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Why is xylem sap acidic and phloem sap alkaline. Give me the detailed answer please

Why is xylem sap acidic and phloem sap alkaline. Give me the detailed answer please

Grade:11

3 Answers

hrithik
11 Points
6 years ago
Sucrose is loaded into the phloem at a source, usually a photosynthesizing leaf. For this to occur, hydrogen ions are pumped out of the companion cell using ATP. This creates a high concentration of hydrogen ions outside the companion cell. Sucrose is loaded (moved into companion cells) by active transport, against the concentration gradient. However, the protein carrier involved in the loading, has two sites, one for sucrose and one for a hydrogen ion. When it is used to pump sucrose into the companion cell, hydrogen will move in the opposite direction, back down its concentration gradient. This is why a high concentration of ions is needed outside the cell. The sucrose can then diffuse down the concentration gradient into the sieve tube element via the plasmodesmata that connects the companion cell with the sieve tube element. This lowers the water potential of the sieve element so water enters by osmosis. At another point sucrose will be unloaded from the phloem into a sink (e.g. root). It is likely that the sucrose moves out by diffusion and is then converted into another substance to maintain a concentration gradient. Again, water will follow by osmosis. This loading and unloading results in the mass flow of substances in the phloem. There is evidence to support this theory; the rate of flow in the phloem is about 10,000 times faster than it would be if it was due only to diffusion, the pH of the phloem sap is around 8 (it is alkaline due to loss of hydrogen ions), and there is an electrical potential difference across the cell surface (negative inside due presumably to the loss of positively charged ions).
Rishi Sharma
askIITians Faculty 646 Points
3 years ago
Dear Student,
Please find below the solution to your problem.

Sucrose is loaded into the phloem at a source, usually a photosynthesizing leaf. For this to occur, hydrogen ions are pumped out of the companion cell using ATP. This creates a high concentration of hydrogen ions outside the companion cell. Sucrose is loaded (moved into companion cells) by active transport, against the concentration gradient. However, the protein carrier involved in the loading, has two sites, one for sucrose and one for a hydrogen ion. When it is used to pump sucrose into the companion cell, hydrogen will move in the opposite direction, back down its concentration gradient. This is why a high concentration of ions is needed outside the cell. The sucrose can then diffuse down the concentration gradient into the sieve tube element via the plasmodesmata that connects the companion cell with the sieve tube element. This lowers the water potential of the sieve element so water enters by osmosis. At another point sucrose will be unloaded from the phloem into a sink (e.g. root). It is likely that the sucrose moves out by diffusion and is then converted into another substance to maintain a concentration gradient. Again, water will follow by osmosis. This loading and unloading results in the mass flow of substances in the phloem. There is evidence to support this theory; the rate of flow in the phloem is about 10,000 times faster than it would be if it was due only to diffusion, the pH of the phloem sap is around 8 (it is alkaline due to loss of hydrogen ions), and there is an electrical potential difference across the cell surface (negative inside due presumably to the loss of positively charged ions).

Thanks and Regards
RISHIKA
550 Points
3 years ago
HEY FRIEND
The sap present in a plant stem is alkaline in nature. Phloem sap involves pumping out of protons which is a cotransport method for the intake of Sucrose in the dive tubes. Hence it makes the phloem sap slightly alkali.
pH of phloem sap is moderately alkaline and acidic sometimes due to the contamination with xylem exudates and bark tissue and a high amount of organic acid is responsible for the slightly acidic pH of phloem sap
THANKS

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