Some peripheral proteins on the surface of intestinal cells, for example, act as digestive enzymes to break down nutrients to sizes that can pass through the cells and into the bloodstream.
All plasma membranes share the characteristic of being selective permeable and containing various proteins. Some membranes, however, have components that are specialized for a specific purpose.
Microvilli are finger-like projections on the surface of some cells. These projections increase surface are for absorption. Cells that line the small intestine contain microvilli. Junctions are another specialized group of proteins that connect to other cells. There are three main types of junctions: tight junctions, desmosomes, and gap junctions. Tight junctions are proteins that hold adjacent cells together very tightly so nothing can penetrate between them.
Cells that line the digestive and urinary tract contains many tight junctions to ensure the contents within those hollow organs do not leak out into the outer layers or body cavity. Desmosomes are sometimes called anchoring junctions. These junctions hold cells together by fibers, which allows movement without separation. Cells that contain desmosomes are found within the muscle tissue and the skin. Gap junctions are open areas within the plasma membrane found between two adjacent cells.
The proteins connect two cells while allowing chemicals to pass between the cells. Cystic fibrosis CF affects approximately 30, people in the United States, with about 1, new cases reported each year. The genetic disease is most well known for its damage to the lungs, causing breathing difficulties and chronic lung infections, but it also affects the liver, pancreas, and intestines.
Only about 50 years ago, the prognosis for children born with CF was very grim—a life expectancy rarely over 10 years. Today, with advances in medical treatment, many CF patients live into their 30s. The symptoms of CF result from a malfunctioning membrane ion channel called the cystic fibrosis transmembrane conductance regulator, or CFTR. In healthy people, the CFTR protein is an integral membrane protein that transports Cl — ions out of the cell.
In a person who has CF, the gene for the CFTR is mutated, thus, the cell manufactures a defective channel protein that typically is not incorporated into the membrane, but is instead degraded by the cell. This characteristic puzzled researchers for a long time because the Cl — ions are actually flowing down their concentration gradient when transported out of cells.
Active transport generally pumps ions against their concentration gradient, but the CFTR presents an exception to this rule. In normal lung tissue, the movement of Cl — out of the cell maintains a Cl — -rich, negatively charged environment immediately outside of the cell. This is particularly important in the epithelial lining of the respiratory system. Respiratory epithelial cells secrete mucus, which serves to trap dust, bacteria, and other debris.
Cilia on the epithelial cells move the mucus and its trapped particles up the airways away from the lungs and toward the outside. In order to be effectively moved upward, the mucus cannot be too viscous; rather it must have a thin, watery consistency.
This is how, in a normal respiratory system, the mucus is kept sufficiently watered-down to be propelled out of the respiratory system. If the CFTR channel is absent, Cl — ions are not transported out of the cell in adequate numbers, thus preventing them from drawing positive ions. While nature figured this out a long time ago, we now make fabrics and medical devices that copy this process.
Gore Industries, one of the big employers in Flagstaff, makes a fabric called "Gore-Tex" which repels large water droplets but allows smaller air molecules to pass through, making the fabric "breathable. The catch: While diffusion works well for the tiny single cell, it does not, by itself, get the job done in a multi-cellular organism where the tissues are buried deep inside the body. Imagine your bicep muscle while you are lifting weights. The tissue, comprised of millions of cells, will quickly run out of oxygen and build up carbon dioxide.
Diffusion through the skin could not keep up. This is where the circulatory system helps out. The smallest blood vessels, the capillaries, run though these tissues. The blood from the lungs releases oxygen to the cells because O 2 is at lower concentration in the tissues , and picks up carbon dioxide because CO 2 is at higher concentration in the tissues and carries it back to the lungs to be exhaled.
This does require energy. It also explains why your breathing rate increases when you exert yourself, and is one of the costs of being multi-cellular. Active Transport : Sometimes diffusion doesn't happen fast enough for the cell's needs, and there are times when nutrients need to be stockpiled or excreted at a higher concentration than would occur naturally by diffusion.
Two solutions that have the same concentration of solutes are said to be isotonic equal tension. When cells and their extracellular environments are isotonic, the concentration of water molecules is the same outside and inside the cells, and the cells maintain their normal shape and function. Osmosis occurs when there is an imbalance of solutes outside of a cell versus inside the cell.
A solution that has a higher concentration of solutes than another solution is said to be hypertonic , and water molecules tend to diffuse into a hypertonic solution Figure 3. Cells in a hypertonic solution will shrivel as water leaves the cell via osmosis. In contrast, a solution that has a lower concentration of solutes than another solution is said to be hypotonic , and water molecules tend to diffuse out of a hypotonic solution.
Cells in a hypotonic solution will take on too much water and swell, with the risk of eventually bursting. Various organ systems, particularly the kidneys, work to maintain this homeostasis. For all of the transport methods described above, the cell expends no energy.
