the solvent for virtually all of biochemistry, ~70% of the mass within each cell is water
Carbon has four electrons in the outer shell - these hybridise into 4 sp3 hybrid orbitals as tetrahedron. If symmetrical, the angle is 109.28 degrees.
WATER AS A SOLVENT (substances such as household sugar dissolve in water, means that their molecules separate from each other, each becoming surrounded by water molecules.)
When a substance dissolves in a liquid, the mixture is termed a solution.
The dissolved substance is the solute, and the liquid that does the dissolving is the solvent.
Water is an excellent solvent for many substances because of its polar bonds.
Inside a molecule
Polar bonds (H2O)
Non-polar bonds (O2)
pH (potential hydrogen)
The acidity of a solution is defined by the concentration of H+ ions it possesses.
pH = -log
pure water pH (7)
[H+] = 10-7 moles/liter
substance, proton (H+) donors
substance, proton acceptors (such as OH-)
Water can act as both a weak acid and a weak base.
Acids in an aqueous environment
proton moves from one molecule to the other
pH is a measure of acidity (<7) or alkalinity (>7).
Higher amounts of protons in a solution
results in a lower pH (acidic)
Lower amount of protons
results in a higher pH (basic, or alkali)
Different Enzymes have different optimal pH according to their environment.
The strength of an acid is measured by its dissociation constant, Ka. The larger the Ka the more it dissociates and the stronger the acid.
The pH of a solution of a weak acid and its conjugate base is related to the concentration of the acid and base and the pKa by the Henderson-Hasselbalch equation.
When ph < pKa, the weak acid predominates. When pH > pKa, the conjugate base predominates.
A solution which pH resists change upon addition of either small amounts of strong acid or strong base are added.
(consist of a weak acid and its conjugate base)
BUFFER CAPACITY - is related to the concentrations of the weak acid and its conjugate base,
The greater the concentration of the weak acid and its conjugate base, the greater the buffer capacity.
4- / HPO
42- is the principal buffer in cells, H
3 / HCO3- is an important buffer in blood.
Buffers work because the concentration of the weak acid and base are kept in the narrow window of the titration curve.
Biological Buffer Systems
Maintenance of intracellular pH is vital to all cells:
1. Enzyme-catalyzed reactions have optimal pH,
2. Solubility of polar molecules depends on H-bond donors and acceptors,
3. Equilibrium between CO2 gas and dissolved HCO3- depends on pH.
Buffer systems in vivo are mainly based on:
1. Phosphate, concentration in millimolar range,
2. Bicarbonate, important for blood plasma,
3. Histidine, efficient buffer at neutral pH.
Buffer systems in vitro are often based on sulfonic acids of cyclic amines:
HEPES, PIPES, CHES.