Types of Transport Process in Sieve Tubes

The metabolites of all the mesophyll cells around the sieve elements join in a common pool to load via the surrounding transfer cells. The path of metabolites and other solutes is shown in Figure. Presumably the metabolites are poured in the sieve tubes by the following three types of transport processes:

1. Symplastic transport via the plasmodesmata connections,
2. Apoplastic transport through the cell wall (passive transport), and
3. Active transport (symport and antiport).