Which of the following causes movement of sugars throughout a plant?

Active transport of water into the phloem to carry the sugar molecules.
Positive pressure at the roots from water entering the plant, which pushes fluid into and up the phloem.
Adhesive and cohesive forces, between water molecules and between water and vessel walls.
Negative pressure in the phloem where sugars are produced that repels sugars away from the area.
The presence of fruit (eg. apple) or tuber (eg. potato).

Hint: use your knowledge of diffusion, osmosis, and how organisms can create concentration gradients to do things.

Question

Which of the following causes movement of sugars throughout a plant?

Active transport of water into the phloem to carry the sugar molecules.
Positive pressure at the roots from water entering the plant, which pushes fluid into and up the phloem.
Adhesive and cohesive forces, between water molecules and between water and vessel walls.
Negative pressure in the phloem where sugars are produced that repels sugars away from the area.
The presence of fruit (eg. apple) or tuber (eg. potato).

Hint: use your knowledge of diffusion, osmosis, and how organisms can create concentration gradients to do things.

BlackTom AI · Accepted Answer

Let’s parse the question first: Which statements describe causes of movement of sugars throughout a plant? Then we review each option in turn.
Option A: 'Active transport of water into the phloem to carry the sugar molecules.' This mixes two concepts incorrectly. Water movement into the phloem is not driven by active transport; water moves by osmosis following solute loading, and sugars move via phloem transport driven by pressure differences, not by active uptake of water into phloem. So this is inaccurate as a description of the mechanism.
Option B: 'Positive pressure at the roots from water entering the plant, which pushes fluid into and up the phloem.' Here, the wording suggests root pressure creates a push into the phloem. In reality, while root pressure can occur in the xylem and can influence sap movement, the classic model of sugar transport in the phloem is the pressure-flow (or mass flow) mechanism driven by solute loading at source tissues, creating turgor pressure that pushes phloem sap along. The phrase 'positive pressure at the roots' is not the right central driver for phloem movement, but there is a kernel of truth in the broader idea that pressure differences drive movement and that water movement helps generate pressure in the phloem. Misstating the location (roots) and the exact source of the pressure can mislead, but as a general alignment with the idea of pressure-driven flow, it contains partial truth.
Option C: 'Adhesive and cohesive forces, between water molecules and between water and vessel walls.' These are classic terms in explaining water movement through xylem via cohesion-tension, not sugar movement in phloem. So while cohesion and adhesion are crucial for xylem transport of water, they do not describe the mechanism by which sugars are moved through the phloem.
Option D: 'Negative pressure in the phloem where sugars are produced that repels sugars away from the area.' Phloem sap movement is produced by positive turgor pressure downstream of loading, not negative pressure that repels sugars. So this statement contradicts the known pressure-flow concept and is misleading.
Option E: 'The presence of fruit (eg. apple) or tuber (eg. potato).' Fruits and tubers act as sinks in the plant, attracting sugars toward themselves because they store sugars. Their presence creates concentration gradients and directs sugar flow from sources (like leaves) to sinks (fruits, tubers). This aligns with the idea that sugar movement is driven by source-sink relationships and concentration gradients, making this a plausible contributor to sugar translocation.
Putting it together, you can see that the mechanism for sugar movement relies on loading sugars at sources to create osmotic gradients, leading to pressure-driven flow through the phloem towards sinks, with sink tissues like fruits and tubers playing a role in directing the flow. Options that describe water movement as active transport (A) or rely on xylem-only water mechanics (C) or propose negative pressure in the phloem (D) misrepresent the primary phloem transport process. Option B captures the idea of pressure pushing fluids (though the exact source/location of the pressure is nuanced), and Option E correctly highlights sinks as destinations that influence sugar transport through concentration gradients.
In summary, the statements that align best with the general understanding of phloem sugar transport are those describing pressure-driven movement and the role of sinks, i.e., B and E. The other options either misstate the mechanism (A, C, D) or apply concepts to the wrong transport system (C referring to xylem).

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