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.
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Send FeedbackA source is a plant region that generates organic compounds, such as amino acids and sugars. Examples of sources are green leaves and tubers.
A sink is a region of the plant that is actively growing. Examples include roots and meristems.
Phloem contains four specialised cell types to carry out its function. These are:
Plant assimilates refer to amino acids and sugars (sucrose).
Fig. 1 - The structure of phloem is shown
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.
Both sieve tubes and companion cells are exclusive to angiosperms (plants that flower and produce seeds enclosed by a carpel).
Table 1. The differences between sieve tubes and companion cells.
| Sieve tubes | Companion cells |
| Relatively large cells | Relatively small cells |
| No cell nucleus at maturity | Contains a nucleus |
| Pores in transverse walls | Pores absent |
| Relatively low metabolic activity | Relatively high metabolic activity |
| Ribosomes absent | Many ribosomes |
| Only a few mitochondria present | Large numbers of mitochondria |
Assimilates, such as amino acids and sugars (sucrose), are transported in the phloem by translocation from sources to sinks.
Take a look at our Mass Transport in Plants article to learn more about the mass flow hypothesis.
Sucrose can move into the sieve tube elements via two pathways:
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).
Cytoplasmic junctions refer to cell to cell or cell to extracellular matrix connections through the cytoplasm.
Fig. 2 - Movement of substances through the apoplast and symplast pathways
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.
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.
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.
Fig. 3 - The differences between xylem and phloem tissue
Table 2. A summary of the comparison between xylem and phloem.
| Xylem | Phloem |
| Mostly non-living tissue | Mainly living tissue |
| Present at the inner part of the plant | Present on the external part of the vascular bundle |
| Movement of materials is uni-directional | Movement of materials is bi-directional |
| Transports water and minerals | Transports sugars and amino acids |
| Provides mechanical structure to the plant (contains lignin) | Contains fibres that will provide strength to the stem (but not in the scale of lignin in the xylem) |
| No end walls between cells | Contains sieve plates |
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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 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.
Where are xylem and phloem located?
Xylem and phloem are arranged in a vascular bundle of a plant.
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