Phloem

Phloem is a specialised living tissue that transports amino acids and sugars from the leaves (source) to the growing parts of the plant (sink) in a process called translocation. This process is bi-directional.

The structure of phloem

Phloem contains four specialised cell types to carry out its function. These are:

• Sieve tube elements - a sieve tube is a continuous series of cells that plays a key role in maintaining the cells and transporting amino acids and sugars (assimilates). They work closely with companion cells.
• Companion cells - cells responsible for transporting assimilates into and out of the sieve tubes.
• Phloem fibres are sclerenchyma cells, which are non-living cells in the phloem, providing structural support to the plant.
• Parenchyma cells are permanent ground tissue that will form the bulk of a plant.

Fig. 1 - The structure of phloem is shown

The adaptations of phloem

The cells that make up phloem have been adapted to their function: sieve tubes, which are specialized for transport and lack nuclei, and companion cells, which are necessary components in the translocation of assimilates. Sieve tubes have perforated ends, so their cytoplasm connects one cell to another. Sieve tubes translocate sugars and amino acids within their cytoplasm.

Sieve tube cell adaptations

• Sieve plates connect them (endplates of the cells) transversely (extending in a cross direction), allowing the assimilates to flow between the sieve element cells.
• They do not have a nucleus and have a reduced number of organelles to maximize the space for the assimilates.
• They have thick and rigid cell walls to withstand the high hydrostatic pressure generated by translocation.

Companion cells adaptations

• Their plasma membrane folds inwards to increase the surface area for material absorption (see our Surface Area to Volume Ratio article to read more).
• They contain many mitochondria to produce ATP for active transport of assimilates between the sources and sinks.
• They contain many ribosomes for protein synthesis.

Table 1. The differences between sieve tubes and companion cells.

The function of phloem

Assimilates, such as amino acids and sugars (sucrose), are transported in the phloem by translocation from sources to sinks.

Phloem loading

Sucrose can move into the sieve tube elements via two pathways:

• The apoplastic pathway
• The symplastic pathway

The apoplastic pathway describes the movement of sucrose through the cell walls. Meanwhile, the symplastic pathway describes the movement of sucrose through the cytoplasm and plasmodesmata.

Plasmodesmata are intercellular channels along the plant cell wall which facilitate the exchange of signalling molecules and sucrose between cells. They act as cytoplasmic junctions and play a key role in cellular communication (due to the transportation of signalling molecules).

Fig. 2 - Movement of substances through the apoplast and symplast pathways

Mass flow

Mass flow refers to the movement of substances down the temperature or pressure gradients. Translocation is described as mass flow and takes place in the phloem. This process involves sieve tube elements and companion cells. It moves substances from where they are made (sources) to where they are needed (sinks). An example of a source is the leaves, and the sink is any growing or storage organs such as roots and shoots.

The mass flow hypothesis is often used to explain the translocation of substances, although it's not fully accepted due to the lack of evidence. We will summarise the processes here.

Sucrose enters the sieve tubes from the companion cells by active transport (requires energy). This causes reduced water potential in the sieve tubes, and water flows in by osmosis. In turn, the hydrostatic (water) pressure increases. This newly created hydrostatic pressure near the sources and lower pressure in the sinks will allow the substances to flow down the gradient. Solutes (dissolved organic substances) move into the sinks. When the sinks remove the solutes, water potential increases, and water leaves the phloem by osmosis. With this, the hydrostatic pressure is maintained.

What is the difference between xylem and phloem?

Phloem are made of living cells supported by companion cells, whereas xylem vessels are made of non-living tissue.

Xylem and phloem are transport structures that together form a vascular bundle. Xylem carries water and dissolved minerals, starting at the roots (sink) and ending at the plant leaves (source). The movement of water is driven by transpiration in a unidirectional flow.

Phloem transports assimilate to the storage organs by translocation. Examples of storage organs include storage roots (a modified root, e.g., a carrot), bulbs (modified leaf bases, e.g., an onion) and tubers (underground stems that store sugars, e.g., a potato). The flow of material within phloem is bi-directional.

Fig. 3 - The differences between xylem and phloem tissue

Table 2. A summary of the comparison between xylem and phloem.