Process of Secondary Metabolism In Plants!
The plants do not possess any mechanism for the disposal of their metabolic wastes. These waste products accumulate in their cells and often serve as starting points for many transformational processes.
Such pathways, producing substances of no significance to the maintenance of a main metabolic pathway, constitute what has come to be called secondary metabolism.
Secondary metabolism leads to the formation of secondary products. The secondary metabolic pathways are genetically and enzymatically regulated. Secondary products or secondary metabolites include some polysaccharides, many antibiotics, steroids, carotenoids, alkaloids flavonoids, etc.
Three types of secondary products are distinguished with respect to metabolic stability:
(1) truly metabolically inert end products,
(2) product whose stability pertains to a physiological or developmental state and
(3) those with a continuous turnover. In the course of the metabolism of these products, several polymeric products are formed: for example, rubber, sporopollenins, lignins.
The formation of these substances is catalysed by phenoloxidases and peroxidases and quite often, the cell wall is the preferred site of formation of these macromolecules. Degradation of secondary products to primary metabolic substrates is effected by a group of enzymes that have low substrate specificity such as methylating monoxygenases and glucosidases.
The quantities of secondary products vary with the developmental state of the plant, the season, the climate and even the time of the day. This is because secondary metabolism is subject to the influence of several regulatory factors.
The significance of the formation of secondary products lies not so much in that they serve as the source material for regeneration of compounds needed for primary metabolism. Even if it happens, the contribution is so minor that it has no impact on metabolism as a whole.
But the formation of secondary products permits the continuance of the primary metabolic event, through the removal of products of the concerned synthetic pathway from the site-of reaction and this is of far greater significance.
Secondary products: Ecological significance:
The secondary products are of special importance in the maintenance of ecological balance. The mutual influence as between the constituent members of an ecosystem is largely traceable to the secondary products generated by the components species of the system.
The allelopathic relations between one plant and another are to a certain extent determined by the nature of the secondary products. For example, the monoterpenes released from the leaves of Salvia teucophylla and Artemisia californica would make the soil that absorbed those substances unsuitable for the generation of the seeds of annual plants.
Secondary products: Plant – animal relations:
Secondary plant products have a determinative influence upon plant – animal relations. They serve the ends of predation, pollination and seed dispersal. Scent plays an important role in attracting the insects concerned with pollination.
Substances involved include monoterpenes such as limonene of citrus and geraniol of roses; amines like the monoamines of Arum and benzoic acid derivatives such as vanillin of Vanilla. Colour of the flower is the expression of pigments, which are also secondary products.
For example, the white colour of certain flower is because of the presence of flavones such as luteolins; yellow flowers are caused by carotenoids, betaxanthins and yellow flavonols and the red and violet colours are caused by betacyanins and anthocyanins. When ripe, fruits have bright-coloured coats and emit characteristic odours.
Both play significant role in attracting animals and birds and eventual dispersal of fruit and seed. The colour and odour originate in secondary products. Many plant products have their flavour and palatability determined by the presence of secondary products. This plays a role in attracting or repelling herbivores.
Secondary products: Disease-resistance and incidence:
Secondary products form a part of general defence system of higher plants against microbial infections. Conversely, the parasites form secondary products that help in overcoming the resistance of host plant.
Secondary products in relation to Man:
Taste, flavour, colour and palatability of foods and beverages is due to the chemical nature of the secondary products. Many of the drugs which man uses also are products of secondary metabolism in plants.
For e.g., D. Sorbital is a sugar alcohol used by diabetics; reserpine, an anti-hypersensitive drug, strychnine which serves as a central stimulant and quinine which is an anti-malarial drug are all Indole derivatives. Narcotics such as hashish and marijuana are monoterpenes from Cannabis sativa.
The beverages of coffee, tea, cocoa contain caffeine, theobromine and theophylline, all purines. Nicotine from tobacco and arecaidine from Areca catechu are nicotinoc acid derivatives. The drug value of the Solanaceous plants like Datura and Atropa is due to the presence of scopolamine and other alkaloids in them.
A large variety of secondary products serve as industrial raw material. Linseed oil employed in paint industry, castor oil used in the manufacture of lubricants and soaps, waxes used in making polishes and varnishes, suberin a constituent of cork used in the production of insulating materials stoppers and filters are all fatty acid derivatives.
Tannins which have an important application in leather industry are polyphenols. Rubber is a polyterpene produced by Ficus elastica, Hevea brasiliensis, etc. The essential oils extensively used in perfumery, cosmetics and preparations of solvents are terpenoids.
