Plant Nutrition and Mineral Metabolism

Carbon is the most abundant element in plants by mass (about 45-50% of dry weight), as plants are primarily composed of carbon-based compounds like cellulose, lignin, and sugars. Nitrogen is important but less abundant (1-5%), while iron and manganese are micronutrients.

Macronutrients are elements plants need in large amounts. Primary macronutrients are N, P, K. Secondary macronutrients include Ca (calcium), Mg, and S. Manganese, chlorine, and copper are all micronutrients needed in trace amounts.

The four most abundant elements in living cells are Carbon, Hydrogen, Oxygen, and Nitrogen (CHON), which together make up about 96% of cellular mass. Phosphorus (P), Sulfur (S), Magnesium (Mg), and Calcium (Ca) are all present but less abundant.

Macronutrients are elements plants need in large quantities. Primary macronutrients are N, P, K. Secondary macronutrients include Ca (Calcium), Mg, and S. Molybdenum (Mo), Manganese (Mn), and Zinc (Zn) are all micronutrients needed only in trace amounts.

The atmosphere (air) contains approximately 78% nitrogen gas (N2), making it by far the largest reservoir of nitrogen on Earth. While the ocean contains dissolved nitrogen, soil contains organic nitrogen compounds, and rocks contain nitrogen in minerals, none of these compare to the vast atmospheric reservoir.

Many algae (especially certain groups like green algae Chlamydomonas) have chloroplasts that lack grana stacks - their thylakoid membranes are unstacked. Higher plants (shrubs, herbs, trees) all have well-developed grana stacks in their chloroplasts for efficient light capture.

Boron plays a crucial role in seed germination, particularly in pollen tube growth and cell elongation during early germination stages. While Iron is important for chlorophyll and Magnesium is the central atom in chlorophyll, and Zinc is involved in enzyme function, Boron specifically supports germination processes.

Active transport of minerals against their concentration gradient requires energy expenditure (ATP). While respiration produces the ATP and root metabolism is involved, the direct requirement for active uptake is the expenditure of energy. Photophosphorylation occurs in chloroplasts during photosynthesis, not directly in mineral uptake.

Microelements (micronutrients like Fe, Mn, Zn, Cu, Mo, B, Cl) primarily function as cofactors for various enzymes in plant metabolism. They are not building blocks of amino acids (that's N, S), not constituents of hormones (though some like B may influence hormone activity), and not binders of cell structure (that's Ca). Their main role is enzyme activation/cofactors.

When phosphorus is deficient, growth is reduced and leaves fall off prematurely. Plants lacking phosphorus produce dull, bluish-green leaves that turn a purplish or bronze colour with brown edges.

Nitrogenase is the key enzyme complex responsible for biological nitrogen fixation - it converts atmospheric N2 into ammonia (NH3). This enzyme is found in nitrogen-fixing bacteria like Rhizobium. Nitrogen deaminase and Nitrodioxidase are not real enzymes involved in this process, and amino acid decarboxylase is involved in amino acid metabolism, not nitrogen fixation.

Mineral absorption in plants primarily occurs through active transport, which requires energy to move ions against their concentration gradients. Diffusion and osmosis are passive processes, while adsorption refers to surface attachment - none of these can explain the selective accumulation of minerals often found in higher concentrations inside roots than in soil.

Rhizobium (misspelled as 'Rhozobium' in the option) is a well-known nitrogen-fixing bacterium that forms symbiotic relationships with legume roots. Acetobacter is acetic acid bacteria, Mycoplasma lacks a cell wall and is not a nitrogen fixer, and Chlamydia is an obligate intracellular pathogen - none of these fix nitrogen.

K+  ions controls the stomatal movement in plants

Nitrification is the two-step process where ammonia (NH3) is first converted to nitrite (NO2-) by bacteria like Nitrosomonas, then nitrite is converted to nitrate (NO3-) by Nitrobacter. The specific conversion mentioned (ammonia to nitrite) is the first step of nitrification. Nitrogen fixation converts N2 to NH3, denitrification converts nitrates back to N2, and eutrophication is nutrient pollution causing algal blooms.

Rhizobia is “the group of soil bacteria that infect the roots of legumes to form root nodules”. Rhizobia are found in the soil and after infection, they produce nodules in the legume where they fix nitrogen gas (N₂) from the atmosphere, turning it into a more readily useful form of nitrogen (N). From here, the nitrogen is exported from the nodules and used for growth in the legume. Once the legume dies, the nodule breaks down and releases the rhizobia back into the soil where they can live individually or reinfect a new legume host.

Pea plant can fix nitrogen from air.

Calcium, a vital component in cell walls of plants, is essential for root growth.

Magnesium has an important role in photosynthesis because it forms the central atom of chlorophyll.