Plant Metabolism and Cell Contents

Ergastic Cell Contents

• Non-living substances present in the cell.
• Can be identified by microscopic examination or chemical and physical tests.
• Found in cytoplasm and vacuole.

Primary Metabolites

• Essential for plant survival
• Organic compounds involved in growth, development, and reproduction of the organism
• Examples include carbohydrates, proteins, lipids
• Absence of primary metabolites would result in immediate death.

Secondary Metabolites

• Not essential for plant survival.
• Often referred to as byproducts of metabolism.
• Examples include alkaloids, volatile oils, tannins, flavonoids.
• Play roles in functions like defense.

Carbohydrates

• Composed of carbon, hydrogen, and oxygen
• Carbon and hydrogen are present in the ratio 2:1, like that of water.
• Classified by number of sugar units:
  • Monosaccharides (e.g. glucose)
  • Disaccharides (e.g. sucrose)
  • Oligosaccharides
  • Polysaccharides (e.g. starch, cellulose)

Carbohydrates Role in Plants

• Source of energy: Energy can be stored as starch (storage polysaccharide)
• Structural Function: Example is cellulose in the plant cell wall
• Building blocks: The 5-carbon monosaccharide ribose is a component of coenzymes and the backbone of RNA. Deoxyribose is a component of DNA.

Starch

• Polysaccharide formed of many glucose units
• Source of energy
• Stored in the form of starch grains
• Starch granules are composed of amylose and amylopectin

Amylose vs Amylopectin

• Amylose: Unbranched chain of glucose units, soluble in water, gives blue color with iodine
• Amylopectin: Branched chain of glucose units, insoluble in water, gives bluish black precipitate with iodine

Starch Tests

• General test for carbohydrates: Iodine test: Suspension of starch gives a dark bluish black color
• Molisch's test: Suspension of starch and alcoholic α-naphthol gives violet ring at the interface upon addition of concentrated H2SO4.

Medicinal Uses of Starch

• Topically as dusting powder
• Antidote for iodine poisoning
• Lubricant for surgeons’ gloves
• Tablet disintegrant
• Suspending agent for barium meals
• Nutrient and demulcent properties

Important Starches

• Potato starch
• Maize starch
• Wheat starch
• Rice starch

Starch Grain Features

• Each grain has a hilum, the point of origin of starch deposition.
• Starch is deposited in layers around the hilum.
• Hilum can be located in the center or periphery.

Other Polysaccharides: Mucilage and Gums

• Mucilage: Viscous sticky material

• Plays a protective role in membranes, forming colloidal solution in water.

• Used as demulcent, emollient, and bulk laxatives

• Gives red color with ruthenium red

• Gums: Viscous secretion of trees and shrubs.

• Hardens on drying but is soluble in water.

• Used in pharmaceutical industry in the preparation of emulsions and suspensions.

• Classified as polysaccharides or salts of polysaccharides

Pectin

• Purified carbohydrate obtained from the rind of citrus fruits.
• Source of dietary fiber.
• Structural polysaccharide in plant cell walls.
• Used in food as a gelling agent.
• Used in pharmacy as:
  • suspending agent
  • in antidiarrheal formulations

Drugs Derived from Starch

• Hetastarch (hydroxyethyl starch): Plasma expander
• Dextrin: Product of partial degradation of starch, used in infant formulas for nutrition, drug delivery, oral disintegrating films
• Dextran: Plasma expander

Proteins

• Classified into:

  • Structural proteins in the cell membrane and cell wall
  • Storage proteins, mostly in seeds
  • Enzymes

• In plants, proteins are stored as amorphous mass called aleurone grains, often containing a crystalloid and a globoid.