Question

Describe and discuss the solute stream in the plant body. Include in answer:
-a description of the cells and tissues of the food production and food storage tissues of flowering plants.
-a description of the special association of sieve elements, companion cells and bundle sheath cells.
-a description of the biomolecules that are transported as food and nitrogen carriers.
-a description of how phloem conducts food from source to sink according to the pressure flow hypothesis including phloem loading in a leaf, the passage of sucrose from palisade mesophyll to sieve element (including polymer trapping), the flow through the phloem, and phloem unloading in a root.

Answer 1

Solute molecules can dissolve in water because water molecules can bind to them via hydrogen bond. Solute potential (Ψ_s), also called osmotic potential, is negative in a plant cell and zero in distilled water. These solute are transported to plant in different form.

Phloem that also known as bast , that have the tissues in plants that conduct foods made in the leaves to all other parts of the plant. Phloem is composed of various specialized cells called sieve tubes, companion cells, phloem fibres, and phloem parenchyma cells. In flowering plant double fertilization occur where one pollen fertilizes the egg, forming a diploid zygote, while the other combines with the two polar nuclei, forming a triploid cell that develops into a food storage tissue called the endosperm.

Sieve elements are specialized cells that are important for the function of phloem, which is a highly organized tissue that transports organic compounds made by the plant where bundle sheaths regulate the movement of substances and companion cell will carry out all the metabolic functions of the sieve-tube member. Without the companion cell, the phloem can not work.

In plant carbohydrate that in form of starch, lipid, proteins are the biomolecule that are transported as food and Some amino acids like glutamine, glutamate, asparagine, and aspartate, which serve as important nitrogen carriers in plants.

According to pressure-flow hypothesis that water containing food molecules flows under pressure through the phloem. Here the dissolved sugars flow from source and are released at sinks where they have to used. This movement of sugars creates a pressure difference between the source and the sink, allowing for continued movement of dissolved sugars in the phloem.

Glucose is produced by photosynthesis in the mesophyll cells of green leaves. Sucrose(Form of starch) is the main form that useful for transportation in plant. Sucrose can reach the sieve tubes through plasmodesmata that allow for its diffusion from cell to cell and conversion of sucrose to large molecule occur. The larger molecules(RFOs) cannot move back to phloem parenchyma cells (PP) and are transferred and accumulated in sieve tubes. Photoassimilate, mainly in the form of sucrose, is loaded and accumulates in the phloem by passing through the apoplast from palisade mesophyll cell to companion cell. This model of phloem loading, called polymer trap mechanism. Polyols can also be transported into the phloem. Sucrose, RFOs and polyols are transported in the sieve tubes to the sink organs in the transport phloem. Sucrose is unloaded into the release phloem where the hydrostatic pressure is supposed to be lower. Phloem unloading in actively growing tissues such as the shoot or root apex occurs through the protophloem.

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