Question

Given figure demonstrates the translocation of organic solutes according to pressure flow hypothesis.

Which out of the following statements is incorrect regarding this ?

• A. A continuous high osmotic concentration is maintained in region P, which loads a sieve tube element by passively transporting solute molecules into it
• B. $R$ shows the movement of $H_{2}O$ into sieve tube elements from nearby xylem vessels by osmosis
• C. Translocation of organic solutes takes place from region P with high turgor pressure to region Q with low turgor pressure
• D. In region Q, material is either consumed or is converted into insoluble storage forms resulting in decreased osmotic pressure

Answer 1

Answer: (A)

In the given figure, region $P$ represents the source or supply i.e., leaves where photosynthesis takes place. Region ${}^{\prime }{Q}^{\prime }$ represents the sink i.e., roots. According to pressure flow mechanism water containing sugar in solution flows under pressure through the phloem.
Glucose is produced by photosynthesis in the mesophyll cells of the green leaves, some of which is converted into non reducing sugar, i.e., sucrose. The sucrose is actively transported to the companion cells of the smallest vein in a leaf and diffuses through the plasmodesmata to sieve tube elements. As a result, concentration of sucrose increases in the sieve tube cells. Water moves by osmosis from the nearby xylem vessels in the leaf vein (shown by ${}^{\prime }{R}^{\prime }$). Hydrostatic pressure moves the sucrose and other substances in the sieve tube cells, and then moves them to sink. At sink region, water moves out of sieve tube cells by osmosis, lowering hydrostatic pressure.
As the sap is pushed down the phloem sugar is removed by the cortex of both stem and root and is consumed or converted into starch. Starch is insoluble and exerts no osmotic effect. Consequently, the osmotic pressure of the contents of phloem decreases. Finally relatively pure water is left in the phloem and this is thought to leave by osmosis or be drawn back into nearby xylem vessels by suction of the transpiration pull.