Phloem is a cool tissue in plants that moves amino acids and sugars from the leaves to the growing parts of the plant. It's called translocation and it goes both ways. A source is a part of the plant that makes organic compounds like amino acids and sugars, and examples are green leaves and tubers. A sink is a part of the plant that is growing, like roots and meristems.

The structure of phloem

Phloem has four special cell types to help it do its job. These are sieve tube elements, companion cells, phloem fibres and parenchyma cells. The sieve tube is a bunch of cells that work together to transport amino acids and sugars (called assimilates) and keep the cells healthy. Companion cells help move assimilates in and out of the sieve tubes. Phloem fibres are cells that don't do anything living, but they give the plant support. Parenchyma cells are part of the plant's tissue and make up most of it. Assimilates are amino acids and sugars, like sucrose.

The adaptations of phloem

Phloem has two important cell types for moving assimilates: sieve tube elements and companion cells. These cells are only found in angiosperms, which are plants that make flowers and seeds in a protective structure called a carpel. Sieve tubes have sieve plates that connect them, which let assimilates flow between cells. They don't have a nucleus or many organelles, so there's more space for assimilates. Sieve tubes also have strong cell walls to handle the pressure of translocation. Companion cells have folded plasma membranes to absorb more material and lots of mitochondria and ribosomes to make ATP and proteins to move assimilates. Table 1 shows how these cells are different.

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The function of phloem

Assimilates like amino acids and sugars, such as sucrose, move through the phloem by translocation from sources to sinks. Check out our Mass Transport in Plants article to learn more about how this works. Sucrose can enter the sieve tube elements through two pathways: the apoplastic pathway and the symplastic pathway. The apoplastic pathway is when sucrose moves through the cell walls. The symplastic pathway is when sucrose moves through the cytoplasm and plasmodesmata, which are channels between cells along the plant cell wall. Plasmodesmata help exchange signaling molecules and sucrose between cells, acting as cytoplasmic junctions that play a key role in cellular communication. Cytoplasmic junctions are connections between cells or between cells and the extracellular matrix, sharing cytoplasmic space for exchange.

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Mass flow is the movement of substances down temperature or pressure gradients. Translocation in the phloem is an example of mass flow and involves sieve tube elements and companion cells moving substances from sources (e.g. leaves) to sinks (e.g. roots or shoots). The mass flow hypothesis explains this process, although it's not fully accepted due to lack of evidence. Here's how it works: Sucrose enters the sieve tubes from companion cells by active transport, creating a lower water potential in the tubes, causing water to flow in by osmosis and increasing hydrostatic pressure. This pressure gradient between sources and sinks allows substances to flow. Solutes move into sinks and, as they're removed, water potential increases, causing water to leave the phloem and maintain hydrostatic pressure.

What is the difference between xylem and phloem?

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.

Transpiration describes the loss of water vapour through the stomata. 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.

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The phloem's primary function is translocation, which involves transporting assimilates to sinks. Phloem consists of four specialised cell types: sieve tube elements, companion cells, phloem fibers, and parenchyma cells. Sieve tubes are responsible for conducting food matter in the plant, and they work closely with companion cells which provide metabolic support. Substances can move through the symplastic pathway (cell cytoplasms) or the apoplastic pathway (cell walls).

Phloem

What does phloem transport?

Amino acids and sugars (sucrose). They are also called assimilates. 

What is phloem?

Phloem is a type of vascular tissue that transports amino acids and sugars.

What is the function of phloem?

To transport amino acids and sugars by translocation from source to sink. 

How are phloem cells adapted to their function?

The sieve tube elements have few organelles to maximise space for assimilates, a rigid cell wall to withstand high hydrostatic pressure and contain sieve plates to allow the flow of assimilates. Companion cells contain a folded membrane which increases the SA:V ratio for transport, many mitochondria to generate ATP for active transport and many ribosomes for protein synthesis.

Where are xylem and phloem located?

Xylem and phloem are arranged in a vascular bundle of a plant.

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