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Phloem

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Biology

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.

A 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.

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.

Plant assimilates refer to amino acids and sugars (sucrose).

The adaptations of phloem

Phloem contains sieve tube elements and companion cells, which are necessary components in the translocation of assimilates. Efficient translocation occurs due to the adaptations of these cells.

Both sieve tubes and companion cells are exclusive to angiosperms (plants that flower and produce seeds enclosed by a carpel).

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 amount of organelles to maximise 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.

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
A large number of ribosomes
Only a few mitochondria present
Large numbers of mitochondria

The function of phloem

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.

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).

Cytoplasmic junctions refer to cell to cell or cell to extracellular matrix connections through the cytoplasm.

Phloem diagram movement of substances through apoplast and symplast pathways StudySmarter

Figure 1. Movement of substances through the apoplast and symplast pathways. Source: Jackacon, vectorised by Smartse, Public domain, via Wikimedia Commons.

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?

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.

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

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

Phloem - Key takeaways

  • The main function of phloem is to transport assimilates to sinks via translocation.
  • Phloem contains four specialised cell types: sieve tube elements, companion cells, phloem fibres and parenchyma cells.
  • Sieve tubes and companion cells work closely together. Sieve tubes conduct food matter in the plant. They are accompanied (literally) by companion cells. Companion cells support sieve tube elements by providing metabolic support.
  • Substances can move via the symplastic pathway, which is through cell cytoplasms, and the apoplastic pathway, which is through cell walls.

Phloem

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

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

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

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.

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

Final Phloem Quiz

Question

What does phloem transport?

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Answer

Sugars and amino acids.

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Question

Xylem and phloem are the two specialised transport structure in the plant. True or False?

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Answer

True.

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Question

What is the direction of transport of substances in the phloem?

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Answer

Substances are transported bi-directionally.

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Question

What is the name of the tissue where xylem, phloem and cambium reside?

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Answer

The vascular bundle.

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Question

What is cambium?

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Answer

An actively dividing bundle of cells between xylem and phloem.

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Question

What is the name of the seed leaf that gives the name to monots and dicots?

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Answer

Cotyledon.

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Question

How are vascular bundles arranged in the stem of a dicot plant?

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Answer

The vascular bundles are arranged in the ring-like structure. Xylem is present in the inner part of the cambium ring and phloem is present at the exterior. 

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Question

What are phloem fibres?

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Answer

Non-living schlerenchyma cells that provides support to the plant.

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Question

What are parenchyma cells?

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Answer

Permanent ground tissue that will form the bulk of a plant.

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Question

What are the three main structural features of sieve tube elements?

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Answer

  1. Connected by sieve plates transversely which allows organic matter flow between sieve element cells.
  2. Sieve tubes have abandoned their nucleus and reduced the amount of organelles they have to focus on translocation (movement) of organic material.
  3. Thick and rigid cell walls which allows the phloem to withstand the hydrostatic pressure.

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Question

Why do companion cells have many mitochondira?

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Answer

In order to produce ATP for active transport of materials between the sources and sinks and sieve tube elements.

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Question

How do companion cells increase the surface area for substance absorption?

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Answer

Plasma membrane folds inwards to increase surface area to volume ratio for substance absorption.

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Question

What is meant by a cytoplasmic junction?

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Answer

Cell to cell or cell to extracellular matrix connection through the cytoplasm.

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Question

Fill in the blanks about transport of materials in phloem.


______ ______ and sugars such as sucrose are transported in phloem. They will be transported in _________ movement from the leaves to the _______ parts of the plant (e.g. shoots and roots), ______ organs (e.g. roots), flowers and fruits. Therefore, materials are transported from ______ to ______.


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Answer

Amino acids and sugars such as sucrose are transported in phloem. They will be transported in bi-directional movement from the leaves to the growing parts of the plant (e.g. shoots and roots), storage organs (e.g. roots), flowers and fruits. Therefore, materials are transported from sources to sinks.


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Question

Give any two differences between xylem and phloem.

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Answer

For example: 


  1. Phloem transports sugars and amino acids while xylem transports water and minerals.
  2. Phloem has sieve plates between cells but xylem has no end walls (continuous flow of material).

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