According to our textbook, Xylem transport is only unidirectional while transport through phloem is multi-directional.
But minerals are taken up by xylem from the surrounding tissues actively, and even though they flow along with the water column, they are actively unloaded near 'sink' sites. Sometimes, minerals are also taken up from aging parts and transported to growing areas of the plant. Then shouldn't mineral transport through xylem be multi directional?
Please Explain
Movement through the xylem is powered by transpiration of water through stomata in leaves and a sustained tension in the xylem itself due to water's cohesive properties. See: https://en.wikipedia.org/wiki/Xylem#Cohesion-tension_theory
The "multi-directional" transport you are talking about does not actually occur in the xylem. Minerals have to be transported OUT of the xylem before they can move other ways. Transport THROUGH the xylem is uni-directional; transport OUTSIDE of the xylem can be multi-directional.
Movement of water through the xylem is caused by root pressure, capillary action and mainly via transpirational pull. All these factors results in a net upward movement of water molecules through the xylem vessel.
In the phloem, movement is multidirectional because of the presence of companion cells. These companion cells have many mitochondria that can carry out aerobic respiration to provide sufficient energy for active transport of substances in the desired direction.
The xylem vessel has no protoplasm, hence water cannot move due to active transport — hence it can only go upwards.
the movement is not multi-directional because the minerals that are transported in the xylem vessel do not need to go anywhere else apart from going up the plant for the leaves to use them this includes the water carried by them.All the leaves are in one direction (which is up the plant)therefore there is no need for the vessels to have a lot of directions which would slow down the plants processes are rather avoided.
Xylem is just an array of emptied dead cells with no inherent directional structure. It will/would pass fluid equally well in either direction. Likewise, phloem is also just a series of tubes, but live ones that are sustained by an adjacent cell. It can and does pass fluids in either direction.
Functionally, flow in xylem is only in one direction primarily because roots are at one end of the tree and evaporation/transpiration is at the other. Water also gets unloaded throughout the tree by osmosis. Tracking the detailed osmotic flow might show water and minerals moving downward locally, there is no significance to this (largely academic) exception.
Functionally, the phloem is actively loaded with the products of photosynthesis. Consequent osmosis generates pressure in the phloem that affects transport away from the source leaf. Importantly, it is via this pressure that nutrients and signalling molecules are supplied to the apical meristem. This is how a new shoot is nourished to grow from buds. This is how flowers are produced. After a new shoot has extended, new foliage will load the associated phloem and 'stuff' will flow the opposite direction. Hence, the phloem is emphasized to be multi-directional.
