Introduction
The vascular plant tissue, phloem, is responsible for distributing and transporting carbohydrates produced during photosynthesis. Phloem can be found in the stem’s vascular bundles, the abaxial region of every leaf’s venations, and the exterior section of the root cylinders because the plant is a continuous unit.
As a fundamental tissue in the plant body, phloem functions extend from its primary process of transporting sugar to include transporting signalling molecules like mRNAs, hormones, defences from biotic and abiotic agents, sustenance for the organs, gas exchange, and storage of numerous waste products, including starch, calcium oxalate crystals, and tannins.
All vascular plants have phloem, which usually contains specialised live conducting cells known as sieve elements. The nucleus, ribosomes, and other organelles of sieve elements degenerate during development, improving sugar transport. As a result, these cells’ survival and functionality will depend on the parenchyma cells, which support the physiological tasks of these sieve elements.
Table of Contents
- Definition of Phloem
- Structure of Phloem Tissue
- Phloem Loading and Unloading in Plants
- Function of Phloem
- Frequently Asked Questions (FAQs)
Definition of Phloem
The complex tissue known as phloem is the transportation system for soluble organic molecules in vascular plants.
The other component of the vascular plant transport system, the non-living xylem, transports water and minerals from the root. The phloem is composed of living tissue that actively transports sugars to plant organs such as the fruits, flowers, buds, and roots using turgor pressure and energy in the form of ATP. Translocation is the term for this movement process.
Phloem, whose name comes from the Ancient Greek word phloiós, which means “bark,” is the innermost layer of the bark in trees. Carl Nägeli invented the term phloem in 1858.
Types of Phloem
There are two types of phloem: Primary Phloem and Secondary Phloem.
- Primary Phloem: It is the type of phloem produced by the primary meristem of a vascular plant. Phloem that originates from the procambium during primary growth is known as primary phloem. Protophloem or metaphloem are the components of the primary phloem.
The primary phloem is present in the primary plant body. As opposed to the secondary phloem, which develops inside the primary phloem, it occurs in the periphery. The secondary phloem has a radial system, whereas the primary phloem does not. The primary phloem has less number of phloem fibres, sieve tubes, and phloem parenchyma compared to the secondary phloem in cellular components. Additionally, the primary phloem often lacks sclereids.
- Secondary Phloem: The secondary meristems of a vascular plant produce the secondary phloem. The meristematic tissue, vascular cambium, is involved in this growth. The secondary phloem develops from the vascular cambium during secondary growth. The increase in plant width, mainly in trees, is caused by secondary growth.
The stems and roots contain the secondary phloem, which develops inside the primary phloem. The secondary phloem has a radial arrangement of phloem rays. Phloem fibres (also known as bast fibres), sieve tubes, phloem parenchyma, and sclereids are all found in more significant numbers in secondary phloem than in primary phloem. The secondary phloem’s sieve tubes are broader but shorter. As a result, photosynthate moves more quickly through secondary phloem sieve tubes than primary phloem sieve tubes.
Structure of Phloem Tissue
Conducting cells, parenchyma cells, and supporting cells comprise three types of cells forming the phloem structure. The conducting cells, also known as sieve elements, are made up of rows of sieve tube cells with holes in their lateral walls, facilitating the movement of nutrients throughout the plant.
Two specialised cells, companion cells, albuminous cells, and unspecialised cells used for storage, constitute the parenchyma. The companion cells, as their name suggests, work alongside the sieve tube cells to support the metabolic processes of the sieve components. Plasmodesmata link the companion cells to the components of the sieve tube.
Sclerenchymatous cells, specifically fibres and sclereids, act as supportive cells. They mainly serve supportive and mechanical purposes. The secondary cell walls of both cells give them rigidity and high tensile strength.
The phloem’s structure is composed of various components. Each component functions together to enhance the transfer of carbohydrates and amino acids from a source to sink tissues, where they are utilised or stored.
Sieve element cells and companion cells comprise most of the cells in phloem sieve tubes.
Sclerenchyma and parenchyma cells are also found in the phloem, filling in extra gaps and offering support.
The Sieve Elements
Sieve element is the most highly specialised plant cell type. They are different in that they lack a nucleus at maturity and other organelles, including cytosol, ribosomes, and Golgi apparatus, which maximises the amount of space that can be used for material translocation. The elongated, narrow cells that make up the sieve elements are connected to form the phloem’s sieve tube structure.
The ‘sieve member,’ found in angiosperms, and the more primitive ‘sieve cells,’ which are connected to gymnosperms, are the two primary forms of sieve elements. Both are derived from a common form of “mother cell”.
Sieve Plates
Sieve plates, modified plasmodesmata, are found at the junctions between sieve member cells. Sieve plates are comparatively large, thin sections of pores that make it easier for materials to move between element cells.
