sievrörsceller

Sievrörsceller, often translated as sieve tube elements, are the principal conducting cells of the phloem in plants. These elongated, living cells are arranged end to end to form sieve tubes, which are responsible for the translocation of sugars, primarily sucrose, throughout the plant. A defining characteristic of mature sieve tube elements is the absence of a nucleus, vacuole, and ribosomes, maximizing the space available for sap flow.

Adjacent to each sieve tube element is a companion cell, which is metabolically active and provides the necessary ATP and proteins for the sieve tube element's function. Companion cells are connected to sieve tube elements via numerous plasmodesmata, allowing for efficient exchange of materials. The end walls of sieve tube elements are perforated, forming sieve plates, which facilitate the passage of phloem sap from one element to the next.

The process of phloem loading, where sugars are actively transported into the sieve tube elements, and phloem unloading, where sugars are released to sink tissues like fruits and roots, are crucial for plant growth and development. Sievrörsceller play a vital role in distributing the products of photosynthesis to all parts of the plant that require energy or building materials. Their unique structure and close association with companion cells are key adaptations for their specialized transport function.