Carrier Proteins : Some substances are able to move down their concentration gradient across the plasma membrane with the aid of carrier proteins. Carrier proteins change shape as they move molecules across the membrane.
An example of this process occurs in the kidney. Glucose, water, salts, ions, and amino acids needed by the body are filtered in one part of the kidney.
This filtrate, which includes glucose, is then reabsorbed in another part of the kidney. Because there are only a finite number of carrier proteins for glucose, if more glucose is present than the proteins can handle, the excess is not transported; it is excreted from the body in the urine.
Channel and carrier proteins transport material at different rates. Channel proteins transport much more quickly than do carrier proteins. Channel proteins facilitate diffusion at a rate of tens of millions of molecules per second, whereas carrier proteins work at a rate of a thousand to a million molecules per second. The sodium-potassium pump maintains the electrochemical gradient of living cells by moving sodium in and potassium out of the cell.
Describe how a cell moves sodium and potassium out of and into the cell against its electrochemical gradient. The primary active transport that functions with the active transport of sodium and potassium allows secondary active transport to occur. The secondary transport method is still considered active because it depends on the use of energy as does primary transport.
Active Transport of Sodium and Potassium : Primary active transport moves ions across a membrane, creating an electrochemical gradient electrogenic transport. The process consists of the following six steps:. Several things have happened as a result of this process. At this point, there are more sodium ions outside of the cell than inside and more potassium ions inside than out.
For every three ions of sodium that move out, two ions of potassium move in. This results in the interior being slightly more negative relative to the exterior. This difference in charge is important in creating the conditions necessary for the secondary process. The sodium-potassium pump is, therefore, an electrogenic pump a pump that creates a charge imbalance , creating an electrical imbalance across the membrane and contributing to the membrane potential.
ABC transporters are a protein superfamily that all have an ATP binding cassette and transport substances across membranes. 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.
This adds to the overall selectivity of the plasma membrane. The exact mechanism for the change of shape is poorly understood. Proteins can change shape when their hydrogen bonds are affected, but this may not fully explain this mechanism.
Each carrier protein is specific to one substance, and there are a finite number of these proteins in any membrane. This can cause problems in transporting enough of the material for the cell to function properly. Carrier Proteins : Some substances are able to move down their concentration gradient across the plasma membrane with the aid of carrier proteins. Carrier proteins change shape as they move molecules across the membrane.
An example of this process occurs in the kidney. Glucose, water, salts, ions, and amino acids needed by the body are filtered in one part of the kidney. This filtrate, which includes glucose, is then reabsorbed in another part of the kidney. Because there are only a finite number of carrier proteins for glucose, if more glucose is present than the proteins can handle, the excess is not transported; it is excreted from the body in the urine.
Channel and carrier proteins transport material at different rates. Channel proteins transport much more quickly than do carrier proteins. Channel proteins facilitate diffusion at a rate of tens of millions of molecules per second, whereas carrier proteins work at a rate of a thousand to a million molecules per second.
Passive transport, such as diffusion and osmosis, moves materials of small molecular weight across membranes. Plasma membranes must allow or prevent certain substances from entering or leaving a cell. In other words, plasma membranes are selectively permeable; they allow some substances to pass through, but not others. If they were to lose this selectivity, the cell would no longer be able to sustain itself, and it would be destroyed.
Some cells require larger amounts of specific substances than other cells; they must have a way of obtaining these materials from extracellular fluids. This may happen passively, as certain materials move back and forth, or the cell may have special mechanisms that facilitate transport. Some materials are so important to a cell that it spends some of its energy hydrolyzing adenosine triphosphate ATP to obtain these materials.
Red blood cells use some of their energy to do this. All cells spend the majority of their energy to maintain an imbalance of sodium and potassium ions between the interior and exterior of the cell. The most direct forms of membrane transport are passive. Passive transport is a naturally-occurring phenomenon and does not require the cell to exert any of its energy to accomplish the movement.
