Transport in the phloem

TRANSPORT IN THE PHLOEM
Phloem is a plant tissue that carries the product of photosynthesis from the leaves to other parts of the plant. Unlike xylem tissue phloem tissue consist of living cells.
Anatomy of phloem
Phloem has several components including sieve tubes, sieve plate and sieve cells.
Sieve element- They are elongated living cell without nuclei in which transport occur, in angiosperm they are connected end to end forming a cellular aggregate called sieve tubes which are separated from one another by perforated end walls called sieve plate.
Structure of the phloem
Sieve cells (Companion cell)- They are spindle shaped cells arranged one above the other. companion cells carry out energy producing metabolic activities for the sieve tube elements, They posses plasmodesmata through which materials pass to the sieve tubes,Phloem parenchyma cells- They are thin walled cells which are used for storage of solute.Phloem fibers- They are thick walled cells that provide strength to that phloem


Structure of phloem tissue in angiosperm

Phloem translocation-Is the movement of dissolved materials such as sugar, amino acids and some mineral salt form one plant part to another. Plants produce carbohydrates (sugar) in the leaves by photosynthesis, but non photosynthetic parts of the plants also require carbohydrates and other organic and non organic materials. For this reason, Nutrients are transported from source (region of excess carbohydrate, primarily mature leaves) to the sink (regions were carbohydrates are needed). Some important sinks are roots, flower, stem and developing leave.
Mechanism of phloem transport
Pressure (Mass) flow mechanism.
This mechanism was proposed by Ernst Munch, a German plant physiologist in 1930. Phloem translocation is generally believed to be driven by positive pressure. He proposed that a passive mass flow of phloem sap through sieve tubes was driven by the osmotically generated pressure gradient. This osmotically generated pressure help to move materials from source to sink.
Source- A plant region with excess carbohydrate, primarily mature leaves and storage organ such as roots.Sink- A plant region were carbohydrates are needed for utilization or storage example growing part and storage organ .
Explanation on pressure flow mechanism.
Source (Loading)- Sugar and other Solutes are passively and actively transported from mesophyll cell of photosynthesizing leaf through symplatic path way to the companion cell of the phloem then they pass through plasmodesmata of these cells to sieve tubes of the phloem and creates a very negative osmotic potential in the phloem.
Water moves into sieve cell along with osmotic gradient generated by accumulating sugar, as sieve cell walls resist deformation a positive pressure (turgor pressure ) is build up.
A mass flow of water occurs down the turgor pressure gradient between the source and the sink. Sugar and other solutes then moves with water to the sink.
Sink (Unloading)- Sugar is pumped to non photosynthesizing cells result in osmotic removal of water from adjacent sieve cells, this reduce the turgor pressure in the sieve cells allowing more flow of solute from the source to the sink.

Mechanism of phloem transport
The unloading process creates a pressure difference between the source and the sink allowing for continued movement of dissolved sugars in the phloem. The pressure difference is sort of like supply and demand. When some of the sugar is used in the sink, it will demand more from the source. This demand for more sugar will help new sugar enter the phloem at the source and move to the sink in order to meet the demand. If the sink does not need sugar, there will be a pressure resistance, which will cause the sugar to move to a different sink. For example, if the stem is growing slowly, it will not need much sugar for energy. This means that rather than going to the growing stem, the sugar will instead go to a storage sink such as a root
90 percent or more of materials translocated in Phloem consists of carbohydrates mostly in non reducing form (sucrose), the reducing form of sugar (glucose and fructose) are not always transported by phloem. Other materials translocated in the phloem include amino acid, organic acid, protein and some mineral nutrient such as potassium and phosphate. The process of sucrose loading into the phloem occur either passively by diffusion or actively by sucrose transporter proteins embedded in the plasma membranes of the sieve elements and companions cells by proton/ sucrose contransport mechanism.
Mechanism of sucrose loading
When the protons are pumped out of the sieve tube through the plasma membrane to the companion cell using carrier protein and ATP, cause the pH of the cell to be lower (more acidic) than inside. One proton binds to the transporter on the extracellular surface to form state 2 then sucrose binds to form the fully loaded transporter (state 3). The fully loaded transporter (state 3) undergoes a conformational change (states 3–4) resulting in H+/sucrose symport (transport-associated currents). In states 4 and 5, the proton and the sucrose molecule dissociate and the empty transporter re-orientates in the membrane to complete the transport cycle.

