Question

Which of the following is true or false regarding the transport of substances in plants?
(i) Mass flow is the movement of substance in bulk as a result of pressure difference.
(ii) Bulk flow is achieved only through a positive hydrostatic pressure gradient.
(iii) The apoplast system is the system of interconnected protoplasts.
(iv) Most of the water flow in the roots occurs via the apoplast since the cortical cells are loosely packed, and offer no resistance to water movement.
(a) (i) and (iv) are true; (ii) and (iii) are false.
(b) (i) and (ii) are true; (iii) and (iv) are false.
(c) (ii) and (iii) are true; (i) and (iv) are false.
(d) (i), (ii), and (iv) are true; (iii) is false.

Answer 1

Hint: Here in the question four statements are given regarding the transport of substances in plants. For transportation, plants have an entire tissue system where different tissues transport different materials to different parts of the plant. Out of four statements, three are true and only one is false.

Complete answer:
(i) The movement of a substance in bulk as a result of pressure difference is named mass flow. Another term given to mention mass flow is “mass transfer” and “bulk flow”, which is the movement of fluids that occurs due to down pressure or pressure gradient, for example, blood circulation and transport of water in vascular plant tissues. Bulk flow in plant biology typically refers to the movement of water from the soil up through the plant to the leaf tissue through the xylem, but can also be used in the transport of larger solutes like sucrose through the phloem. It depends on the pressure gradients of the medium itself.
(ii) A positive hydrostatic pressure gradient usually leads to mass flow. It depends on the pressure gradients of the medium itself. Solute flow is driven by a difference in hydraulic pressure created from the unloading of solutes within the sink tissues. That is by active or passive transport, as the solutes are off-loaded into sink cells, the density of the phloem liquid decreases locally creating a pressure gradient.
(iii) The apoplast system is that the system of interconnected protoplasts.
In the apoplast pathway, water is transported from the plant organs to the xylem through the cell membrane of intervening cells. A Casparian strip of endodermal cells blocked the apoplastic route. Hence, the symplastic route is employed to deliver water and ions over the cortex. The apoplastic route is least affected by the metabolic state of the root because it is made up of non-living components.
(iv) The amount of the water that flows in the roots of plants occurs through the apoplasts because the cortical cells are loosely packed and they do not offer any resistance to water movement. Water transport in the xylem mainly depends upon the cohesion of water molecules to each other and the adhesion of the vessel's wall via hydrogen bonding. These are combined with the high water pressure of the plant's substrate and low pressure of the acute tissues usually of leaves.
From the above explained all the four points it is clear that the points first, second, and fourth are true and the third one is false.

So, the correct answer is '(i), (ii) and (iv) are true; (iii) is false.'

Note:
-Mass flow refers to the movement of water from the soil up through the plant to the leaf tissue through the xylem, but can also be applied to the transport of larger solutes (e.g. sucrose) through the phloem.
-Solute flow is driven by a difference in hydraulic pressure created from the unloading of solutes within the sink tissues.
-In the apoplast pathway, water is transported from the plant organ to the xylem through the cell membrane of intervening cells.
-Water transport in the xylem relies upon the cohesion of water molecules to each other and adhesion to the vessel's wall via hydrogen bonding combined with the high water pressure of the plant's substrate and low of the acute tissues.