Mass flow occurs

Question

Mass flow occurs

BlackTom AI · Accepted Answer

Starting with the topic of mass flow in phloem transport, let's evaluate each option in turn and see which one aligns best with the mechanism involved.
Option 1: 'when water follows sugars into storage.' This statement hints at water moving with sugars into storage sites, but it is too vague about where this occurs and doesn’t accurately describe the continuous mass flow from sources to sinks through the phloem. It also doesn’t specify the driving force or the directionality typical of mass flow in phloem.
Option 2: 'due to high sugar concentration in the xylem.' Mass flow in phloem is driven by differences in sugar concentration and water potential, but the xylem is primarily involved in water transport driven by transpiration, not the phloem’s sugar transport. Saying high sugar in the xylem as the cause confuses the two tissues and their roles, making this option misleading.
Option 3: 'due to high water concentration in the phloem.' Phloem transport is not driven by a high water concentration alone; rather, it is driven by the loading and unloading of sugars into the sieve tubes, which alters osmotic potential and draws in water from the xylem by osmosis. Simply citing high water concentration in the phloem doesn’t capture the osmotic mechanism that follows sugar loading.
Option 4: 'when water osmoses into the phloem where sugars are pumped.' This option describes the essential mechanism: sugars are actively loaded (pumped) into the phloem at source cells, increasing the osmotic potential. Water then follows by osmosis into the phloem, creating a high turgor pressure that drives the bulk flow of sap toward sinks. It directly links sugar loading with osmotic water movement and the mass flow process, which is consistent with the classic model of phloem transport.
In summary, options 1–3 each miss or misstate key elements of the mass flow mechanism—whether it’s the location, the driving tissue, or the osmotic trigger—whereas option 4 correctly ties sugar loading in the phloem to osmotic water uptake, producing the pressure-driven bulk movement characteristic of mass flow.

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