The Chemical Basis of Life

Chapter 2

Matter

• Substance that has mass and takes up space– Compose all living things

• Generally found in 1 of 3 states

• Composed of 1 or more elements

Elements• 92 occur naturally on Earth

• 4 make up 96% of the human body (CHON)

• Composed of atoms

Atoms

• Smallest particles that retains properties of an element• Made up of subatomic particles:– Protons (+) in nucleus– Electrons (-) orbits nucleus– Neutrons (no charge) in nucleus

• Protons and neutrons– Mass of about 1

• Electrons– Mass is negligible (1/2000)

Reading A Periodic Table

• Elements differ depending on the number of subatomic particles

• Atomic symbol– 1st letter or 2 (usually)

• Atomic number– Determined by number of protons – Element specific

• Mass number– Determined by number of protons + neutrons

Isotopes

• Atoms with different number of neutrons– Effects mass number how?– Effects atomic number how?

• Can be stable or unstable (radioactive)• Behave the same as respective element (electrons are key)• Occur naturally as a mix in elements

– Living cells can’t distinguish between them– Applications

• Dating fossils• Biological tracers• Brain scanning• Cancer treatments

• Dangers– Radioactive atoms give off energy that destroys chemical bonds when

they collide

• Electrons are key– Move in orbitals called shells– Repel one another, but attracted to protons

• Electron shells– Outermost determines chemical properties

• Closer to the nucleus = lower energy and are filled first

– Holds up to 2 or 8 electrons• Filled are unreactive• Unfilled are reactive

– Number differs between atoms

Chemical Properties of Atoms

HYDROGEN1p+ , 1e-

HELIUM2p+ , 2e-

CARBON6p+ , 6e-

OXYGEN8p+ , 8e-

SODIUM11p+ , 11e-

CHLORINE17p+ , 17e-

NEON10p+ , 10e-

electron

proton

neutron

Electron Shell Models

Chemical Bonds

• Hold 2 or more atoms together– Complete outer shells

– By sharing, donating, or receiving electrons

• Form molecules (H2, I2, and O2) or compounds

(H20, NaCl, C6H12O6)

– Demonstrates emergent properties

• 2 H+ (gas) + O- (gas) = H2O (water)

• Na+ (metal) + Cl- (poisonous gas) = NaCl (table salt)

Ionic Bond• One atom loses electrons cation (charge?)• Another atom gains these electrons anion

(charge?)• Charge difference attracts the two– Very weak bond– Table salt (NaCl)

cation anion

Covalent Bond• Atoms share outer pair or pairs of valence

electrons– Single, double, or triple covalent bond

• Non-polar covalent– Electrons shared equally– E.g.: hydrogen gas/ H2/ H-H

• Polar covalent– Electrons spend more time near most electronegative

nucleus– E.g.: water/ H20

• electrons more attracted to O nucleus than to either H nuclei

Hydrogen Bond

• Positive charge on H attracts negative charge on another atom

• Individually weak, but often numerous = strong

• Important to many biological compounds– E.g. water

• Makes up 70 – 90% of all living things• Bonds create unique properties

Properties of Water

• Solubility– Solvent: dissolving agent– Solute: substance that is dissolved– Solution: liquid mix of 2+ substances

• Aqueous solution when water is solvent

• Polarity– ‘Like dissolves like’– Hydrophilic

• Sugar or salt and water– Hydrophobic

• Oil and water

Properties of Water (cont.)• Water movement– Cohesion

• Creates surface tension– Overfilling a glass, a ‘belly flop’, or beading of water

– Adhesion • Water moves up plants against gravity or paper towels

• Temperature stabilization– Molecule mov’t affects temp– Hard to change H-bonds

• Increase ice formation• Reduce evaporation• Vary internal temp.

pH Scale

• Measures [H+] of a solution (acidity)

• Change of 1 on scale means 10X change in [H+] (logarithmic scale)

Highest H+ Lowest H+

0---------------------7-------------------14Acidic Neutral Basic

pH Scale (cont.)

• Acids

– Increase [H+]– pH less than 7

• Bases– Decrease [OH-]– pH greater than 7

• Neutrals– Equal [H+] and [OH-]– pH of 7.0

Buffer Systems

• Minimize shifts in pH

• Can accept H+ when in excess and donate H+ when depleted– Form water

• Common in biological fluids– Human blood at 7.4, slight deviations can be

deadly

Making and Breaking Bonds• Chemical reactions are often reversible– Chemical equilibrium: forward and reverse reactions

occurring at the same rate (no net change)

• Starting materials are reactants, ending are products

• # of atoms are conserved– 1st law of thermodynamics