(a) Differentiate between enzymes and coenzymes and describe their mechanisms of action. (10+10=20 marks) (b) Define secondary…

Enzymes versus Coenzymes and their Mechanisms of Action

Enzymes and coenzymes function cooperatively in cellular catalysis but exhibit fundamental structural and functional differences. Enzymes are high-molecular-weight, heat-labile macromolecular biocatalysts that are almost exclusively proteinaceous in nature (excluding catalytic RNAs/ribozymes), possessing distinct primary-to-quaternary conformations with specialized active sites. In contrast, coenzymes are low-molecular-weight, dialysable, heat-stable, non-protein organic or metallo-organic accessory molecules, frequently derived from water-soluble vitamins, that transiently or stably associate with the inactive apoenzyme to constitute the catalytically active holoenzyme. Unlike enzymes, which remain structurally intact post-reaction, coenzymes participate as stoichiometric co-substrates, undergoing transient chemical modifications before being enzymatically regenerated.

Enzymatic catalysis operates by stabilizing the transition state and dramatically lowering the free energy of activation (Δ G^ddagger) without altering the equilibrium constant. Substrate binding at the active site occurs either through rigid steric complementarity (Fischer’s Lock and Key model) or dynamic structural reconfiguration upon binding (Koshland’s Induced Fit model), utilizing precise catalytic strategies such as acid-base catalysis, covalent intermediate formation, and electrostatic stabilization. Conversely, the mechanism of coenzymes involves chemical group or electron transfers. Redox coenzymes like Nicotinamide Adenine Dinucleotide (NAD⁺) and Flavin Adenine Dinucleotide (FAD) act as reversible acceptors of hydride ions (H⁻) and hydrogen atoms (H⁺ + e⁻), respectively, in dehydrogenase-mediated reactions. Transfer coenzymes such as Coenzyme A (CoA-SH) and Pyridoxal Phosphate (PLP) function by covalently forming reactive intermediates to shuttle acyl groups and amino groups between donor and acceptor substrates.

Secondary Metabolites: Definition, Classification, and Plant Significance

Secondary metabolites are diverse organic compounds derived from primary metabolic pathways (shikimate, mevalonate, and acetate pathways) that are not directly required for fundamental vegetative growth, development, or cellular respiration, and exhibit restricted taxonomic distribution among plant lineages. They are broadly categorized into: Terpenoids (e.g., azadirachtin from Azadirachta indica), Phenolics (e.g., curcumin from Curcuma longa, anthocyanins, and lignin), and Nitrogen-containing Alkaloids (e.g., morphine from Papaver somniferum and reserpine from Rauvolfia serpentina).

Ecologically and physiologically, secondary metabolites are essential for plant survival and adaptation. Terpenoids and bitter alkaloids serve as constitutive or inducible chemical defenses against herbivory and microbial phytopathogens. Phenolics like flavonoids absorb damaging UV-B radiation, protecting photosynthetic machinery, while anthocyanins and volatile terpenes impart floral pigments and scents to attract specific pollinators and seed dispersers. Lignin provides structural rigidity to tracheary elements, facilitating water transport under negative pressure. Furthermore, exudates such as juglone mediate allelopathic suppression of competing neighboring flora, and select alkaloids function as mobile nitrogen storage pools, which secondarily provide invaluable pharmacophores in modern therapeutics.

Phloem Structure and Hypotheses of Translocation

Phloem is the specialized, complex living vascular tissue responsible for the multi-directional translocation of photoassimilates (chiefly sucrose) from source to sink. In angiosperms, it comprises enucleated sieve tube elements interconnected end-to-end via perforated sieve plates, metabolically coupled nucleated companion cells, storage-oriented phloem parenchyma, and supportive sclerenchyma fibres.

Several hypotheses have been formulated to explain phloem translocation. The Münch Pressure-Flow Hypothesis remains the most widely accepted mechanism. It postulates that active proton-coupled symport of sucrose at source tissues lowers the sieve-tube water potential (Ψ_w), driving osmotic water influx from adjacent xylem. This generates a high localized hydrostatic turgor pressure (P). Concurrent sucrose unloading and water efflux at sink tissues generate a low hydrostatic pressure, driving bulk mass flow of the aqueous solution along the physical pressure gradient.

Alternative hypotheses include the Electro-osmotic Hypothesis (Spanner), which suggests that potassium ion (K⁺) circulation across sieve plates maintains an electrical potential gradient that propels polar water and sugar molecules. The Cytoplasmic Streaming Hypothesis (de Vries) attributes assimilate distribution to ATP-dependent cyclosis within sieve elements. The Contractile Protein Hypothesis (Thaine) proposes transcellular peristaltic movement facilitated by undulating P-protein filaments, while Interfacial Flow Hypotheses rely on spreading across phase boundaries.

Experimental evidence from aphid stylet exudation, radioactive ¹⁴CO₂ tracing, and measured positive turgor gradients conclusively establishes that while P-proteins and cytoplasmic streaming aid structural sealing and short-distance mobilization, osmotically driven bulk pressure flow represents the true physiological mechanism of long-distance phloem transport.