RV

Molecular Compounds for Plants 3

Molecular Compounds in Living Organisms

Essential Elements for Life

  • CHONPS: The foundational elements for molecular compounds in living organisms, particularly highlighted in plants:

    • Carbon (C)

    • Hydrogen (H)

    • Oxygen (O)

    • Nitrogen (N)

    • Phosphorus (P)

    • Sulphur (S)

Primary and Secondary Metabolism in Plants

  • Organic Molecules Utilized by Plants:

    • Plants can utilize thousands of different organic molecules; a single plant cell may contain over 10,000 distinct types.

    • Almost all cellular dry weight comprises compounds associated with primary metabolism.

  • Primary Metabolism:

    • Involves compounds essential for basic survival and growth.

    • Key categories include:

      • Carbohydrates

      • Lipids

      • Proteins

      • Nucleic acids

  • Secondary Metabolites:

    • Organic compounds that are not directly involved in the normal growth, development, or reproduction of an organism.

    • They often provide a selective advantage, such as defense mechanisms or attracting pollinators.

Carbohydrates

  • General Characteristics:

    • The most abundant organic molecule found in nature.

    • Serve as primary energy-storage molecules.

    • Are hydrophilic, meaning they readily dissolve in water.

  • Monomers and Polymers:

    • Polymer: A substance made up of many small, similar, or identical subunits.

    • Monomer: Each individual subunit that makes up a polymer.

    • Polymerization: The chemical process by which monomers link together to form polymers.

  • Types of Carbohydrates:

    • Monosaccharides:

      • Single sugar molecules, serving as fundamental building blocks and primary energy sources.

      • Examples include glucose, fructose, and ribose.

      • Described by the general formula (CH2O)n, where the number of carbon atoms n typically ranges from 3 ext{ to } 7.

      • Structurally, they consist of a carbon chain, a hydroxyl group (-OH), and a carbonyl group (C=O).

      • Glucose is particularly important for transport in animals.

      • Can exist in both chain and ring forms (e.g., eta-glucose and ext{alpha}-glucose).

    • Disaccharides:

      • Composed of two sugar subunits linked together via a covalent bond.

      • Function in transport and energy transfer.

      • Examples: sucrose (glucose + fructose), maltose, lactose.

      • Formation (Dehydration Synthesis / Condensation Reaction):

        • Two monomers are joined together.

        • One molecule of water is produced.

        • This process requires an input of energy.

      • Breakdown (Hydrolysis):

        • The opposite reaction of dehydration synthesis.

        • A disaccharide is broken apart, releasing energy.

    • Polysaccharides:

      • Composed of many sugar subunits linked covalently.

      • Primarily serve roles in energy storage and structural support in plants.

      • Must be hydrolyzed into mono- or disaccharides before they can be utilized as an energy source.

      • Starch:

        • The primary energy storage polysaccharide in plants.

        • Exists in two forms: amylose and amylopectin.

        • Stored in specialized structures called starch grains.

      • Fructans:

        • Polymers of fructose, stored in some plants like wheat, rye, and barley.

      • Cellulose:

        • The principal structural component of a plant cell wall.

        • Composes approximately 50 ext{%} of the total organic carbon in the living world, highlighting its abundance.

        • It is particularly difficult to break down because it only contains eta-glucose subunits.

        • Strong hydrogen bonds form between the hydroxyl groups of adjacent cellulose chains, which makes the structure incredibly rigid and protected, hindering hydrolysis.

        • Only specific microorganisms (e.g., fungi, prokaryotes, protozoa, silverfish) possess the enzymes necessary to hydrolyze cellulose.

  • Why Carbohydrates are a Good Energy Source for Plants:

    • Plants can produce their own carbohydrates through photosynthesis.

    • They do not rely on consuming other animals for energy.

    • Plants lack a digestive system typical of animals.

    • Carbohydrates are water-soluble, facilitating transport within the plant.

Lipids

  • General Characteristics:

    • Unlike carbohydrates, lipids are hydrophobic, meaning they are insoluble in water.

    • Used for energy storage and structural purposes.

    • While large, they are not technically macromolecules because they are not formed through the polymerization of repeating monomers.

    • Essential components of biological membranes (e.g., chloroplasts, mitochondria).

    • Used by certain plants, especially in seeds and fruits (e.g., olives) for energy reserves.

  • Types of Lipids:

    • Triglycerides:

      • A class of lipids that includes both fats and oils.

      • Chemically, they consist of three fatty acid molecules esterified to a glycerol molecule.

      • Lack polar groups, contributing to their hydrophobic nature.

      • Saturated Triglycerides: Contain only single bonds between carbon atoms in their fatty acid chains.

      • Unsaturated Triglycerides: Possess one or more double bonds between carbon atoms in their fatty acid chains.

      • The differentiation between fats and oils is based on the ratio of saturated to unsaturated fats and their physical state at room temperature (fats are solid, oils are liquid).

    • Phospholipids:

      • Modified triglycerides where one of the fatty acid groups is replaced by a phosphate head group.

      • Are integral components of all cellular membranes.

      • The phosphate head is negatively charged and hydrophilic, while the fatty acid tails are hydrophobic.

