A vascular plant’s phloem serves as a transport system for soluble organic compounds. The phloem is made up of living tissue, which uses turgor pressure and energy in the form of ATP to actively transport sugars to the plant organs such as the fruits, flowers, buds and roots; xylem, the other component of the vascular plant transport system, moves water and minerals from the root and is not alive.
Function of Phloem
Phloem is responsible for translocating photoassimilates from the leaves, where they are produced by photosynthesis, to the rest of the plant through translocation.
Through active transport, sugars are transported from the source to the phloem. A pressure flow hypothesis explains the next step, the translocation of photoassimilates.
Osmotic gradients are created when organic substances (in this case sugar) are present within cells at high concentrations. Water is passively drawn over the gradient from the adjacent xylem, creating a sugar solution and high turgor pressure in the phloem. As a result of the high turgor pressure, water and sugars move into the sink tissues (e.g. the roots, growing tips of stems and leaves, flowers and fruits).
As soon as the sink receives the sugar solution, the sugars are used for growth and other processes. The concentration of sugars in the solution decreases as the water influx from the xylem drops, resulting in low pressure in the phloem at the sink. Photoassimilates and water are consistently moved around the plant in both directions in areas of high and low pressure.
Structure of Phloem
There are several components that make up the phloem structure. Together, these components facilitate the transport of sugars and amino acids from sources to sink tissues for consumption or storage.
The Sieve Elements
The sieve elements are elongated, narrow cells that make up the sieve tube structure of the phloem. There are several types of sieve element cells found in plants, but sieve element cells are the most highly specialized. At maturity, they lack a nucleus, ribosomes, cytosol, and Golgi apparatus, maximizing space for material translocation.
Two types of sieve elements exist: the ‘sieve member’ in angiosperms, and the more primitive ‘sieve cells’ in gymnosperms. Both are derived from a single ‘mother cell’.
Sieve plates, which are modified plasmodesmata, connect sieve member cells. Materials exchange between element cells is facilitated by sieve plates, which have relatively large, thin pores.
When the phloem is damaged by insects or herbivorous animals, the sieve plates also serve as a barrier to prevent sap loss. A unique protein called “P-protein” (Phloem-protein), which is formed inside the sieve element, is released from its anchor site and forms a “clot” on the pores of the sieve plate, preventing sap loss.
The sieve elements in gymnosperms are more primitive than in angiosperms, and instead of sieve plates they have numerous pores at the tapered ends of the cell walls for direct passage of materials.
The Companion Cells
In gymnosperms, each sieve element cell is closely associated with an albuminous cell or ‘Strasburger cell’.
Nuclei and dense cytoplasm are present in companion cells, as well as many ribosomes and mitochondria. Due to the lack of organelles in the sieve element, the companion cells can carry out metabolic reactions and perform other cellular functions. Companion cells are therefore essential for the survival and function of sieve elements.
Via plasmodesmata, a microscopic channel connecting the cytoplasm of cells, sucrose, proteins, and other molecules can be transferred between the sieve tube and companion cells. Companion cells facilitate the transport of materials around the plant and to the sink tissues, as well as the loading of sieve tubes with photosynthesis products and the unloading of the tubes at the sink tissues.
Additionally, companion cells produce and transmit signals, such as defense signals and phytohormones, which are transported through the phloem.
The parenchyma is a collection of cells that make up the ‘filler’ of plant tissues. The walls are made of cellulose, which is thin and flexible. A plant’s parenchyma stores starch, fats, proteins, tannins, and resin in the phloem.
The sclerenchyma is the main support tissue of the phloem, providing the plant with stiffness and strength. A sclerenchyma consists of fibers and sclereids; both have a thick secondary cell wall and are usually dead when they mature.
Bast fibers enable the phloem to be flexible while supporting tension strength. They are narrow, elongated cells with thick walls of cellulose, hemicellulose, and lignin and a narrow lumen (inner cavity).
Sclereids are shorter, irregularly shaped cells, which provide compression strength to the phloem, though they can restrict flexibility. By generating a gritty texture when chewed, screreids protect themselves from herbivory.
Related Biology Terms
- Xylem – Within vascular plants, xylem transports water from the roots to the leaves and shoots.
- Photosynthesis – The process which most plants use to convert energy from the sunlight, water and carbon dioxide into oxygen and carbohydrates.
- Photoassimilates – The biological compounds (usually energy-storing monosaccharaides) which are produced by photosynthesis.
- ATP – Adenosine triphosphate is the high-energy molecule that transports energy for metabolism within cells.
Phloem is a type of plant tissue that is responsible for transporting sugars, amino acids, and other organic compounds from the leaves to other parts of the plant. Phloem is composed of several different cell types, including sieve elements, companion cells, and parenchyma cells.
Phloem transport occurs through a process called translocation, which involves the movement of organic compounds from a source (such as the leaves) to a sink (such as the roots or developing fruits). Translocation is driven by a combination of pressure gradients, metabolic processes, and active transport mechanisms.
The two main types of phloem cells are sieve elements and companion cells. Sieve elements are elongated cells that form the main transport pathway in the phloem. They are characterized by their sieve plates, which are perforated regions of the cell wall that allow for the exchange of materials between adjacent cells. Companion cells, on the other hand, are specialized cells that provide metabolic support and energy to the sieve elements.
Phloem plays a crucial role in plant growth and development, by supplying organic compounds to the various parts of the plant that require them. Some specific functions of phloem include the transport of photosynthates from the leaves to the rest of the plant, the distribution of nutrients and signaling molecules, and the facilitation of long-distance communication between different plant organs.