How is sugar transported from leaves to other parts of a plant?

Sugar is transported from leaves to other parts of a plant through the phloem in a process called translocation.

In more detail, the process of translocation involves the movement of organic compounds, such as sugars, from the leaves where they are produced (source) to other parts of the plant (sink) that require these nutrients for growth and development. This process is facilitated by the phloem, a type of vascular tissue in plants.

The sugars, primarily sucrose, are produced in the leaves during photosynthesis. These sugars are then actively transported into the phloem cells, known as sieve tube elements, by companion cells. This active transport creates a high concentration of sugars in the phloem, causing water to move into the cells by osmosis from the adjacent xylem vessels, which have a lower solute concentration. This increase in pressure forces the sugar solution to move through the phloem to areas of lower pressure, such as the roots and other non-photosynthesising parts of the plant.

The movement of sugars in the phloem is bidirectional, meaning it can move both up and down the plant, unlike the unidirectional movement of water in the xylem. This allows the plant to distribute sugars to where they are needed most, whether that be to developing flowers, fruits, roots or storage organs.

The process of translocation is a vital aspect of a plant's life cycle. It ensures that all parts of the plant receive the necessary nutrients for their growth and development, even if they are not directly involved in photosynthesis. Without this efficient transport system, plants would not be able to sustain their growth or reproduce effectively.