Biology Notes FA FSc Chapter No 2 Biological Molecules

Biological Molecules FA and FSc Note Chapter No 2 for kpk, sindh, Punjab and fbise Short Question, Long Question Exercise, Examples, MCQs. Unique Professor Notes for Class 1st Years.

Short Questions Biological Molecules


Definition: “it is the branch of biology which deaths with the structure, composition, function and chemical processes of molecules and compounds that are present in the bodies of a living organism.”

The most common element by number in the body is hydrogen, next is oxygen.


Definition: “When two or more atoms of the same elements or different elements are rounded together, they form a structure called molecules.


The molecules occurring in the bodies of living organisms are of two types.

Organic Molecules:


1)      Those molecules which contain carbon as a basic element, bounded with hydrogen atoms are known as organic molecules.”

Examples: The organic molecules present in living organisms are carbohydrates, protein, lipids, nucleic acids, enzymes hormones etc.

2) Inorganic molecules:


Those molecules which do not contain carbon as basic elements or hydrogen is not directly bonded with carbon are known as inorganic molecules.”

Exhales: The Inorganic molecules present in the body of living organisms is water minerals (ions).

Read more: Cell structure and function Biology Class 11 Notes Chapter 1

Q.2 ii) What are different kinds of carbohydrates? Give two examples of each.



   “Carbohydrates stand polyhydroxy aldehydes or ketones or importance which yield such combinations on hydrolysis. Carbohydrates contain either aldehyde or ketone as functional groups attached to one of the carbon atoms.”

Types of carbohydrates:

     Carbohydrates contain three major categories of molecules which are:

1- Monosaccharides:

     Monosaccharides are simple sugars which are not hydrolyzed (broken down by the addition of water) into more simple units.


    In general, the basic molecular formula is (CH2O)n.

  Monosaccharide functions as a source of energy for organisms.


     Glucose, galactose, and fructose.

2- Oligosaccharides:

     “Oligosaccharides are hydrolyzed to form (breakup) from two to ten simple monosaccharide units. The units or monomers are bonded together by glycosidic bonds.”

  The oligosaccharides which consist of two monosaccharides joined together by a covalent bond are called disaccharides. Disaccharides function as a nutritional source of monosaccharides.


   Sucrose (table sugar), maltose, and lactose.

3- Polysaccharides:

  Polysaccharides are polymers of monosaccharides. The addition of new monosaccharides could continue indefinitely making a huge molecule a long (and branched via the 6- carbon) chain of glucose molecules. This long chain is known as a polysaccharide. Polysaccharides are an easily accessible storage form of glucose.


  Starch and glycogen, cellulose, chitin etc.

Q.2 iii) Compare the isomers and stereoisomers of glucose.



  “Isomers live molecules or molecular mixes that are similar in that they have the exact molecular formula, however, have different configurations of the atoms or levels of atoms (functional groups) involved.”


  Glucose, fructose, and galactose are hexoses which have the same empirical formula (C6H12O6) but different structural formula so they are isomers of each other.


   As an example, glucose, while maintaining its basic structure, can arrange its atoms or functional groups in a number of different spatial arrangements, forming a number of isomers of glucose, with different properties.

Q.2 iv) Give the chemical nature of the glycosidic bond.


Glycosidic bonds:   

   “A glycosidic bond or glycosidic link is a style of covalent glue that joins a carbohydrate (sugar) molecule to another body, which may or may not be another carbohydrate.”

   Glycosidic bonds are formed between a sugar molecule (i.e. carbohydrate) and -OR group. There are multiple forms of glycosidic bonds such as C-, O-, N-, and S-. These documents are differentiated by the atom (carbon, oxygen, nitrogen, or sulfur) bonding the sugar and -OR group together. The ‘-OR’ refers to when an oxygen atom is attached to an R group, which is any molecule containing a carbon and hydrogen atom. Examples of -OR groups include ketone and its distant forms carboxylic acid and ester. 

