The sieve cells are those that conduct sap with sugars and nutrients in the phloem of non-angiosperm vascular plants. They are homologous to the sieve tube elements of angiosperms. Both types of cells remain alive despite having lost the nucleus and several essential organelles.
The sieve cells are long and narrow, with overlapping ends. On their entire lateral surface they have small porous areas (sieves) in contact with albuminous cells, sometimes called Strasburger cells..
Screen tube elements are short and wide. They form continuous tubes. Near their ends they have porous plates in contact with companion cells.
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Like most phloem cells, sieves have a cell wall made up of cellulose, hemicellulose, and pectin. Sieves are depressions with pores up to 15 μm in diameter. These can be observed using an optical microscope.
The pores are crossed by bridges, or cytoplasmic tubules, between adjacent sieve and albuminous cells, which create continuity between the protoplasms of both.
Each of these bridges is surrounded by a cylinder of callose composed of a β-Hyaline-looking, densely packed glucan. This prevents leakage of the content of the bridges.
In contrast to the elements of the sieve tube, the adjacent sieve and albuminous cells are generally not derived from the division of the same parental cell..
The structures of the cell walls that by means of bridges establish communication between the protoplasms of cribrifying and albuminous cells are called plasmodesmata..
Vascular plants possess two types of complex conductive tissue organized in parallel vascular bundles along the cortex of roots, stems, branches, and leaf veins..
On the one hand, the xylem distributes water and mineral solutes taken from the soil. On the other, phloem transports water, sugars produced by photosynthesis, and nutrients previously stored in other cells..
Like xylem, phloem is derived from a growth region of the stem called the vascular cambium. Its main component is the sieve cells or the elements of the sieve tube..
The phloem also contains sclerenchymal cells, with supporting function, idioblasts, secretory function, and parenchymal cells, with storage function..
Albuminous cells are also parenchymal. Like the companion cells of angiosperms, they have a protoplasm with abundant ribosomes and mitochondria, an extensive rough endoplasmic reticulum, plastids with starch grains, and a nucleus that can be lobed. They can also have a large vacuole.
Lacking essential nuclei and organelles, sieve cells need, to stay alive, the metabolic machinery, proteins and ribonuclear protein complexes, other nutrients, ATP, signaling molecules, and albuminous hormones..
The movement of these compounds within the plant would not be possible without albuminous cells.
The movement of water and dissolved substances in the phloem can happen in different directions at different times. Even certain solutes can move in opposite directions simultaneously. This capacity is due to the fact that the phloem is composed of living cells, capable of carrying out various metabolic processes..
From albuminous cells, the sugars produced in photosynthetic tissues are loaded into sieve cells. The elevation of the concentration of sugars in these cells decreases the osmotic potential of the sap, attracting water from the adjacent xylem. This increases the turgor of the sieve cells..
The increased pressure of the sap causes it to passively move towards the target tissues.
As the sugars are discharged in these tissues, the turgor of the sieve cells lowers, which causes the water to be returned to the xylem. This process is repeated cyclically, producing the continuous shipment of sugars through the phloem and its discharge into the target tissues..
In some plants, the discharge of sugars in the sieve cells against a concentration gradient requires the enzyme adenosine triphosphate.
Unloading sugars in flowers and fruits implies an additional energy expenditure because the transport must take place against a gradient of sucrose, fructose and glucose.
During the periods of greatest plant growth, the main active sieve cells are those that form part of the phloem of the starch storage organs and the growing apical, root and axillary meristems..
During periods of intense photosynthetic activity, the main active sieve cells are those of the phloem of the leaves and the storage organs..
Viruses that attack plants often use sieve cell systems or sieve tube elements as a channel to invade the entire organism..
The screened cells obliterate the lesions that suffer rapidly by deposition of callosal. Aphids have specially adapted mouthparts to neutralize this defense, so they can continuously suck sap for hours. These and other sap-eating insects transmit viruses that attack plants.
When sieve cells die, so do their associated albuminous cells. This is an indication of the close interdependence of both types of microorganisms..
It is not known why large amounts of tubular endoplasmic reticulum can cause occlusion of the sieve pores in the sieve cells of gymnosperms..
Xylem and phloem solved the problem of water and nutrient transport in terrestrial environments, allowing the evolution of large plants and therefore the appearance of forests and the generation of the enormous biodiversity that they harbor worldwide..
With respect to the sieve tube elements and their companion cells, the associated sieve and albuminous cells are considered primitive. This is the point of the fact that sieve cells are found in all vascular plants without flowers, and only in some phylogenetically basal angiosperms..
Angiosperms are thought to have originated from gymnosperms. This would be the evolutionary reason why sap transport systems based on sieve tube elements are similar to those based on sieve cells. In other words, both systems would be homologous.
As proof of this homology, it can be mentioned that both systems show remarkable similarities, especially in the characteristics of the protoplast (loss of the nucleus and of the organelles themselves) and of the screening system..
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