Protoplasm: Meaning, Physical and Chemical Nature | Cell

In this article we will discuss about:- 1. Meaning of Protoplasm 2. Physical Nature of Protoplasm 3. Chemical Nature.

Meaning of Protoplasm:

The “living structure” present inside the cell referred to as protoplasm differentiates the living beings from the non-living beings. Huxley (1868) defined it as “Physical basis of life”. First of all, Felix Dujardin (1835) described this homogenous jelly like mass of the cell and gave the name Sarcode.

Later on, in 1839, Purkinje termed it as Protoplasm. Protoplasm is the ground substance of living matter found within membrane of a cell. Von Mohl (1946) emphasized its role in cell division including other metabolic processes of the cell.

The existence of similarities between the protoplasm of plants and animals was discovered by Max Schultz (1861) who proposed the famous “Protoplasm theory”. According to this theory, protoplasm is the main part of the cells, performing all the physiological activity of the cell. It includes the ground substance cytoplasm having nucleus and limited by cell membrane.

Physical Nature of Protoplasm:

It is optically homogenous, elastic, colourless, jelly like, semi-transparent to transparent and semi-viscous substance. There is about 80-90% of water in protoplasm. It appears granular containing many vacuoles. It responses well to external stimuli like electric shock, heat, cold, chemicals etc. Stimulation is its natural quality or distinct characteristics.

Many soluble materials like salts, sugars and minerals are suspended in protoplasm in form of ions or molecules. Besides it, many-many organic substances like fats, proteins remain soluble which particles are bigger in size than molecules. Such type of solution is referred to as colloidal solution and the soluble particles are said as colloids. It is evident that protoplasm is a crystallocolloidal solution.

Basically the protoplasm is a polyphasic colloidal system. The colloidal structure of protoplasm was explained by Fisher and Hardy in 1894 and 1899 respectively. Viscosity of protoplasm has been found greater than that of water, because of this reason protoplasm exhibits Brownian and amoeboid movement including cytoplasmic streaming (cyclosis).

The Brownian movement is characterized by the zigzag motion of the particles occurring due to the bombardment of one molecule or particle by other. The amoeboid movements are the result of viscosity and continued change of sol gel. The cytoplasmic streaming or cyclosis is the circulation of protoplasm within the cell.

This streaming movement is of two types:

(a) Rotation:

The movement of protoplasm is in one direction (clock-wise or anticlockwise) around vacuole inside a cell, e.g., leaf cells of the Hydrilla, Vallisneria etc.

(b) Circulation:

The protoplasm moves in different directions around different vacuoles inside a cell e.g., staminal hairs of Tradescantia. The contractility of protoplasm is essential for various vital activities. The absorption and removal of water by the cells is due to the contractility of cytoplasm. The particles of protoplasm are adhered with each other by Vanderwaal’s forces, that hold long chains of molecules together.

Protoplasm is found in solution and gel states which are reversible or inter-convertible (sol gel). A gel is a group of suspended particles in a jelly like state. The molecules of a gel are held together by various bonds of varying strength.

This gel may become in watery condition and the liquid state as sol. This process is called solution. This gel solution condition of colloidal system are the fundamental basis for mechanical behaviour of cytoplasm.

The various components give appearance of protoplasm of different kinds:

(a) Reticular Theory:

According to this theory, particles form a network of fibrils in a liquid medium. It was proposed by Klein, Corno etc.

(b) Granular Theory:

According to this theory, the protoplasm is composed by minute granules suspended in a fluid said as bioplast or elementary organism. It was proposed by Altmann (1893).

(c) Fibrillar Theory:

According to this theory, the protoplasm consists of fibres embedded in the inner mass of matrix. The fibrillae are said as spongioplasm and the ground substance as hyaloplasm. It was proposed by Flemming.

(d) Alveolar Theory:

According to this theory, the protoplasm consists of numerous suspended droplets or alveoli similar to foam or emulsion, also said as uritomes and inter-alveolar as mioplasma. This is also known as emulsion theory which states that the solid portion is in the form of foam and the liquid portion taking place in the bubbles of the foam.

According to modern concept protoplasm is a colloidal substance exhibiting the presence of large number of minute particles held in suspension in the liquid portion. The organic substances which consist of proteins and carbohydrates in suspension may be hydrophilic or water loving. pH value of protoplasm is variable. It ranges between 5.8 to 6.8.

Chemical Nature of Protoplasm:

Chemically, protoplasm consists of inorganic and organic matters. Among inorganic parts are included water, gas, salts etc. and organic matters are grouped into various proteins, carbohydrates, fats and other regulatory substances. The dry protoplasm shows 45% proteins, 25% carbohydrates, lipids 25% and 5% other substances.

