Ch 6Plant Water Relation — Class 12 Biology, concept-first.

A plant obtains a variety of substances from its surroundings - water, minerals, nutrients, food and gases such as oxygen and carbon dioxide - and of these it is the availability of water that most of…

Water shows the following properties, which together give it its great biological importance:

Root : The root is the main organ through which a land plant absorbs water and dissolved minerals from its surroundings.

Even though a root is surrounded by soil, not every drop of water in that soil is actually available for the plant to absorb.

Once a root hair is in contact with capillary soil water, getting that water into the living cytoplasm of the cell is not a single step but a short sequence of three distinct physical processes, each…

a. Imbibition : Imbibition is the very first step by which a root hair takes up water, and it is a purely physical, non-living process: it is the swelling of a hydrophilic (water-loving) colloidal mat…

b. Diffusion : Diffusion is the second step in a root hair's uptake of water, and it is defined generally as the net movement of ions, atoms or molecules of a substance from a region of their higher c…

c. Osmosis : Osmosis is the third and final step of water absorption by a root hair — the process by which water actually crosses the selectively permeable plasma membrane and enters the living cytopl…

According to the principle of thermodynamics, every component of a physical system possesses a certain amount of free energy that is available to do work; the free energy per molecule of a substance i…

Plasmolysis is what happens when a living plant cell is placed in a hypertonic external solution — one more concentrated than the cell's own sap.

Once water has entered a root hair cell through the imbibition–diffusion–osmosis sequence, it still has to cross the width of the root — through the cortex, past the endodermis, and into the pericycle…

Water absorption by roots can also be classified by a different criterion altogether: whether the living cells of the root have to spend metabolic energy to bring the water in.

a. Passive absorption : Passive absorption is the principal way water enters a plant through its roots, and by far the larger contributor — about 98% of all water absorbed by a plant is taken up passi…

b. Active absorption : Active absorption is water uptake driven directly by the metabolic activity of living root cells, rather than by any pull from the shoot.

Translocation of water, also called ascent of sap, is the long-distance transport of water together with its dissolved minerals from the root upward to the stem, leaves and other aerial parts of the p…

a. Root Pressure Theory (Vital Theory) : The root pressure theory (also called the vital theory, because it credits the phenomenon to the living activity of root cells) was first proposed by J.

b. Capillarity theory (physical force theory) : The capillarity theory, also called the physical force theory, was proposed by Bohem in 1863.

c. Cohesion-tension theory (Transpiration pull theory) : The cohesion-tension theory, also known as the transpiration pull theory, is the theory most widely accepted today for explaining ascent of sap…

Soil is the plant's main source of mineral nutrition. Minerals themselves are the solid, crystalline inorganic materials naturally present in the earth's crust, and plants absorb them from the soil in…

Every part of a plant needs a continuous supply of food (the products of photosynthesis, or photosynthate) to support its nutrition and growth.

A plant absorbs water constantly and continuously, but it puts hardly 5% of everything it takes in to actual use, for cell expansion and growth.

i. Cuticular transpiration: Cuticular transpiration is water loss occurring by simple diffusion straight through the cuticle — a layer of the waxy substance cutin that coats the outer surface of epide…

ii. Lenticular transpiration: Lenticular transpiration is water loss occurring through lenticels — small, slightly raised patches on the bark made up of loosely arranged complementary cells with unusu…

iii. Stomatal transpiration: Stomatal transpiration is water loss occurring through the stomata — minute pores in the epidermis, each formed by a pair of guard cells together with their surrounding ac…

A typical stomatal apparatus in the leaf epidermis is built from three components: a pair of guard cells, the stoma (the pore they enclose between them), and a set of surrounding accessory (or subsidi…

Opening and Closing of Stoma : Opening and closing of the stoma is controlled entirely by the turgor of its two flanking guard cells, and that turgor itself rises and falls over the course of the day…

Significance of Transpiration :

Q.1 of the textbook exercise (p132): ten multiple-choice questions.

Q.2 of the textbook exercise (p132): nine very-short-answer questions.

Q.3 of the textbook exercise (pp132-133): ten questions; question 10 has two parts (a, b).

Q.4 of the textbook exercise (p133): five long-answer questions.

The textbook's end-of-chapter Project (p133): prepare a PowerPoint presentation on the different types of transpiration. A guided approach is given.

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