Ergastic matters i.e., Non-living cell contents(reserve materials)-digieduco

Besides the protoplasmic substances i.e. organelles, several other non-protoplasmic i.e. non-iiving matters as by products of metabolism are also found within the cell. Various kinds of solid organic and inorganic materials, includin g food materials, mineral crystals, oils, gums, resins, mucilages, tannins, alkaloids, rubber etc. often occur in the cytoplasm or in the vacuoles. Those subtances are termed as ergastic matters. They are found mainly either in the form of granules, droplets or crystals.

Ergastic matter occur in the vacuoles and in the cell wall, they may be also associated With the protoplasmic components of the cell. When present in the vacuole, these substances are held in solution in the sap.

Ergastic matters are divided into three groups, viz. (i) Reserve materials, (ii) Secretory Materials and (iii) Excretory materials.

I. RESERVE MATERIALS

These are the reserve food matters and stored for the future use of plants. These matters are concerned with the nutrition of the plants, Reserve materials are of two types. e.g. (1) non-nitrogenous, e.g. carbohydrates, fats and oils, and (2) nitrogenous materials e.g. proteins. '

1. Non-nitrogenous reserve materials :

A. Carbohydrates

Carbohydrates are organic non-nitrogenous compounds consisting only of carbon, hydrogen and oxygen ; hydrogen and oxygen occur in the ratio of 2 : 1 as in water. The empirical formula
of carbohydrate is CnH2nOn. Carbohydrates are of several types
e.g. sugars, starch, glycogen, cellulose, inulin etc.

(a) Sugars-Sugars are soluble carbohydrates and sweet to taste,
Monosaccharide sugars-Glucose and fructose are the examples of monosaccharides, having the formula C6H12O6. Due to the presence of six carbon atoms, they are called hexose sugars. They are also known as reducing sugars as on heating with Bennedict’s solution, these sugars settle down as a red precipitate. Monosaccharide sugars occur in dissolved state in the cell sap of many fruits such as grape, mango etc.

Test-When Fehling’s (A and B) or Bennedict's solution in equal volumes are added to glucose or fructose (grape juice) and heated over bunsen flame orange red colour (cuprous oxide) is precipitated.

Disaccharide sugars-Sucrose, maltose etc. are the examples of disaccharides, they have the chemical formula C12H22O11. They are formed by the condensation of two molecules of monosaccharides and eliminating one molecule of water. Sucrose occurs in storage organs of some plants like sugarcane, beet root etc.; maltose is commonly found to occur in germinating seeds.

Test-Cane sugar does not respond to Fehling’s test, but when treated with normal HCi or H2SO4 and heated with Fehiing’s solution it gives brick red colouration.

(b) Laminarin is a kind of soluble carbohydrate found in the cell sap of brown algae in the dissolved state, it consists of linked glucose units. Similarly mannitol, found in some brown algae, is a reserve soluble carbohydrate and a hexahydric alcohol.

(c) Inulin-It is a type of polysaccharide carbohydrate which occurs as a storage material in colloidal state in the cell sap of the vacuoles of the tubers of some Compositae [Dahlia sp., Artichoke (Helianthus tuberosus)], Campanulaceae etc.

Tests-(i) When tuber of Artichoke is treated with dilute glycerine or 95% alcohol for a long time, inulin is precipitated in the form of beautiful fan-shaped crystals within the cell. These crystals are formed at the comers or sides of the cell walls. (ii) Inulin solution and acid phloroglucin form yellowish brown colour.

(d) Starch grains (Polysaccharides)-Starch grains are complex insoluble carhohydrates and most commonly found as solid particles. This carbohydrate (starch) is composed of long-chain molecules, whose basic units are anhydrous glucose residues of the formula (C6H10O5). They occur as reserved food material and form an important food for the future use of plants. Starch grains may be found In all parenchymatous tissue, but they are specially found in storage organs such as tubers, corms, rhizomes, endosperm or cotyledons or seeds. Starch grains are formed almost exclusively in plastids, mainly in leucoplasts and chloroplasts.Assimilatory starch is synthesised by chloroplasts while storage starch is produced by leucoplasts-one or more starch grains may arise in one plastid. They are mainly composed of two components, e.g. amylose and amylopectin. Structurally starch grains vary greatly, their diameter ranges from 12u to 100u.

There are two kinds of starch grains viz. (i) assimilatory starch and (ii) storage or reserve starch.