Membrane proteins that aid in the passive transport of substances do so without the use of ATP. During primary active transport, ATP is required to move a substance across a membrane, with the help of membrane protein, and against its concentration gradient. One of the most common types of active transport involves proteins that serve as pumps. Similarly, energy from ATP is required for these membrane proteins to transport substances—molecules or ions—across the membrane, against their concentration gradients from an area of low concentration to an area of high concentration.
The activity of these pumps in nerve cells is so great that it accounts for the majority of their ATP usage. Active transport pumps can also work together with other active or passive transport systems to move substances across the membrane. For example, the sodium-potassium pump maintains a high concentration of sodium ions outside of the cell. Therefore, if the cell needs sodium ions, all it has to do is open a passive sodium channel, as the concentration gradient of the sodium ions will drive them to diffuse into the cell.
In this way, the action of an active transport pump the sodium-potassium pump powers the passive transport of sodium ions by creating a concentration gradient.
When active transport powers the transport of another substance in this way, it is called secondary active transport. Symporters are secondary active transporters that move two substances in the same direction.
Since cells store glucose for energy, glucose is typically at a higher concentration inside of the cell than outside; however, due to the action of the sodium-potassium pump, sodium ions will easily diffuse into the cell when the symporter is opened.
The flood of sodium ions through the symporter provides the energy that allows glucose to move through the symporter and into the cell, against its concentration gradient. Conversely, antiporters are secondary active transport systems that transport substances in opposite directions.
Other forms of active transport do not involve membrane carriers. Once pinched off, the portion of membrane and its contents becomes an independent, intracellular vesicle. A vesicle is a membranous sac—a spherical and hollow organelle bounded by a lipid bilayer membrane. Endocytosis often brings materials into the cell that must to be broken down or digested.
Many immune cells engage in phagocytosis of invading pathogens. Like little Pac-men, their job is to patrol body tissues for unwanted matter, such as invading bacterial cells, phagocytize them, and digest them. Phagocytosis and pinocytosis take in large portions of extracellular material, and they are typically not highly selective in the substances they bring in. Cells regulate the endocytosis of specific substances via receptor-mediated endocytosis.
Receptor-mediated endocytosis is endocytosis by a portion of the cell membrane which contains many receptors that are specific for a certain substance. Iron, a required component of hemoglobin, is endocytosed by red blood cells in this way.
Iron is bound to a protein called transferrin in the blood. Specific transferrin receptors on red blood cell surfaces bind the iron-transferrin molecules, and the cell endocytoses the receptor-ligand complexes. Many cells manufacture substances that must be secreted, like a factory manufacturing a product for export. These substances are typically packaged into membrane-bound vesicles within the cell.
When the vesicle membrane fuses with the cell membrane, the vesicle releases its contents into the interstitial fluid. The vesicle membrane then becomes part of the cell membrane. Specific examples of exocytosis include cells of the stomach and pancreas producing and secreting digestive enzymes through exocytosis Figure 3. The addition of new membrane to the plasma membrane is usually coupled with endocytosis so that the cell is not constantly enlarging. Through these processes, the cell membrane is constantly renewing and changing as needed by the cell.
Cystic fibrosis CF affects approximately 30, people in the United States, with about 1, new cases reported each year. The genetic disease is most well-known for its damage to the lungs, causing breathing difficulties and chronic lung infections, but it also affects the liver, pancreas, and intestines.
Only about 50 years ago, the prognosis for children born with CF was very grim—a life expectancy rarely over 10 years. Today, with advances in medical treatment, many CF patients live into their 30s.
In healthy people, the CFTR protein is an integral membrane protein that transports Cl— ions out of the cell. In a person who has CF, the gene for the CFTR is mutated, thus, the cell manufactures a defective channel protein that typically is not incorporated into the membrane, but is instead degraded by the cell. This puzzled researchers for a long time because the Cl— ions are actually flowing down their concentration gradient when transported out of cells.
Active transport generally pumps ions against their concentration gradient, but the CFTR presents an exception to this rule. In normal lung tissue, the movement of Cl— out of the cell maintains a Cl—-rich, negatively charged environment immediately outside of the cell.
This is particularly important in the epithelial lining of the respiratory system. Respiratory epithelial cells secrete mucus, which serves to trap dust, bacteria, and other debris. Cilia on the epithelial cells move the mucus and its trapped particles up the airways away from the lungs and toward the outside. In order to be effectively moved upward, the mucus cannot be too viscous, rather, it must have a thin, watery consistency. In a normal respiratory system, this is how the mucus is kept sufficiently watered-down to be propelled out of the respiratory system.
If the CFTR channel is absent, Cl— ions are not transported out of the cell in adequate numbers, thus preventing them from drawing positive ions.
0コメント