Plant Metabolites and Their Functions

Chapter 6: Production of Chemical from Plants

• Metabolism & Metabolites: Metabolism encompasses the chemical processes within a living organism to sustain life. Metabolites are the intermediates and products of metabolism, typically small molecules with diverse functions (structure, signaling, stimulatory, defense).

Metabolic Engineering in Plants

• Plants are highly productive for small molecules.
• Over 100,000 metabolites were identified in 2004 (possibly only 10% of the total).
• Plants are the richest organisms in secondary metabolites (5,000–25,000 per plant) compared to other kingdoms.

Plant Metabolism

• Illustrates photosynthesis, respiration, leaf water exchange, and sugar transport in a plant.

Plant Metabolism Outline

• Photosynthesis: Driven by light energy, uses carbon dioxide and water to produce sugars. Sugars are stored as starch.
• Respiration: Releases energy from sugars for plant growth, uses oxygen.
• Translocation: Movement of sugars (photosynthate) from leaves to other plant parts.
• Production of other Compounds: Sugars are also converted into other molecules like pectin, cellulose, lignin, fats, proteins, enzymes, pigments, vitamins, alkaloids, tannins and other protective substances.

Primary Metabolites

• Necessary for plant growth, development, and reproduction.
• Main types are carbohydrates, proteins, nucleic acids, and lipids.
• Some are precursors for secondary metabolites.

Secondary Metabolites

• Not directly involved in plant growth, development, and reproduction.
• Often have crucial ecological roles, like defense, attracting pollinators, or seed dispersal.
• Examples include lignin, anthocyanins, phytoalexins, and alkaloids.

Carbohydrates

• Sugars formed from glucose and its isomers.
• Come in various sizes:
  • Monosaccharides: Single sugar units (e.g., glucose, fructose).
  • Disaccharides: Two sugar units (e.g., sucrose—glucose and fructose).
  • Polysaccharides: Polymers of more than two sugar units.

Polysaccharides

• Structural: Used for plant support (e.g., cellulose—major component of cell walls).
• Storage: Used to store energy (e.g., starch stored in roots and tubers).

Cellulose

• Very strong due to its chemical structure.
• Composed of long chains of beta-glucose molecules (100–15,000 glucose molecules).
• Main structural polysaccharide in plants, forming 40–60% of cell walls.

Proteins

• Make up a significant portion of plant biomass.
• Composed of one or more polypeptides of amino acids.
• Plants create amino acids from photosynthesis products through complex processes involving nitrogen uptake and energy (ATP and NADPH).
• Structural and storage proteins serve roles in maintaining and promoting plant growth.
• Structural proteins form a major percentage of plant cell wall and membranes.
• Storage proteins are concentrated in seeds and are essential for seed growth.

Enzymes

• Catalyze biochemical reactions in plants.
• Many enzymes are also beneficial for human health.
• Some are commercially valuable because their activity persists when isolated from plants.
• Examples include papain and bromelain, isolated from papaya and pineapple respectively.

Nucleic Acids

• Complex biological polymers forming RNA and DNA.
• Contain bases (adenine, thymine, guanine, cytosine), which are similar in all plants..
• Essential for genetic information and protein synthesis.

Lipids

• Include various substances that dissolve in nonpolar solvents (ether, chloroform, acetone) but not in water.
• Some store energy.
• Primary components of biological membranes.
• Some act as hormones, antioxidants, and pigments.

Major Lipid Classes

• Fatty Acids: Essential components of lipids.
• Triacylglycerols: Major energy storage lipids.
• Waxes: Protective coatings on leaves and fruits.
• Phospholipids: Essential components of biological membranes.

Oils

• Found in various parts of plants, but are often concentrated in seeds.
• Commercially important due to substantial oil content in certain seeds (e.g., palm, cotton, sesame).
• Common fatty acids in plant oils: oleic, linoleic, and linolenic.
• Linoleic and linolenic are essential fatty acids, meaning humans cannot synthesize them.

Waxes

• Complex mixtures of fatty acids and long-chain alcohols.
• Form protective layers on leaves, stems, and fruits, reducing water loss.
• Examples include cutin (major component of the cuticle), suberin (found in cork cells), and epicuticular waxes.

Plant Secondary Metabolites (Part 2)

• Diverse group of organic compounds produced in plants.
• Not directly involved in plant growth, development, or reproduction but often serve important ecological functions.
• Important for plant interactions, such as defense mechanisms against herbivores and microbes.
• Plants use secondary metabolites for functions such as attracting pollinators, dispersing seeds, coloration, and fragrance.
• Examples: morphine, caffeine, nicotine, menthol, and rubber.

Classification of Plant Secondary Metabolites

• Grouped based on similar structures, biosynthetic pathways, or plant type.
  • Alkaloids: Often toxic to other organisms. Pharmaceutical and recreational uses (e.g., morphine, nicotine).
  • Terpenoids: Contribute to scents and flavors (e.g., eucalyptus, cinnamon, cloves).
  • Phenolics: Diverse group with various functions, including lignin (provides structural support), anthocyanins (pigments), and tannins (astringency).

Enhancing Product Yield in Major Crops Through Biotechnology

• Breeding Strategies: Selecting plants with desirable traits for propagation (e.g. DNA fingerprinting.)
• Plant Cloning (Tissue Culture): Generating copies of plants from plant cells
• Genetic Engineering/Genetic Modification: Introducing new traits through genetic modification (e.g., enhancing yield in oil palm).
• Biofertilizers: Enhancing nutrient supply to plants through microorganisms (e.g. mycorrhizae).
• Biocontrol: Introducing beneficial microorganisms to control pests.