Mass flow occurs

Question

Mass flow occurs

BlackTom AI · Accepted Answer

The question snippet is about when mass flow occurs in plant transport. 
Option 1: 'when water follows sugars into storage.' This is not accurate as a general description of mass flow in phloem transport; mass flow is driven by pressure differences due to loading and unloading of sugars, not simply by water following sugars into storage. 
Option 2: 'when water osmoses into the phloem where sugars are pumped.' This matches the standard description of phloem transport: active loading of sugars into the phloem creates a high osmotic pressure that draws water by osmosis into the phloem, generating a mass flow toward sink tissues. Although the wording 'osmoses' is unconventional, the core idea aligns with the mechanism of sugar loading driving water influx and the bulk flow of phloem sap. 
Option 3: 'due to high sugar concentration in the xylem.' Mass flow in question is phloem-based; the driving pressure comes from sugar loading in the phloem, not high sugar concentration in the xylem. Therefore this statement misattributes the location and mechanism. 
Option 4: 'due to high water concentration in the phloem.' Phloem transport depends on sugar loading creating osmotic differences and water movement into the phloem, but the essential trigger is the active loading of sugars, not simply 'high water concentration' in the phloem. This option oversimplifies and misstates the driving factor. 
Overall, option 2 best captures the mechanism: active sugar loading in the phloem causes water to enter osmotically, enabling mass flow toward sinks. The other choices either misstate the vascular tissue involved (xylem vs phloem) or confuse the driving forces (water concentration vs sugar loading).

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