BIOL 1107 - Water and Life Key Concepts

Water is a _____ molecule

polar

Why is water considered a polar molecule?

O is more electronegative (attracts electrons more than H), which leads to 2 partial negative charges

-each H develops a partial positive charge

What does polarity allow?

-polarity allows water molecules to form hydrogen bonds between H from a molecule and O from a neighboring molecule

_________ between water molecules are the root of four essential properties of water for living organisms

Hydrogen Bonds

List the 4 essential properties of water

-Cohesive behavior

-Moderation of temperature

-Expansion upon freezing

-Versatile solvent

Property 1: Cohesive behavior

-Hydrogen bonds hold water molecules together. This is cohesion that results in high surface tension (difficulty in stretching or breaking the surface)

-It plays an essential role in sap circulation for example:

• cohesion makes water molecules behave like they were attached together

  • water evaporation in leaves creates a low pressure that attracts water molecules. Due to cohesion, molecules pull each other upwards (against gravity)

Cohesion

Hydrogen bonds holding water molecules together that results in high surface tension

Property 2: Moderation of temperature

-water can absorb a large amount of heat with only a slight change in its own temperature

-when heated, most of the heat is used to break hydrogen bonds. upon cooling, the reforming of hydrogen bonds releases a large amount of heat. this is responsible for the thermic inertia of water (i.e., it takes time to increase water temperature, and it takes time to cool down)

-this thermic inertia is due to its high specific heat (among the highest on earth)

-it helps climate regulation: ocean absorbs heat during the winter (and warms up the surrounding atmosphere)

-water (shower or sweat) can quickly absorb body temperature. Water evaporation absorbs a large amount of heat (heat of vaporization). The remaining surface cools, which is evaporative cooling

-evaporative cooling helps stabilize temperatures for organisms and bodies of water

Property 3: Expansion upon freezing

-hydrogen bonds impose a certain distance between molecules

-in liquid water, hydrogen bonds form and break at any moment. some molecules are close together (no H-bonds) or more distant (H-bonds)

-in solid water (ice), water molecules are locked into a crystalline lattice. each molecule form H-bonds with its neighbors. the distance between molecules is fixed

-therefore, the number of molecules within a given volume is higher in liquid water than in solid water. thus, water is less dense as a solid than as a liquid

-water reaches its maximal density at 4ºC (i.e., temperature at the bottom of the ocean)

-ice floats at the surface, which prevents further freezing

• water below the surface is cold, but still liquid and allows the survival of living organisms

  • if ice sank, all bodies of water would eventually freeze solid, which would make life impossible on earth

Property 4: Versatile solvent

-due to its polar properties, water is a versatile solvent

-molecules with ionic (charges) or polar (partial charges) regions can interact with water molecules

• the molecule is surrounded by a layer of water molecules: hydration shell and dissolve into water

• this is true from small ions up to very large molecules (e.g., sugar, protein)

-polar molecules can interact with water: they are hydrophilic

non-polar molecules cannot interact with water (e.g., lipids): they are hydrophobic. water and non-polar molecules repel each other

Solvent

dissolving agent of a solution

-in case of water as a solvent, we talk about an aqueous solution

pH: acid and basic solutions

-a hydrogen atom in a hydrogen bond between two water molecules can shift from one to the other

-the hydrogen atom leaves its electron behind and is transferred as a proton, or hydrogen ion (H+)

-the molecule that lost the proton is now a hydroxide ion (OH-)

-the molecule with the extra proton is now a hydronium ion (H3O+ though it is often represented as H+)

-This is called the dissociation of water molecules

-the phenomenon is rare but has huge effects on organisms because H+ and OH- are very reactive

-an acid increases H+ concentration of a solution

-a base reduces H+ concentration of a solution

-strong acids and bases dissociate completely in water

-most acids and bases in living organisms are weak acids and bases

• they are not totally dissociated in water

  • the reaction is reversible (equilibrium)

pH (potential of hydrogen)

- a measure of the acidity of a solution

• pH = 7: neutral

• pH < 7: acid

• pH > 7: base

What is the pH for blood

-the pH for blood is 7.4

-slight change of pH is detrimental and often lethal

-blood contains buffers

-many biological systems have buffers that maintain pH within a specific range of values

Buffers

• buffers are weak acids/bases that can absorb an excess of H+ (if pH decreases) or release h+ (if pH increases)