NEET MDS Lessons
Biochemistry
The Bicarbonate Buffer System
This is the main extracellular buffer system which (also) provides a means for the necessary removal of the CO2 produced by tissue metabolism. The bicarbonate buffer system is the main buffer in blood plasma and consists of carbonic acid as proton donor and bicarbonate as proton acceptor :
H2CO3 = H+ + HCO3–
If there is a change in the ratio in favour of H2CO3, acidosis results.
This change can result from a decrease in [HCO3 − ] or from an increase in [H2CO3 ]
Most common forms of acidosis are metabolic or respiratory
Metabolic acidosis is caused by a decrease in [HCO3 − ] and occurs, for example, in uncontrolled diabetes with ketosis or as a result of starvation.
Respiratory acidosis is brought about when there is an obstruction to respiration (emphysema, asthma or pneumonia) or depression of respiration (toxic doses of morphine or other respiratory depressants)
Alkalosis results when [HCO3 − ] becomes favoured in the bicarbonate/carbonic acid ratio
Metabolic alkalosis occurs when the HCO3 − fraction increases with little or no concomitant change in H2CO3
Severe vomiting (loss of H+ as HCl) or ingestion of excessive amounts of sodium bicarbonate (bicarbonate of soda) can produce this condition
Respiratory alkalosis is induced by hyperventilation because an excessive removal of CO2 from the blood results in a decrease in [H2CO3 ]
Alkalosis can produce convulsive seizures in children and tetany, hysteria, prolonged hot baths or lack of O2 as high altitudes.
The pH of blood is maintained at 7.4 when the buffer ratio [HCO3 − ] / [ H2CO3] becomes 20
BIOLOGICAL ROLES OF LIPID
Lipids have the common property of being relatively insoluble in water and soluble in nonpolar solvents such as ether and chloroform. They are important dietary constituents not only because of their high energy value but also because of the fat-soluble vitamins and the essential fatty acids contained in the fat of natural foods
Nonpolar lipids act as electrical insulators, allowing rapid propagation of depolarization waves along myelinated nerves
Combinations of lipid and protein (lipoproteins) are important cellular constituents, occurring both in the cell membrane and in the mitochondria, and serving also as the means of transporting lipids in the blood.
Thyroid Hormones
Thyroid hormones (T4 and T3) are tyrosine-based hormones produced by the follicular cells of the thyroid gland and are regulated by TSH made by the thyrotropes of the anterior pituitary gland, are primarily responsible for regulation of metabolism. Iodine is necessary for the production of T3 (triiodothyronine) and T4 (thyroxine).
A deficiency of iodine leads to decreased production of T3 and T4, enlarges the thyroid tissue and will cause the disease known as goitre.
Thyroid hormones are transported by Thyroid-Binding Globulin
Thyroxine binding globulin (TBG), a glycoprotein binds T4 and T3 and has the capacity to bind 20 μg/dL of plasma.
Diseases
1. Hyperthyroidism (an example is Graves Disease) is the clinical syndrome caused by an excess of circulating free thyroxine, free triiodothyronine, or both. It is a common disorder that affects approximately 2% of women and 0.2% of men.
2 Hypothyroidism (an example is Hashimoto’s thyroiditis) is the case where there is a deficiency of thyroxine, triiodiothyronine, or both.
The basic characteristics of enzymes includes
(i) Almost all the enzymes are proteins and they follow the physical and chemical reactions of proteins (ii) Enzymes are sensitive and labile to heat
(iii) Enzymes are water soluble
(iv) Enzymes could be precipitated by protein precipitating agents such as ammonium sulfate and trichloroacetic acid.
COPPER
The normal serum level of copper is 25 to 50 mg/dl.
Functions of copper
(a) Copper is necessary for iron absorption and incorporation of iron into hemoglobin.
(b) It is very essential for tyrosinase activity
(c) It is the co-factor for vitamin C requiring hydroxylation
(d) Copper increases the level of high density lipo protein and protects the heart.
Wilson’s disease
In case of Wilson’s disease ceruloplasmin level in blood is drastically reduced.
Wilson’s disease leads to
(i) Accumulation of copper in liver leads to hepatocellular degeneration and cirrhosis
(ii) Deposition of copper in brain basal ganglia leads to leticular degeneration
(iii) Copper deposits as green pigmented ring around cornea and the condition is called as Kayser-Kleischer ring
Over accumulation of copper can be treated by consumption of diet containg low copper and injection of D-penicillamine, which excretes copper through urine.
Menke’s kidney hair syndrome
It is X-linked defect. In this condition copper is absorbed by GI tract, but cannot be transported to blood. The defect in transport of copper to blood is due to absence of an intracellular copper binding ATPase.
VITAMINS
Based on solubility Vitamins are classified as either fat-soluble (lipid soluble) or water-soluble. Vitamins A, D, E and K are fat-soluble
Vitamin C and B is water soluble.
B-COMPLEX VITAMINS
Eight of the water-soluble vitamins are known as the vitamin B-complex group: thiamin (vitamin B1), riboflavin (vitamin B2), niacin (vitamin B3), vitamin B6 (pyridoxine), folate (folic acid), vitamin B12, biotin and pantothenic acid.
CLASSIFICATION OF LIPIDS
Lipids are classified as follows:
1. Simple lipids: Esters of fatty acids with various alcohols.
(a) Fats: Esters of fatty acids with glycerol. Oils are fats in the liquid state. A long-chain carboxylic acid; those in animal fats and vegetable oils often have 12–22 carbon atoms.
(b) Waxes: Esters of fatty acids with higher molecular weight monohydric alcohols. Waxes are carboxylic acid esters, RCOOR’ ,with long, straight hydrocarbon chains in both R groups
2. Complex lipids: Esters of fatty acids containing groups in addition to an alcohol and a fatty acid.
(a) Phospholipids: Lipids containing, in addition to fatty acids and an alcohol, a phosphoric acid residue. They frequently have nitrogen containing bases and other substituents,
Eg glycerophospholipids the alcohol is glycerol
sphingophospholipids the alcohol is sphingosine.
(b) Glycolipids (glycosphingolipids): Lipids containing a fatty acid, sphingosine, and carbohydrate. These lipids contain a fatty acid, carbohydrate and nitrogenous base. The alcohol is sphingosine, hence they are also called as glycosphingolipids. Clycerol and phosphate are absent
e.g., cerebrosides, gangliosides.
(c) Other complex lipids: Lipids such as sulfolipids and aminolipids. Lipoproteins may also be placed in this category.
3. Precursor and derived lipids: These include fatty acids, glycerol, steroids, other alcohols, fatty aldehydes, and ketone bodies, hydrocarbons, lipid soluble vitamins, and hormones. Because they are uncharged, acylglycerols (glycerides), cholesterol, and cholesteryl esters are termed neutral lipids
4. Miscellaneous lipids: These include a large number of compounds possessing the characteristics of lipids e.g., carotenoids, squalene, hydrocarbons such as pentacosane (in bees wax), terpenes etc.
NEUTRAL LIPIDS: The lipids which are uncharged are referred to as neutral lipids. These are mono-, di-, and triacylglycerols, cholesterol and cholesteryl esters.