Oct 08, Recommended for you. For stem cells, bigger doesn't mean better Nov 12, Study shines a light into 'black holes' in the Arabidopsis genome Nov 11, Nov 11, Nov 10, Load comments 0. Let us know if there is a problem with our content. Your message to the editors. Your email only if you want to be contacted back. Send Feedback. Thank you for taking time to provide your feedback to the editors. Your friend's email.
Your email. I would like to subscribe to Science X Newsletter. Learn more. Your name. Note Your email address is used only to let the recipient know who sent the email. Your message. Your Privacy This site uses cookies to assist with navigation, analyse your use of our services, collect data for ads personalisation and provide content from third parties. Ok Cookie options. E-mail newsletter. It appears that you are currently using Ad Blocking software.
ATP itself is formed through secondary active transport using a hydrogen ion gradient in the mitochondrion. Endocytosis is a type of active transport that moves particles, such as large molecules, parts of cells, and even whole cells, into a cell. There are different variations of endocytosis, but all share a common characteristic: The plasma membrane of the cell invaginates, forming a pocket around the target particle. The pocket pinches off, resulting in the particle being contained in a newly created vacuole that is formed from the plasma membrane.
Phagocytosis is the process by which large particles, such as cells, are taken in by a cell. For example, when microorganisms invade the human body, a type of white blood cell called a neutrophil removes the invader through this process, surrounding and engulfing the microorganism, which is then destroyed by the neutrophil Figure 3.
A variation of endocytosis is called pinocytosis. In reality, this process takes in solutes that the cell needs from the extracellular fluid Figure 3. Figure 3. Three variations of endocytosis are shown. A targeted variation of endocytosis employs binding proteins in the plasma membrane that are specific for certain substances Figure 3. The particles bind to the proteins and the plasma membrane invaginates, bringing the substance and the proteins into the cell.
If passage across the membrane of the target of receptor-mediated endocytosis is ineffective, it will not be removed from the tissue fluids or blood.
Instead, it will stay in those fluids and increase in concentration. Some human diseases are caused by a failure of receptor-mediated endocytosis. In the human genetic disease familial hypercholesterolemia, the LDL receptors are defective or missing entirely.
People with this condition have life-threatening levels of cholesterol in their blood, because their cells cannot clear the chemical from their blood. Figure 4. In exocytosis, a vesicle migrates to the plasma membrane, binds, and releases its contents to the outside of the cell. In contrast to these methods of moving material into a cell is the process of exocytosis.
Exocytosis is the opposite of the processes discussed above in that its purpose is to expel material from the cell into the extracellular fluid. A particle enveloped in membrane fuses with the interior of the plasma membrane. This fusion opens the membranous envelope to the exterior of the cell, and the particle is expelled into the extracellular space Figure 4. The combined gradient that affects an ion includes its concentration gradient and its electrical gradient.
Living cells need certain substances in concentrations greater than they exist in the extracellular space. Summarize the function of the three major ABC transporter categories: in prokaryotes, in gram-negative bacteria and the subgroup of ABC proteins.
ATP-binding cassette transporters ABC-transporters are members of a protein superfamily that is one of the largest and most ancient families with representatives in all extant phyla from prokaryotes to humans. ABC transporters are transmembrane proteins that utilize the energy of adenosine triphosphate ATP hydrolysis to carry out certain biological processes including translocation of various substrates across membranes and non-transport-related processes such as translation of RNA and DNA repair.
They transport a wide variety of substrates across extra- and intracellular membranes, including metabolic products, lipids and sterols, and drugs. ABC transporters are involved in tumor resistance, cystic fibrosis and a range of other inherited human diseases along with both bacterial prokaryotic and eukaryotic including human development of resistance to multiple drugs.
Bacterial ABC transporters are essential in cell viability, virulence, and pathogenicity. ABC transporters are divided into three main functional categories.
In prokaryotes, importers mediate the uptake of nutrients into the cell. The substrates that can be transported include ions, amino acids, peptides, sugars, and other molecules that are mostly hydrophilic.