Mechanism of Stomatal Movement:
Stomata function as turgor-operated valves because their opening and closing movement is governed by turgor changes of the guard cells. Whenever, the guard cells swell up due to increased turgor, a pore is created between them. With the loss of turgor the stomatal pores are closed. Stomata generally open during the day and close during the night with a few exceptions.
The important factors which govern the stomatal opening are light, high pH or reduced CO2 and availability of water. The opposite factors govern stomatal closure, viz., darkness, low pH or high CO2 and dehydration. There are three main theories about the mechanism of stomatal movements:
1. Hypothesis of Guard Cell Photosynthesis (Schwendener, 1881):
Guard cells contain chloroplasts. During day the chloroplasts perform photosynthesis and produce sugar. Sugar increases osmotic concentration of guard cells. It causes absorption of water from nearby epidermal cells. The turgid guard cells bend outwardly arid create a pore in between. However, photosynthetic activity of guard cell chloroplasts seems to be negligible.
2. Classical Starch Hydrolysis Theory:
The main features of the theory were spelled out by Sayre (1923). It was modified by Steward (1964). The guard cells contain starch. At low carbon dioxide concentration (in the morning achieved through photosynthesis by mesophyll and guard cells), pH of guard cells rises. It stimulates enzyme phosphorylase. Phosphorylase converts starch into glucose 1-phosphate. The latter is changed to glucose 6-phosphate which undergoes hydrolysis to produce glucose and phosphoric acid.
Glucose increases osmotic concentration of guard cells. On account of it, the guard cells absorb water from neighbouring cells, swell up and create a pore in between them. Evening closure of stomata is brought about by increased carbon dioxide content (due to stoppage of photosynthesis) of leaf. It decreases pH of guard cells and brings about phosphorylation of glucose. In the presence of phosphorylase, glucose 1-phosphate is changed into starch.
As a result, osmotic concentration of guard cells falls. They lose water to adjacent epidermal cells. With the loss of turgidity, the guard cells shrink and close the pore in between them.
Objections, (i) Glucose is not found in guard cells at the time of stomatal opening, (ii) Starch <===> Sugar changes are chemically slow while opening and closing of stomata are quite rapid. (iii) Wide changes in pH of guard cells cannot be explained on the basis of carbon dioxide concentration. (iv) Onion and some of its relatives do not possess starch or related polysaccharide that can be hydrolysed to the level of glucose, (v) Blue light has been found to be more effective than other wavelengths for opening of stomata. The same cannot be explained by starch hydrolysis theory, (vi) Hydrolysis of starch theory cannot account for high rise in osmotic pressure found in guard cells.
3. Malate or K+ ion Pump Hypothesis (Modern Theory):
The main features of the theory were put forward by Levitt (1974). According to this theory, pH of the guard cell can rise due to active H+ uptake by guard cell chloroplasts or mitochondria, C02 assimilation by mesophyll and guard cells. A rise in pH causes hydrolysis of starch to form organic acids, especially phosphoenol pyruvate. Starch Hexose Phosphate —> Phosphoglyceric Acid —> Phosphoenol Pyruvate.
Phosphoenol pyruvate can also be formed by pyruvic acid of respiratory pathway. With the help of PEP carboxylase (PEP case), it combines with available CO2 to produce oxalic acid which gets changed into malic acid. Malic acid dissociates into H+ and malate.
H+ ions pass out of the guard cells in exchange for K+ ions. Some K+ ions (accompanied by C I– ions) are also absorbed actively from adjacent epidermal cells with the help of cAMP, ATP and cytokinins. Guard cells maintain their electroneutrality by balancing K+ with malate and CI–.
In the combined state they pass into the small vacuoles and increase the osmotic concentration of the guard cells. As a result guard cells absorb water from the nearby epidermal cells through endosmosis, swell up and create a pore in between them.
During stomatal closure, the H+ ions diffuse out of the guard cell chloroplasts. It decreases pH of the guard cell cytoplasm. Any malate present in the cytoplasm combines with H+ to form malic acid. Excess of malic acid inhibits its own biosynthesis.
High CO2 concentration also has a similar effect. Undissociated malic acid promotes leakage of ions (Jackson and Taylon, 1970). As a result K+ ions dissociate from malate and pass out of the guard cells. Formation of abcisic acid (as during drought or midday) also promotes reversal of H+ <===> K+ pump and increases availability of H+ inside the guard cell cytoplasm.
Loss of K+ ions decreases osmotic concentration of guard cells as compared to adjacent epidermal cells. This causes exosmosis and hence turgidity of the guard cells decreases. It closes the pore between the guard cells. Simultaneously the organic acids are metabolised to produce starch.