Mechanism of sucrose loading into the sieve tube

Unlike xylem transport phloem transport is bi-direction it can be either from upward to downward and vice versa. For example when sugar is transferred from storage part (root) to growing part (young leaves).



Importances of phloem translocation
It help to Transfer photosynthetic product such as sucrose from one plant part to another
It transfer other organic compound such as metabolites and hormone throughout the plant.
It transfer some mineral nutrient such as Nitrogen and phosphorus within the plant.
Factors which control the rate phloem translocation
Temperature:  Organic solutes that move out of leaves towards their respective destinations are thermo-regulated.  Increase in the temperature of soil increases the rate of solute utilization which in turn increase the demand of solute which eventually lead to increased movement of materials towards roots decrease in the temperature results in the fall in the rate of movement of solutes towards the root.Light and darkness has many effects on biological systems of which photosynthesis is very important. Translocation of food in plants is also influenced by the presence or absence of light.  Dark allow more translocation because most of stored starch is converted to sucrose at night and transported out of the leaves, though more food material move towards roots. Light favors movement of organic solutes towards shoot apex ;The action spectrum of the light that favors translocation has been found to be red and blue which are acts as the action spectrum for photosynthetic reactions.
Effect of Phytohormones:   Growth promoting hormones like indole acetic acid Gebberllins and cytokinins are found to accelerate the rate of translocation.  As the above said hormones activate cellular metabolic activities, cell division and cell elongation in the apexes and other growing tissues, the said structures need greater amount of organic solutes energy and structural components hence they accelerate the rate of movement of materials from the source to the sink.
Mineral supply:  Among all the essential nutrients, boron has been found to facilitate the movement of sucrose.  The absence of boron reduces the rate of translocation significantly.  Whether boron forms an ionizable complex with sucrose or facilitate the formation of sucrose it is not clear.  Phosphate facilitates the formation of more of sucrose from starch hence increase the process of translocation. Even K+ ions and other metallic ions have thought to play an important role in electro-osmosis
Effect of pressure Gradient
Translocation between the supply end and the receiving end is always governed by the concentration gradient.  Steeper the gradient, the greater the rate of translocation and vice versa.  Such a gradient mediated movement of solutes, it can be demonstrated by cutting off the leaf which acts as the supply end.  By removing the leaves, the gradient will be abolished and automatically the transportation comes to stand still. Generally at the source end, sieve tubes contents show greater positive pressure and the receiving ends like stem apex, root apex, developing flower or fruit show negative pressure.  This provides a motive force for the rapid movement of organic solutes from leaves to the sinks.
Note
The phloem sap changes significantly in its composition while underway from leaves to sink such as developing fruits and tubers. Phloem sap entering the developing fruit is more dilute in sucrose and much richer in certain amino acids than the sap exported by the leaf. Apparently as the sap move through stem sucrose is lost to the adjacent tissue and amino acids are loaded to the phloem. Hence the composition of phloem is not always constant it changes depending on the sink


REFERENCES
Lincoln Taiz and Eduardo Zeiger. (2006) Plant Physiology, 4th ed. Sinauer Associates, Inc. Sunderland
Horst Marschner (1986) Mineral Nutrition of Higher Plants, 2nd. Ed. Academic Press, San Diego,CA:

Lincoln Taiz and Eduardo Zeiger. (2006) Plant Physiology, 4th ed. Sinauer Associates, Inc. Sunderland
Horst Marschner (1986) Mineral Nutrition of Higher Plants, 2nd. Ed. Academic Press, San Diego, CA