(i) Assimilatory starch-During the photosynthetic process starch is manufactured temporarily within the mesophyll cells of the leaves by the help of chloroplasts, this temporary formed starch is called assimilatory starch.Assimilatory starch grains are granular in form and temporary in duration as they may be again converted into soluble sugar during metabolism.

(ii) Storage starch-Soluble sugar is distributed into the different regions of the plant body, some quantity of it reaches the underground parts, where by the action of the larger leucoplastids (amyloplasts) and in the absence of sunlight they are converted into permanent starch grains. They have definite shape and are bigger in size. These insoluble starch grains can also be converted into soluble sugar by the action of enzymes amylase or diastase whenever required.

Structure and shape of the starch grain-

Starch grains are stratified in appearance. The stratification is formed round a dark refractive and definite spot, called hilum-this hilum portion is first formed within the starch grain, and therefore hilum is the centre of origin of the starch grain Around the hilum alternate layers of water and carbohydrate are formed giving the starch grain a stratified appearance. The starch molecules are arranged in the grain in such a way as to exhibit some crystalline properties-this can be seen by viewing the grain in a microscope with crossed polarizers when it appears luminous except for a dark cross, the arms of which intersect at the hilum of the grain. Starch grains may be oval (e.g. potato), globose irregular (e.g. pea), polygonal (e.g. maize. rice), dumbell-shaped (e.g. in the latex of the laticiferous tissues of Opuntia sp.) etc. in shape.

Classification of the starch grain-

According to the position of hilum, starch grains may be (i)eccentric and (ii) concentric or centric.

(i) Eccentric-when hilum is situated at one end in the grain, as seen in potato (Solanum tuberosum) starch.

(ii) Centric-When hilum is centrally situated in the grain as seen in pea (Pisum sativum), wheat (Triticum aestivum) starch.

The starch grains may be simple, semi-compound or compound :

(i) Simple starch grain-When grains are quite free and separate from each other, generally found in potato (Solanum tuberosum).

(ii) Compound starch grain-When two or more grains are united with each other hence compound grains are provided with two or more hila. This type is characteristic of some plants e.g. rice (Oryza sativa), sweet potato (Ipomoea batatas) etc.

(iii) Semi-compound or Half-compound starch grain-When a compound starch grain is surrounded by common line of stratification composed of starchy material.

Commercial starches are obtained from various parts of the plant e.g. sago starch from the stem of Metroxylon sago ; tapioca starch from fleshy roots of Manihot esculenta; arrowroot starch from rhizomes of Maranta arundinacea and Curcuma angustifolia ; wheat, maize and rice starches from seeds etc.

Tests-(l) Starch grains stain bluish-black with a solution of iodine in potasium iodide. (2) Starch on heating with water forms a semi-viscous translucent solution in paste. (3) Starch grains turns deep blue with Schultze’s solution (chlor-zinc iodine solution) but the brightness of the colour is lost gradually.

(e) Glycogen-This substance is another insoluble polysaccharide (a-glucopyranose polymer like starch) having the formula (C6H10O5)n. It occurs in the cells of many fungi, bacteria and in some algae. Glycogen is also called animal starch because it is present in sufficient quantity within the muscles and liver of the animal. Glycogen can be converted into sugar (glucose) by the action of the enzyme glycogenase.

Test-With iodine solution glycogen turns brown colouration.

(f) Reserve cellulose-It is also an insoluble polysaccharide carbohydrate with the formula (C6H10O5)n and is composed of long chain-like molecules. Plant cell wall is composed of this substance. Reserve cellulose is also called hemi-cellulose. It is found on the cell walls of the endosperm of the seeds of Phoenix sylvestris, Areca catechu etc. Reserve cellulose, though indigestable, is used as food.

Test-Reserve cellulose is insoluble in water but soluble in alkalies.

(g) Pectins-This is another type of reserve carbohydrate found mainly in the cell walls of many fruits. The middle-lamella of the cell wall is composed of pectins. Pectins are used as food.

B. Fats and Oils : Fats and oils (storage lipids) are present as reserve materials in the cells of the endoperm, perisperm and also in the cotyledon-tissues of many seeds, in spores, in meristematic cells, and sometimes in differentiated tissues of the vegetative body. They occur as solid bodies or often as fluid droplets of various sizes, either dispersed in the cytoplasm or aggregated in larger masses.

