TRANSPORT OF FOOD BY PHLOEM

TRANSPORT OF FOOD  BY PHLOEM

 

·         Food is manufactured in the leaves and green stems of the plant during the process of photosynthesis in the form of glucose; it is then translocated upwards, downwards and laterally to all parts of the plant through the phloem.

Structure of Phloem:

·         Phloem consists of sieve tube and companion cells. Sieve tubes are living cells which contain cytoplasm but do not have a nucleus. So, its function is supported by companion cell

·         The conducting cells of the phloem are cylindrical cells called sieve tubes, which have sieve like partitions at both of the ends. These partitions are called sieve plates.

·         A continuous column from the leaves to other parts of the plant is formed by the arrangement of sieve tubes one above the other. Besides sieve tubes the phloem also has companion cells and phloem parenchyma.

Function of phloem:

·         Phloem also conducts some other substances such as amino acid.

·         Phloem vessel in the vascular bundles will transport sucrose and amino acid from the leaf. Sucrose is the main form of carbohydrate i.e. translocated in plants with the expenditure of energy.

·         When sucrose is synthesized in the leaf cells, the osmotic pressure of the cell increases. As a result, water from the surrounding cells is forced to flow into the leaf cells by osmosis. This causes sucrose to be translocated from the point of its synthesis to the receiving end.

·         For example, in the flowering season, when vegetative activity is more at the apex of the plant, sugar in the leaves will be readily consumed. This is the reason for the translocation of sugar to the buds from the storage regions during spring. Excess food is translocated to the storage regions when vegetative activity of the plant is reduced.

 

Food is prepared in the mesophyll cells of the leaf. The palisade mesophyll cells is just below the upper epidermal layer, it contain more chloroplast than the spongy mesophyll layer. The spongy mesophyll layer is below the palisade mesophyll layers that have large air sacs. There is a slight separation between the cells to provide absorption of CO2. This separation must be very little to support capillary active for water distribution. The movement of water and dissolved minerals in xylem is always upward from the soil to the leaves. The movement of food can be upward as well as downward depending upon the need of plants.

Phloem transports food and certain other solutes from the sites of production (sources) to the site of utilization (sink). The movement of dissolved substances through the phloem is not completely understood. However, it appears that the phloem transports can be explained by bulk flow of phloem sap; under pressure by way of sieve like end walls of the sieve tube elements.

According to mass or passive flow hypothesis, two features are important for phloem transport of substances. These are:

        I.            The active (energy driven) transport of sugars and other solutes into the sieve types in sources areas.(leaves)

      II.            An active removal of solutes where the sieve tubes enters sinks (roots). These features result in a higher concentration of solutes at the sources end of the sieve tubes. This causes water to enter the sieve tube at that end. In turn the entry of water causes greater turgidity (hydrostatic pressure) at the source end. So that the entire fluid content of the sieve tube is squeezed toward the sink level of the tube.

According to another theory protoplasmic streaming) substances are transported through phloem by the streaming movements of protoplasmic material of the sieve tube elements. The pressure of actively streaming transcellular strands of cytoplasm forming a continuous link between linearly arranged sieve tube elements.

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