Plants transport organic molecules over long distances within the phloem sap. These molecules are transported from source tissues to sink tissues. The transport process is known as translocation. Translocation in biological systems are active.
This transport is active, and can take various forms such as primary active transport (uniport), antiport and symport. Uniport transports one molecule in one direction, while symport transports two different molecules in opposite directions.
Relationship Between Translocation in Biological Systems and Active Transport
A translocation in genetics is a process in which the genetic information in chromosomes changes. This occurs when a part of one chromosome breaks off and attaches to another chromosome. This mixing of genetic material leads to mutations. These mutations can affect the way a cell functions and cause many diseases such as leukemia, breast cancer, schizophrenia, muscular dystrophy, and Down syndrome. In the human body, there are 23 pairs of chromosomes. The two parts of each chromosome are joined together at a point called the centromere. If a pair of chromosomes has a mutation in this area, it will have a different genetic structure and may contain extra or missing information. The mutations in these chromosomes are called translocations. There are two types of translocations: reciprocal and Robertsonian. A reciprocal translocation involves a swap of genetic information between two different chromosomes, while a Robertsonian translocation occurs when an entire chromosome is attached to another chromosome at the centromere.
A significant fraction of the proteins synthesized in a cell are transported across a membrane. These transport systems are complex and energy-coupled. They have several tasks including selecting the correct precursors, providing a pathway through which they can cross the membrane without compromising its permeability barrier, and assisting in protein folding. The best-studied protein translocation systems are located in the endoplasmic reticulum (ER) and mitochondrial membranes. These systems use energy derived from ATP, a molecule formed by cellular respiration.
Back to Translocation in biological systems: The most common type of translocation in biological systems is Active Transport. It uses proteins to move ions and molecules across a selectively permeable membrane against a concentration gradient. This process is energy-driven and uses a large amount of ATP. It also requires a certain amount of lipids to form the membrane permeability barrier. The lipids can be provided by the cell itself or by another source, such as a fatty acid synthesized from triglycerides in the mitochondria.
In addition to active transport, there are also group translocations. These transport groups of solutes into cells by undergoing chemical modification. Glucose, for example, appears inside the cell as glucose-6-phosphate. During this process, the transmembrane EIIA molecule transfers P to EIIB, which then takes up glucose from the surrounding cytoplasm and phosphorylates it. The resulting glucose-6-phosphate then enters the cell where it is metabolized.
The phloem of plants is a major route for the transport of organic molecules from sources to sinks. The majority of the constituents that move through phloem are sugars and amino acids. A number of other substances, such as enzymes, hormones, and toxins, also travel between the phloem and xylem. In some cases, these molecules can be transported over long distances between graft unions. Research into the role of long-distance signaling in plant development has focused on low molecular weight peptides that appear to be transported by this mechanism.
Mechanism of Active Transport and Translocation in Biological Systems
Biological systems use a process called active transport to move ions and other small molecules across the cell membrane. This process uses energy produced from adenosine triphosphate (ATP). The ATP is broken down into ADP and phosphate, which are used to carry the molecules across the membrane. Active transport also moves sugars, amino acids, and other large molecules into and out of cells. It can also be used to transport lipids and proteins.
A number of different types of active transport and translocation in biological systems have been identified, but most involve protein-coupled ion channels or pumps. These proteins are able to change their conformation, which creates a channel through the cell membrane and allows ions to pass through. Molecules of ATP bind to the protein and cause it to change shape, creating a vesicle and allowing the ions to enter the cell.
Plant physiologists have long considered translocation of major solutes in phloem and xylem to be the primary way that a plant communicates with other parts of its body. This is because the major constituents of phloem and Xylem move at rates far in excess of their mass flows, and are considered to carry information about plant hormones and other signaling molecules.
These signaling molecules are expected to be transported by phloem at long distances, and to convey the message in a coherent way. However, little is known about the mechanism of signaling in phloem. Some researchers have postulated that low-macromolecules can act as signals in phloem, but this remains controversial.
In addition to active transport, cellular structures such as cell walls and vesicles help with transport. The role of these structures is to protect the cytoplasm from outside substances, while providing a route for the transport of organic molecules. Another important transport mechanism involves endocytosis, which is the process by which a cell takes in materials from the surrounding environment.
This mechanism is a type of group translocation in biological systems, and is used by bacteria for the uptake of sugars. The molecule undergoes a series of chemical changes, including phosphorylation, cleavage, and folding. It is then converted to Glucose-6-Phosphate, a form that can’t escape the bacterium. This process is also referred to as PEP group translocation or the phosphotransferase system.