Long Questions biological molecules Chapter 2 for KPK

Explain how the properties of water make it important for life?

Water is essential for life. There is no existence of life without water. Allah the Almighty has created all living organisms from the water. The bodies of living organisms contain 70 % to 90 % water. It is essential for the existence of protoplasm because protoplasm cannot survive if its water content is reduced as low as 10 percent. Because of its immense importance water is present in sufficient amounts in most of the places on earth.

Chemical and physical properties of water:
    The chemical and physical properties of water are so designed that they are absolutely important for the vital processes of life.

i. Polar nature of water:
Water is a polar molecule. The oxygen end of the molecule is electronegative bearing a partial negative charge and the hydrogen end is electropositive bearing a partial positive charge. Water molecules are bonded to one another through hydrogen bonds. Hydrogen bonds are much weaker than covalent bonds but they still cause water molecules to remain attached together.

ii. Universal solvent:
       Due to the polar nature of water, it dissolves almost all types of polar substances and is therefore regarded as a universal solvent. This facilitates chemical reactions both inside and outside the living cell. Water supplies the medium for most chemical responses in cells.

   When sodium chloride is put into water, the electronegative ends of water molecules are attracted to the sodium ions and the electropositive ends of water molecules are attracted to the chloride ions. As a result, the sodium and chloride ions separate and dissolve in water. Water dissolves all minerals present in soil which are absorbed by plant roots and transported to other tissues.

iii. Cohesive and adhesive force of water:
    The water molecules remain attached together and do not separate because of hydrogen bonding. This develops cohesive force among them and therefore water flows freely without breaking apart. Water molecules also adhere (stick) to surfaces. It can fill a tubular vessel and still flows so that dissolved molecules are evenly contributed throughout a system.

iv. High specific heat:
      Water has high specific heat. Specific heat is the amount of heat energy required to raise the temperature of one gram of water by one degree Celsius. It means that water absorbs or releases large quantities of heat energy with little change in temperature. This is why the temperature of the water rises and falls more slowly as compared to other liquids. This property helps the organisms to maintain body mental temperature and protect them from rapid temperature changes.

v. High heat of vaporization:
Water also has a high heat of vapourization. The heat of vapourization helps animals and plants to get rid of excess body heat during sweating and transpiration respectively.
    The presence of hydrogen bonds among the water molecules cause water to remain liquid rather than change to ice or steam. Without hydrogen bonds, water would boil at
-80oC and would freeze at -100oC. In such conditions, life for living organisms would become impossible.

vi. Water expands at low temperature:
        Water has a unique property, as it expands when the temperature falls below 4°C. Water is most heavy at 4oC. Therefore ice (solid water) is less dense than liquid water and this is the reason that ice floats in liquid water. The water body freezes on the surface at low temperature.

vii. High surface tension:
          Water has a high surface tension. In living cells this part of surface tension allows a thin movie of water to cover membranes and to keep them moist.

viii. Ionization of Water:

Water molecules may ionize into hydrogen ions (H+) and hydroxyl ions (OH). Very few molecules out of a very large number may ionize. The presence of ions is important for the normal functioning of enzymes.

Q. 3 ii) Describe the properties and roles of disaccharides.



   The oligosaccharides that are hydrolyzed into two simple units are called disaccharides, those hydrolyzed into three units are trisaccharides and so on. Disaccharides are the most common oligosaccharides.


   Common disaccharides are sucrose, lactose, and maltose. Sucrose is present in sugarcane and is hydrolyzed into glucose and fructose.

    Sugar + water + sucrase (enzyme)  →Glucose + fructose + sucrose


The covalent bond that is formed between two monosaccharide units is called a glycosidic bond. Lactose is found in milk that contains galactose and glucose.


            Lactose + Water       →          Glucose + Galactose

   Maltose is disaccharide found in fruits. It is composed of two glucose units and is found in our digestive tract as a result of starch digestion.


