Ch 11Transport in Plants — Class 11 Botany, concept-first.

Around 450 million years ago, during the Ordovician period, plants began moving from their long-established aquatic world onto newly formed land.

Transport is the process by which water, minerals and manufactured food are moved to every part of the plant body, with the conducting tissues xylem and phloem carrying out most of this work.

Cell-to-cell transport, also called short-distance transport, covers only a small area and a handful of cells at a time - moving a substance, say, from one root-hair cell to its immediate neighbour.

Passive transport covers every route by which a substance crosses a membrane using only its own kinetic energy, moving down a concentration gradient without any cost in cellular ATP.

Active transport is defined by two things happening together: molecules move against (uphill on) their concentration gradient, and this uphill movement is paid for with cellular energy from ATP made d…

Water underpins almost everything a plant does. Protoplasm itself is roughly 60-80% water, so a plant's whole structure - both externally and internally - depends on adequate water supply.

Imbibition is the adsorption of water onto the surface of dry, hydrophilic colloidal materials - substances like gum, starch, proteins, cellulose, agar and gelatin, collectively called imbibants - cau…

Water potential, given the symbol Ψ (the Greek letter psi) and measured in Pascals, is a way of expressing how much potential energy water has in a given system compared with pure water at the same te…

Whenever a solution is separated from its pure solvent by a semipermeable membrane, the dissolved solute creates a real, measurable pressure in the solution called osmotic pressure (OP, symbol π) - sp…

When a plant cell is placed in pure water or any hypotonic solution, water enters the cell by endosmosis and presses the cell membrane outward against the surrounding, rigid cellulose cell wall - this…

Diffusion Pressure Deficit, a term coined by Meyer in 1938, measures how much lower a solution's diffusion pressure is compared with that of its pure solvent at the same temperature and atmospheric pr…

Osmosis is a specific, restricted form of diffusion in which only the solvent - water, in a living plant - crosses a selectively permeable membrane, always moving from the side with higher water poten…

Because terrestrial plants cannot draw water directly from a surrounding aquatic medium the way their ancestors once did, they must actively absorb it from the soil to maintain turgidity, keep their m…

The organs responsible for absorbing water are, almost exclusively, young roots - and within a root, absorption is concentrated specifically at the zone of root hairs, a region close to the growing ti…

Once water has been absorbed at the root hair, it must travel radially inward, crossing successive tissue layers - the cortex, the endodermis and the pericycle - before it finally reaches the xylem el…

Kramer (1949) identified two genuinely separate mechanisms by which roots take up water, operating independently of one another.

Water and the mineral ions dissolved in it, once they have reached the xylem, are together called sap, and the process of carrying that sap upward through the plant - against gravity, and in the case…

Because the plant's vascular tissue actually contains two separate conducting systems, xylem and phloem, a direct experiment is needed to establish which one is responsible for carrying water upward.

Vital force theories share the basic claim that living cells must be actively involved to lift sap upward - water could not rise on its own through purely dead tissue, in this view.

Root pressure is a real, measurable positive pressure that builds up inside the tracheary elements of the xylem, generated purely by the root's own metabolic activity rather than by any pull exerted f…

Physical force theories try to explain the ascent of sap through the dead xylem vessels alone, without invoking any living-cell activity.

Water taken up by the roots ultimately reaches the leaves and is released into the atmosphere as vapour - a loss called transpiration - and the scale of this loss is genuinely large: under about 5% of…

Water vapour can leave a plant by three anatomically distinct routes, and these three differ enormously in how much of the total water loss each one actually accounts for.

A stoma is a microscopic pore found in the epidermis of leaves and green stems, typically about 10-40 micrometres long and 3-10 micrometres wide, and a single mature leaf can carry anywhere from aroun…

Stomatal movement - the opening and closing of the pore between a pair of guard cells - is, at the most basic mechanical level, always caused by changes in guard-cell turgor pressure: rising turgor st…

Two broad groups of factors govern how fast a plant transpires. External or environmental factors include atmospheric humidity (a very humid atmosphere sharply slows transpiration, while drier air spe…

Antitranspirants are substances deliberately applied to plants to slow down transpiration, ideally without interfering with the gas exchange (CO2 and O2) that the plant still needs to carry out.

Guttation is the exudation of excess water, as visible liquid droplets, from the edges or tips of leaves - and crucially, unlike ordinary transpired vapour, this liquid is never pure water but a dilut…

Because transpiration itself is hard to measure directly, plant physiologists rely on indirect methods that track a proxy quantity instead.

Transpiration causes a plant to lose water on a genuinely large scale - roughly 95% of everything a plant absorbs through its roots is eventually lost again as transpired vapour - which on the surface…

Leaves manufacture food through photosynthesis and, in the short term, store the surplus as starch grains inside the chloroplast; when that food is actually needed elsewhere in the plant, the starch i…

It is now firmly established that phloem, and not xylem, is the tissue responsible for translocating dissolved organic solutes throughout the plant.

The ringing (or girdling) experiment removes every tissue lying outside the vascular cambium of a woody stem - the bark, the cortex, and specifically the phloem - while deliberately leaving the xylem…

Unlike the ascent of sap in the xylem, which moves strictly upward from root to shoot, the phloem is capable of translocating its organic solutes in essentially any direction the plant's needs require…

Every phloem translocation pathway connects a source to a sink. A source is any organ capable of exporting organic food to areas of metabolism or storage elsewhere in the plant - typically a mature, a…

Phloem loading is the process by which the sugars made by photosynthesis move out of the mesophyll cells of a mature leaf and into the sieve elements of the phloem, ready for long-distance transport,…

Phloem unloading is essentially the reverse process at the far, receiving end of translocation, moving sucrose out of the sieve elements and into a sink organ such as a root, tuber, flower or fruit, a…

Several competing hypotheses have tried to explain the actual physical mechanism by which phloem moves its dissolved solutes.

Mineral nutrients present in soil exist in one of two states - either dissolved directly in the soil solution, or adsorbed onto the surface of colloidal clay particles - and their movement into living…

Passive mineral absorption needs no metabolic energy from the plant and works chiefly through ion exchange between the soil solution and the surface of the root, explained by two competing theories.

Active mineral absorption moves ions against their own concentration gradient, and, because that is thermodynamically uphill, it must be powered by metabolic energy - a requirement demonstrated by the…

Some ions inside a living cell, called fixed ions, become effectively trapped and never diffuse back out across the membrane once they are inside - yet their electrical charge still has to be balanced…

The Evaluation exercise closes the chapter with ten items. The first five are single-best-answer MCQs testing DPD/turgor-pressure arithmetic in a fully turgid cell, the correctness of statements about…

Sample papers and previous-year board questions for this subject.