Another type of transport involves endocytosis, a process in which the cell folds its cell membrane around a foreign substance and fuses it with a vesicle that can digest it. For example, when white blood cells recognize a pathogen, they engulf it by folding their cell membranes around the bacteria and merging them with a lysosome, or digestive vesicle. Other types of endocytosis include the secretory transport of low-macromolecules, such as bioactive peptides, to and from the cell. These are thought to play an important role in metabolic and transcriptional gene regulation.
Importance of Translocation in the Uptake of Solutes
The movement of solutes in plants is essential for growth and development. The most important solutes, such as sugars and amino acids, move from sites of their synthesis (sources) to those where they are used for growth and development (sinks) through the plant body’s conducting tissue, called phloem. The importance of translocation has been emphasized in the study of plant genetics and in plant breeders’ efforts to increase crop yields through the selection of individuals with a desirable balance of chromosomes.
The phloem also conveys signals that regulate growth and development, such as the systemic transport of low molecular weight peptides. These are thought to travel with assimilates, and thus can be transmitted over long distances at rates far in excess of mass flow of solutes. The ability to transport such signals over long distances has a major impact on the regulation of gene expression in plants.
In a cellular sense, translocation involves a chromosomal break and the joining of fragmented chromatin to other chromosomes. Such chromosome rearrangements are common in human cancers, but the mechanistic steps by which they form have been difficult to understand. Recent experiments have shed light on the formation of chromosomal translocations. These studies show that non-random spatial genome organization, DNA repair pathways and chromatin features such as histone marks are important for determining translocation frequency and pairing.
Moreover, the ability of the phloem to translocate solutes is essential for the uptake of these solutes by cells that use them for growth and cell division. In order to better understand this process, scientists have developed a technique known as autoradiography. This involves placing a cross-section of a plant on X-ray film. The phloem will blacken the film, but not other tissues, which helps to identify its role in transferring solutes.
Scientists have also been able to use radioactive carbon isotopes to trace phloem transport. They do this by placing a controlled plant in an environment that contains the radioactive carbon isotope 14C. The plant then converts the 14C into sugars during photosynthesis. This allows the researchers to track these sugars as they move through the phloem.
The phloem is also the main source of sugars and other organic molecules, such as proteins, that are transported to different parts of the plant. It is a complex conducting tissue that includes a wide range of cell types and is composed of a number of tubules. The phloem is a major determinant of the “source-sink” relationship that determines harvest index, which is the ratio of grain yield to total biomass at the time of harvest. Among the most important solutes that are transported by the phloem are carbon and nitrogenous assimilates. In legumes, sucrose and the amides Gln and Asn predominate in phloem, while in xylem assimilates such as ureides, allantoin and citrulline predominate.
Scientists agree that the soluble products of photosynthesis which are formed in the photosynthetic tissue enter the sieve tube by a process known as active transport. However, the cause of the movement of materials once they are in the sieve tubes is unknown. It is known that sugars and amino acids tend to move along concentration gradients and the speed at which they travel makes it impossible to conclude that diffusion is solely responsible.
A possible explanation for this is that there is a mass flow of solutes through the phloem due to a turgor pressure gradient. This turgor pressure is highest in the leaves(the source) where the sugars are formed and lowest in the roots(the sink). The high turgor pressure in the leaves is due to the high sugar concentration and consequently high osmotic pressure present here. These mass flow of solutes have been shown to move at different speeds as sugars and amino acids do. These substances can also move in opposite directions within the same sieve tubes.
The deferring movements in speed of the solutes can be accounted for by the fact that they do posses different molecular properties and also by the fact that the walls of the sieve tubes are not equally impermeable to all solutes resulting in the difference in the ratio of concentration of the various solutes which is expected to change as they move along the sieve tube leading to changes in their rate of flow.
However, there is an objection to the mass flow hypothesis. Experimental calculations reveal that the gradients of turgor pressure which exist would be insufficient to overcome the considerable resistance imposed by the sieve pores considering the fact that the pores are bunged up with numerous filaments.
This discovery has led to the assumption that mass flow might be aided at the sieve plates by electro-osmosis which is defined as the passage of water across a charged membrane. It is therefore argued that an electrical potential might be maintained across the sieve plate, the lower side of the plate being negative while the upper side being positive.