              Maltose  +  Water               Glucose  +  Glucose

  Physical properties of Disaccharide:

   Depending on the monosaccharide constituents, disaccharides show a variation of the following properties

  • Crystalline
  • Sometimes water-soluble
  • Sometimes sweet-tasting and sticky-feeling.

Common Disaccharides:

   Some of the common disaccharides are as follows.

1- Sucrose (table sugar, cane sugar, beet sugar): 

       Sucrose is a white, odorless crystalline powder with a sweet taste. It is best known for its role in human nutrition. Sucrose is made up of glucose (monosaccharide) and fructose (monosaccharide).

2. Lactose (milk sugar): 

     Lactose is found in milk. Lactose is formed from galactose and glucose. Lactose has a complex molecular structure, and so some people are unable to digest it properly.

3. Maltose: 

      Maltose (malt sugar) is formed from bonding between two units of glucose. It is used in the making of soft candies such as chocolates and fruit based-treats.

Role of disaccharides in human health:

   Disaccharides (double sugar) play important functions in the human diet. Disaccharides are a kind of carbohydrate that includes two sugar molecules called monosaccharides that are connected together in a compound. Human body digests disaccharides in foods and breaks them into the two individual sugar molecules that are then absorbed through your small intestine.

Q. 3 iii) Classify proteins. List examples and roles of structural and functional proteins.



  Proteins are macromolecules (polymers) formed of units (monomers) called amino acids. A large number of amino acids are known. Of these, only twenty different types of amino acids combine in different numbers and different sequences forming hundreds and thousands of different types of protein molecules.

   Proteins are the most abundant organic compounds of the cell. They play the most important role in cellular functions. They contain elements of carbon, hydrogen, oxygen, and nitrogen. Some proteins also contain sulfur.

Structure of Amino Acids:

   Amino acids are carboxylic acids having amino groups. Each amino acid has a central carbon atom called alpha carbon. There are four different groups attached to the alpha carbon. These are an amino group, carboxyl group, hydrogen of alpha carbon and an R group. The former three groups attached to the alpha carbon are constant members and are present in all amino acids while the fourth one i.e. R group is variable. It is either a hydrogen or alkyl group. Due to this variable, the amino acids are different from one another.

Peptide bond:

   A bond called peptide bond links amino acids in a protein molecule to each other. The peptide bond is formed between an amino group of one amino acid and a carboxyl group of another amino acid. This is a dehydration or condensation reaction in which one water molecule is formed.

                                  General structure of an Amino acid
General structure of an Amino acid

Dipeptide chain:
   A chain containing three amino acids and two peptide bonds is known as dipeptide chain.
Tripeptide chain:
   A chain with four amino acids and three peptide bonds is called tripeptide chain|
Polypeptide chain:
  A chain with many amino acids and many peptide bonds is called polypeptide.
 Most protein molecules are usually formed of two or many polypeptide chains e.g. haemoglobin and insulin.

Example of polypeptide chain:
   Haemoglobin is an oxygen-carrying protein in the red blood cells which consists of four polypeptide chains while an insulin molecule is consists of two polypeptide chains. The number and sequence of amino acids in a protein molecule is highly specific for its normal function. If an amino acid is not occupying its specific position in a protein molecule it will fail to perform its function.

  If one out of 574 amino acids in a haemoglobin molecule is not present in its specific position then haemoglobin changes its normal globular shape and becomes sickle-shaped. As a result of the disc-shaped red blood cells also become sickle-shaped. In sickle cell, haemoglobin molecule glutamic acid is replaced by valine. Such type of haemoglobin cannot perform its function and the person with sickle cell haemoglobin dies. The size of the protein molecule depends upon the number and kinds of total amino acids present in the molecule.

