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the solvent for virtually all of biochemistry, ~70% of the mass within each cell is water
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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.
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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.)
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When a substance dissolves in a liquid, the mixture is termed a solution.
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The dissolved substance is the solute, and the liquid that does the dissolving is the solvent.
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Water is an excellent solvent for many substances because of its polar bonds.
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Covalent bond
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Inside a molecule
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Hydrogen bond
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between molecules
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Polar bonds (H2O)
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uneven charge
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Non-polar bonds (O2)
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even charge
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The acidity of a solution is defined by the concentration of H+ ions it possesses.
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pH scale
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pH = -log
10[H+] |
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pure water pH (7)
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[H+] = 10-7 moles/liter
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acids
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substance, proton (H+) donors
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bases
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substance, proton acceptors (such as OH-)
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Water can act as both a weak acid and a weak base.
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Acids in an aqueous environment
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proton moves from one molecule to the other
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pH is a measure of acidity (<7) or alkalinity (>7).
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Higher amounts of protons in a solution
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results in a lower pH (acidic)
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Lower amount of protons
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results in a higher pH (basic, or alkali)
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Different Enzymes have different optimal pH according to their environment.
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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.
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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.
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When ph < pKa, the weak acid predominates. When pH > pKa, the conjugate base predominates.
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A solution which pH resists change upon addition of either small amounts of strong acid or strong base are added.
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(consist of a weak acid and its conjugate base)
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BUFFER CAPACITY - is related to the concentrations of the weak acid and its conjugate base,
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The greater the concentration of the weak acid and its conjugate base, the greater the buffer capacity.
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H
2PO4- / HPO42- is the principal buffer in cells, H2CO3 / HCO3- is an important buffer in blood. |
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Buffers work because the concentration of the weak acid and base are kept in the narrow window of the titration curve.
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Maintenance of intracellular pH is vital to all cells:
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1. Enzyme-catalyzed reactions have optimal pH,
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2. Solubility of polar molecules depends on H-bond donors and acceptors,
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3. Equilibrium between CO2 gas and dissolved HCO3- depends on pH.
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Buffer systems in vivo are mainly based on:
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1. Phosphate, concentration in millimolar range,
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2. Bicarbonate, important for blood plasma,
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3. Histidine, efficient buffer at neutral pH.
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Buffer systems in vitro are often based on sulfonic acids of cyclic amines:
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HEPES, PIPES, CHES.
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