Once sugars have been loaded into phloem cells, water moves into the cells due to ________, pushing the sugars through the phloem elements down a concentration gradient.

Question

BlackTom AI · Accepted Answer

Consider what drives water movement into the phloem after sugars are loaded. 
Option 1: hydrolysis. This is a chemical reaction that breaks bonds, not a mechanism for water movement into cells, so it does not explain the sap-loading process. 
Option 2: transpiration. This term refers to water loss from leaves mainly affecting xylem water transport, not the phloem loading that occurs in companion cells and sieve elements; thus it’s not the correct mechanism here. 
Option 3: active transport. While active transport can move solutes against gradients, the question specifically ties water entry to a gradient created by loading sugars, which is more characteristically handled by osmosis rather than direct energy-dependent water pumping. 
Option 4: osmosis. This is the movement of water across a semipermeable membrane from higher water potential to lower water potential, driven by solute concentration. Loading sugars into the phloem lowers the water potential inside phloem cells, drawing water in by osmosis and generating turgor pressure to push the sugars through the phloem down the gradient. 
In light of these explanations, you can see why osmosis is the mechanism that explains water entry and sap movement in the phloem in this scenario.

Once sugars have been loaded into phloem cells, water moves into the cells due to ________, pushing the sugars through the phloem elements down a concentration gradient.单项选择题

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