The membrane-spanning region of the ABC transporter protects hydrophilic substrates from the lipids of the membrane bilayer thus providing a pathway across the cell membrane. In gram-negative bacteria, exporters transport lipids and some polysaccharides from the cytoplasm to the periplasm. Eukaryotes do not possess any importers. Exporters or effluxers, which are both present in prokaryotes and eukaryotes, function as pumps that extrude toxins and drugs out of the cell.
The third subgroup of ABC proteins do not function as transporters, but rather are involved in translation and DNA repair processes. This alternating-access model was based on the crystal structures of ModBC-A. In bacterial efflux systems, certain substances that need to be extruded from the cell include surface components of the bacterial cell e. They also play important roles in biosynthetic pathways, including extracellular polysaccharide biosynthesis and cytochrome biogenesis.
Siderophores are classified by which ligands they use to chelate the ferric iron, including the catecholates, hydroxamates, and carboxylates. Iron is essential for almost all living organisms as it is involved in a wide variety of important metabolic processes. However, iron is not always readily available; therefore, microorganisms use various iron uptake systems to secure sufficient supplies from their surroundings.
There is considerable variation in the range of iron transporters and iron sources utilized by different microbial species. Siderophores are small, high-affinity iron chelating compounds secreted by microorganisms such as bacteria, fungi, and grasses.
Iron is essential for almost all life, because of its vital role in processes like respiration and DNA synthesis.
This ion state is the predominant one of iron in aqueous, non-acidic, oxygenated environments, and accumulates in common mineral phases such as iron oxides and hydroxides the minerals that are responsible for red and yellow soil colours. Hence, it cannot be readily utilized by organisms.
Many siderophores are nonribosomal peptides, although several are biosynthesised independently. Because of this property, they have attracted interest from medical science in metal chelation therapy, with the siderophore desferrioxamine B gaining widespread use in treatments for iron poisoning and thalassemia.
Synthesis of enterobactin : Enterobactin also known as Enterochelin is a high affinity siderophore that acquires iron for microbial systems. It is primarily found in Gram-negative bacteria, such as Escherichia coli and Salmonella typhimurium.
Iron is tightly bound to proteins such as hemoglobin, transferrin, lactoferrin, and ferritin. There are great evolutionary pressures put on pathogenic bacteria to obtain this metal. For example, the anthrax pathogen Bacillus anthracis releases two siderophores, bacillibactin and petrobactin, to scavenge ferric iron from iron proteins. While bacillibactin has been shown to bind to the immune system protein siderocalin, petrobactin is assumed to evade the immune system and has been shown to be important for virulence in mice.
In eukaryotes, other strategies to enhance iron solubility and uptake are the acidification of the surrounding e. The most effective siderophores are those that have three bidentate ligands per molecule, forming a hexadentate complex and causing a smaller entropic change than that caused by chelating a single ferric ion with separate ligands. Siderophores are usually classified by the ligands used to chelate the ferric iron. The majors groups of siderophores include the catecholates phenolates , hydroxamates and carboxylates e.
Citric acid can also act as a siderophore. Group translocation is a protein export or secretion pathway found in plants, bacteria, and archaea.
With some exceptions, bacteria lack membrane-bound organelles as found in eukaryotes, but they may assemble proteins onto various types of inclusions such as gas vesicles and storage granules. Bacteria may have a single plasma membrane Gram-positive bacteria or an inner membrane plus an outer membrane separated by the periplasm Gram-negative bacteria.
Proteins may be incorporated into the plasma membrane. They can also be trapped in either the periplasm or secreted into the environment, according to whether or not there is an outer membrane. The basic mechanism at the plasma membrane is similar to the eukaryotic one. In addition, bacteria may target proteins into or across the outer membrane. Systems for secreting proteins across the bacterial outer membrane may be quite complex.
The systems play key roles in pathogenesis. These systems may be described as type I secretion, type II secretion, etc. In most Gram-positive bacteria, certain proteins are targeted for export across the plasma membrane and subsequent covalent attachment to the bacterial cell wall.
A specialized enzyme, sortase, cleaves the target protein at a characteristic recognition site near the protein C-terminus, such as an LPXTG motif where X can be any amino acid , then transfers the protein onto the cell wall. PEP group translocation, also known as the phosphotransferase system or PTS, is a distinct method used by bacteria for sugar uptake where the source of energy is from phosphoenolpyruvate PEP.
0コメント