What is the cause for sugar cannot move down stem?

Sucrose is the main sugar transported in plants. Plants produce sucrose in the leaves during photosynthesis and transport it to other plant parts, such as roots and developing seeds, for use as an energy source. Sucrose is transported through the phloem, a specialized tissue that runs throughout the plant.

Sucrose cannot move down the stem by diffusion because the concentration of sucrose is higher in the leaves than in the stem. Diffusion is the movement of molecules from an area of high concentration to an area of low concentration. In order for sucrose to move down the stem by diffusion, the concentration of sucrose in the leaves would need to be lower than the concentration of sucrose in the stem.

Sucrose is transported down the stem by active transport. Active transport is the movement of molecules against a concentration gradient, from an area of low concentration to an area of high concentration. Active transport requires energy, which is provided by ATP (adenosine triphosphate).

The active transport of sucrose down the stem is driven by a proton gradient. Protons (H+) are pumped from the phloem cells into the surrounding cells, creating a higher concentration of protons outside the phloem cells. This proton gradient provides the energy for the active transport of sucrose into the phloem cells.

Once sucrose is transported into the phloem cells, it is transported down the stem by mass flow. Mass flow is the movement of molecules in a fluid due to a pressure gradient. The pressure gradient in the phloem is created by the active transport of sucrose into the phloem cells. The pressure gradient forces the sucrose solution down the stem, to areas of lower pressure.