The sieve plates also serve as a barrier to stop sap loss when the phloem is cut or harmed, which is frequently done by an insect or herbivore.
Gymnosperms contain more primitive sieve elements than angiosperms do. Instead of sieve plates at the tapering end of the cell walls, they have numerous pores that allow material to pass through freely.
The Companion Cells
Each sieve element cell generally has a “companion cell” in angiosperms and an albuminous cell (“Strasburger cell”) in gymnosperms.
Companion cells possess a nucleus, a thick cytoplasm, a large number of ribosomes, and a large number of mitochondria. This indicates that because the sieve element lacks the necessary organelles, the companion cells can carry out the metabolic processes and other cellular tasks.
Thus, companion cells are responsible for supplying energy for the movement of materials throughout the plant and the sink tissues, and for the facilitation of loading sieve tubes with photosynthesis-related products and unloading at the sink tissues.
Phloem Parenchyma
A group of cells called the parenchyma serves as the “filler” in plant tissues. They contain cellulose walls that are thin but flexible. Starch, lipids, proteins, tannins and resins in some plants are stored primarily by the parenchyma within the phloem.
Phloem Sclerenchyma
Sclerenchymatous cells are found in phloem fibres. The phloem, which gives the plant stiffness and strength, is mainly supported by the sclerenchyma. Both fibres and sclereids, two types of sclerenchyma, have a strong secondary cell wall and are often dead when they reach maturity.
The bast fibres are thin, elongated cells with thick cellulose, hemicellulose, and lignin walls and a limited lumen (inner cavity) that support the tension strength while allowing the phloem to be flexible.
Phloem Loading and Unloading in Plants
The mesophyll cells of the leaf produce sugar as a result of photosynthesis. These sugars are translocated through the sieve tube elements of phloem. Phloem loading refers to the transfer of sugar from mesophyll cells (source) to sieve tube elements, and phloem unloading refers to the transfer of sugar from sieve tube elements to roots or other storage cells.
Phloem Loading
There are two different types of phloem loading mechanisms.
- Active Phloem Loading: It is also known as the sucrose-H+ cotransporter mechanism. This method involves an energy-driven movement of sugars from apoplast, or cell walls outside the plasma membrane, to sieve phloem tubes.
- Passive Phloem Loading: Passive phloem loading: Organic solutes move freely from mesophyll cells through symplast (i.e. from cell to cell) to sieve tubes of phloem element via companion cells through plasmodesmata.
The following steps involve active phloem loading:
- Sucrose is produced in the leaf’s photosynthetic tissue.
- Sucrose is transported to the apoplast by diffusion from photosynthesising tissue.
- H+ ions are actively pushed through the plasma membrane of companion cells and into the apoplast by a carrier protein. This procedure makes use of ATP.
- Co-transportation takes place here. A proton gradient is formed as the concentration of H+ in the apoplast increases, and H+ then diffuses back to companion cells along with sucrose. This procedure also involves a cotransporter protein.
After phloem loading, sucrose is translocated to the consumption end or sink organs; and the sugar is transported to sink organs at the consumption end. The term “phloem unloading” refers to this procedure.
Phloem Unloading
Phloem unloading occurs similar to phloem loading, either by symplast or apoplast. When sugar arrives at the receiving end, it is unloaded from the filter tube into the cells or sink. There are three types of phloem unloading mechanisms.
- Sieve Element Unloading: In this procedure, imported sugars leave sink tissue sieve components.
- Short Distance Transport: A short-distance pathway, also known as post-sieve element transfer, is now being used to transport the sugars to the cells in the sink.
- Storage and Metabolism: Carbohydrates are finally stored or metabolised in the cells of the sink.
Generally, when sucrose consumption rates are very high and sink cells are metabolically very active, as in the meristematic tissue of developing roots, fruits, leaves, etc., symplast is used for phloem unloading. When storage organs like fruits (grapes, oranges, etc.) and roots have sink cells, sucrose unloading happens through the apoplast.
Function of Phloem
Water-based sap contains a lot of carbohydrates produced during photosynthesis. These sugars are sent to storage organs like tubers or bulbs or non-photosynthetic plant sections like the roots by phloem. The phloem, which transports sap, comprises still-living cells compared to the mostly-dead xylem.
Phloem is a class of complex permanent tissue that develops into a conductive or vascular system in the plant’s body. It transports the prepared nutrients from the leaves to the growing areas and storage organs. It is also considered that vascular plants’ phloem sap contributes to the transmission of informative signals.
Related Links:
- Difference between Xylem and Phloem
- Transportation in Plants
- Difference between Tracheids and Vessels
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