structure of Haemoglobin

The shape of Proteins:
   As regards the shape, proteins are classified into two types; fibrous proteins and globular proteins
a. Fibrous proteins:
      The molecules of fibrous proteins are composed of one or more polypeptide chains, which are linearly arranged in the form of fibers. They are water-insoluble. Some of these may form sheet-like structures.
Examples of fibrous proteins:
  Keratin found in hairs, nails, fur, outer skin, myosin present in muscle cells, collagen which is the most abundant protein in higher vertebrates found in skin, ligaments, tendons, bones and in the cornea of the eyes.
b. Globular Proteins:
      Globular proteins, as the name indicates, are globular or spherical in shape due to the folding of polypeptide chains. They are usually water-soluble.
Examples of globular proteins:
  Haemoglobin, albumen of egg white, enzymes, antibodies and the proteins of cell membranes.
Levels of Structure (organization):
   There are four levels of organization of protein molecules. This is because each type of polypeptide chain bends, folds and twists in a particular way within a protein molecule. This gives protein molecule a characteristic structure that classifies protein into four different types. The primary structure is the sequence of the amino acids joined together by peptide bonds. Sanger in 1951 was the first person who determined the sequence of amino acids in the insulin molecule.
a. Primary structure:
      A polypeptide chain having a linear sequence of amino acids is called the primary structure.
b. Secondary structure:
      When a polypeptide chain of amino acids become spirally coiled, the structure is called a secondary structure of the protein.
c. Tertiary structure:
    When the secondary structure of a protein is arranged into a three-dimensional structure, it is called a tertiary structure.
d. Quaternary structure:
      When two or more polypeptide chains are arranged into a large-sized molecule, it is called a quaternary structure.
Functions of Proteins:
    Proteins perform the most important functions in the life of living organisms.

  • Proteins are the structural and building materials of cellular membranes called lipoprotein membranes.
  • All enzymes are proteins. They speed up biochemical reactions inside the body of living organisms.
  • The digestive enzymes are important for the process of digestion. Without their presence, food cannot be digested.
  • Some hormones such as insulin are proteins which regulate biochemical processes.
  • Myosin and actin fibers play an important role in the contraction of muscles and movements.
  • Haemoglobin is an oxygen-carrying protein of red blood cells.
  • In animals’ proteins form most structures such as skin, nails, hairs, claws, hooves etc.
  • In plants, proteins are stored in most seeds for the future need of the embryos e.g. bean, pulses, pea etc.

Q. 3 iv) Describe the properties and roles of acylglycerol, terpenes and phospholipids.

1- Acylglycerol:
      “Acylglycerol is a form of lipids which are composed of glycerol and fatty acids.”
   The most common acylglycerol are triglycerides containing one glycerol molecule and three fatty acids.
   Glycerol is a three-carbon compound, to each carbon, a hydroxyl group is attached. Hydroxyl groups are polar and therefore glycerol is soluble in water. The acid portions of three fatty acids react with three hydroxyl groups of the glycerol so that triglyceride and three water molecules are formed. This reaction is condensation. The triglyceride molecule can be hydrolyzed into its components i.e. glycerol and three fatty acids. Triglycerides are stored in animals as fats. 

Formation of triglyceride from fatty acid and glycerol

Fatty acid:
A fatty acid consists of a long hydrocarbon chain with a carboxyl (acid) group (-COOH) at one end. Most of the fatty acids in cell contain 16-18 carbon atoms per molecule. Fatty acids may be saturated or unsaturated.
i. Saturated fatty acids:
     Saturated fatty acids have no double bond between carbon atoms. Such molecules cannot accommodate any more hydrogen atoms if added to them. Acylglycerol with saturated fatty acids such as palmitic acids are called fats and are solid at room temperature. Saturated fatty acids are stored in animals as fats.
Unsaturated fatty acids:
   Unsaturated fatty acids have one or more double bonds between some carbon atoms (C=C). In such molecules, the number of hydrogen is less than two per carbon atom. Any more hydrogen can be added to these molecules. Unsaturated fatty acids such as oleic acids are stored in plant seeds. Acylglycerol with unsaturated fatty acids is usually liquid at room temperature.
  The triglyceroids have high caloric value and usually yield twice as much energy per gram as that of carbohydrate.

