Active transport phloem

Plants possess a kind of circulatory system by which fluids are transported from the roots upward in the xylem and downward from the leaves through the phloem. Many compounds are carried between cells in this manner, while others are transported across cell membranes and against concentration gradients by active transport. A number of compounds that move between cells in either of these two manners have been classified as hormones.366-369 The major plant... 

The movement of most organic compounds throughout the plant takes place in the other vascular tissue, the phloem. A portion of the photosynthetic products made in the mesophyll cells of the leaf diffuses or is actively transported across cellular membranes until it reaches the conducting cells of the leaf phloem. By means of the phloem, the photosynthetic products— which then are often mainly in the form of sucrose—are distributed throughout the plant. The carbohydrates produced by photosynthesis and certain other substances generally move in the phloem toward regions of lower... 

Many solute properties are intertwined with those of the ubiquitous solvent, water. For example, the osmotic pressure term in the chemical potential of water is due mainly to the decrease of the water activity caused by solutes (RT In aw = —V ri Eq. 2.7). The movement of water through the soil to a root and then to its xylem can influence the entry of dissolved nutrients, and the subsequent distribution of these nutrients throughout the plant depends on water movement in the xylem (and the phloem in some cases). In contrast to water, however, solute molecules can carry a net positive or negative electrical charge. For such charged particles, the electrical term must be included in their chemical potential. This leads to a consideration of electrical phenomena in general and an interpretation of the electrical potential differences across membranes in particular. Whether an observed ionic flux of some species into or out of a cell can be accounted for by the passive process of diffusion depends on the differences in both the concentration of that species and the electrical potential between the inside and the outside of the cell. Ions can also be actively transported across membranes, in which case metabolic energy is involved. 

Water enters and leaves the phloem by passively moving toward regions of lower water potential ( P = P - n + pwgh Eq. 2.13a). The conducting cells of the xylem generally have a low and relatively constant osmotic pressure (here 0.1 MPa). Solutes either diffuse or are actively transported into and out of the sieve elements, leading to a high phloem osmotic pressure of 1.7 MPa in the leaf and a decrease to 0.7 MPa in the root the much lower n in the sink leads to a lower P there, which favors the delivery of more solutes. 

Such a large osmotic pressure, caused by the high concentrations of sucrose and other solutes, suggests that active transport is necessary at some stage to move certain photosynthetic products from leaf mesophyll cells to the sieve elements of the phloem. From the definition of water potential, = P — II + pwgh (Eq. 2.13a), we conclude that the hydrostatic pressure in the phloem of a leaf that is 10 m above the ground is... 

In our current example, the osmotic pressure of the phloem solution decreases from 1.7 MPa in the leaf to 0.7 MPa in the root (Fig. 9-18). Such a large decrease in n is consistent with the phloem s function of delivering photosynthetic products to different parts of a plant. Moreover, our calculations indicate that flow is in the direction of decreasing concentration but that diffusion is not the mechanism. (Although the total concentration decreases in the direction of flow, the c - of every solute does not necessarily do so.) Finally, we note the importance of removing solutes from the phloem solution at a sink, either by active transport or by diffusion into the cells near the conducting cells of the phloem. 

The known involvement of metabolism in translocation in the phloem could be due to active transport of solutes into the phloem of a leaf or other source, which is often referred to as loading, and/or to their removal, or unloading, in a root or other sink, such as a fruit (Fig. 9-l7a). Indeed, loading often involves proton-sucrose cotransport (Fig. 3-l4a) via a carrier located in... 

Sucrose is loaded into the phloem by active transport. 

In animals, hormones are produced by specific groups of cells or glands but in plants all living cells appear to be potentially capable of PGR synthesis. As PGRs are synthesized within a cell, they may modify metabolic activities within that cell or in surrounding cells. They, also, may be transported from one part of a plant to another part of the plant by simple diffusion or by energy-consuming active transport via xylem and phloem tissue (4). 

Transport in both the xylem and the phloem is a long distance transport. We have just mentioned that the sieve plates still raise problems for the mass flow hypothesis. According to some hypotheses transport over the short distance through the sieve plates is not accomplished by mass flow as in the sieve tubes but by an active transport. This active transport is over a short distance and its essential characteristic is that it requires the expenditure of energy. 

This one-directional transport behavior results in a leaf treated with fungicide being protected as a whole. Active substance is transported into the apical region of newly developing leaves however, redistribution into these leaves is limited because the active substance must first be transported downward in the phloem of the treated developed leaf before it can be transported via the xylem to the apical region of the newly developing leaf. For fungicidal compounds like triadimenol and triadimefon, which are predominantly if not exclusively transported in the xylem, this type of transport... 

The functions of potassium in the plant are manifold. This element serves to activate or catalyze a host of enzyme actions, to facilitate the transport of nutrients and assimilates in the xylem and phloem, to maintain the structural integrity of the plant cell, to regulate turgor pressure, to mediate the fixation of nitrogen in leguminous plant species, and to protect plants to some degree from certain plant diseases. 

Insecticides with systemic action are taken up relatively quickly by the plants and transported into the vascular system. According to the type of application, uptake occurs through the roots or the parts of the plant above ground. Distribution is chiefly by the xylem, but is also possible by the phloem and by diffusion from cell to cell. The persistence of activity is dependent on the type of substance, the intensity of breakdown in the plant or the soil, and environmental conditions. A much longer period of protection can be maintained if, by application of granulates at drilling or planting out, a depot of the substance is created in the soil from which the active substance is released slowly and taken up by the plants. 

The most metabolic activity of plants is carried out in the tissue called parenchyma, which generally makes up the bulk of the volume of all soft edible plant structures. The epidermis, which sometimes is replaced by a layer of corky tissue, is structurally modified to protect the surface of the organ. The highly specialized tissues collenchyma and sclerenchyma provide mechanical support for the plant. Water, minerals, and products of metabolism are transported from one part to another of the plant through the vascular tissues, xylem and phloem, which are the most characteristic anatomical features of plants on the cross section. 

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