Xylem in plants is responsible for following function :

Question

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Correct — A, Transport of water and dissolved minerals. Xylem is the water-conducting half of a vascular plant's plumbing, and its traffic is one-way: water taken in at the root, with the mineral ions dissolved in it, carried up through the stem to every leaf. It is built from four cell types — tracheids and vessel elements, which do the conducting, plus xylem parenchyma for storage and short-range lateral movement and xylem fibres for mechanical strength. The two conducting types are dead at maturity: the protoplasm disintegrates and what remains is a hollow tube with a lignified wall, which is exactly what a low-resistance pipe under suction has to be. Vessels — short cells stacked end to end with their end walls perforated into a continuous pipe — are an angiosperm innovation; gymnosperms such as pine and deodar conduct through tracheids alone. The lifting mechanism is the transpiration pull of the cohesion-tension theory put forward by H. H. Dixon and J. Joly in 1894: water evaporating from the wet mesophyll cell walls inside the leaf lowers the water potential there and puts the whole column under tension, typically of the order of −1 to −2 megapascals; because water molecules hydrogen-bond to one another (cohesion) and to the lignified vessel wall (adhesion), that tension drags an unbroken thread of water upward instead of snapping it. The minerals simply ride that stream. Root hairs absorb nitrate, phosphate, potassium, calcium, magnesium and sulphate from the soil solution; the ions cross the root cortex and are actively loaded past the endodermis — whose Casparian strip of suberin blocks the cell-wall route and forces every ion through a living membrane — into the xylem, after which they travel wherever the water goes. That is why the option pairs the two: in xylem, water is the vehicle and the dissolved minerals are the cargo. The scale is unintuitive — the great bulk of the water a root absorbs is lost again as vapour at the leaf surface, and it is precisely that loss that pays for the lift.

Why the other options are wrong — (b) This is the other vascular tissue's job, and it is the trap the question is built around. Food — sucrose made in the leaf — is translocated by the phloem, through sieve tube elements assisted by companion cells, on the pressure-flow (Münch, 1930) mechanism. Phloem is alive, moves sap in both directions from any source to any sink, and its sap is a syrup; xylem is dead, moves in one direction, and its sap is almost pure water.  (c) Plants have no circulatory system for gases at all, so no tissue can be named. Oxygen and carbon dioxide move by simple diffusion — in and out through the stomata on leaves, through lenticels on woody stems and roots, and then through the continuous intercellular air spaces of the spongy mesophyll and cortex. Because no plant cell is far from an air space, diffusion is fast enough on its own; a transport tissue is unnecessary.  (d) Two things are wrong here. 'Essential amino acids' is a term from animal nutrition — the ones an animal cannot make and must eat; a plant synthesises all twenty protein amino acids itself, so the category does not apply to it. And organic nitrogen in a plant, amino acids included, is redistributed mainly in the phloem, not the xylem, which is why nitrogen is remobilised out of old leaves before they are shed.

Concept: Land plants are split by one anatomical fact: whether or not they have vascular tissue. Mosses and liverworts do not, which is why they stay small and damp-bound. Tracheophytes — ferns, gymnosperms and angiosperms — have a conducting system of two complementary tissues, and every question of this type is really asking you to keep them apart. Xylem carries water and dissolved minerals upward from root to shoot; its conducting cells are dead, hollow and stiffened with lignin, and the flow is driven by tension generated at the leaf, not by any pump. Phloem carries the products of photosynthesis from wherever they are made or stored (the source) to wherever they are needed (the sink); its sieve tube elements are alive but have lost their nucleus, and they are kept working by adjacent companion cells. The two are packed together in vascular bundles: in a dicot stem the bundle is conjoint and collateral, xylem on the inner side and phloem on the outer, with cambium between; in a monocot stem the bundles are scattered and closed; in a root the two tissues alternate on separate radii. When the cambium of a dicot or a gymnosperm keeps dividing, it adds new xylem inwards year after year — and that accumulated secondary xylem is what we call wood.

State PCS 2025 Q146 — Science & Technology PYQ with Answer

State PCS 2025 Q146 — Science & Technology PYQ with Answer