Question

Explain the role of phloem in Plant transport. Use the words ‘source and sink’ in your answer.

Answer 1

Hint: Xylem and phloem are conducting tissues of plants, together they form a vascular bundle and hence known as vascular tissues. Plants having vascular tissues are known as Tracheophytes. There are two main types of vascular tissues namely xylem and phloem.

Complete answer: A xylem is water and a mineral conducting tissue, whereas the phloem is the food conducting tissue. Phloem is living tissue made up of companion cells, sieve tubes, phloem parenchyma and phloem sclerenchyma. Phloem conducts photoassimilate produced in a leaf. The site where photoassimilate is synthesized is called a source. Here, leaf cells act as a source. Loading of photoassimilate occurs from the source cell. Whereas, the site where unloading of photoassimilate occurs is called a sink. Root cells act as a sink. Leaves are the source for carbohydrate synthesis and storage organs are referred to as sink. Phloem sap has multi-directional flow. The movement through the phloem tissue is carried out by positive hydrostatic pressures. This process is termed as translocation.
The number of plasmodesmata and membrane transport proteins determines the loading and unloading patterns. Plants synthesize glucose using carbon dioxide, sunlight and water, which is required for the growth and development of plants.

Note: Usually during the spring season or in bud condition of plant root, which is storage organ act as the source for carbohydrate and growing parts of plants act as a sink. The movement in phloem sap in phloem tissue is multidirectional.