Chemistry of Cells

Chapter 3

= Protoplasm =

Definition

"The basic living substance of all living bodies is called Protoplasm."

The name of protoplasm was suggested by Purkingi, an Italian biologist and Huxly described it as the "Physical basis of Life."

Physical Properties
  1. Living protoplasm is a colourless, semitransparent matter with a lustrous appearance.
  2. It is fluid to slight viscous in consistency.
  3. It is elastic in nature and can be stretched into threads.
  4. Living Protoplasm shows a streaming movement along the inner surface of the cell, generally in a circular way called "Cyclosis".
  5. Protoplasm remains alive between 0 - 60° C and at higher temperature it dies and coagulates into a solid mass.
  6. It has got high affinity for water and remains saturated.
  7. Viscosity of protoplasm changes with the water contents.

Chemical Properties

Being a living and dynamic matter, it is difficult to estimate and analyze the composition of protoplasm with certainty. During the course of analysis protoplasm dies and decomposes. However some of the compounds and their amounts found in protoplasm are as follows:

Water (H2O) –- 65 to 95 %

Proteins –-10 to 20%

Lipids –- 2 to 3%

Carbohydrates –- 1%

Inorganic salts –- 1%

Nucleic Acids (DNA) –- 0.4%

Nucleic Acids (RNA) –- 0.7%
  1. Water

    It is the most abundantly found compound in protoplasm. Chemically, water is the best solvent in which proteins, carbohydrates and different organic and inorganic salts are dissolved. These substances take part in different metabolic processes in dissolved state. Moreover, viscosity and fluidity of protoplasm is water dependent. Water is also responsible for the transportation of organelles and substances from one part of the cell to another.

    Water exists in the protoplasm in two form i.e. Free water, which is readily available for metabolic processes and makes protoplasm fluid for cyclosis. Bounded water, which is held by the colloidal particles such as proteins, it is not readily available. Almost one part of protein can hold 40 parts of water in colloid.
  2. Proteins

    Proteins is the basic material of protoplasm. It forms 10 – 20% of protoplasm by weight.

    Chemically protein is composed of carbon, hydrogen, oxygen and nitrogen, some of the proteins also contain sulphur and phosphorus. Each protein molecule is a complex and large chain of basic units called "Amino Acids". Each Amino Acid is made up of an Alkyl Radical attached with acid group (-COOH) and an amino group (NH2) at the same carbon atom. There are more than 20 amino acids, which form proteins the simplest amino acid is Glycine as shown below:

    Peptide Linkage

    The bond established between two amino acid molecules by the removal of one water molecule is called Peptide Linkage.

    By close observation we can see that Amino Acid carries replaceable H+ ion of Amine at one end and a replaceable OH- ion of acid at the other end. By replacing these ions from two amino acid molecules and combining them, we can obtain a larger molecule composed of two Amino acid units called Dipeptide molecule.

    It exists between Nitrogen atom of Amine and Carbon atom of Carboxylic acid group (C – N). If elimination of water molecule is continued, a large number of amino acid molecules can be linked together by means of Peptide linkage forming a chain called Protein.

    Functions of Proteins

    Proteins perform two main functions i.e. structural and catalytic. Structurally, proteins form cell membrane, Nuclear membrane and membranes of the organelles. It also forms the bulk of protoplasm by its colloidal property. Proteins are the antijens and antibodies of each specific immune system, blood group and tissue type. They also serve as stored food in different seeds, fruits, vegetables and animals organs.

    Catalytically, proteins are the bio-catalysts or enzymes. All hormones are also proteins. Enzymatic proteins catalyze the rate of chemical reaction going on within the cell. Thus we can say that it is the essential component of protoplasm. Proteins are water soluble and Hydrophillic.
  3. Lipids

    Lipids are organic compounds composed of carbon, hydrogen and oxygen. They are in the form of oils, fats, Triglyceride and Cholesterol. All of them are insoluble in water and lighter than water. They are 2-3% in protoplasm.

    The structural unit of lipid i.e. triglyceride is fatty acid. When three molecules of fatty acids combine with one molecule of Glycerol, one molecule of Triglyceride is obtained. Triglyceride further converts into oils, cholesterol or animal fat.

    Functions of Lipids

    Lipids serve as stored food and can be utilize whenever there is need of energy. They yield much more energy one oxidation as compared to Carbohydrates.

    All membranes of cell, such as plasma membrane contain "Phospho lipid" as its structural component.

    Cuticle of insects and plants contains lipid, which serves to protect them by making cuticle waterproof. Cuticle also serves to prevent transpiration in plants. A layer of lipid beneath the skin of animals, stored in a dispose tissue serves as an insulator and prevents heat loss.
  4. Carbohydrates

Carbohydrates are also organic compounds containing carbon, hydrogen and oxygen but the ratio of hydrogen and oxygen is the same as in water i.e. 2:1 respectively. Carbohydrates constitute 1% of protoplasm by weight.