In passive transport, substances move from an area of higher concentration to an area of lower concentration. A physical space in which there is a range of concentrations of a single substance is said to have a concentration gradient.
Passive Transport : Diffusion is a type of passive transport. Diffusion through a permeable membrane moves a substance from an area of high concentration extracellular fluid, in this case down its concentration gradient into the cytoplasm. The passive forms of transport, diffusion and osmosis, move materials of small molecular weight across membranes. Substances diffuse from areas of high concentration to areas of lower concentration; this process continues until the substance is evenly distributed in a system.
In solutions containing more than one substance, each type of molecule diffuses according to its own concentration gradient, independent of the diffusion of other substances. Many factors can affect the rate of diffusion, including, but not limited to, concentration gradient, size of the particles that are diffusing, and temperature of the system.
In living systems, diffusion of substances in and out of cells is mediated by the plasma membrane. Some materials diffuse readily through the membrane, but others are hindered; their passage is made possible by specialized proteins, such as channels and transporters. The chemistry of living things occurs in aqueous solutions; balancing the concentrations of those solutions is an ongoing problem.
In living systems, diffusion of some substances would be slow or difficult without membrane proteins that facilitate transport. The sodium-potassium pump maintains the electrochemical gradient of living cells by moving sodium in and potassium out of the cell. Describe how a cell moves sodium and potassium out of and into the cell against its electrochemical gradient. The primary active transport that functions with the active transport of sodium and potassium allows secondary active transport to occur.
The secondary transport method is still considered active because it depends on the use of energy as does primary transport. Active Transport of Sodium and Potassium : Primary active transport moves ions across a membrane, creating an electrochemical gradient electrogenic transport. The process consists of the following six steps:. Several things have happened as a result of this process. At this point, there are more sodium ions outside of the cell than inside and more potassium ions inside than out.
For every three ions of sodium that move out, two ions of potassium move in. This results in the interior being slightly more negative relative to the exterior.
This difference in charge is important in creating the conditions necessary for the secondary process. The sodium-potassium pump is, therefore, an electrogenic pump a pump that creates a charge imbalance , creating an electrical imbalance across the membrane and contributing to the membrane potential. Define an electrochemical gradient and describe how a cell moves substances against this gradient.
Electrochemical Gradient : Electrochemical gradients arise from the combined effects of concentration gradients and electrical gradients. Simple concentration gradients are differential concentrations of a substance across a space or a membrane, but in living systems, gradients are more complex.
Because ions move into and out of cells and because cells contain proteins that do not move across the membrane and are mostly negatively charged, there is also an electrical gradient, a difference of charge, across the plasma membrane. The interior of living cells is electrically negative with respect to the extracellular fluid in which they are bathed.
The situation is more complex, however, for other elements such as potassium. The combined gradient of concentration and electrical charge that affects an ion is called its electrochemical gradient. To move substances against a concentration or electrochemical gradient, the cell must use energy. Active transport mechanisms, collectively called pumps, work against electrochemical gradients.
Small substances constantly pass through plasma membranes. Active transport maintains concentrations of ions and other substances needed by living cells in the face of these passive movements.
Two mechanisms exist for the transport of small-molecular weight material and small molecules. Primary active transport moves ions across a membrane and creates a difference in charge across that membrane, which is directly dependent on ATP.
Secondary active transport describes the movement of material that is due to the electrochemical gradient established by primary active transport that does not directly require ATP. An important membrane adaption for active transport is the presence of specific carrier proteins or pumps to facilitate movement. There are three types of these proteins or transporters: uniporters, symporters, and antiporters. A uniporter carries one specific ion or molecule.
A symporter carries two different ions or molecules, both in the same direction. An antiporter also carries two different ions or molecules, but in different directions. All of these transporters can also transport small, uncharged organic molecules like glucose. These three types of carrier proteins are also found in facilitated diffusion, but they do not require ATP to work in that process.
