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.
The absence of ions in the secreted mucus results in the lack of a normal water concentration gradient. Thus, there is no osmotic pressure pulling water into the mucus. The inner membrane is loaded with the proteins that make up the electron transport chain and help generate energy for the cell. The double membrane enclosures of mitochondria and chloroplasts are similar to certain modern-day prokaryotes and are thought to reflect these organelles' evolutionary origins.
This page appears in the following eBook. Aa Aa Aa. Cell Membranes. Figure 1: The lipid bilayer and the structure and composition of a glycerophospholipid molecule. A The plasma membrane of a cell is a bilayer of glycerophospholipid molecules. Figure 2: The glycerophospholipid bilayer with embedded transmembrane proteins. What Do Membranes Do? Figure 3: Selective transport. Specialized proteins in the cell membrane regulate the concentration of specific molecules inside the cell.
Figure 4: Examples of the action of transmembrane proteins. Transporters carry a molecule such as glucose from one side of the plasma membrane to the other. How Diverse Are Cell Membranes? Membranes are made of lipids and proteins, and they serve a variety of barrier functions for cells and intracellular organelles. Membranes keep the outside "out" and the inside "in," allowing only certain molecules to cross and relaying messages via a chain of molecular events. Cell Biology for Seminars, Unit 3.
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Change LearnCast Settings. Scitable Chat. Register Sign In. Water molecules tend to diffuse into a hypertonic solution because the higher osmotic pressure pulls water Figure 3. If a cell is placed in a hypertonic solution, the cells will shrivel or crenate 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. Cells in a hypotonic solution will take on too much water and swell, with the risk of eventually bursting, a process called lysis.
When cells and their extracellular environments are isotonic , the concentration of water molecules is the same outside and inside the cells, so water flows both in and out and the cells maintain their normal shape and function. Various organ systems, particularly the kidneys, work to maintain this homeostasis.
A common example of facilitated diffusion is the movement of glucose into the cell, where it is used to make ATP. Although glucose can be more concentrated outside of a cell, it cannot cross the lipid bilayer via simple diffusion because it is both large and polar.
To resolve this, a specialized carrier protein called the glucose transporter will transfer glucose molecules into the cell to facilitate its inward diffusion. There are many other solutes that must undergo facilitated diffusion to move into a cell, such as amino acids, or to move out of a cell, such as wastes. Because facilitated diffusion is a passive process, it does not require energy expenditure by the cell.
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 active transport, ATP is required to move a substance across a membrane, often with the help of protein carriers, and usually 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, usually against their concentration gradients from an area of low concentration to an area of high concentration.
These pumps are particularly abundant in nerve cells, which are constantly pumping out sodium ions and pulling in potassium ions to maintain an electrical gradient across their cell membranes. An electrical gradient is a difference in electrical charge across a space. In the case of nerve cells, for example, the electrical gradient exists between the inside and outside of the cell, with the inside being negatively-charged at around mV relative to the outside.
This process is so important for nerve cells that it accounts for the majority of their ATP usage. 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.
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