Phloem Functions, Cells, Tissue and Structure

What is Phloem?

Phloem is the vascular tissue that transports and distributes organic nutrients that are made in the leaves to other parts of the plants. The phloem and xylem make up the transport system in vascular plants.

Xylem function in the transport of water and dissolved nutrients from the root to other parts of the plant whereas, the function of phloem is to transport food and organic nutrients from the site of photosynthesis which is the leaves to other parts of the plant. In order to understand the differences and similarities of these vascular tissues, you can read the article xylem and phloem.

The site at which the nutrients are produced is called the source cell and the site at which the nutrient is taken to for use is called the sink cell. A typical example of a source cell is the leaves where nutrients are produced through the process of photosynthesis whereas examples of the sink could be fruits and flowers.

This vascular tissue is composed of living tissue and makes use of turgor pressure and energy in form of ATP in order to transport sugar to the flowers, buds, fruits, and roots of the plant. It is a pathway to signaling molecules and is composed of 3 types of cells which include the parenchyma, sieve elements, and sclerenchyma.

All the phloem cell types vary in their distribution and morphology. The main function of the sieve elements is specialized in transport and they have lost their nuclei as well as other organelles as a result of this. Therefore, in order for the sieve elements to sustain all their physiological function and activities, they rely on specialized neighboring parenchyma cells.

There are two types of phloem which include the primary and secondary phloem. Primary phloem is derived from the procambium while secondary phloem is derived from the cambium. The primary phloem can be found in vascular plant lineages such as ferns, lycophytes, and monocotyledon. In these taxa of plants, the sieve elements are long-living. As for the secondary phloem, it is formed in plants with secondary growth where the primary phloem collapses. Then, the new secondary phloem is formed constantly and as a result, the longevity of sieve elements is much more reduced in the body of the secondary plant.

The function of the phloem is to transport organic molecules and nutrients from the leaves to other parts of the plant
Photo credit: https://www.sbs.utexas.edu/

What does the Phloem do?

The phloem transport photoassimilates (energy-storing monosaccharides produced by photosynthesis) in the form of proteins and sucrose sugars through the system of translocation. These photoassimilates are produced in the leaves through the process of photosynthesis and are transported from the leaves to the other part of the plant.

Through active transport, sugars are moved from the leaves (source cell) to the phloem and then the translocation of the photoassimilates takes place. The pressure-flow hypothesis explains the translocation of the photoassimilates.  This pressure-flow hypothesis, therefore, proposes that the water that contains food molecules flow through the phloem under pressure. This pressure is a result of the difference between the water concentration of the solution in phloem and the pure water in the neighboring xylem ducts.

This means an osmotic gradient is created once there is a high concentration of sugar within the cells. Therefore, there is a passive transport of water from the adjacent xylem over the gradients to form a sugar solution and a high turgor pressure in the phloem tissues. It is this high turgor pressure that causes the sugar and water to move through the phloem tubes into sink tissues such as fruits, roots, flowers, and growing tips of stems and leaves.

The sugar solution is then received by these sink tissues and used for growth and other processes. However, the amount of water influx from the xylem reduces as the concentration of sugars reduces in the solution. This leads to low pressure in the phloem at the sink. The photoassimilates and water are moved around the plant in both directions (bidirectional) where there are areas of low and high pressure.

Phloem structure

The phloem structure is made up of 3 types of phloem cells which include the sieve elements, parenchyma cells, and sclerenchyma cells. However, the sclerenchyma cells may be absent sometimes in primary and secondary phloem. How these cells are arranged in the tissues, their presence, and their quantities all vary.

Generally, the phloem consists of several components such as conducting cells, generally called sieve elements, parenchyma cells, specialized companion cells or albuminous cells, fibers, and sclereids. Each of these components functions together to aid the conduction of amino acids and sugars from a source tissue to the sink tissues. The sugar and amino acids are then stored and consumed in the sink tissue.

Sieve elements

The term ‘sieve element‘ encompasses all conducting cells of the phloem tissue including the sieve cells and sieve tube elements. They are called sieve elements due to the strainer appearance the cells have because of the numerous pores that cross their bodies. These sieve pores are normally lined up with callose and they have specialized plasmodesmata of wider diameter.