Protoplasm contains varying quantities of water in different parts of an organism. There is about 80%—90% of water which is about 3 parts of the total protoplasm. Water forms the main suspension medium in which various metabolic processes are carried on. Despite it, water acts as a best natural solvent. It helps in reactions through hydrolysis and dehydration.

It has a high specific heat, thus, it provides protection against fluctuation in temperatures. It acts as a medium of dispersion of substance in the protoplasm. The water has also high surface tension which provides the protoplasm a consistency. Water is essential in metabolism since the enzymes act exclusively in the presence of water.

Various organic compounds which remain in the protoplasm are proteins, fats, carbohydrates, enzymes, hormones, vitamins, nucleic acids and inorganic matters which are found in the protoplasm are Carbon (C), Hydrogen (H), Oxygen (O, more quantity). Nitrogen (N), Phosphorus (P), Sulphur (S) as macro element and Zinc (Z), Copper (Cu), Manganese (Mn), Molybdenum (Mo), Vanadium (Vd) and Selenium (Se) as micro or trace element.

Proteins are the main components of protoplasm, made up of many complexes of physiological materials.

The following elements are found in proteins:

The most important feature of the protein is the hydrolysis, i.e., they get broken up into its smaller components by hydrolyzing agencies like water, hydrolytic enzymes etc. The primary product of protein hydrolysis is the amino acid. Emil Fischer gave his view that the amino acids are “building stores” of the proteins. Carbohydrates are the compounds that constitute about 12% of protoplasm. They are made up of C,H and O.

It has been found that three main kind of carbohydrates are present in protoplasm:

(1) Monosaccharides—glucose and fructose

(2) Disaccharides— sucrose, lactose and maltose and

(3) Polysaccharides— cellulose, glycogen and starch.

When carbohydrates are broken down, they form glucose which provides energy. In animal, cells store glucose in form of glycogen which is reconverted into glucose. Lipids are fats or fat like lipid substances.

These are also formed of C, H and O, but the amount of oxygen is small in it and are associated with phosphates and sulphates. It is soluble in organic solvents but insoluble in water. On hydrolysis they yield fatty acids and glycerol and provide energy. Notable examples are tripalmitin, stearin, oleic acid etc.

Lipids have complex molecular arrangement and are main parts of the cell membrane, hormones and vitamins of the cell. These are present in the cell as a storage product and vitamins of the cell and supply energy. Lipids may be simple, complex or derived.

The true fats, some times called neutral fats, are the simple fats and occur as triglycerides of the fatty acids. Complex lipids mainly regulate cell permeability, blood coagulation and fat metabolism.

Lecithin, plasmogen, glycolipids, cephalins, sphingolipids and lipoproteins come under the category of complex lipids. The derived lipids yield simple and complex lipids along with some other derived products like aldehyde, ketones etc. The steroids come under this category which are characterized by the presence of ring structure in their molecules. The steroids include vitamin D and sex hormones.

Nucleic acid, an important organic substances, is found in nucleus and cytoplasm. These acids are associated with chromosomes and thus control the important metabolic activities of the cell and carry hereditary information from one generation to another generation.

The types of nucleic acids present within the cell is RNA (Ribonucleic acid) and DNA (Deoxyribonucleic acid). Both are polymers of nucleotides. RNA contains phosphoric acid, ribose sugar and nitrogen bases (adenine, guanine, cytosine and uracil). DNA has the same structure as in RNA except that the thymine is present in place of uracil and the sugar is deoxyribose.

There are some additional substances which regulate the metabolic activities of the cell. These are hormones, vitamins and respiratory pigments. Hormones and vitamins are required in very small amount or in traces and are essential for growth and development. The respiratory pigment like haemoglobin are the carriers of O₂ and CO₂ during respiration.

Inorganic salts constitute 1-4% of the protoplasm and found in form of carbonates, chlorides, phosphates, sulphates of calcium, potassium, magnesium and iron etc.

They are essential for maintaining proteins in the protoplasmic solution. For example, phosphorus and calcium are essential for the skeleton. Cl is surplus in blood and tissue fluids. PO₄ is associated with phospholipids and phosphoprotein etc. Magnesium is required for formation of bones.

Some most significant biological properties of protoplasm includes respiration, growth, reproduction, metabolism, irritability, excretion, digestion, movement etc. Inorganic salts occur in solution in ionic states and equilibrium is maintained between them.

They number about 30 and important among them are Na, Ca, K, Mg, P, S, Cl, Mn etc. Gases are by-products like CO₂ and O₂ etc. They occur in diffusion state and go out during excretion and respiration process. In the last, thus, we can say that protoplasm is a mixture of various chemical compounds which represent reversible crystallocolloid nature with changing viscosity.