How is sugar moved from sources to sinks in plants?

Sugar is moved from sources to sinks in plants through a process called translocation in the phloem.

Translocation is a fundamental process in plants that involves the movement of organic compounds, primarily sugars, from the areas of production (sources) to the areas of utilisation or storage (sinks). The sources are typically the leaves where photosynthesis occurs, producing glucose. The sinks can be various parts of the plant such as roots, fruits, seeds, and non-photosynthesising leaves that require the sugars for growth and development.

The process of translocation begins with the active transport of sugars into the sieve-tube elements of the phloem in the source tissues. This creates a high concentration of sugars in the phloem, causing water to move into the sieve tubes by osmosis from the adjacent xylem vessels. This results in a pressure gradient, with the source end having a higher pressure than the sink end. The pressure difference drives the bulk flow of sugars towards the sink tissues.

Upon reaching the sink tissues, the sugars are actively transported out of the phloem and into the cells where they are utilised or stored. This reduces the sugar concentration in the phloem at the sink end, causing water to move out of the sieve tubes and back into the xylem. This maintains the pressure gradient, allowing for continuous translocation of sugars from the source to the sink.

It's important to note that the direction of translocation is not always from the leaves to the roots. It can change depending on the plant's needs. For instance, in the spring, stored sugars in the roots can be translocated upwards to the budding leaves to support their growth. This dynamic process ensures that all parts of the plant receive the necessary nutrients for their function and development.