NEET MDS Lessons
Biochemistry
BIOLOGICAL BUFFER SYSTEMS
Cells and organisms maintain a specific and constant cytosolic pH, keeping biomolecules in their optimal ionic state, usually near pH 7. In multicelled organisms, the pH of the extracellular fluids (blood, for example) is also tightly regulated. Constancy of pH is achieved primarily by biological buffers : mixtures of weak acids and their conjugate bases
Body fluids and their principal buffers
Body fluids Principal buffers
Extracellular fluids {Biocarbonate buffer Protein buffer }
Intracellular fluids {Phosphate buffer, Protein }
Erythrocytes {Hemoglobin buffer}
Ampholytes, Polyampholytes, pI and Zwitterion
Many substances in nature contain both acidic and basic groups as well as many different types of these groups in the same molecule. (e.g. proteins). These are called ampholytes (one acidic and one basic group) or polyampholytes (many acidic and basic groups). Proteins contains many different amino acids some of which contain ionizable side groups, both acidic and basic. Therefore, a useful term for dealing with the titration of ampholytes and polyampholytes (e.g. proteins) is the isoelectric point, pI. This is described as the pH at which the effective net charge on a molecule is zero.
For the case of a simple ampholyte like the amino acid glycine the pI, when calculated from the Henderson-Hasselbalch equation, is shown to be the average of the pK for the a-COOH group and the pK for the a-NH2 group:
pI = [pKa-(COOH) + pKa-(NH3+)]/2
For more complex molecules such as polyampholytes the pI is the average of the pKa values that represent the boundaries of the zwitterionic form of the molecule. The pI value, like that of pK, is very informative as to the nature of different molecules. A molecule with a low pI would contain a predominance of acidic groups, whereas a high pI indicates predominance of basic groups.
Enzymes are protein catalyst produced by a cell and responsible ‘for the high rate’ and specificity of one or more intracellular or extracellular biochemical reactions.
Enzymes are biological catalysts responsible for supporting almost all of the chemical reactions that maintain animal homeostasis. Enzyme reactions are always reversible.
The substance, upon which an enzyme acts, is called as substrate. Enzymes are involved in conversion of substrate into product.
Almost all enzymes are globular proteins consisting either of a single polypeptide or of two or more polypeptides held together (in quaternary structure) by non-covalent bonds. Enzymes do nothing but speed up the rates at which the equilibrium positions of reversible reactions are attained.
In terms of thermodynamics, enzymes reduce the activation energies of reactions, enabling them to occur much more readily at low temperatures - essential for biological systems.
Riboflavin: Vitamin B2
Riboflavin, or vitamin B2, helps to release energy from foods, promotes good vision, and healthy skin. It also helps to convert the amino acid tryptophan (which makes up protein) into niacin.
RDA Males: 1.3 mg/day; Females: 1.1 mg/day
Deficiency : Symptoms of deficiency include cracks at the corners of the mouth, dermatitis on nose and lips, light sensitivity, cataracts, and a sore, red tongue.
Factors regulating blood calcium level
(i) Vitamin D
(a) Vitamin D and absorption of calcium: Active form of calcium is calcitriol. Calcitriol enters intestinal wall and binds to cytoplasmic receptor and then binds with DNA causes depression and consequent transcription of gene code for calbindin. Due to increased availability of calbindin, absorption of calcium increases leading to increased blood calcium level.
(b) Vitamin D and Bone: Vitamin D activates osteoblast, bone forming cells & also stimulates secretion of alkaline phosphatase. Due to this enzyme, calcium and phosphorus increase.
(c) Vitamin D and Kidney: Calcitriol increase reabsorption of calcium and phosphorus by renal tubules.
(ii) Parathyroid hormone (PTH)
Normal PTH level in serum is 10-60ng/l.
(a) PTH and bones: In bone, PTH causes demineralization. It also causes recreation of collagenase from osteoclast leads to loss of matrix and bone resorption. As a result, mucopolysacharides and hydroxyproline are excreted in urine.
(b) PTH and Kidney: In kidney, PTH causes increased reabsorption of calcium but decreases reabsorption of phosphorus from kidney tubules.
(iii) Calcitonin Calcitonin decreases serum calcium level. It inhibits resorption of bone. It decreases the activity of osteoclasts and increases osteoblasts.
Hyper Calcemia When plasma Ca2+ level is more than 11mg/dl is called Hypercalcemia. It is due to parathyroid adenoma or ectopic PTH secreting tumor. calcium excreted in urine decreases excretion of chloride causing hyperchloremic acidosis.
Hypocalcemia Plasma calcium level less than 8mg/dl is called hypocalcemia. Tetany due to accidental surgical removal of parathyroid glands or by autoimmune disease. In tetany, neuromuscular irritability is increased. Increased Q-7 internal in ECG is seen. Main manifestation is carpopedal spasm. Laryngismus and stridor are also observed.
Titration of a weak acid with a strong base
• A weak acid is mostly in its conjugate acid form
• When strong base is added, it removes protons from the solution, more and more acid is in the conjugate base form, and the pH increases
• When the moles of base added equals half the total moles of acid, the weak acid and its conjugate base are in equal amounts. The ratio of CB / WA = 1 and according to the HH equation, pH = pKa + log(1) or pH = pKa.
• If more base is added, the conjugate base form becomes greater till the equivalance point when all of the acid is in the conjugate base form.
The pH scale
An acidic solution is one in which [H+ ] > [OH- ]
•In an acidic solution, [H+ ] > 10-7 , pH < 7.
•A basic solution is when [OH- ] > [H+ ].
•In a basic solution, [OH- ] > 10-7 , pOH < 7, and pH >7.
• When the pH = 7, the solution is neutral.
•Physiological pH range is 6.5 to 8.0