Other mechanisms transport much larger molecules. 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.
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. Both are pumps. In primary active transport, the energy is often - though not exclusively - derived directly from the hydrolysis of ATP. Often, primary active transport, such as that shown below, which functions to transport sodium and potassium ions allows secondary active transport to occur discussed in the section below.
The second transport method is still considered active because it depends on the use of energy from the primary 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.
Visit the site to see a simulation of active transport in a sodium-potassium ATPase. Secondary active transport brings sodium ions, and possibly other compounds, into the cell. As sodium ion concentrations build outside of 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. Osmosis is the movement of water through a semipermeable membrane according to the concentration gradient of water across the membrane, which is inversely proportional to the concentration of solutes. While diffusion transports material across membranes and within cells, osmosis transports only water across a membrane and the membrane limits the diffusion of solutes in the water.
Not surprisingly, the aquaporins that facilitate water movement play a large role in osmosis, most prominently in red blood cells and the membranes of kidney tubules. Osmosis is a special case of diffusion. Water, like other substances, moves from an area of high concentration to one of low concentration. An obvious question is what makes water move at all? Imagine a beaker with a semipermeable membrane separating the two sides or halves.
On both sides of the membrane the water level is the same, but there are different concentrations of a dissolved substance, or solute , that cannot cross the membrane otherwise the concentrations on each side would be balanced by the solute crossing the membrane. If the volume of the solution on both sides of the membrane is the same, but the concentrations of solute are different, then there are different amounts of water, the solvent, on either side of the membrane.
To illustrate this, imagine two full glasses of water. One has a single teaspoon of sugar in it, whereas the second one contains one-quarter cup of sugar. If the total volume of the solutions in both cups is the same, which cup contains more water? Because the large amount of sugar in the second cup takes up much more space than the teaspoon of sugar in the first cup, the first cup has more water in it.
Returning to the beaker example, recall that it has a mixture of solutes on either side of the membrane. A principle of diffusion is that the molecules move around and will spread evenly throughout the medium if they can.
However, only the material capable of getting through the membrane will diffuse through it. In this example, the solute cannot diffuse through the membrane, but the water can. Water has a concentration gradient in this system. Thus, water will diffuse down its concentration gradient, crossing the membrane to the side where it is less concentrated. This diffusion of water through the membrane—osmosis—will continue until the concentration gradient of water goes to zero or until the hydrostatic pressure of the water balances the osmotic pressure.
Osmosis proceeds constantly in living systems. Tonicity describes how an extracellular solution can change the volume of a cell by affecting osmosis. A solution's tonicity often directly correlates with the osmolarity of the solution.
Osmolarity describes the total solute concentration of the solution. A solution with low osmolarity has a greater number of water molecules relative to the number of solute particles; a solution with high osmolarity has fewer water molecules with respect to solute particles.
In a situation in which solutions of two different osmolarities are separated by a membrane permeable to water, though not to the solute, water will move from the side of the membrane with lower osmolarity and more water to the side with higher osmolarity and less water.
This effect makes sense if you remember that the solute cannot move across the membrane, and thus the only component in the system that can move—the water—moves along its own concentration gradient. An important distinction that concerns living systems is that osmolarity measures the number of particles which may be molecules in a solution.
Therefore, a solution that is cloudy with cells may have a lower osmolarity than a solution that is clear if the second solution contains more dissolved molecules than there are cells. Three terms—hypotonic, isotonic, and hypertonic—are used to relate the osmolarity of a cell to the osmolarity of the extracellular fluid that contains the cells.
In a hypotonic situation, the extracellular fluid has lower osmolarity than the fluid inside the cell, and water enters the cell in living systems, the point of reference is always the cytoplasm, so the prefix hypo - means that the extracellular fluid has a lower concentration of solutes, or a lower osmolarity, than the cell cytoplasm. It also means that the extracellular fluid has a higher concentration of water in the solution than does the cell.
In this situation, water will follow its concentration gradient and enter the cell. The asymmetry lies here within the membrane and is artificially produced by gluing together an ordinary collodion membrane with one previously impregnated with a basic dyestuff or an alkaloid. National Center for Biotechnology Information , U. Journal List J Gen Physiol v. J Gen Physiol. Author information Article notes Copyright and License information Disclaimer.
This creates a pressure termed osmotic pressure. Cells cannot actively move water, it must follow osmotic gradients. Solutions that have a greater solute concentration will pull water via osmotic pressure. This depends on the total number of solutes, not the type.
Note that some water can pass through the cell membrane but most water passes through protein membrane channels termed aquaporins. Cells may find themselves in three different sorts of solutions. The terms isotonic, hypertonic, and hypotonic refer to the concentration of solutes outside the cell relative to the solute concentration inside the cell.
In an isotonic solution, solutes and water are equally concentrated within and outside the cell. The cell is bathed in a solution with a solute concentration that is similar to its own cytoplasm. Many medical preparations saline solutions for nasal sprays, eye drops, and intravenous drugs are designed to be isotonic to our cells. Distilled pure water is the ultimate hypotonic solution. If a cell is placed in a hypotonic solution, it will tend to gain water. A hypertonic solution has a high solute concentration lower water concentration compared to the cell cytoplasm.
Very salty or sugary solutions brines or syrups are hypertonic to living cells. If a cell is placed in such a solution, water tends to flow spontaneously out of the cell. Filtration is another passive process of moving material through a cell membrane. While diffusion and osmosis rely on concentration gradients, filtration uses a pressure gradient.
Molecules will move from an area of higher pressure to an area of lower pressure. Filtration is non-specific. If molecules are small enough to pass through the membrane, they will. The force that pushes the molecules is termed hydrostatic pressure. One example of filtration is making coffee. Think of the coffee filter as the cell membrane and the coffee grounds, flavor and caffeine as the molecules.
The pressure is exerted by the water from the machine. It forces materials through the coffee filter into the coffee pot. Small molecules like caffeine, water, and flavor pass through the filter but the coffee grounds do not. They are too big. If you poked holes in the filter, the coffee grounds would end up in your coffee! The coffee filter represents the filtration membrane which is typically a layer of cells.
Filtration is one of the main methods used for capillary exchange. Blood pressure provides the driving force or hydrostatic pressure to force materials out of capillaries to cells or to form the filtrate fluid in the nephron of the kidney. Hydrostatic pressure is countered by osmotic pressure. Remember osmotic pressure is created due to increased solute concentration and will pull water toward the area of higher solutes. These two pressures must be in balance for homeostasis of fluid volumes.
In our body large molecules such as plasma proteins and red blood cells should not pass out of the blood through the cell membranes lining the capillaries. If they pass through and end up in in the tissues or in the kidney and later the urine it is abnormal and a sign of disease. In active methods the cell must expend energy ATP to do the work of moving molecules. Active transport often occurs when the molecule is being moved against its concentration gradient or when moving very large molecules into our out of the cell.
There are 3 main types of active processes. Read More. Generation of resting membrane potential. Skip to content Main Navigation Search. Dictionary Articles Tutorials Biology Forum. Cell Structure A typical eukaryotic cell is comprised of cytoplasm with different organelles, such as nucleus, endoplasmic reticulum, Golgi apparatus, mitochondria, and so on.
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