Molecular base of life Water
Water is the most abundant liquid on the surface of the Earth and life as we know
it demands water. Most cells are around 65% water by volume and 70% by mass,
Water drives folding and assembly of biopolymers and supports the formation of
membranes and other biological structures. Similarly, it is well documented that water
is produced, consumed, altered, or utilized transiently during enzymatic reactions
about one third to one half of known enzymatic reactions consume or produce water.
Water accounts for 70% or more of the weight of most organisms. In human adults
also, total body water accounts for about 70% of the lean body mass (Lean body mass
is a component of body composition, calculated by subtracting body fat weight from
total body weight: total body weight is lean plus fat. In equations: LBM = BW − BF).
In obese males, water constitutes a lower percentage of body weight (45–60%)
than in lean individuals (55 – 70%). Adult lean females have low water content (45 –
60%) and the value in infants can be in the range 65 – 75%.
Distribution of water in the body
Structure of water
-Asymmetry of a water molecule and distribution of electrons result in a dipole
structure with the oxygen end of the molecule negatively charged and the hydrogen
end of the molecule positively charged.
-Dipole structure of water molecule produces an electrostatic bond (hydrogen
bond) between water molecules
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-Hydrogen bonds form when the positive end of one water molecule bonds to the
negative end of another water molecule.
Physiochemical properties of water.
Unique properties of water include:
1- Higher melting and boiling point than other hydrogen compounds.
2- High heat capacity (Amount of heat to raise T of 1 g by 1C0 ) Water has high heat
capacity (1 calorie). Water's high heat capacity is a property caused by hydrogen
bonding among water molecules. When heat is absorbed, hydrogen bonds are broken
and water molecules can move freely. When the temperature of water decreases, the
hydrogen bonds are formed and release a considerable amount of energy.
3-Greater solvent power than any other substance.
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Self-ionization of water
H2O(l) + H2O(l) H3O+ (aq) + OH- (aq)
K = [H3O+ ][OH- ]/[H2O]2 K[H2O]2 = [H3O+ ][OH- ]
* activity of pure water is 1
Kw = [H3O+ ][OH- ] Kw is called ion product of water At 25 °C, Kw = 1.0 x 10-14
* Valid also for dilute aqueous solutions
Value of Kw varies with temperature
A “neutral” solution is defined as a solution where [H3O+ ] = [OH- ] = 1.0 x 10-7
When [H3O+ ] > [OH- ] > 1.0 x 10-7, the solution is acidic
When [H3O+ ] < [OH- ] < 1.0 x 10-7, the solution is basic
Note that acidic solutions does not mean [OH- ] is zero and vice versa
-The pH and pOH Scale
The hydronium ion concentration is a measure of a solution’s acidity
Usually small numbers
The pH scale is used to avoid these numbers pH = -log[H3O+ ] [H3O+ ] = 10-pH
Likewise pOH = -log[OH- ] [OH- ] = 10-pOH
Consider a solution at 25 °C [H3O+ ][OH- ] = 1.0 x 10-14
Take the negative log of both sides
-log([H3O+ ][OH- ]) = -log(1.0 x 10-14) -log[H3O+ ] + -log[OH- ] = 14
pH + pOH = 14
pH can be measured using a pH meter or by an indicator
-Weak acids and bases
Weak acids and bases are weak electrolytes
HA(aq) + H2O(l) H3O+ (aq) + A- (aq)
Ka = [H3O+ ][A- ]/[HA] Ka is called the dissociation or ionization constant
similar to the acidity scale pKa = -log Ka
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The larger the value of Ka the smaller the value of pKa and the stronger the acid
For a weak base B
B(aq) + H2O(l) BH+ (aq) + OH- (aq)
Kb = [BH+ ][OH- ]/[B]
As in the case of weak acids, the larger the value of Kb the stronger the base
pKb = -log Kb
Buffers and the common ion effect
Common ion effect: Describes the behavior of solutions in which the same ion is
produced by two different compounds. The resulting solution is called a buffer
Two common ways to produce this effect:
1. Weak acid + soluble ionic salt of the weak acid
2. Weak base + soluble ionic salt of the weak base
1-Weak acid + salt of weak acid buffer
Acetic acid - Sodium acetate solution
CH3COONa CH3COO- + Na+
CH3COOH + H2O CH3COO- + Na+
Sodium acetate dissolves completely to produce a high concentration of acetate
ions which causes equilibrium to shift left CH3COOH does not dissociate and pH
is higher
Solutions containing a weak acid and one of its ionic salts, are always less acidic
than solutions containing the same concentration of the weak acid alone
Consider the dissociation of our weak acid HA Ka = [H3O+ ][OH- ]/[HA]
