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⭕ Explain why Option (a) is incorrect

Asked by jhajuhi19 27th December 2019, 12:06 AM
Answered by Expert
  • In the given figure , region P represents the source or supply i.e., leaves where photosynthesis takes place. Region Q represents the sink i.e., roots. 
  • According to the pressure flow hypothesis, water containing sugar in solution flows under pressure through the phloem.
  • Glucose is produced during 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 R).
  • Hydrostatic pressure moves the sucrose and other substances in the sieve tube cells, and then moves them to the 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.
Answered by Expert 30th December 2019, 12:51 PM
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