What are sieve tubes in phloem?

Sieve tubes in phloem are elongated cells that transport sugars and other nutrients throughout the plant.

Sieve tubes are a crucial component of the phloem, the vascular tissue in plants responsible for the transport of organic nutrients. These elongated cells are arranged end-to-end, forming a continuous tube-like structure. The end walls of these cells, known as sieve plates, have pores that allow for the movement of substances.

The primary function of sieve tubes is to transport sugars, primarily sucrose, and other organic nutrients from the leaves, where they are produced during photosynthesis, to other parts of the plant. This process is known as translocation. The movement of these nutrients occurs from areas of high concentration, typically the leaves, to areas of lower concentration, such as the roots or developing fruits.

Interestingly, sieve tubes are living cells, but they lack a nucleus, ribosomes, and other typical cell structures. This is because these structures would obstruct the flow of nutrients. Instead, each sieve tube element is associated with a companion cell, which is a normal plant cell with all the usual cell structures. The companion cell helps to maintain the sieve tube and assists in the loading and unloading of sugars and other nutrients.

In summary, sieve tubes in phloem are a vital part of the plant's nutrient transport system. They form a network of tubes that allow for the efficient movement of sugars and other organic substances from the leaves to other parts of the plant.