2. Phospholipids:
       “Phospholipids are composed of one glycerol molecule, two fatty acids and one phosphoric acid molecule usually linked to some nitrogen group.”
   A triglyceride molecule is converted into phospholipid when a fatty acid is replaced by one phosphate. A phospholipid molecule has two parts. A phospho-head which is polar and soluble in water (hydrophilic), and two hydrocarbon tails which are non-polar and insoluble in water (hydrophobic). Phospholipids arrange themselves in a double layer in the presence of water in the plasma membrane of the cells.

Diagram shows the assemblage of phospholipids in plasma membrane

3. Terpenoids:
       Terpenoids are lipids that like the steroids do not contain fatty acids. They are lipid soluble and water-insoluble substances. The terpenoids are formed of units called isoprenoid units. They join by the process of condensation and give rise to different types of compounds such as terpenes, rubber, carotenoids, etc.

Q. 3 v) Define conjugated molecule and describe the roles of common conjugated molecules.

Conjugated Molecules:
   “A conjugated molecule is formed by the combination of two different molecules belonging to different categories.”
   When a carbohydrate molecule combines with protein, a conjugated molecule called glycoprotein is formed. Other examples are nucleoproteins, glycolipids, and lipoproteins.
Types of conjugated molecules:
1. Lipoproteins:
       Lipoprotein is formed by the combination of lipids and proteins. Lipoproteins are the basic structural framework of the plasma membrane and all other types of membranes in the cell.
2. Nucleoproteins:
    Nucleoprotein is formed by the combination of nucleic acids with proteins. A eukaryotic chromosome is basically a nucleoprotein that is formed by the DNA and protein. These are slightly acidic and soluble in water.
3. Glycolipid:
       Glycolipid is formed by the combination of carbohydrates and lipids. Glycolipids are an important component of brain and plasma membrane.
4. Glycoprotein:
    Glycoproteins are formed by the combination of carbohydrates and proteins.  Glycoproteins are an integral component of the plasma membrane. They are also present in egg albumin.

Q. 3 vi) Explain the double helical structure of DNA as proposed by the Watson and Crick.

Deoxyribonucleic Acid (DNA):

  In 1962, James Watson, Francis Crick, and Maurice Wilkins jointly received the Nobel Prize in physiology or medicine for their 1953 determination of the structure of deoxyribonucleic acid (DNA).
  According to this model, the DNA molecule is a double helix. The double helix can be visualized as spiral stair case wound around a central axis. Watson and Crick suggested that base pair always consists of purine pointing toward pyrimidines, keeping the molecule diameter at a constant 2nm. The base pair is flat with a distance of 0.34 nm between them.

Watson and Cricks Model of DNA

DNA contains pentose sugar as deoxyribose. It is formed of four different types of nucleotides. These nucleotides are named after the base present in them they are Adenine deoxyribonucleotide, Guanine deoxyribonucleotide. Thymine deoxyribonucleotide and Cytosine deoxyribonucleotide.

    These four types of nucleotides are used as building blocks of DNA molecule. The nucleotides in the DNA molecule are bonded to one another in such a manner that the sugar of one nucleotide is linked to the phosphate group of the next one. In this way, the nucleotides form a linear molecule called a strand in which the backbone is made up of sugar alternating with the phosphate group. The bases are projected to one side of the strand. The sequence of the nucleotide in one type of DNA is constant while it is different from another type of DNA molecule.

    A DNA molecule consists of two strands. The two strands twist about one another in the form of a double helix. The two strands run in opposite direction to each other in the double helix. They are held together by hydrogen bonds between purine and pyrimidine bases. Thymine in one strand is always paired with adenine in the opposite strand and guanine is always paired with cytosine.
     There are two hydrogen bonds between adenine and thymine and three hydrogen bonds between guanine and cytosine.

Structure of purine and pyrimidine

Biology Class 11 Unique Professor Notes for Pdf 2021


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