      • This amphipathic nature leads to the formation of a phospholipid bilayer in aqueous environments, with hydrophilic heads facing the watery exterior and interior, and hydrophobic tails forming an impermeable interior barrier.

    • Important Lipids in Plants:

      • Cuticle: A protective layer on the epidermis of leaves and stems, composed of wax embedded in cutin (a structural lipid).

      • Epicuticular Wax: Covers the cuticle, providing additional protection against water loss.

      • Suberin: A major component found in the cork cells of the outer layer of bark, contributing to its protective and impermeable properties.

    • Steroids:

      • Easily distinguished by their characteristic structure of four fused hydrocarbon rings.

      • Primarily function in signaling within plants.

      • Examples include brassins (brassinosteroids), which act as plant hormones regulating growth and development.

  • Lipids in Plants - Roles and Considerations:

    • Water Loss Protection: Cuticle and suberin prevent desiccation.

    • Seed Production: Lipids provide concentrated energy storage for developing embryos in seeds.

    • Why Plants Don't Produce Lots of Fat: Fat can solidify and become heavy, making it hard for plants to move or transport these reserves efficiently, unlike their more soluble carbohydrate counterparts.

Proteins

  • General Characteristics:

    • In most organisms, proteins constitute over 50 ext{%} of the dry weight; however, in plants, this percentage is lower due to their high cellulose content.

    • Exhibit immense diversity in function and structure, yet fundamentally share a common foundational structure.

    • Are polymers of nitrogen-containing amino acids, which are linked together by peptide bonds to form polypeptide chains.

    • Proteins are large and complex molecules.

  • Protein Structure Levels:

    • Primary Structure: The unique linear sequence of amino acids in a polypeptide chain. This sequence dictates all subsequent levels of structure.

    • Secondary Structure: Local folded structures that form within a polypeptide via hydrogen bonds between the backbone atoms.

      • Alpha-helix ( ext{alpha}-helix): A common coiled structure, maintained by hydrogen bonds.

      • Beta-pleated sheet (eta-pleated sheet): A common folded structure where polypeptide chains lie parallel to each other, also held by hydrogen bonds.

      • Fibrous proteins, which often have structural roles, frequently exhibit these secondary structures.

    • Tertiary Structure: The overall three-dimensional shape of a single polypeptide chain, resulting from interactions between the side chains (R-groups) of amino acids within the protein.

    • Quaternary Structure: The arrangement of multiple polypeptide chains (subunits) in a multi-subunit protein complex.

  • Key Roles of Proteins in Plants:

    • Cross-membrane proteins: Facilitate transport of substances across cellular membranes.

    • Enzymes: Act as biological catalysts, accelerating biochemical reactions necessary for plant metabolism and growth.

Nucleic Acids

  • General Characteristics:

    • Polymers responsible for carrying genetic information and involved in protein synthesis.

    • Composed of monomeric units called nucleotides, each consisting of a phosphate group, a five-carbon sugar, and a nitrogenous base.

  • Types of Nucleic Acids:

    • Ribonucleic Acid (RNA):

      • Its sugar subunit is ribose.

      • Primarily functions as a blueprint for protein synthesis, carrying genetic instructions from DNA to the ribosomes.

    • Deoxyribonucleic Acid (DNA):

      • Its sugar subunit is deoxyribose.

      • Serves as the carrier of genetic messages, organized into genes.

      • Typically exists as a double helix, with two antiparallel polynucleotide strands.

  • Nitrogenous Bases:

    • Purines: Adenine (A), Guanine (G).

    • Pyrimidines: Cytosine (C), Thymine (T) - only in DNA, Uracil (U) - only in RNA.

  • Base Pairing and Stability:

    • In DNA, Adenine (A) always pairs with Thymine (T) via two hydrogen bonds.

    • Guanine (G) always pairs with Cytosine (C) via three hydrogen bonds.

    • The presence of three hydrogen bonds between G-C pairs makes this pairing stronger than A-T pairing, contributing to the overall stability of the DNA molecule.

  • Why DNA is Superior to RNA for Genetic Storage:

    • More Protected Structure: DNA's double-helix shape offers greater protection for the genetic information within, as the bases are sequestered in the interior of the helix.

    • Deoxyribose vs. Ribose: Deoxyribose sugar in DNA lacks a hydroxyl group at the 2' position, making DNA inherently more stable and less reactive than RNA, which has a hydroxyl group at that position.

    • Thymine vs. Uracil Stability: The presence of Thymine in DNA (which has a methyl group) makes it more stable against degradation compared to Uracil in RNA.

  • Roles of Nucleic Acids in Plants:

    • Protection of Genetic Information: DNA protects the plant's inheritable traits and instructions.

    • Protein Synthesis: RNA is crucial for translating genetic information into functional proteins.

Adenosine Triphosphate (ATP)

  • Principal Energy Carrier:

    • ATP is the primary currency of energy in all living organisms, including plants.

    • It stores and transfers energy within cells.

  • Energy Release and Recharge:

    • Energy is released from ATP through the hydrolysis of a phosphate group, typically forming ADP (adenosine diphosphate) and an inorganic phosphate.

    • ATP is constantly