Biological Chemistry

Water • 3/4 of Earth’s surface • Present as solid, liquid and gas • Solid state of water (ice) floats on liquid • Life began in water • Cells are 70-95% water

Polar covalent bonds in water molecules result in hydrogen bonding • H2O molecule shaped like wide V – 2 H atoms joined to O atom by single covalent bonds • Covalent bond – Strong chemical bond in which two atoms share one or more pairs of valence electrons Oxygen is more electronegative than hydrogen, so electrons of covalent bonds spend more time near oxygen – Polar covalent bonds

• Electronegativity – Attraction of a given atom for the electron of a covalent bond

Polar molecule – Overall charge is unevenly distributed • O partial negative charge (δ-) • 2 Hs partial positive charge (δ+)

Slight positive H of one molecule attracted to slight negative O of another molecule = Molecules held together by hydrogen bonds

Four properties of water contribute to Earth’s suitability for life

  1. Cohesive behavior

  2. Ability to moderate temperature

  3. Expansion upon freezing

  4. Versatility as a solvent

  5. Cohesion of water molecules • Water molecules stay close to one another because of hydrogen bonding • Cohesion helps with transport of water and dissolved nutrients against gravity in plants → H-bonds between cell walls and water (adhesion) → Evaporating water pulls more up (upward pull)

• Surface tension – Measure of how difficult it is to stretch or break the surface of a liquid • Water has greater surface tension than other liquids – Ex: overfilled glass of water, spider, waterstrider

  1. Moderation of temperature by water • Absorbs heat from warmer air • Releases heat to cooler air • Can absorb or release a relatively large amount of heat with only a slight change in its own temperature • The faster a molecule moves, the greater its kinetic energy • HEAT → depends in part on volume – A measure of the matter’s TOTAL kinetic energy due to motion of its molecules
    • TEMPERATURE → Not dependant on volume – A measure of heat intensity that represents the AVERAGE kinetic energy of molecules • Pot of coffee vs. swimming pool – Pot of coffee: higher temperature – Pool: more heat Heat passes from warmer to cooler objects until two equalize • Unit of heat measurement – Calorie – Joule

Water has a high SPECIFIC HEAT – Amount of heat that absorbed or lost for 1 g of a substance to change its temperature 1°C – A measure of how well a substance resists changing its temperature in response to heat absorption/loss • Water – 1 cal/(g·° C) • Why does water have a high specific heat? – H bonds • Does not change temperature much when absorbing/releasing heat

Proof in nature: Coastal areas have milder climates • Water stabilizes ocean temperatures • Organisms made of lots of water can better resist changes in temperature

Evaporative cooling • Molecules moving fast enough to overcome bonds can escape and enter the air as a gas • Liquid to gas – Evaporation/vaporization • Heat of vaporization – Quantity of heat a liquid must absorb for 1 g of it to be converted from liquid to a gaseous state

Water’s high heat of vaporization due to H bonds • Considerable amount of solar heat absorbed by oceans is consumed during evaporation of surface water • Tropical air moves toward the poles, cools, releases heat, condenses to rain

As liquid evaporates, surface of liquid left behind cools down – Evaporative cooling occurs as the hottest molecules (those with greatest kinetic energy) are most likely to leave

Helps keep us cool as the hottest molecules leave → Ex: Sweating, Transpiration

  1. Ice floats on liquid water • One of the few substances that is less dense as a solid than a liquid – Due to H bonding • Temps > 4°C – Water behaves like other liquids – Expands when warm, contracts when cool • 4°C-0°C – Water freezes because molecules too slow to break H bonds • At 0 ° C, molecules locked into lattice – Each molecule H-bonded to 4 other molecules

H bonds keep molecules at a slightly farther distance apart, so less dense If ice sank – All water bodies would freeze • Floating ice insulates water underneath, letting life exist • Provides solid habitat for some animals

  1. Water: the solvent of life • A liquid that is a homogeneous mixture of ≥2 substances – Solution • Ex. sugar in water • Solvent – Dissolving agent • Solute – Substance dissolved • Aqueous solution – Water is the solvent

• Water is versatile solvent, due to polarity of molecules • Salt in water – O- regions attracted to Na+ regions – H+ attracted to Cl- regions • Hydration shell: Sphere of water molecules around each dissolved ion • Water does this working inward, dissolves whole salt cube

• Non-ionic polar molecules also dissolve in water – Ex: sugar • Hydrophilic – Substance that likes water • Not all hydrophilic substances dissolve in water – Ex: Will not dissolve if too big, like cotton • May remain suspended in liquid – Stable suspension of fine particles in liquid

• Hydrophobic – Repel water – Non-ionic and non-polar substances (Can’t form hydrogen bonds) • Ex: Vegetable oil and vinegar