[3] Organic molecules such as sugars, amino acids, certain hormones, and messenger RNAs are known to be transported in the phloem through the cells called sieve tube elements.
The phloem sugar is consumed by cellular respiration or converted into starch, which is insoluble and exerts no osmotic effect.
Additionally, when viruses or growth chemicals are applied to an actively photosynthesising leaf, they are translocated downwards to the roots.
Some argue that mass flow is a passive process, while sieve tube vessels are supported by companion cells.
Moreover, amino acids and sugars (examples of organic solutes) are translocated at different rates, contrary to the hypothesis’s assumption that all materials being transported would travel at a uniform speed.
If the mechanism of translocation operates according to pressure flow hypothesis, bidirectional movement in a single sieve tube is not possible.
[5] In this model, small sugars such as sucrose move into intermediary cells through narrow plasmodesmata, where they are polymerised to raffinose and other larger oligosaccharides.
As larger molecules, they are unable to move back but can proceed through wider cell wall channels (plasmodesmata) into the sieve tube element.
The actively transported apoplastic phloem loading is viewed as more advanced, as it is found in the later-evolved plants, particularly those in temperate and arid conditions.