Phloem Transport

Food is synthesized in the green parts of a plant. The non-green parts are depended on the photosynthetic cells for nourishment. The food in the form of sucrose is transported by the vascular tissue phloem. Let us learn a bit more about phloem transport. The transportation occurs in the direction of the source to sink. Transport of organic solutes from one part of the plant to the other through phloem sieve tubes is called translocation of organic solvents.

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Source and Sink- The Bi-directional flow

The photosynthetic part usually acts as the source and the part in which the food is stored acts as the sink. But in Early Spring when the leaves are shed, the sugar stored in roots mobilize the organic material towards the growing Buds. Here we can see that the direction of the source and sink is reversed. Hence we can say that bidirectional flow of food occurs in the phloem.

Browse more Topics under Transport In Plants

• Means of Transport
• Transpiration

Evidence to Support that Translocation occurs through the Phloem

1. Ringing or Girdling Experiment

In a healthy potted plant, all the tissue outer to the xylem including bark, cortex, and phloem is removed from a small portion of the woody stem (girdling). The upper and lower part of the plant is now attached only through the xylem.

After a few days, it is observed that the food material is accumulated just above the girdling. Also, the roots die first in the girdled plant. This may happen because the food is not transported to the roots. This experiment proves that phloem is responsible for translocation of organic material.

2. Mechanism of Phloem Transportation 

Sieve tubes in the phloem form long columns with holes in the end walls. Cytoplasmic strands pass through these holes forming a continuous channel.

3. Pressure flow or Mass flow Hypothesis

This theory was proposed by Munch and elaborated by Mars and others. It is the most accepted mechanism for translocation of Sugars in higher plants. It occurs in the following steps.

• The glucose prepared in the leaves is converted into sugar.
• The sugar in the form of sucrose is moved into the companion cells and then into the living phloem sieve tube cells by active transport.
• This creates a hypertonic condition in the phloem.
• Water in xylem vessels adjacent to phloem moves through endosmosis.
• Osmotic pressure rises and phloem SAP moves from an area of higher osmotic pressure to the area of low pressure.
• Osmotic pressure is maintained low at the sink.
• At the sink again active transport is required to move the sugar out of the phloem SAP into the cell where the sugar is used to release energy by the process of respiration.

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Solved Questions for You

Question 1: Movement of substances in xylem is unidirectional while in phloem it is bidirectional. Explain.

Answer: Xylem transports water. Since transportation of water always takes place from roots to leaves, the direction of transport always remains in the upward direction. Translocation of organic material occurs from source to sink. Since the source and the sink may change their position, the movement is bidirectional.

Question 2: Differentiate between diffusion and translocation in plants.

Answer Diffusion is the passage of substances from the region of their higher concentration to the region of lower concentration due to the kinetic energy of the particles. It usually occurs in all directions. Translocation is a bulk transport of materials in solutions from inside the plant channels in a particular direction caused by forces other than the kinetic energy of the particles.