Phloem and Sugar Transport

Photosynthesis produces sugar in plant leaves. But sugar is often needed in distant parts of the plant. Translocation is the process that distributes the products of photosynthesis to other plant tissues.

Leaves are typically called sugar sources, which are sites that produce more sugar than they consume. Roots, stems, and fruits are typically sugar consumers, called sugar sinks.

Sugar transport between a source and a sink happens in a plant tissue called phloem. In angiosperms, phloem is made up of specialized cells called sieve-tube elements. These cells are arranged end to end to form long tubes. Companion cells are found alongside each sieve-tube element.

Sugar can reach the phloem through several routes. It can travel through cell walls and extracellular spaces in the apoplastic pathway, or it can move through plasmodesmata. Once sugar reaches the phloem, companion cells actively load it into the sieve-tube elements.

A watery solution called phloem sap flows through the sieve-tube elements. It contains sugar, amino acids, hormones, and minerals.

According to the pressure-flow hypothesis, water follows sugar into the phloem by osmosis. This increases pressure within the phloem and drives phloem sap movement along the pressure gradient.

Phloem sap then flows to sink tissues, which have a low sugar concentration. Sugar either diffuses or is actively transported out of the phloem. As sugar is unloaded, water follows by osmosis and reduces pressure within the phloem.

Through translocation in the phloem, plants can distribute resources to the tissues where they are needed at a particular time or season.