Phloem and Sugar Transport

Sugar sources are plant tissues that produce more sugar than they consume, with leaves being the primary example through photosynthesis. Sugar sinks are tissues that require more sugar than they can make, including roots, stems, fruits, and flowers. Translocation through the phloem distributes sugar from sources to sinks based on plant needs.

According to the pressure-flow hypothesis, water follows sugar into the phloem by osmosis, increasing internal pressure and driving phloem sap toward sink tissues with lower sugar concentration. As sugar is unloaded at the sink, water follows by osmosis and relieves pressure in the phloem. This pressure gradient creates continuous flow from source to sink regions.

Phloem in angiosperms consists of sieve-tube elements arranged end to end to form long transport tubes, joined by plasmodesmata. Companion cells accompany each sieve-tube element and facilitate phloem health and loading of solutions. Together, these specialized cells create the vascular tissue responsible for distributing sugar and other resources throughout the plant.

Sugar reaches the phloem through apoplastic or symplastic routes. The apoplastic route involves movement through extracellular space and cell walls, while the symplastic route uses plasmodesmata that directly connect the cytoplasm of neighboring cells. Plants can use passive or active transport via these pathways, and may switch between loading modes depending on water and energy demands.

Phloem sap is a watery solution containing sugar, amino acids, hormones, and minerals. This nutrient-rich fluid moves through sieve-tube elements to distribute essential resources to all plant tissues. The composition of phloem sap allows plants to transport not only energy in the form of sugar but also signaling molecules and building blocks needed for growth and development.

Companion cells are located alongside sieve-tube elements and facilitate phloem health and loading of solutions into the phloem from surrounding tissues. These cells actively participate in moving sugar from source tissues into the phloem through active or passive transport mechanisms. Their metabolic activity supports the energy-dependent processes required for efficient phloem loading and translocation.

Translocation distributes photosynthetic products and other resources to tissues that cannot produce them, enabling plants to support growth in roots, shoots, flowers, and fruits. By moving sugar from leaves to distant sinks, plants can allocate resources based on seasonal needs and developmental priorities. This process allows specialized tissues to function efficiently without producing their own energy.