Phloem Transport: Definition, Structure, Mechanism

Introduction :

A living tissue in plants, called phloem, is responsible for the transport of soluble organic compounds made during photosynthesis to the parts of the plant where they are needed. These compounds are referred to as photosynthates. Phloem forms the innermost layer of the bark of trees.

Structure of phloem :

Conducting cells called sieve elements and parenchyma cells, companion cells, albuminous cells, unspecialized cells and supportive cells such as fibers and sclereids form the phloem tissue.

• Sieve elements do transport sugar throughout the plant. After their maturation, they have no nucleus and have very few organelles and hence depend upon companion or albuminous cells for their metabolic needs. At this stage, they do contain endoplasmic reticulum, found at the plasma membrane, which connects them to their companion or albuminous cells
• Companion cells are a special type of parenchyma cells that carry all the cellular functions of a sieve-tube element. The sieve tube element is connected to the dense cytoplasm of the companion cells by plasmodesmata
• Albuminous cells play a similar role as companion cells. The only difference between them is that albuminous cells are only associated with sieve cells and are found in seedless vascular plants and gymnosperms
• Supportive cells also provide a mechanical support system to the plant instead of only helping plants in the transport of sugars. These fall into two categories: fibers and sclereids. Both of these contain a secondary cell wall and die at maturity

1. Bast fibers provide tensile strength without limiting flexibility. They are long, narrow supportive cells found in the xylem and form the main component of many textiles such as paper, linen, and cotton.
2. Sclereids add to the compression strength but may reduce the flexibility to some extent. They are hard and irregularly shaped and hence serve as anti-herbivory structures. Sclereids are responsible for the gritty structures of pear.

Functions of phloem :

• Phloem transports photosynthetically prepared food materials from the leaves to the storage organs and later from storage organs to the growing regions of the plant body
• Phloem tissue helps to transport carbohydrates from sources to sinks through the sieve elements

• If the phloem gets destroyed, nutrients won’t be able to reach the roots, and consequently, the tree will die

Evidence supporting the fact that translocation occurs through phloem:

• Ringing or Girdling experiment :

All the tissue outer to the xylem, including bark, cortex, and phloem, is removed from a small portion of the woody stem in a healthy potted plant. This woody stem is called girdling. Now, the xylem only connects the lower and upper parts of the plant. It can be noted after a few days that the food material is collected near the girdling. Also, the roots die in a girdled plant. This happens because the food isn’t transported to the roots done by phloem. This proves that phloem supports the translocation of organic material.

• Mechanism of translocation

Long columns with holes in the end walls are formed by sieve-tubes in the phloem. To form a continuous channel, cytoplasmic strands pass through these holes.

• Pressure flow or mass flow hypothesis : 

Münch proposed this theory, and it is the most accepted theory that tells us how the sap flows through the phloem. The glucose prepared during photosynthesis is converted into sugar, which in the form of sucrose, moves to the companion cells. Further, active transport moves into the phloem sieve-tube cells, which creates a hypertonic condition in the phloem. Endosmosis helps in the movement of water through the xylem parallel to the phloem. Phloem sap moves from an area of higher osmotic pressure to the area of low osmotic pressure due to the rise in osmotic pressure, which is maintained low at the sink. Active transport is again required at the sink 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.

Conclusion : 

Phloem mass flow is driven by osmotically generated pressure gradients. This forms the Münch theory, and debates regarding its hypothesis still go on. The vascular tissue phloem transfer food in the form of sucrose to the non-photosynthetic parts of the plant. This transport occurs in the direction of the source to sink. This transport of organic solute through sieve-tube elements from one part of the plant to another is called translocation.