The areas
where the nutrients are stored is known as sinks, examples are
the roots, flowers, fruits and stems. While where they originate is a source,
the leaves are the only source in the plant.
The movement of food is always
from the source to the sink. In the source, the food from the leaf is prepared
in the form of glucose from carbon dioxide and amino acids. Glucose is converted
into sucrose as it is more chemically stable, makimg it easier to transport.
Later it enters into the phloem at the expense of energy.
The
osmotic concentration of phloem will increase. Water will then enter into the
sieve tubes by the process of osmosis due to which the hydrostatic pressure in
the phloem tissue rises. This high pressure produced in the phloem tissues allow
translocation to all parts of the plants to have low pressure in their tissues.
At sinks, sucrose move from the phloem into the storage site or growing parts
of the plants. Water will also move out from the phloem tube. Hydrostatic
pressure will decrease in the phloem at sink. A pressure gradient is set up in
the phloem with high pressure at source and low pressure at sink so that the
phloem sap with food will move from source to the sink. The phloem transports
food according to the need of the plant.
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