The sieve elements are the type of phloem cells that transport sugar throughout the plant. They lack a nucleus at maturity and have very few organelles. As a result, for most of their metabolic needs, they depend on companion cells or albuminous (Strasburger)cells. Sieve elements lack organelles like ribosomes, Golgi apparatus, and cytosol and therefore utilize the available space for the translocation of materials. They are the most highly specialized cell type in plants.

The structure of the sieve element is an elongated and narrow form. These cells are connected together to form the phloem sieve tube structure. The two main types of sieve elements include the sieve member in angiosperm and the more primitive sieve cells in gymnosperms. These two types are both derived from a common ‘mother cell’ form.

Sieve tube cells

Before maturity, the sieve tube cells possess vacuoles and other organelles like ribosomes. However, at maturity, these organelles move to the cell wall and dissolve ensuring there is little to impede the movement of fluids. The few organelles that the sieve tube cells possess include the rough endoplasmic reticulum which can be located at the cell membrane usually near the plasmodesmata that connects them to their albuminous cell or companion cells. There is a group of pores called the sieve areas that grows at the end of all sieve cells from modified and enlarged plasmodesmata.

Sieve Plates

The sieve plates are modified plasmodesmata at the connections between sieve members. They are large thin areas of pores that aid the exchange of materials between element cells. However, instead of sieve plates, gymnosperms have numerous pores at the tapered end of their cell walls for the direct passage of materials.

Also, when the phloem is damaged or cut, they act as a barrier in order to prevent the loss of sap. A distinct protein known as P-protein is released once the phloem is damaged or injured by an insect or herbivorous animal. This protein is formed within the sieve elements and released from its anchor site. Hence, in order to prevent the loss of sap at the damage site, the P-protein accumulates to form a clot on the pores of the sieve plate.

Sieve cells

The structure of sieve cells is an elongated form with tapering ends. Sieve cells lack an area in the sieve elements where the pores are of a wider diameter. Therefore, they lack sieve plates and in all stages of development lack P-protein. In the terminal parts of the sieve cells, the sieve areas may be more abundant. The pores in these terminal parts, however, are of the same diameter as the pores of the lateral areas of the sieve elements.

Furthermore, specialized parenchyma cells in close contact with the sieve elements carry the maintenance and living of the sieve cells. These specialized parenchyma cells are called albuminous cells. The albuminous cell maintains the physiological functioning of the sieve cells as well as the loading and unloading of products of photosynthesis (photosynthates).

Parenchyma cells

The parenchyma in plants is a collection of cells that makes up the filler of plant tissues. Their walls are made of cellulose and are thin but flexible. The parenchyma’s primary function in the phloem is to store starch, proteins, fats, tannins, and resins in plants.

There is only one type of parenchyma in the primary phloem and it intermingles with the sieve elements. In the secondary phloem, two parenchyma types exist which are the axial parenchyma and ray parenchyma. The axial parenchyma is derived from the fusiform of the cambium whereas, the ray parenchyma is derived from the ray initials of the cambium.

In conifers, the axial parenchyma is arranged in concentric alternating layers. These cells have a number of phenolic substances that are defense mechanisms against bark attackers. The axial parenchyma in Gnetales intermingles with the sieve cells. Furthermore, some of these axial parenchyma cells act as Strasburger cells.

The way the axial phloem parenchyma is actually distributed in angiosperms is actually more varied. It may appear in bands and radial rows or feature as a background tissue where other cells are dispersed or appear as sieve tube centric. The abundance of fibers or sclereids is a related determining factor for the distribution of the axial phloem parenchyma. For instance, there is a more organized arrangement of the parenchyma in plant species with more fibers.

Companion cells

The companion cells are a specialized form of parenchyma cell. Sieve-tube members depend on a close association with these companion cells in order to function metabolically. The smaller companion cell carries virtually all the cellular functions of a sieve-tube element. A companion cell normally has a large number of mitochondria and ribosomes.

The plasmodesmata connect the dense cytoplasm of a companion cell to the sieve-tube element. A companion cell and a sieve tube element share a common sidewall that has a large number of plasmodesmata. In angiosperms, each sieve element cell is associated closely with a companion cell while in gymnosperms, it is associated with an albuminous cell or Strasburger cell.

There are two types of companion cells which include:

  1. Ordinary companion cells: These cells have smooth walls. Apart from the sieve tube cells, these cells have few or no plasmodesmatal connections to cells.
  2. Transfer cells: These cells have much-folded walls that are adjacent to non-sieve cells. This eventually creates and allows larger areas of transfer. Transfer cells are specialized in scavenging solutes.

