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Sieve cells are long, conducting cells in the phloem that do not form sieve tubes. The major difference between sieve cells and sieve tube members is the lack of sieve plates in sieve cells. [1] They have a very narrow diameter and tend to be longer in length than sieve tube elements as they are generally associated with albuminous cells. [4]
The presence of sieve plates greatly increases the resistance along the pathway, thereby generating and maintaining substantial pressure gradients in the sieve elements between source and sink. The movement in phloem is multi-directional, unlike in xylem cells, where the flow is upwards only.
All of the cellular functions of a sieve-tube element are carried out by the (much smaller) companion cell, a typical nucleate plant cell except the companion cell usually has a larger number of ribosomes and mitochondria. The dense cytoplasm of a companion cell is connected to the sieve-tube element by plasmodesmata. [10]
Callose is a plant polysaccharide. Its production is due to the glucan synthase-like gene (GLS) in various places within a plant. It is produced to act as a temporary cell wall in response to stimuli such as stress or damage. [1] Callose is composed of glucose residues linked together through β-1,3-linkages, and is termed a β-glucan.
Between the sieve-tube members are sieve plates, which have pores to allow molecules to pass through. Sieve-tube members lack such organs as nuclei or ribosomes , but cells next to them, the companion cells , function to keep the sieve-tube members alive.
The plants have both short-term and long-term mechanisms to prevent sieve element sap loss. [2] The short-term mechanism involves sap proteins, and the long-term mechanism involves callose, which helps to close the open channels in broken sieve plates .
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Sieve-tube members that are alive contain a polymer called callose, a carbohydrate polymer, forming the callus pad/callus, the colourless substance that covers the sieve plate. Callose stays in solution as long as the cell contents are under pressure. Phloem transports food and materials in plants upwards and downwards as required.