Transpiration and Translocation Processes

Two transport journeys in a plant

A plant needs water and mineral ions taken up by its roots, while its leaves make sugars by photosynthesis. These substances must reach other parts of the plant. Two transport tissues provide different routes: xylem carries water with dissolved mineral ions, such as nitrate ions, from roots towards leaves; phloem carries dissolved sugars from places that supply them to places that need or store them.

Transpiration: replacing water lost from leaves

Water enters through the roots and travels up the stem in xylem. At the leaves, some water evaporates from the surfaces of cells inside the leaf. The resulting water vapour diffuses out through open stomata, the small pores used for gas exchange. This loss of water vapour is transpiration. More water moves up the xylem to replace what has been lost, carrying dissolved mineral ions with it. The continuous movement from roots towards leaves is called the transpiration stream.

Notice the difference between the two parts of that account: transpiration is the loss of water from the plant, while the transpiration stream is the movement of replacement water through xylem. The detailed structure of stomata and how to observe them are covered separately.

Translocation: delivering sugars

A leaf making sugars by photosynthesis can be a source. A growing root, flower or other part that uses or stores those sugars can be a sink. Translocation moves sugars dissolved in water through phloem from source to sink. Unlike the root-to-leaf journey in xylem, the direction in phloem depends on where the source and sink are; it is not always downwards.

At a source, sugars are loaded into the phloem by active transport, which uses energy. This makes the solution inside the phloem more concentrated, so water enters it by osmosis. The incoming water pushes the sugar solution along the tube towards a sink, where sugars leave the phloem for use or storage. In this way, sugars made in a photosynthesising leaf can reach a root that cannot photosynthesise.

Transpiration

Water and dissolved mineral ions move from roots towards leaves in xylem. Water evaporates inside leaves, then water vapour diffuses out through open stomata. That loss is transpiration; the replacement flow through xylem is the transpiration stream.

Translocation

Phloem transports dissolved sugars from a source to a sink. Sugars are loaded by active transport; water then enters by osmosis and pushes the solution along. Direction depends on the positions of source and sink, not simply on whether a part is above or below another.