The companion cells possess a nucleus and are stuffed with dense cytoplasm. They contain many mitochondria and ribosomes. Sieve elements lack the appropriate organelles and cannot perform metabolic reactions and cellular functions. Since companion cells can carry out these metabolic reactions and other cellular functions, sieve elements depend on them for their survival and functioning.

The microscopic channel (plasmodesmata) that connects the companion cells and sieve-tube cells allows the transfer of protein, sucrose, and other molecules to the sieve elements. Hence, companion cells play a role in fuelling the transport of materials around the plant and the sink tissues. Also, it aids the loading of sieve tubes with photosynthetic products, and at the sink tissues facilitate the unloading. In addition to this, companion cells can produce and transmit signals e.g phytohormones and defense signals. These signals are transported to sink organs through the phloem.

Albuminous cells

These cells play similar roles to the companion cells but are only associated with sieve cells. They are only found in seedless vascular plants and gymnosperms. Albuminous cells are also called Strasburger cells.

The maintenance and functioning of the sieve cells are carried by specialized parenchyma cells known either as these albuminous cells or Strasburger cells. These cells in close contact with the sieve elements, and with numerous plasmodesmata, maintain the physiological functioning of the sieve cells, as well as the loading and unloading of photosynthates.

These cells were initially named albuminous due to the proteinaceous appearance of their cell’s contents. However, these cells were later named Strasburger cells after its discoverer Eduard Strasburger, due to the fact that the high protein content of the cell is not always present. These cells being called Strasburger are recommended over being termed albuminous cells.

In the secondary phloem, Strasburger cells can be axial parenchyma cells as in Ephedra or ray parenchyma cells as in conifers. However, the presence of conspicuous connection is the only reliable feature to distinguish a Strasburger cell from an ordinary cell. In primary phloem, the parenchyma cells that act as Strasburger cells are the ones next to the sieve cells.

Sclerenchyma cells

The sclerenchyma cells are cells with thick lignified secondary walls which when present in the phloem, give structure to it. These cells, however, can be absent in the phloem. They are supportive tissues of the phloem that give strength and stiffness to the plant.

Phloem is called stratified when it has concentric layers of sclerenchyma. The bands of this tissue in Leguminosae are associated with the concentric fiber bands. Phloem that is older tends to show more sclerification than the younger ones.

The phloem sclerenchyma function as a barrier to bark attackers. It is classified into two groups that differ in form and size and are distinguished based on their origin. The group of sclerenchyma are:

  1. Bast Fibers
  2. Sclereids

Fibers

Bast fibers allow the flexibility of the phloem and support the tension strength, They are narrow and elongated cells that have a thick secondary wall and are normally dead as they attain maturity. Their thick walls contain a narrow lumen,  hemicellulose, cellulose, and lignin. Fibers can be seen in xylem too and are the main elements of several textiles such as linen, cotton, and paper.

Sclereids

These cells are shorter and irregular in shape with a thick secondary wall. Sclereids add compression strength to the phloem and serve as a protective mechanism from herbivory as they give a gritty texture when chewed by herbivores. However, they may restrict or reduce the flexibility of the phloem to an extent. These cells are usually dead as they attain maturity. They are responsible for the gritty texture in pears.

Diagram

longitudinal section of a phloem cell
Longitudinal section of a phloem cell
Photo credit: https://www2.estrellamountain.edu/

Types

  1. Primary phloem
  2. Secondary phloem

Primary phloem

The primary phloem is similar to the primary xylem as it is divided into protophloem and metaphloem. It is derived from the embryo in the seed and the procambium from the organ’s apices. As the plant is still growing, the protophloem is usually differentiated first while the metaphloem is differentiated last.

The sieve elements of the protophloem lack companion cells sometimes, like in the case of Arabidopsis. Sieve elements in such cases are sustained by other neighboring parenchyma cells. In plants without secondary growth as in monocotyledons, the metaphloem is conducted during the life of the plant.

The primary phloem can be found in vascular plant lineages such as ferns, lycophytes, and monocotyledon. There are different arrangements of the primary phloem and xylem in different vascular plant lineages based on the stele type. The two main types of stele that exist are the protostele and siphonostele. Primary phloem tissue is formed mainly by the parenchyma cells and sieve elements. It is, therefore, simpler than the secondary phloem.

