Question

Explain why xylem transport is unidirectional and phloem transport is bidirectional.

Answer 1

Hint: The vascular plants consist of two types of tissues for the transportation. One is xylem, which is responsible for the transportation of water, minerals, and other micronutrients. The other is, phloem, which is responsible for the transportation of food and macronutrients.

Complete answer:
The unidirectional movement means the movement of molecules in only one direction. Xylem is unidirectional in terms of transportation, meaning the flow of water molecules takes place from the roots towards the top of the plant. Bidirectional movement means the movement of molecules in two opposite directions. Phloem is bidirectional in terms of transportation, meaning the flow of food is in both the directions.

There are two main reasons due to which the xylem is unidirectional, namely positive root pressure and transpiration pull. Positive root pressure means it is against the gravitational pull, and the transpiration pull acts as a suction pump for the xylem tube. Thus, the water rises upwards.

The phloem tissue gets accumulated with sucrose and it absorbs water, which creates high turgor pressure. In the phloem the food flows from the source to the sink. The other areas contain low turgor pressure at the moment because of the loss of water molecules. Thus, the water flows from a region of high turgor pressure towards a region of low turgor pressure. This is the reason why the direction of the phloem is both upwards and downwards, depending on the gradient formed.

Note: The cells of the vascular tissues are slender and long. The individual cells present in the phloem are connected end to end in pipe like manner. The vascular tissues are arranged in discrete, long strands known as vascular bundles.