By rearranging the above equation we arrive at the Henderson-Hasselbalch equation:

pH = pK_a + log[A^-]/[HA]

It should be obvious now that the pH of a solution of any acid (for which the equilibrium constant is known, and there are numerous tables with this information) can be calculated knowing the concentration of the acid, HA, and its conjugate base [A^-].

At the point of the dissociation where the concentration of the conjugate base [A^-] = to that of the acid [HA]:

pH = pK_a + log[1]

The log of 1 = 0. Thus, at the mid-point of a titration of a weak acid:

pK_a = pH

In other words, the term pK_a is that pH at which an equivalent distribution of acid and conjugate base (or base and conjugate acid) exists in solution.

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.

LIPOPROTIENS

Lipoproteins Consist of a Nonpolar Core & a Single Surface Layer of Amphipathic Lipids

The nonpolar lipid core consists of mainly triacylglycerol and cholesteryl ester and is surrounded by a single surface layer of amphipathic phospholipid and cholesterol molecules .These are oriented so that their polar groups face outward to the aqueous medium. The protein moiety of a lipoprotein is known as an apolipoprotein or apoprotein,constituting nearly 70% of some HDL and as little as 1% of Chylomicons. Some apolipoproteins are integral and cannot be removed, whereas others can be freely transferred to other lipoproteins.

There  re five types of lipoproteins, namely chylomicrons, very low density lipoproteins(VLDL)  low density lipoproteins (LDL), high density Lipoproteins (HDL) and free fatty acid-albumin complexes.

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.

PHOSPHOLIPIDS

These are complex or compound lipids containing phosphoric acid, in addition to fatty acids, nitrogenous base and alcohol 

There are two  classes of phospholipids

1. Glycerophospholipids (or phosphoglycerides) that contain glycerol as the alcohol.

2. Sphingophospholipids (or sphingomyelins) that contain sphingosine as the alcohol

Glycerophospholipids

Glycerophospholipids are the major lipids that occur in biological membranes. They consist of glycerol 3-phosphate esterified at its C1 and C2 with fatty acids. Usually, C1 contains a saturated fatty acid while C2 contains an unsaturated fatty acid.

In glycerophospholipids, we refer to the glycerol residue (highlighted red above) as the "glycerol backbone."

Glycerophospholipids are Amphipathic

Glycerophospholipids are sub classified as

1. Phosphatidylethanolamine or cephalin also abbreviated as PE is found in biological membranes and composed of ethanolamine bonded to phosphate group on diglyceride.

2. Phosphatidylcholine or lecithin or PC which has chloline bonded with phosphate group and glycerophosphoric acid with different fatty acids like palmitic or hexadecanoic acid, margaric acid, oleic acid. It is a major component of cell membrane and mainly present in egg yolk and soy beans.

3. Phosphatidic acid (phosphatidate) (PA)

It consists of a glycerol with one saturated fatty acid bonded to carbon-1 of glycerol and an unsaturated fatty acid bonded to carbon-2 with a phosphate group bonded to carbon-3.

4.Phosphatidylserine (PS)

This phospholipid contains serine as an organic compound with other main components of phospholipids. Generally it found on the cytosolic side of cell membranes.

5. Phosphoinositides

It is a group of phospholipids which are negatively charged and act as a a minor component in the cytosolic side of eukaryotic cell membranes. On the basis of different number of phosphate groups they can be different types like phosphatidylinositol phosphate (PIP), phosphatidylinositol bisphosphate(PIP2) and phosphatidylinositol trisphosphate (PIP3). PIP, PIP2 and PIP3 and collectively termed as phosphoinositide.

6. Cardiolipin :

lt is so named as it was first isolated from heart muscle. Structurally, a cardiolipin consists of two molecules of phosphatidic acid held by an additional glycerol through phosphate groups. lt is an important component of inner mitochondrial membrane. Cardiolipin is the only phosphoglyceride that possesses antigenic properties.

General structure of amino acids

• All organisms use same 20 amino acids.
• Variation in order of amino acids in polypeptides allow limitless variation.
• All amino acids made up of a chiral carbon attached to 4 different groups      

 - hydrogen
 - amino group
 - carboxyl
 - R group: varies between different amino acids

• Two stereoisomers (mirror images of one another) can exist for each amino acid. Such stereoisomers are called enantiomers. All amino acids found in proteins are in the L configuration.
• Amino acids are zwitterions at physiological pH 7.4. ( i.e. dipolar ions). Some side chains can also be ionized

Structures of the 20 common amino acids

• Side chains of the 20 amino acids vary. Properties of side chains greatly influence overall conformation of protein. E.g. hydrophobic side chains in water-soluble proteins fold into interior of protein
• Some side chains are nonpolar (hydrophobic), others are polar or ionizable at physiological pH (hydrophilic).
• Side chains fall into several chemical classes: aliphatic, aromatic, sulfur-containing, alcohols, bases, acids, and amides. Also catagorized as to hydrophobic vs hydrophilic.
• Must know 3-letter code for each amino acid.

Aliphatic R Groups

• Glycine: least complex structure. Not chiral. Side chain small enough to fit into niches too small for other amino acids.
• Alanine, Valine, Leucine, Isoleucine
  • no reactive functional groups      
  • highly hydrophobic: play important role in maintaining 3-D structures of proteins because of their tendency to cluster away from water
• Proline has cyclic side chain called a pyrolidine ring. Restricts geometry of polypeptides, sometimes introducing abrupt changes in direction of polypeptide chain.

Aromatic R Groups

• Phenylalanine, Tyrosine, Tryptophan
  • Phe has benzene ring therefore hydrophobic.  
  • Tyr and Trp have side chains with polar groups, therefore less hydrophobic than Phe.
  • Absorb UV  280 nm. Therefore used to estimate concentration of proteins.

Sulfur-containing R Groups

• Methionine and Cysteine)
  • Met is hydrophobic. Sulfur atom is nucleophilic.
  • Cys somewhat hydrophobic. Highly reactive. Form disulfide bridges and may stabilize 3-D structure of proteins by cross-linking Cys residues in peptide chains.

Side Chains with Alcohol Groups

• Serine and Threonine
  • have uncharged polar side chains. Alcohol groups give hydrophilic character.
  • weakly ionizable.

Basic R Groups

• Histidine, Lysine, and Arginine.
  • have hydrophilic side chains that are nitrogenous bases and positively charged at physiological pH.
  • Arg is most basic a.a., and contribute positive charges to proteins.

Acidic R Groups and their Amide derivatives

• Aspartate, Glutamate
  • are dicarboxylic acids, ionizable at physiological pH. Confer a negative charge on proteins.
• Asparagine, Glutamine
  • amides of Asp and Glu rspectively
  • highly polar and often found on surface of proteins
  • polar amide groups can form H-bonds with atoms in other amino acids with polar side chains.