Water
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. |
Covalent bond |
Inside a molecule |
Hydrogen bond |
between molecules |
Polar bonds (H2O) |
uneven charge |
Non-polar bonds (O2) |
even charge |
Henderson-Hasselbalch equation
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pH (potential hydrogen)
The acidity of a solution is defined by the concentration of H+ ions it possesses. |
pH scale |
|
pure water pH (7) |
[H+] = 10-7 moles/liter |
acids |
substance, proton (H+) donors |
bases |
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. |
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Buffers
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. |
H 2
PO 4-
/ HPO 4
2- is the principal buffer in cells, H 2
CO 3
/ HCO 3- 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. |
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