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Transportation in Plants: Phloem Tissue

An article about phloem tissue in plants, covering its role and what cells form the tissue. It also covers how the different cells work together to achieve one fuction.This is a short revision bite suitable for students of High School/Senior School level. If you have any questions please feel free to message me.

The role of phloem tissue in a plant is to transport soluble products produced during photosynthesis from the leaves where they were made to where they are needed in the plant. It is used to transport organic materials, e.g. sucrose from the leaves. Unlike in the xylem, in a phloem transport is bidirectional and it is an active process, and so requires energy. Phloem is used to transport:

  • Sugars
  • Amino Acids
  • Minerals

Phloem are said to carry substances from source to sink. This means from a place where the substance was produced/is being produced to a place where the material is being used.

Phloem tissue is formed from 2 main types of cell; sieve tube elements and companion cells. Like xylem, the phloem tissue is formed from a small cell which elongates, and is then joined end to end with another cell to form a long tube. However, the end walls do not completely disintegrate, but instead a sieve plate is formed which allows the transport of minerals up and down the plant. Next to each sieve plate is a companion cell. This cell carries out the metabolic activities, such as respiration, which the phloem cell cannot.

The 4 main cells found in phloem tissue are:

  1. Sieve Tube Elements- Living cells arranged in a tube shape, made of cells linked end to end. Both ends of each cell have small holes, and are called sieve plates. These allow for movement of solutes in both directions. These cells still contain a small amount of cytoplasm; however it is arranged on the outside of the cell. Like xylem cells the cell has no nucleus and very few organelles to allow the solutes the easiest path through the vessel. Any organelles are concentrated around the periphery of the cell.
  2. Companion Cell- This cell is responsible for all the metabolic activities of the phloem cell. It has a very dense cytoplasm packed with organelles. They have a very high metabolic activity rate, and they produce all the ATP needed for the active transport of solutes. The cells are linked to the sieve tube by small holes called plasmodesmata through which all materials are exchanged. Some companion cells are used to move materials in and out of the sieve tubes. These cells are called transfer cells.
  3. Phloem Parenchyma- These are unspecialised cells which pack materials into and out of the phloem. If they are full, or  turgid the help to provide support, but they can also be used to store food, or others become specialised to be transfer cells.
  4. Phloem Fibres- These fibres give the phloem support to prevent collapse.

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