Classification

Carbohydrates are classified into three types namely,

  1. Monosaccharides
  2. Disaccharides
  3. Polysaccharides

(i) Monosaccharides

Definition

"The carbohydrates, which cannot be further hydrolyzed or simplified, are called Monosaccharides or simple sugar. "

Examples of Monosaccharides

Examples of Monosaccharides are Glucose, Fructose and Glactrose.

Classification of Monosaccharides

Monosaccharides can further be classified according to the number of carbon atoms present in each molecule. These are:

Triose ® Three carbon atoms ® C3H6O3

Tetrose ® Four carbon atoms ® C4H8O4

Pentose ® Five carbon atoms ® C5H10O5

Hexsose ® Six Carbon atoms ® C6H16O6

Properties of Monosaccharides

Monosaccharides are solid, crystalline and white in colour. They are sweet in taste and soluble in water.

Monosaccharides are found in Honey, fruits like Grapes and Milk.

(ii) Disaccharides

Definition

"The carbohydrates which on hydrolysis yeild two moleculesu of Monosaccharides are called Disaccharides."

The two monosaccharides are united by means of condensation by elimination of one water molecule.

C12H22O11 + H2O ® C6H12O6 + C6H12O6

Sucrose Glucose Fructose

Examples of Disaccharides

Examples of disaccharides are Sucrose, Lactose and Maltose.

Properties of Disaccharides

Disaccharides are white crystalline solid compounds and are water-soluble. They are obtained from sugarcane, Beet (sucrose), milk (Lactose and Matose).

(iii) Polysaccharides

Definition

"Those carbohydrates which on hydrolysis yeild a large number of Monosaccharide molecules are called Polysaccharides."

Explanation

Polysaccharide + nH2O ® n Monosaccharides

n(C6H10O5) + nH2O ® n(C6H12O6)

Where n= any natural number. Polysaccharides are in the form of long, straight and branched chains in which monosaccharides molecules are joint together by means of condensation or Glycosidic linkage.

Properties of Polysaccharides

Polysaccharides are solid in the form of grains, fibres etc. They are insoluble in water. They are found in cell wall as structural component like cellulose and Pectin, or as a reserved food in the form of starch in potato, sweet potato, wheat rice and corn. In animals and bacteria, Polysaccharides occur in the form of Glycogen stored in cytoplasm, Liver and muscles.

Functions of Polysaccharides

Mono and Disaccharides serve as readily available source of energy. They oxidize into CO2 and H2O and yield a large amount of energy. This process is called Respiration. Polysaccharides serve as reserved food as well as structural component of plant cell wall.
  1. Inorganic Salts

    Protoplasm contains almost 1% inorganic salts. These salts are soluble in water and therefore found in ionic state. Ions are of two types, Cations and Anions. Cations are Na+, K+, Ca++, Mg++, Fe++ etc. Some cations are found in a small quantity like Zn++, Cu++, Mn++ and Mo++. Anions are Cl-, S--, SO4--, PO4---, F- and I-, HCO3- and CO3--.

    Most of the salts exert osmotic pressure and therefore they are responsible for the homeostasis. Some of the ions catalyze different reaction while some especially Na+ and K+ are responsible for conduction of impulse or sensation. Ca++ regulates the movement of components and membrane permeability.
  2. Nucleic Acids

As the name suggests, these compounds are present in Nucleus. The unit of Nucleic acids is called "Nucleotides". These are of two types:

  1. RNA or Ribonucleic Acid
  2. DNA or Deoxy Ribonucleic Acid

Each of them is made up of three compounds; i.e:

  1. A pentose sugar
  2. Nitrogen base
  3. Phosphoric Acid (H3PO4)
  1. Pentose Sugar

    It is a five carbon sugar with a general formula (C5H10O5). It is called Ribose and contains one oxygen lesser i.e. C5H10O4 then is called Deoxy Ribose. Ribose is the back bone of RNA and deoxy Ribose is the back bone of DNA.
  2. Nitrogen Base

    These are of two types namely Purine and Pyrimidine base. Purine contains two amino acids i.e. Adenine and Guanine while Pyridine contains two amino acids namely Thymine and Cytosine in DNA and Urocil and cytosine in RNA. Nitrogen bases are attached with the long chain of sugar in an alternate manner.
  3. Phosphoric Acid

It provides phosphate (PO4---) which serves to bind sugar molecules with each other to form back bone of Nucleotides by means of 3-5 linkage DNA is the hereditary material found in chromosomes , it is limited to Nucleus. RNA is concerned with protein, synthesis in ribosomes and is found in Nucleolus and Ribosome.

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