[H3O+ ] = Ka [HA]/[OH- ]
Henderson-Hasselbalch Equation
Assume the salt is monovalent and its concentration along with the acid is
reasonable [A- ] = [salt] [H3O+ ] = Ka [acid]/[salt]
Take the negative log of both sides
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-log [H3O+ ] = -log([acid]/[salt])
- log Ka pH = log([salt]/[acid]) + pKa
This is the Henderson-Hasselbalch Equation
2-Weak base + salt of weak base buffer
Ammonia- ammonium acetate buffer
NH4Cl NH4 + + Cl-
NH3 + H2O NH4 + + OH-
Most of the NH4 + comes from NH4Cl Shifts the equilibrium for ammonia to the
left. This results in more NH3 and less OH-
pOH = log ([salt]/[base]) + pKb
pH = log ([conjugate base]/[acid]) + pKa
pOH = log ([conjugate acid]/[base]) + pKb
Buffers
A buffer solution resists changes in pH because it is able to react with added acid
or base
Consider sodium acetate/acetic acid buffer
CH3COONa CH3COO- + Na+ (1)
CH3COOH + H2O CH3COO- + Na+ (2)
Add a strong acid produces H3O+
Equilibrium (2) shifts to the left. This reaction occurs to a great extent
The net reaction is H3O+ + CH3COO- CH3COOH + H2O
If strong base is added [OH- ] increases
Affects the autoionization of water 2H2O H3O+ + OH-
Equilibrium shifts left decreasing [H3O+ ] allows more CH3COOH to dissociate
Net result is OH- + CH3COOH CH3COO- + H2O
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Addition of alkali (NaOH) or acid (HCl): Salt is formed, but no free H+ or OH- will
be available.
CH3COOH + CH3COONa + NaOH 2CH3COONa+ H2O
CH3COOH + CH3COONa + HCl NaCl + 2CH3COOH
How does the ammonium chloride-ammonia buffer work (homework)
Biological buffers
- Almost every biological process is pH-dependent; a small change in pH
produces a large change in the rate of the process.
- Principal buffers of:
- ECF: Bicarbonate buffer, Protein buffer
- ICF: Phosphate buffer, Protein buffer
- RBC: Hemoglobin buffer
1- The Bicarbonate buffer system
-
-Principal buffer in blood plasma. Consists of H2CO3 (proton donor) and HCO3
(proton acceptor).
Neutralizes stronger dietary acids and metabolic acids (HA) by converting them to
weak bases (A- ) and increase in H2CO3 .
Strong base (B) Weak acids (BH+) with rise in HCO3 - .
HA + HCO3 - ↔ A- + H2CO3
Formation of H2CO3 depends on dissolved CO2 conc.
B + H2CO3 ↔ BH+ + HCO3 - . which in turn depends on gaseous CO2 .
-
[HCO3 ]/[H2CO3 ] of 20:1 is required to maintain pH of plasma at 7.4.
Neutralization of any acid or base and subsequent change in buffer ratio or blood pH
is neutralized by respiratory elimination of H2CO3 as CO2 or urinary elimination of
HCO3 - .
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Acidosis
Accumulation of acids or loss of alkali lead to fall of [HCO3-]/[H2CO3] below 20.
Ratio in favor of H2CO3.
Two types:
1. Metabolic acidosis: Conc. of plasma HCO3-. decreased due to excessive loss of
bases in renal failure, diabetic ketosis and severe diarrhea.
2. Respiratory acidosis: Retention of CO2 due to hypoventilation lead to rise in
H2CO3, lowers the ratio.
Occurs due to chronic respiratory obstructive airway disease (asthma), prolonged
anesthesia, unconsciousness, etc.
Alkalosis
Accumulation of alkali or loss of acids Increase in ratio Rise in pH.
1. Metabolic alakalois: High intake of alkaline substances, severe vomiting,
indiscriminate use of antacids, etc.
2. Respiratory alakalosis: Excess removal of CO2 from blood due to
hyperventilation lead to decrease in H2CO3
2- The Protein buffer system
Very important in plasma and ICF
Proteins exist as anions serving as conjugate bases (Pr- ) at blood pH 7.4 and form
conjugate acids (HPr) accepting H+ .
Have the capacity to buffer some H2CO3 in blood.
H2CO3 + Pr- HCO3 - + HPR
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Recommended daily fluid intake:
The amount of fluid you need depends on many things including the weather, how
much physical activity you do and your age, but European recommendations suggest
1.6L of fluid per day for women (about 8 200ml glasses) and 2L of fluid per day for
men. And at regular intervals, 8 times a day (before, during and in-between meals).
Whether it comes from an orange, an apple or an egg (1 whole cooked egg weighs
about 50g has 37g water. 74%), your body can pull out water from everything you
consume. on average food provides about 20% of total fluid intake.
The two main mechanisms for maintaining water balance are:
Thirst - this tells us when we need to take in more fluid
Urine output - the kidneys regulate the surplus, or deficit, of the water we
consume by either emptying it into the urinary bladder or holding onto it in the
blood plasma.
