Write short notes on the following in about 150 words each : 10×5=50 (a) Explain what is secondary active transport. Discuss its…

Secondary Active Transport Secondary active transport is the movement of ions or molecules across a membrane, driven by the electrochemical gradient of another ion, typically H⁺, rather than direct ATP hydrolysis. This process relies on the proton-motive force generated by primary active transport via H⁺-ATPases. It operates through two main mechanisms: symport (cotransport) and antiport (counter-transport). In symport, the transported solute moves in the same direction as the driving ion; for instance, H⁺/NO₃⁻ symporters in root cells utilize the inward flux of protons to accumulate nitrate against its concentration gradient. In antiport, solutes move in opposite directions, such as H⁺/Na⁺ antiporters that expel toxic sodium while importing protons. This mechanism is critical for plant ion acquisition, enabling roots to uptake essential nutrients like K⁺, NO₃⁻, and H₂PO₄⁻ from dilute soil solutions. By coupling nutrient uptake to proton gradients, plants efficiently concentrate ions in the cytosol, ensuring adequate supply for metabolic processes even under low external nutrient availability.

GOGAT Glutamate Synthase (GOGAT) is a key enzyme in the glutamine synthetase-glutamate synthase (GS-GOGAT) cycle, responsible for the final step of nitrogen assimilation. It catalyzes the reductive amination of 2-oxoglutarate using glutamine as the nitrogen donor, producing two molecules of glutamate. This reaction is coupled with the reduction of 2-oxoglutarate to glutamate. GOGAT exists in two isoforms: NADH-GOGAT, located in plastids and mitochondria, which uses NADH as the electron donor, and Ferredoxin-GOGAT (Fd-GOGAT), found in chloroplasts, which uses reduced ferredoxin generated by photosynthesis. Fd-GOGAT is particularly significant in C3 and C4 plants, linking nitrogen metabolism directly to photosynthetic electron transport. The catalytic function involves a complex mechanism where glutamine donates its amide nitrogen to 2-oxoglutarate, facilitated by the reduction of the enzyme’s active site. This cycle is vital for synthesizing amino acids, nucleotides, and other nitrogenous compounds, ensuring efficient nitrogen utilization in plant growth and development.

Phytohormone Regulation of Seed Dormancy Seed dormancy is a state of arrested development, regulated by a complex interplay of phytohormones, primarily Abscisic Acid (ABA) and Gibberellins (GA). ABA maintains dormancy by inducing the expression of seed-specific genes and transcription factors such as ABI3, ABI4, and ABI5, which promote desiccation tolerance and quiescence. The DOG1-dependent pathway, involving ABA signaling, reinforces this state. Conversely, GA breaks dormancy by promoting the degradation of DELLA proteins, which are growth repressors. This signaling cascade, mediated by the GID1 receptor, leads to the activation of genes involved in embryo growth and mobilization of reserves. Additionally, GA weakens the seed coat and endosperm layers, facilitating radicle emergence. Ethylene and brassinosteroids also play supportive roles in germination. The balance between ABA and GA levels, influenced by environmental cues like temperature and light, determines the transition from dormancy to germination. This hormonal antagonism ensures that seeds germinate only under favorable conditions, maximizing survival and fitness.

Metallophytes Metallophytes are plants that thrive in soils with high concentrations of heavy metals. They are physiologically classified into three categories: metal excluders, which restrict metal uptake to the roots; metal accumulators, which tolerate moderate metal levels; and hyperaccumulators, which concentrate metals to exceptionally high levels in their shoots. Examples include Rinorea bengalensis, a nickel hyperaccumulator, and Pteris vittata, which accumulates arsenic. These plants employ mechanisms such as enhanced metal chelation, sequestration in vacuoles, and efficient root exclusion to tolerate toxicity. Their practical importance lies in phytoremediation, where they are used to clean up contaminated soils, and phytomining, where they are harvested for metal recovery. In India, Rinorea bengalensis populations in the Andaman Islands and other regions are studied for their potential in nickel extraction. Metallophytes also serve as biomonitoring tools, indicating soil contamination levels. Their ability to accumulate metals makes them valuable for environmental restoration and sustainable mining practices, offering an eco-friendly alternative to conventional remediation methods.

Invasive Alien Species Invasive Alien Species (IAS) are non-native organisms that establish and spread, causing significant harm to biodiversity, ecosystem services, and human health. In India, examples include Lantana camara, Parthenium hysterophorus, and Eichhornia crassipes. Lantana forms dense thickets, reducing native plant diversity and altering soil chemistry. Parthenium produces allergenic pollen, causing respiratory issues, and competes with crops. Eichhornia covers water bodies, aggravating eutrophication and deoxygenation, which harms aquatic life. These species often lack natural predators, allowing rapid proliferation. They disrupt food webs, hybridize with native species, and reduce agricultural yields. The economic cost includes losses in crop production, increased maintenance of waterways, and healthcare expenses. Management strategies include mechanical removal, chemical control, and biological control. However, prevention through strict quarantine and public awareness is crucial. Addressing IAS requires integrated approaches involving policy enforcement, research, and community participation to protect India’s rich biodiversity and ecological balance.