RV

Secondary Metabolites in Plants 4

Metabolites

Primary Metabolites

  • Definition: These are fundamental organic molecules essential for the function and life of all plant cells.

  • Composition: They include carbohydrates, lipids, proteins, and nucleic acids.

  • Distribution: Found universally in all plant cells.

Secondary Metabolites

  • Definition: Organic molecules characterized by a restricted distribution among different plant species and even within a single plant.

  • **Functions:

    • Chemical Signals: They act as chemical signals, often playing a crucial role in defense mechanisms against herbivores or pathogens.

    • Allelopathy: These are chemicals produced by genes that exhibit inhibitory effects, allowing a plant to control its root space and prevent other plants from growing nearby. This grants the producing plant a competitive advantage.

  • Synthesis and Storage: Synthesized at various sites within the plant, but frequently stored in vacuoles.

  • Major Classes: There are three major classes of secondary metabolites:

    • Alkaloids

    • Terpenoids

    • Phenolics

Alkaloids

  • Characteristics: These are alkaline (basic) and nitrogenous compounds.

  • Significance: Highly important for their pharmacological and medicinal effects, often exhibiting dramatic physiological and psychological properties.

  • Prevalence: Over 10,000 alkaloids have been identified and isolated.

  • **Examples:

    • Morphine:

      • Identification: The first alkaloid to be identified, in 1806.

      • Source: Derived from Papaver somniferum (Opium poppy).

      • Properties: Functions as an analgesic (pain reliever) and a cough suppressant.

    • Cocaine:

      • Source: Isolated from the leaves of Erythroxylem coca (coca).

      • Historical Use: Historically used by the Inca civilization to reduce hunger pangs and combat fatigue.

      • Properties: Possesses anesthetic and energetic qualities. Chewing the leaves is not harmful and can numb the gums.

    • Caffeine:

      • Source: A common stimulant found in many plants, but prominently associated with Coffea arabica (coffee), Camellia sinensis (tea), and Theobroma cacao (cocoa).

      • Plant Function: Provides protection against herbivory by insects and fungi, particularly in seedlings, which produce the highest concentrations of caffeine. It also acts via allelopathy to prevent other plants from growing in its area.

    • Nicotine:

      • Source: A stimulant obtained from Nicotiana tabacum (tobacco plant).

      • Synthesis and Transport: Synthesized in the roots and subsequently transported to the leaves.

      • Storage: Stored primarily in vacuoles.

      • Toxicity: Highly toxic to herbivores and insects, serving as a protective mechanism.

      • Phytoalexin: Functions as an antimicrobial compound, released by the plant via its roots.

Terpenoids

  • Characteristics: This is the largest class of secondary metabolites found in plants, with over 22,000 described.

  • Basic Unit: The smallest unit is the hydrocarbon isoprene.

  • **Classification by Isoprene Units:

    • Monoterpenoid: Composed of 2 isoprene units.

    • Sesquiterpenoids: Composed of 3 isoprene units.

    • Diterpenoids: Composed of 4 isoprene units.

  • Isoprene Emission: Isoprene is emitted from leaves, contributing to the characteristic blue "mist" observed over forests. This occurs because isoprene molecules reflect blue light back, which may help stabilize photosynthetic membranes on hot days and prevent overheating.

  • **Examples:

    • Essential Oils:

      • Composition: A large grouping consisting mainly of monoterpenoids and sesquiterpenoids.

      • Properties: Highly volatile, contributing significantly to plant fragrance.

      • Functions: Can deter herbivores, exhibit anti-microbial and anti-fungal properties, and act allelopathically. Terpenoids in flowers are also crucial for attracting pollinators.

      • Storage: In plants like Mentha (mint), compounds like menthol and menthone are stored in trichomes, which are extensions of the epidermis (fuzzy, little spikes).

    • Taxol:

      • Classification: A diterpenoid.

      • Original Source: Originally isolated from Taxus brevifolia (Pacific Yew tree).

      • History: An early anti-cancer medication. Its initial harvesting was destructive to the yew tree.

      • Current Sources: Now also found in other Taxus plants, some yew-associating fungi, and can be synthesized in laboratories, although this process is very expensive.

    • Rubber:

      • Classification: The largest terpenoid, containing an astounding 400 to 10,000 isoprene units.

      • Source: Isolated from Hevea brasiliensis (Pará rubber tree).

      • Mechanism: The tree excretes latex from a series of connected cells forming tubes called laticifers.

      • Harvesting: Obtained by damaging the bark.

      • Defense: Highly effective against herbivores. Insects can get caught in the sticky latex, and while birds might eat these insects, the latex can make them sick as it is indigestible.

    • Cardiac Glycosides:

      • Classification: Sterol derivatives that can cause cardiac arrest.

      • Function: Act as a protective mechanism against herbivory by being toxic if ingested.

      • Example: Digitalis (Foxglove) contains digitoxin and digoxin.

      • Adaptation: Some insects, such as the Monarch caterpillar, have adapted to consume Milkweed (Asclepias spp.), which contains these compounds. They can break down these toxins or store them in their bodies for their own defense against predators.

Phenolics

  • Characteristics: A broad grouping of compounds, all characterized by possessing a hydroxyl group ( ext{–OH}) bound to an aromatic ring.

  • Function: Despite being universally present in plants, their exact functions are largely unknown.

  • **Examples:

    • Flavonoids:

      • Characteristics: Pigmented and water-soluble phenolics, with over 3,000 described.

      • Dietary Significance: Found in red wine and grape juice, and have been shown to lower cholesterol.

      • **Types and Colors:

        • Anthocyanins: Responsible for red, purple, and blue colors.

        • Flavones & Flavonols: Impart yellow and ivory colors, with some types being colorless.

      • Other Roles: Also act as messengers for symbiosis.

    • Tannins:

      • Function: Considered the most important deterrent for herbivores in angiosperms (flowering plants).

      • Properties: Characterized by a bitter taste. Unripe fruits often have high concentrations of tannins.

      • Mechanism: Used to denature proteins.

      • Storage: Sequestered in vacuoles.

      • Historical Relevance: Used historically to tan animal skin, preventing bacterial infiltration.

    • Lignins:

      • Deposition: Uniquely deposited in plant cell walls.

      • Abundance: The second most abundant organic compound on Earth, after cellulose (1^ ext{st}).

      • Composition: A complex polymer made from three different types of monomers: p-coumaryl, coniferyl, and sinapyl alcohols. The abundance of these monomers differs based on the plant type (e.g., gymnosperms).

      • Primary Roles: Crucial for providing structure and "stiffness" to plants.

      • Evolutionary Significance: Played a huge role in plant evolution by providing strength, waterproofing, and defense against fungi, particularly in stems.

    • Salicylic Acid:

      • Historical Use: Valued as an analgesic by ancient Greek and indigenous peoples, often consumed as a tea from willow (Salix) bark.

      • Plant Function: In plants, it plays a vital role in the development of systemic acquired resistance (SAR). This refers to a localized response to an attack by bacteria, fungi, or viruses, effectively kicking off cell replication and defense mechanisms within the plant.