The chemical context of life

matter occupies space and has mass Matter is composed of chemical elements lelements cannot be broken down compound = two or more elements in a fixed ratio - such as NaCl (salt) Life is composed of matter sodium chlorine NaCl salt + =

“honc” Trace elements

May be needed in only small amounts but may cause harm if absent (iodine) goiter

Atoms zero charge; one dalton mass neutron proton +1 charge; one dalton mass electron -1 charge; minor mass NUCLEUSNUCLEUS

Atoms

He 2 Atomic Number and Weight atomic number 4 atomic weight (protons) (protons+neutrons) In a neutral atom, # protons = # electrons Helium

Examples Na (11 prot, 11 elect, 12 neut) C 6 12 (carbon-12, stable isotope) C 6 14 (carbon-14, unstable and radioactive)

The number of neutrons vary among isotopes of an element

Radioactive isotope 1. The number of protons is unique to each element (C = 6) 2. Isotopes have different numbers of neutrons 3. The nucleus of a radioactive isotope decays and gives off energy

Radioactive isotopes MRI Very useful in biology and medicine but must be handled with extreme care

Electron shells Can be represented by a variety of shapes

Valence shell The chemical behavior of an atom depends mostly on the number of electrons in its outermost shell

Chemical bonds Atoms combine to form molecules 1. Covalent bond - sharing a pair of valence electrons by two atoms nonpolar covalent polar covalent 2. Ionic bonds - one atom “steals” electrons from another

Covalent bonds share valence electrons

...but they don’t always share evenly Nonpolar covalent - the atoms have the same electronegativity and do share electrons equally Example: methane

Polar covalent One atom is more electronegative than another and keeps the electrons near itself more of the time Ex. water

Non-polar covalent -vs- Ionic bonds Non polar covalent bonds Atoms still share electrons, even if they are not evenly distributed (  - ). Ionic bonds An atom “steals” electrons from another atom.

Ionic bonds often complete an atom’s valence shell

anion An ionic bond is formed between: - charge + charge cation

Sodium chloride Ionic compounds are salts

Water is the solvent of life

Unique properties of water Polar Molecule –Cohesive Stabilizes Temperature –Solvent of Life

Like no other common substance, water exists in nature in all three physical states: –as a solid –as a liquid –as a gas

Water molecules form hydrogen bonds Hydrogen atoms in one polar covalent molecule are attracted to an electronegative atom in another polar molecule This creates H-bonds Very important for function in biological molecules

Water’s polarity leads to hydrogen bonding and other unusual properties Hydrogen bonds are weak; forming and breaking constantly - but strong in high numbers If you could stop water in an instant most molecules would be H-bonded to another These characteristics (polarity & H-bonding) give water the properties of cohesion, temperature stability, and make it a universal solvent

Cohesion: Water molecules stick together surface tension

Cohesion: water molecules can move great distances

Adhesion- Unique ability of water to “stick” to other things.

Ice floats! Water molecules in ice (solid water) are farther apart than the molecules in liquid water

Ice is therefore less dense than liquid water, causing it to float.. Why is this a good thing? –If ice sank, it wouldn’t thaw –Creates a blanket of insulation

Water is the universal solvent Solution - a homogenous mixture of two or more substances : aqueous solutions Solvent - dissolving agent (water) Solute - substance dissolved Polar or charged solutes can “stick” to water molecules (i.e. Dissolve)

Na + Cl – – – – – – – – – – – –

The chemistry of life is sensitive to acids and bases A substance that releases H + ions in solution is an acid, and one that accepts H + ions in solution is a base Acidity is measured on the pH scale: –0-6 is acidic –8-14 is basic –pH of 7 is neutral: Pure water

The pH scale

pH  pH = -(log [H + ])  [H + ] = Concentration of Hydrogen ions  For [H + ] = 10 -2, pH = 2  Each change in pH represents 10x change in [H + ] and [OH - ]  pH of 7 is neutral; [H + ] = [OH - ]  pH [OH - ]  pH > 7 is basic; [H + ] < [OH - ]

buffers Buffers are substances that resist pH change –They accept H + ions when they are in excess and donate H + ions when they are depleted –carbonic acid in bloodstream to keep pH at H 2 CO 3 HCO H + Cells are kept close to pH 7 by buffers