Zeefbuizen

Zeefbuizen, also known as sieve tubes, are specialized cells found in the phloem of vascular plants. They are the primary conducting cells responsible for the translocation of sugars, primarily sucrose, throughout the plant. These elongated cells are arranged end to end, forming continuous sieve tubes that can extend for considerable distances.

A characteristic feature of mature sieve tube elements is the absence of a nucleus, vacuole, and ribosomes. Instead, their cytoplasm is highly modified, containing sieve areas which are clusters of pores in the cell walls connecting adjacent sieve tube elements. These pores allow for the passage of phloem sap from one cell to the next. Companion cells, which are closely associated with sieve tube elements, provide metabolic support and load sugars into the sieve tubes.

The process of translocation in sieve tubes is driven by a pressure-flow mechanism. Sugars are actively loaded into the sieve tube elements at source tissues (like leaves) and then move passively through the sieve tubes to sink tissues (like roots or fruits) where they are unloaded. This unidirectional flow is essential for distributing energy resources to all parts of the plant. Zeefbuizen are vital for plant growth, development, and overall survival.