Water Flashcards
(25 cards)
Basic role of water in living systems
To provide a fluid system in which biochemical reactions can take place.
Physical Properties of Water
-At 4 degrees C ice floats in water; there is life in frozen seas.
-Water has a low vapor pressure
-H2O is a liquid at room temperature MW = 18 amu’s
-For comparison, CH4 is a gas at room temperature MW = 16 amu’s
-Melting and boiling points of water are too high when compared to hydrides of group VI elements. H2O, H2S, H2Se and H2Te, which are gases at RT.
MW in regards to MP, BP
In general, as MW increases, MP and BP increase
Why tho?
The physical properties of water (i.e. low vapor pressure, mp, bp) are due to its molecular structure which allows it to form and extensive network of Hydrogen bonds.
-Tetrahedral
-Angular
-Have 2 unshared pair of e-‘s
-Polar
-Water is very polar.
-It is considered a dipole.
-Dissolves polar (hydrophilic)
compounds and ionic solutes.
-Water forms hydrogen bonds:
-Electrostatic attractions between d- O and d+ H
In liquid water at room temperature and atmospheric pressure
each water molecule hydrogen-bonds with an average of 3.4 other water molecules.
In ice, it’s 4 bonds, the max possible
As a point of comparison, Methanol only has 2.3 H-bonds on average and so is more easy to melt/boil (lower MP/BP)
SOLVENT PROPERTIES OF WATER
-Ionic compounds (e.g., KCl) and low MW polar covalent compounds (e.g., C2H5OH and CH3COCH3) tend to dissolve in water
-The underlying principle is electrostatic attraction of unlike charges; the positive dipole of water for the negative dipole of another molecule, etc.
-ion-dipole interaction: e.g., NaCl dissolved in H2O
-dipole-dipole interactions: e.g., ethanol or acetone dissolved in H2O
-dipole induced-dipole interactions: weak and generally do not lead to solubility in water.
Covalent bonding
Major stabilizing factor in organic compounds. Holds atoms in geometric patterns.
Non-covalent bonding
Stabilizing and organizing forces in nature.
-Participate in the organization and stabilization of 3-D structure of a molecule.
-Allow molecules to undergo changes in conformation.
-Provide specific patterns of binding interactions between substances that combine and react with each other.
H-bonding
Not unique to water.
-Weak but cooperative.
-H-bonds are important because they are weak.
Lewis Acid
Atom that looks for or accepts 2 electrons (hydrogen in water) (notes)
Lewis Base
Atom that makes available 2 electrons (oxygen in water) (notes)
Bronsted Acid
H+ Donor (notes)
Bronstead Base
H+ Acceptor (notes)
Hydrophobic forces/interactions
-Forces that hold non-polar regions of molecules together.
-They result from the tendency of water to exclude hydrophobic groups.
-Relatively weak compared to H bonds.
Dispersion of lipids in water
Each lipid molecule forces surrounding water molecules to become highly organized
For clusters of lipid molecules, only lipid portions at the edge of the cluster force the ordering of water molecules. Fewer water molecules are ordered like this, and so entropy is increased.
Van der Waals Interactions (Dispersion Forces)
-attractive forces between 2 atoms when ~ 3 to 4 Ao apart.
-Weaker and less specific. Transient electric dipole.
-As two nuclei draw closer together, their e- clouds begin to repel each other. At the point where the van der Waals attraction exactly balances this repulsive force, the nuclei are said to be in van der Waals contact.
-Each atom has a van der Waals radius.
IONIZATION OF WATER, WEAK ACIDS AND BASES
Water has a small degree of ionization (reversible), which is crucial to the role of water in cellular functions.
H2O <—> H+ + OH-
Ionization information
at 25 oC [H2O] = 55.5 M essentially constant
Kw = 1 X 10-14 Ion product of water
When [H+] = [OH-] = 1 X 10-7 Neutral pH (add up to the 1 x 10 -14 Kw)
pH affects the structure and activity of macromolecules.
Ka
Dissociation (ionization) constant
-constant for a given acid at T = C
The greater the Ka the stronger the acids and vise versa.
Since Ka values can be expressed as
pKa = - logKa —> the smaller the pKa the stronger the acid
The pH of a solution is solely dependent on the equilibrium concentrations of the conjugate acid and base.
pH = pKa + log [conj base]
[conj acid]
If the concentration of the conjugate base and acid are the same then CB/CA = 1 and log of 1 = 0 so it’s eliminated from equation and pH = pKa
(Henderson-Hasselback equation, applies only to weak acids and bases)
When this is true the solution has maximum buffering capacity (flat line on curve in notes), is the point at which the acid is half ionized.
When the pH < pka (flatline on curve) the acid predominates. (The H+ is on).
When the pH > pka (flat line on curve) the base predominates. (The H+ is off)
Buffers
-Aqueous solutions that tend to resist changes in pH when small amounts of acid or base are added (water is not a good buffer, pH changes easily with additions of h+ or OH-)
-Do NOT prevent changes in pH.
-Effective +/- 1 pH unit from pKa
-Buffers have limited capabilities. (capacities)
-The ability to resist changes in pH is directly proportional to the total [Acid] and [base].
-Maximal when pH = pKa.
-a weak acid plus its (strong) conjugate base
-a weak base plus its (strong) conjugate acid
Buffer effectiveness
A solution remains a buffer until the ratio is = 1/10 (pH change = 1 unit), once the ratio is < 1/10 the pH ofvthe solution decreases rapidly (for acids/H+)
The solution remains a buffer until the ratio is = 10/1 (pH change = 1 unit), once the ratio is >10/1 the pH of the solution increase rapidly (for bases when OH- is added)
BUFFERING IN CELLS
Need to keep pH in cells at 7.4, this is done (by/in part) with inorganic phosphate buffer H3PO4. It has 3 inflection points/flat lines on curve due to its 3 protons, and therefore 3 different pK values/buffer zones
It’s pK2 is the only one that matters/is relevant at physiological conditions due to it being 6.85 due to it being close to intracellular pH, as cytoplasmic pH = 6.9 – 7.4.
BICARBONATE BUFFER
-CO2 produced by the oxidation of food stuffs.
-Most of it is exhaled but some of is used in buffering of blood.
-Dissolved CO2, which exchanges with gaseous CO2 in the alveolar tissues of the lungs.
CO2(g) <---> CO2(d)
-Carbonic anhydrase in red blood cells (erythrocytes) and in the kidney tube cells hydrates CO2(d) into carbonic acid
CO2(d) + H2O <—> H+ + HCO3-1 pKa = 6.31
pH = pKa + log [HCO3-1 ]
[CO2(d)]
7.4 = 6.31 + log [HCO3-1 ] = 10.7/1 normal
[CO2(d)]
