out of of the liquid unless it is continually shaken.

– colloid: a liquid that contains non-dissolved materials that do not settle out of liquid. (Water and proteins inside the cell).

Solution Concentrations• Solutions = solvent (the liquid portion) + solutes (substances

dissolved in the solvent).• Concentration - expressed in a number of different ways:

– percent solution (10% solution = 10 g solute / 100 ml of solvent)

– Osmolarity (1 osmole = 1 mole (Avegadro’s # of particles) in 1 kilogram of water).

• Osmolarity is a reflection of the number of particles in a solution and not the type of particle in a solution.

– The osmolarity of body fluids = 300 mOsm• important because it influences the movement of water in

and out of cells.• Note: difference between molality and osmolality is that

osmolality takes into acount the number of particles a molecule breaks into when it goes into solution.

Acids and BasesWhen water ionizes or

dissociates, it releases a small (107 moles/liter) but equal number of H+ and OH ions; Thus its ph is neutral.

Acid: a proton donor.Base: a proton acceptor.

• Take up hydrogen ions or release hyroxide ions.

Stong vs Weak acids. -The more freely the acid or base dissociates the stronger it is.

•Strong acid : HCl H+ Cl- dissociates almost completely •Weak Acid : Acetic acid CH3COOH CH3COO- + H +

•Some dissociates and some does not.

•The pH scale indicates acidity and basicity (alkilinity) of a solution.•Measure of free hydrogen ions as a negative logarithm of the H+ concentration (-log [H+]).

–logarithmic scale - a change of pH by one unit represents a 10 fold change in the concentration of H+.

pH Scale

pH Scale continued

• pH values range from 0 (100 moles/liter; most acidic) to 14 (1014 moles/liter; most basic).

• One mole of water has 107 moles/liter of hydrogen ions; therefore, has neutral pH of 7.

– Acid has a pH less than 7– Base has a pH greater than 7.

• Normal Range in body = 7.35 - 7.45– Acidosis - Nervous system depression,

disorientation and coma.– Alkalosis - N. S. overexcited, convulsion -

possibly fatal.

Buffers• Buffers keep pH steady and within normal limits in

living organisms. – Buffers stabilize pH of a solution by taking up

excess hydrogen or hydroxide ions. – Carbonic acid helps keep blood pH within normal

limits: – H2CO3 H

+ + HCO3-.

• The chemical nature of many molecules changes as the pH of a solution in which they are dissolved changes.

– Most enzymes work within a narrow pH range.– Survival of an organism depends on its ability to

maintain it pH level within a narrow range.

The End.