L•ds are nonpolar organic molecules that are insolubfr- in polar water but s luble in nonpolar organic solvents like ether, alcoh( , and chloroform. P ospholipids and cholesterol are lipids. They are important constituents of cell m mbranes. The most common lipids in animals are fats.

F ts

F ts are esters of fatty acids and glycerol. Fats build cell parts and supply e ergy for cellular activities. Fats have following properties:

  1. Lipid molecules are primarily composed of carbon, hydrogen, and oxygen atoms. Some lipids also contain small amounts of phosphorus and nitrogen. They contain a much smaller proportion of oxygen than carbohydrates. For example the formula of the fat tristearin is C57H1,006.
  2. The building blocks of fat molecules are fatty acids and glycerol. Fatty acids contain long hydrocarbon chains bonded to carboxyl (C001-1) groups. Glycerol is a three-carbon alcohol, with each carbon bearing a hydroxyl (OH) group. Three fatty acid molecules combine with one glycerol molecule. They are attached to the three carbon atoms in the glycerol backbone.3. The fats with three fatty acids are called triglyceride neutral fat or triacylglycerol. The glycerol portion of every fat molecule is the same. But there are many kinds of fatty acids. Therefore, there are many kinds of fats. Fatty acid molecules differ in the length of their carbon chains and in the ways the carbon atoms combine. The most common are even-numbered chains of 14 to 20 carbons.Types of fats

    There are two types of fats:

    (a)    Unsaturated fats: They have double bonds. Their chains bend at the double bonds. So the fat molecules cannot align closely with one another. Therefore, they have low melting points. Thus the fat may be fluid at room temperature. A liquid fat is called oil. Most plant fats are unsaturated. Fatty acids with one double bond are monounsaturated. The fatty acids with numerous double bonds are polyunsaturated.

    (b)   Saturated fatty acids: They do not have double bonds. Animal fats are often saturated. They occur as hard or solid fats. In this case, the carbon atoms join by single carbon-carbon bonds. Each carbon atom binds to many hydrogen atoms.

  3. bn
  4. PhospholipidsA phospholipids molecule is similar to a fat molecule. It contains a glycerol portion and fatty acid chains. But phospholipids have only two fatty acid chains. The nitrogen containing groups replace the third chain. There are two parts of phospholipids:
    • Head: The polar phosphate and nitrogen groups are soluble in water (hydrophilic). They form the “head” of the molecule.
    • Tail: The insoluble (nonpolar, hydrophobic) fatty acid portion forms the “tail.” Phospholipids have double tendency. They are soluble at one end and insoluble at the other end. So they are the major structural components of cell membranes.


    They are naturally occurring, lipid-soluble molecules. They are composed of four fused carbon rings. It forms a rigid structure. Three of the rings are six-sided. The fourth is five-sided. The four rings contain a total of 17 carbons. Cholesterol is an important biologically active steroid. Similarly some important steroids are vitamin D and hormones of the adrenal gland (e.g. aldosterone), the ovaries (e.g., estrogen), and the testes (e.g., testosterone).


  5. ch




Proteins are polymers of amino acids. Proteins always contain atoms of

arbon, hydrogen, nitrogen, oxygen, and sometimes sulfur. Proteins have

ollowing physiological functions:

Proteins are structural material.

They act as energy sources.

Proteins are used against disease ih higher animals.

Proteins act as chemical messengers (hormones).

They act as receptors on cell membranes. Some proteins act as enzymes. Enzymes play important roles in metabolic reactions.

7. They enter and speed up specifib chemical reactions.

Amino Acids

Amino acids are the building blocks of proteins. Amino acids contain an amino group (NH2), a

carboxyl group (COOH), a hydrogen atom, and                      H2N—C—COOH

a functional group R. They all are bonded to a

central carbon atom

The nature of the R group determines the chemical properties of each amino acid. Twenty different amino acids commonly occu; in animals. Peptide bonds join amino acids in chains. Peptide bonds are covalent bonds. The carboxyl group of one amino acid bonds to the amino group of another amino acid in peptide bond. One molecule of water is released. The chain of amino acids can

vary from fewer than 50 to more than 2000,       amino acids in different proteins.

(a)      Two amino acids bond to form dipeptide.

(b)     Three bonded amino acids form a tripeptide.

(c)      Many amino acids bond to form a chain called a polypeptide.

H       H                         H H                                            H 0


e °     I   I                         I °    It

N—CC                       N—CC                              N—C —C—N—C—C

I       I                           I        I

/I                                I        I


H       R                        OH H R           OH                        R              H H           OH




Fig: Levels of protein structure (a) Primary structure, (b) Secondary structure, (c) tertiary structure.

(d) Quaternary structure

tructure of protein

ach type of protein contains a specific number and kind of amino acids. These mino acids are arranged in a particular sequence. The protein molecule may be oiled and folded. They may interact with other protein molecules and form a nique three dimensional structure. Different kinds of protein molecules have ifferent shapes. Their shapes are related to their particular functions in life rocesses. A protein molecule has several different levels of structure.

  • Primary structure: The linear sequence of amino acids in the polypeptide chains is called primary structure.
  • Secondary structure: The secondary structure of a protein is formed by a repeating pattern of bonds (hydrogen bonds) between amino acids. It commonly takes the shape of:
  • Alpha helix
  • Pleated sheet

3 Tertiary structure: Tertiary structure of protein is formed by folding of helix into a three dimensional shape.

4 Quaternary structure: In this case, the protein chains join to form a larger protein. This structure is called quaternary structure,


Ncleic acids are polynucleotides. They are composed of monomers

n  cleotides.


Ncleotides are small organic compounds. They are essential for life. Each

ücleotide is composed of three substances:

  1. Nitrogen containing organic base, Adenine, Guanine, cytosine, thymine and uracil.
  2. A five-carbon sugar (ribose or deoxyribose)
  3. Phosphate.
  • enosine phosphates and nucleotide coenzymes are nucleotides. The nuleotides perform three important roles in the cell. These are:

1. Role in transformation of energy: ATP, ADP, and. AMP are small molecules.. They are energy transferring molecules.

2 Chemical messenger: Cyclic adenosine monophosphate (cAMP) acts as a chemical messenger.

3. Acts as coenzyme: Some nucleotides like NAD and FAD acts as coenzymes. They work with the enzymes.

Nucleic acids

Nu leic acids are large single or double-stranded chains of nucleotide subunits. Th two nucleic acids are:

(a) eoxyribonucleic acid (DNA): DNA makes up the chromosomes in the

nucleus of cell. It can replicate itself. It contains the genetic (hereditary) information of the cell.

(b) Ribonucleic acid (RNA): RNA is present in the nucleus and cytoplasm It carries information between the DNA and ribosome for protein synthesis.




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