Plant Secondary Metabolites Quiz

Plant Secondary Metabolites (PSM)

• PSM are complex organic compounds produced by plants in small quantities.
• Approximately 100,000 known structures have been identified, but only a small percentage (5-15%) of plant structures have been chemically studied.
• They play a vital role in plant survival and evolution.
• Unlike primary metabolites, which are directly involved in growth and development, PSMs indirectly support plant survival.

Defensive Role of PSM

• PSMs play crucial defensive roles through:
  • Mechanical defenses: Thorns, trichomes, thick epidermis, and sticky/smooth surfaces make the plant less desirable to herbivores.
  • Chemical defenses: Alkaloids, phenols, terpenoids, iridoid glycosides, cardenolides, and cyanogenic glycosides act as toxic substances deterring herbivores.

Biosynthesis of PSM

• PSMs are synthesized from precursor molecules, including amino acids (for alkaloids), activated isoprenes, and shikimic acid.
• Biosynthetic pathways primarily occur within the cytoplasm, plastids, endoplasmic reticulum, and mitochondria.
• Enzyme diversity and genetic variations allow for multiple compounds to potentially be synthesized from one enzyme or pathway.

Classification of PSM

• PSMs are categorized into major groups based on structure and function:
  • Terpenoids/Isoprenoids (>25,000 compounds): Synthesized from isoprene units via mevalonate or methylerythritol phosphate pathways. Examples include essential oils, bitters, saponins, and carotenoids.
  • Phenylpropanoids (~2,000 compounds): Derived from amino acids, primarily through the shikimate-chorismate pathway. Examples include tannins and lignin, which play a role in plant structure and defense.
  • Alkaloids (~12,000 compounds): Nitrogen-containing compounds synthesized from amino acids. Diverse functions, including affecting the nervous system. Examples include nicotine and morphine.

Temporal Changes in PSM

• PSM concentrations can change throughout the plant's lifespan (ontogenetic changes) and seasonally (seasonal changes).
• Factors influencing seasonal changes include temperature, light, water availability, and nutrient conditions.

Small Group 1: Slimes (Mucilaginous Substances)

• Mucilaginous substances are polysaccharides with a high water retention capacity, swelling into colloidal solutions or gels.
• They have various applications, for example, protecting the mucosa in inflammatory diseases (GIT, throat) and as lubricants (emollients), laxatives, and antidiarrheal agents.
• Examples include marshmallow root, mallow flower, and flaxseed.

Small Group 2: Glycosides

• Glycosides are compounds featuring a sugar linked to a non-sugar moiety (aglycone).
• Different types of glycosides are formed with various linkages, like the O-glycosidic, S-glycosidic, and C-glycosidic bonds.
• They display a wide spectrum of therapeutic functions varying with the aglycone structure, including diuretic, cardiotonic, and anti-inflammatory properties.
• Examples include salicin (from willow bark), arbutin, cyanogenic glycosides (HCN release), cardiac glycosides (e.g., digitoxin), and anthraquinone glycosides (laxatives).

Large Group 1: Terpenoids (Isoprenoids)

• Terpenoids are derived from isoprene units via synthesis pathways.
• Their structure contains numerous conjugated double bonds, making them susceptible to oxidation.
• Key roles include antioxidant activity, defense against herbivores/pathogens (e.g. carotenoids, lycopene, saponins, e.g., licorice root).

Large Group 2: Phenolics

• Phenolic compounds are aromatic metabolites with hydroxyl groups attached to aromatic rings.
• Common biosynthesis pathways are shikimate-chorismate (from amino acids) and acetate-malonate pathways (polyketides).
• Types range from simple phenols, to phenols with additional carbons, to those with multiple rings (like xanthones, stilbenes, and flavonoids).
• Phenolics display properties including but not limited to strengthening capillary walls, reducing vein permeability, and exhibiting anti-inflammatory and diuretic effects.
• Tannins, a related group, are categorized as hydrolyzable (from simple phenols) or condensed (from flavonoids).

Large Group 3: Alkaloids

• Alkaloids are nitrogen-containing compounds derived from amino acids.
• Subtypes include "true" alkaloids (derived from amino acids), protoalkaloids (amino acid derivatives), and pseudoalkaloids (derived from terpenoids or polyketides).
• They noticeably affect the nervous system, serving as stimulants, sedatives, or spasmolytics.
• Examples include morphine, nicotine, and caffeine.

Practical Applications

• PSMs have potential medicinal applications, including treating chronic diseases, serving as a complement to synthetic medicines, and potentially reducing side effects.
• They exhibit diverse therapeutic potentials.

Test your knowledge on plant secondary metabolites (PSM) and their roles in plant defense and survival. This quiz covers the synthesis, types, and functions of these important compounds in plants. Challenge yourself and discover how much you know about these fascinating organic substances!