During fasting or carbohydrate starvation, oxaloacetate is depleted in liver because it is used for gluconeogenesis. This impedes entry of acetyl-CoA into Krebs cycle. Acetyl-CoA then is converted in liver mitochondria to ketone bodies, acetoacetate and b-hydroxybutyrate.

 Three enzymes are involved in synthesis of ketone bodies:

b-Ketothiolase. The final step of the b-oxidation pathway runs backwards, condensing 2 acetyl-CoA to produce acetoacetyl-CoA, with release of one CoA.

HMG-CoA Synthase catalyzes condensation of a third acetate moiety (from acetyl-CoA) with acetoacetyl-CoA to form hydroxymethylglutaryl-CoA (HMG-CoA).

HMG-CoA Lyase cleaves HMG-CoA to yield acetoacetate plus acetyl-CoA.

 b-Hydroxybutyrate Dehydrogenase catalyzes inter-conversion of the ketone bodies acetoacetate and b-hydroxybutyrate.

Ketone bodies are transported in the blood to other tissue cells, where they are converted back to acetyl-CoA for catabolism in Krebs cycle

MAGNESIUM

The normal serum level of Magnesium is 1.8 to 2.2. mg/dl.

Functions of Magnesium

(a) Irritability of neuromuscular tissues is lowered by Magnesium

(b) Magnesium deficiency leads to decrease in Insulin dependent uptake of glucose

(c) Magnesium supplementation improves glucose tolerance

Causes such as liver cirrhosis, protein calorie malnutrition and hypo para thyroidism leads to hypomagnesemia

The main causes of hypermagnesemia includes renal failure, hyper para thyroidism, rickets, oxalate poisoning and multiple myeloma.

CHOLESTEROL AND ITS IMPORTANCE

Cholesterol is an important lipid found in the cell membrane. It is a sterol, which means that cholesterol is a combination of a steroid and an alcohol .

It is an important component of cell membranes and is also the basis for the synthesis of other steroids, including the sex hormones estradiol and testosterone, as well as other steroids such as cortisone and vitamin D.

In the cell membrane, the steroid ring structure of cholesterol provides a rigid hydrophobic structure that helps boost the rigidity of the cell membrane.

Without cholesterol the cell membrane would be too fluid. In the human body, cholesterol is synthesized in the liver.

Cholesterol is insoluble in the blood, so when it is released into the blood stream it forms complexes with lipoproteins.

Cholesterol can bind to two types of lipoprotein, called high-density lipoprotein (HDL) and low-density lipoprotein (LDL).

A lipoprotein is a spherical molecule with water soluble proteins on the exterior. Therefore, when cholesterol is bound to a lipoprotein, it becomes blood soluble and can be transported throughout the body.

HDL cholesterol is transported back to the liver. If HDL levels are low, then the blood level of cholesterol will increase.

High levels of blood cholesterol are associated with plaque formation in the arteries, which can lead to heart disease and stroke.

K_eq, K_w and pH

As H₂O is the medium of biological systems one must consider the role of this molecule in the dissociation of ions from biological molecules. Water is essentially a neutral molecule but will ionize to a small degree. This can be described by a simple equilibrium equation:

H₂O <-------> H⁺ + OH^-

This equilibrium can be calculated as for any reaction:

K_eq = [H⁺][OH^-]/[H₂O]

Since the concentration of H₂O is very high (55.5M) relative to that of the [H⁺] and [OH^-], consideration of it is generally removed from the equation by multiplying both sides by 55.5 yielding a new term, K_w:

K_w = [H⁺][OH^-]

This term is referred to as the ion product. In pure water, to which no acids or bases have been added:

K_w = 1 x 10^-14 M²

As K_w is constant, if one considers the case of pure water to which no acids or bases have been added:

[H⁺] = [OH^-] = 1 x 10^-7 M

This term can be reduced to reflect the hydrogen ion concentration of any solution. This is termed the pH, where:

pH = -log[H⁺]

Sphingosine is an amino alcohol present in sphingomyelins (sphingophospholipids).  They do not contain glycerol at all.

Sphingosine is attached by an amide linkage to a fatty acid to produce ceramide. The alcohol group of sphingosine is bound to phosphorylcholine in sphingomyelin structure. .

Sphingomyelins are important constituents of myelin and are found in good quantity in brain and nervous tissues.

CALCIUM

Total calcium in the human body is 1 to 1.5kg, out of which 99% is seen in bone and 1% in extracellular fluid. The main source of calcium is milk.

The daily requirement of calcium for child is 1200mg/day and for adult it is 500mg/day. During pregnancy /lactation the calcium requirement is 1500mg/day.

The absorption of calcium takes place in 1st and 2nd part of deuodenum. Calcium absorption requires carrier protein, helped by Ca²⁺ - dependent ATpase.

Factors responsible for increase in calcium absorption include Vitamin D, Parathyroid hormone, acidity and amino acids. Factors such as phytic acid,oxalates, malabsorption  syndromes and Phosphates decreases calcium absorption. The normal calcium level in blood is 9-11mg/dl.