Both of these are antiporter carrier proteins. Uniporters, Symporters, and Antiporters : A uniporter carries one molecule or ion. A symporter carries two different molecules or ions, both in the same direction. An antiporter also carries two different molecules or ions, but in different directions. In secondary active transport, a molecule is moved down its electrochemical gradient as another is moved up its concentration gradient.
Unlike in primary active transport, in secondary active transport, ATP is not directly coupled to the molecule of interest. Instead, another molecule is moved up its concentration gradient, which generates an electrochemical gradient. The molecule of interest is then transported down the electrochemical gradient.
While this process still consumes ATP to generate that gradient, the energy is not directly used to move the molecule across the membrane, hence it is known as secondary active transport. Both antiporters and symporters are used in secondary active transport. Co-transporters can be classified as symporters and antiporters depending on whether the substances move in the same or opposite directions across the cell membrane.
Secondary active transport brings sodium ions, and possibly other compounds, into the cell. As sodium ion concentrations build outside the plasma membrane because of the action of the primary active transport process, an electrochemical gradient is created.
If a channel protein exists and is open, the sodium ions will be pulled through the membrane. This movement is used to transport other substances that can attach themselves to the transport protein through the membrane.
Many amino acids, as well as glucose, enter a cell this way. This secondary process is also used to store high-energy hydrogen ions in the mitochondria of plant and animal cells for the production of ATP. The potential energy that accumulates in the stored hydrogen ions is translated into kinetic energy as the ions surge through the channel protein ATP synthase, and that energy is used to convert ADP into ATP.
Secondary Active Transport : An electrochemical gradient, created by primary active transport, can move other substances against their concentration gradients, a process called co-transport or secondary active transport. Endocytosis takes up particles into the cell by invaginating the cell membrane, resulting in the release of the material inside of the cell. 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 intracellular vesicle formed from the plasma membrane. Phagocytosis : In phagocytosis, the cell membrane surrounds the particle and engulfs it. For example, when microorganisms invade the human body, a type of white blood cell called a neutrophil will remove the invaders through this process, surrounding and engulfing the microorganism, which is then destroyed by the neutrophil.
In preparation for phagocytosis, a portion of the inward-facing surface of the plasma membrane becomes coated with a protein called clathrin, which stabilizes this section of the membrane. The coated portion of the membrane then extends from the body of the cell and surrounds the particle, eventually enclosing it.
Once the vesicle containing the particle is enclosed within the cell, the clathrin disengages from the membrane and the vesicle merges with a lysosome for the breakdown of the material in the newly-formed compartment endosome. What is one of the questions no one likes to be asked? Sometimes the cell needs help moving things as well, or facilitating the diffusion process.
And this would be the job of a special type of protein. What happens if a substance needs assistance to move across or through the plasma membrane? Facilitated diffusion is the diffusion of solutes through transport proteins in the plasma membrane.
Facilitated diffusion is a type of passive transport. Even though facilitated diffusion involves transport proteins, it is still passive transport because the solute is moving down the concentration gradient. Small nonpolar molecules can easily diffuse across the cell membrane. However, due to the hydrophobic nature of the lipids that make up cell membranes, polar molecules such as water and ions cannot do so.
Instead, they diffuse across the membrane through transport proteins. A transport protein completely spans the membrane, and allows certain molecules or ions to diffuse across the membrane. Channel proteins, gated channel proteins, and carrier proteins are three types of transport proteins that are involved in facilitated diffusion. A channel protein , a type of transport protein, acts like a pore in the membrane that lets water molecules or small ions through quickly.
Water channel proteins aquaporins allow water to diffuse across the membrane at a very fast rate. Ion channel proteins allow ions to diffuse across the membrane. A gated channel protein is a transport protein that opens a "gate," allowing a molecule to pass through the membrane. Gated channels have a binding site that is specific for a given molecule or ion. A stimulus causes the "gate" to open or shut.
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