They are the compounds of fatty acids and glycerols. These substances are formed directly by the elaioplasts within the cytoplasm, or by small organelles bounded by a unit membrane called spherosomes (Cutter, 1978). Like carbohydrates, fats and oils are composed of carbon, hydrogen and oxygen, but the ratio of hydrogen and oxygen is not 2:1. The amount of oxygen is very low in relation to carbon and hydrogen. Chemically, fats and oils are similar ; fats are usually solid and oils liquid at normal temperature (10°C; 20°C). Waxes, cutin, suberin etc. which occur in and upon the cell wall as excretory products are also fatty. Phosphatides and sterols are also related to fats.

Fats and oils occur as highly refractive globules in the cytoplasm. Fats may also occur in the cytoplasm as irregularly-shaped soft grains, e.g., vegetable butters and in wax of various seeds ; fats occur as minute refractive granules in the endosperm cells of Ricinus i.e. castor seeds. They may even be crystalline occurring as needle-like crystals in the endosperm cells of the seeds of Bertholletia excelsa (Brazil nut, Lecythidaceae).

In a plant body fats usually occur in the form of oils which are non-volatile. These are known as fixed oils. A permanent grease spot is formed over a piece of paper when fixed oils are placed on it.

Tests for fats and oils--(1)Fats and oils turn black when treated with 1% osmic acid solution in water. (2) They turn reddish when treated with Sudan III or IV. (3) They are soluble in petroleum. chloroform. acetone and ether. (4) Insoluble in alcohol and water, some oils dissolve in high grade
of alcohol e.g. castor oil.

Properties of fat-(1) lighter than water. (2) solid at room i.e. normal temperature, (3) specific gravity varies from 8.875 to 9.070 and (4) insoluble in water but soluble in petroleum, chloroform, acetone and ether.

2. Nitrogenous reserve materials .'

These materials contain nitrogen and sulphur in addition to carbon, hydrogen and Oxygen. Phosphorus is also present in some cases. Nitrogenous reserve materials are present in every living cell either in liquid or solid state.

Nitrogenous matter fall into two categories :--(1) simple soluble form 1. e. amino acids and (2) complex insoluble form 1. e. proteid grains or proteins. Amino acids are the main building blocks of the protein molecules--they are mostly found 1n colloidal solution In Water. Each amino acid contains at least one amino group (-NH ) and one carboxyl group (-COOH). Examples of commonly occurring amino acids in plant cells are tryptophan, tyrosine, histidine, aspartic acid etc.

C. Proteins-These are complex insoluble nitrogenous reserve materials formed from ammo acids and are found in all living plant cells.

Storage proteins may occur in amorphous form, crystal-like form or in the form of definite bodies called aleurone grains. A well known amorphous protein is gluten, which is found together with starch in the endosperm of wheat grain. Crystal like protein in the form of small cubes is found in the cells of potato tuber and in the cotyledonary cells of many pulses.

Aleurone grains sometimes combine both amorphous and crystalloid forms of protein. Aleurone grains are found in the cells of the endosperm and embryo of many seeds. In case of the grains of cereals, the outermost layer or layers of the endosperm just beneath the coat of the caryopsis form the aleurone layer. Aleurone grains are bounded by a proteinaceous membrane which in the simplest type only encloses a mass of amorphons protein. The more complex types have various inclusions viz., crystalloids, globoids, crystals of calcium oxalate etc. In the endosperm cells of the castor oil plant (Ricinus communis), aleurone grains are formed within the vacuoles. The contents of the vacuole are rich in soluble protein, finally they become hardened into irregularly-shaped bodies called protein crystals ; this crystalline pan is known as crystalloid : in addition to crystalloid, small globular body is formed within or attached closely with the grain called globoid. This globoid is not a pure protein but protein in combination with inorganic salts as double phosphates of Ca and Mg. So an aleurone grain of Ricinus consists of a ground substance of proiein in which lie embedded one (sometimes more) crystalloid and one globoid. The aleurone grains of nutmeg(Myristica fragrans) contains only crystalloid. Rosette crystal of calcium oxalate is present in the aleurone grains of Umbelliferae. Generally, aleurone grains are small, even smaller than starch grains.

Tesls-(1)Aleurone grains stain brown with iodine in potassium iodide and yellow with an alcoholic solution of picric acid.
(2) Xantho-protein test---When protein substances are treated with strong nitric acid-a yellew colour is formed. This yellow colour again changes to orange on addition Of a few drops of strong ammonium hydroxide (NH4OH) solution. (3) Biuret test-Protein substances can be detected by their black or violet colouration when treated with 20% sodium hydroxide solution (NaOH) and a few drops of 1% copper sulphate (CuSO4) solution.