Secondary Phloem

The secondary phloem is derived from the cambium and shares a lot of characteristics with the secondary xylem. Like the secondary xylem, the secondary phloem can be storied or non-storied based on whether the cambial mother cells are organized in tiers or not. This phloem type is divided into an axial and radial system. The axial system is made up of sieve elements, fibers, and axial parenchyma. Whereas, the radial system is composed of rays that are parenchymatic.

In temperate and tropical regions, there are growth rings in some trees with early and late phloem. Sometimes, the fiber bandwidth illustrates the presence of growth rings. It also gives a hint on the formation of very small sieve elements in the late phloem. The secondary phloem is formed in plants with secondary growth where the primary phloem collapses. New secondary phloem is formed constantly and as a result, the longevity of sieve elements is much more reduced in the body of the secondary plant.

Functions of Phloem

  • One of the main functions of phloem is the transportation of sap and organic molecules such as sugars, amino acids, certain hormones, and even messenger RNAs within the plant.
  • The phloem function in the process of translocation.
  • It also serves as a site for oviposition and breeding of insects.
  • In vascular plants, phloem sap sends informational signals throughout the plants.

Transportation of organic molecules and sap

The phloem is made of living cells that transport sap. Phloem sap is a water-based solution that is rich in sugars. This sugar is made at the site of photosynthesis (leaves) and is transported by the phloem to non-photosynthetic parts of the plant, such as the roots, tubers, or bulbs. However, in spring during the plant’s growth period, the storage organs like the roots are the sugar source tissue and the growing areas of the plants are the sugar sink tissues.

In xylem cells, movement is in one direction (unidirectional) whereas, in phloem cells, the movement is both ways (multidirectional). Therefore, phloem functions in the transportation of sap and organic molecules such as sugars, amino acids, certain hormones, and even messenger RNAs. They are transported through the sieve tube elements of the phloem.

Translocation

The process of translocation within plants involves the transport of nutrients and molecules throughout the plant. This occurs within the phloem pathway or transport system. Hence, the phloem function as the primary food-conducting tissue in vascular plants. In the phloem, nutrients are translocated as solutes in a solution. This solution is called the phloem sap and the predominant nutrients are sugars, minerals, and amino acids. Sugar happens to be the most concentrated solute in the sap.

Water and mineral move by negative pressure through the xylem but in the phloem, the movement is driven by positive hydrostatic pressure. Hence, translocation is accomplished by the process of phloem loading and unloading. This vascular tissue transport photoassimilates in the form of proteins and sucrose sugars through the system of translocation. These photoassimilates are produced in the leaves through the process of photosynthesis and are transported from the leaves to the other part of the plant.

Through active transport, sugars are moved from the leaves (source cell) to the phloem and then the translocation of the photoassimilates takes place. The pressure-flow hypothesis explains the translocation of the photoassimilates.  This pressure-flow hypothesis, therefore, proposes that the water that contains food molecules flow through the phloem under pressure. This pressure is a result of the difference between the water concentration of the solution in phloem and the pure water in the neighboring xylem ducts.

A site for oviposition and breeding of insects

Another function of the phloem is that it serves as an oviposition and breeding site for insects. This is particular for insects that belong to the order Diptera e.g fruit fly Drosophila montana. These insects in their adult stage congregate in breeding areas that are usually on decomposing tree phloem and areas of sap flux.

It is in these breeding areas, during the mating period that multiple males of the insect species pursue a single female simultaneously. Hence, the location of the oviposition for D. montana coincides with their breeding sites. Their substrates for oviposition are the phloem and sap flux yeast growths on birch trees.

Xylem and phloem are not only essential for plants, they also play a role in the survival of other species like the aphids that take advantage of the sugar in the phloem. They feed on the phloem and obtain the amino acids they need.

Phloem vs Xylem

Xylem and phloem are the two main transport tissues of vascular tissues. They both carry out the transportation process in plants. However, the contrast of the xylem and phloem can be seen in their movement and function.

In the phloem of plants, movement is bidirectional while movement is unidirectional in the xylem of plants.

In vascular plants, xylem transport water and provide support to the plant while the phloem transport nutrients such as sugar, organic molecules, and proteins which enable the plant to stay alive and reproduce.

The phloem transport nutrients and food from the leaves of the plant to other parts of the plant while the xylem transports water and minerals from the roots to other parts of the plants.

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