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. Cells regulate the endocytosis of specific substances via receptor-mediated endocytosis. Receptor-mediated endocytosis is endocytosis by a portion of the cell membrane that 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.
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. However, water-soluble materials—like glucose, amino acids, and electrolytes—need some assistance to cross the membrane because they are repelled by the hydrophobic tails of the phospholipid bilayer.
All substances that move through the membrane do so by one of two general methods, which are categorized based on whether or not energy is required. Passive transport is the movement of substances across the membrane without the expenditure of cellular energy. In contrast, active transport is the movement of substances across the membrane using energy from adenosine triphosphate ATP. In order to understand how substances move passively across a cell membrane, it is necessary to understand concentration gradients and diffusion.
A concentration gradient is the difference in concentration of a substance across a space. When molecules move in this way, they are said to move down their concentration gradient, from high concentration to low concentration. Diffusion is the movement of particles from an area of higher concentration to an area of lower concentration. A couple of common examples will help to illustrate this concept. Imagine being inside a closed room.
If a bottle of perfume were sprayed, the scent molecules would naturally diffuse from the spot where they left the bottle to all corners of the room, and this diffusion would go on until the molecules were equally distributed in the room.
Another example is a spoonful of sugar placed in a cup of tea. Eventually the sugar will diffuse throughout the tea until no concentration gradient remains. In both cases, if the room is warmer or the tea hotter, diffusion occurs even faster as the molecules are bumping into each other and spreading out faster than at cooler temperatures.
Visit this link to see diffusion and how it is propelled by the kinetic energy of molecules in solution. How does temperature affect diffusion rate, and why? Whenever a substance exists in greater concentration on one side of a semipermeable membrane, such as cell membranes, any substance that can move down its concentration gradient across the membrane will do so. If the substances can move across the cell membrane without the cell expending energy, the movement of molecules is called passive transport.
Consider substances that can easily diffuse through the lipid bilayer of the cell membrane, such as the gases oxygen O 2 and carbon dioxide CO 2. These small, fat soluble gasses and other small lipid soluble molecules can dissolve in the membrane and enter or exit the cell following their concentration gradient. This mechanism of molecules moving across a cell membrane from the side where they are more concentrated to the side where they are less concentrated is a form of passive transport called simple diffusion.
O 2 generally diffuses into cells because it is more concentrated outside of them, and CO 2 typically diffuses out of cells because it is more concentrated inside of them. Before moving on, it is important to realize that the concentration gradients for oxygen and carbon dioxide will always exist across a living cell and never reach equal distribution. This is because cells rapidly use up oxygen during metabolism and so, there is typically a lower concentration of O 2 inside the cell than outside.
As a result, oxygen will diffuse from outside the cell directly through the lipid bilayer of the membrane and into the cytoplasm within the cell. On the other hand, because cells produce CO 2 as a byproduct of metabolism, CO 2 concentrations rise within the cytoplasm; therefore, CO 2 will move from the cell through the lipid bilayer and into the extracellular fluid, where its concentration is lower. Figure 3. Large polar or ionic molecules, which are hydrophilic, cannot easily cross the phospholipid bilayer.
Charged atoms or molecules of any size cannot cross the cell membrane via simple diffusion as the charges are repelled by the hydrophobic tails in the interior of the phospholipid bilayer. Solutes dissolved in water on either side of the cell membrane will tend to diffuse down their concentration gradients, but because most substances cannot pass freely through the lipid bilayer of the cell membrane, their movement is restricted to protein channels and specialized transport mechanisms in the membrane.
A common example of facilitated diffusion using a carrier protein 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, and therefore, repelled by the phospholipid membrane.
To resolve this, a specialized carrier protein called the glucose transporter will transfer glucose molecules into the cell to facilitate its inward diffusion. The difference between a channel and a carrier is that the carrier usually changes shape during the diffusion process, while the channel does not.
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. A specialized example of facilitated transport is water moving across the cell membrane of all cells, through protein channels known as aquaporins.
Osmosis is the diffusion of water through a semipermeable membrane from where there is more relative water to where there is less relative water down its water concentration gradient Figure 3.
On their own, cells cannot regulate the movement of water molecules across their membrane, so it is important that cells are exposed to an environment in which the concentration of solutes outside of the cells in the extracellular fluid is equal to the concentration of solutes inside the cells in the cytoplasm. 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. Introduction: Substances, such as water, ions, and molecules needed for cellular processes, can enter and leave cells by a passive process such as diffusion.
Diffusion is random movement of molecules but has a net direction toward regions of lower concentration in order to reach an equillibrium. Simple passive diffusion occurs when small molecules pass through the lipid bilayer of a cell membrane. Facilitated diffusion depends on carrier proteins imbedded in the membrane to allow specific substances to pass through, that might not be able to diffuse through the cell membrane.
Importance: The rate of diffusion is affected by properties of the cell, the diffusing molecule, and the surrounding solution. We can use simple equations and graphs to examine how particular molecules and their concentration affect the rate of diffusion.
We can also compare simple and facilitated diffusion. Question: How do rates of simple and facilitated diffusion differ in response to a concentration gradient? Method: The rate of simple diffusion can be expressed by a modification of Fick's Law for small, nonpolar molecules. We can describe the rate of diffusion as directly proportional to the concentration gradient by the following equation:.
P is a constant relating the ease of entry of a molecule into the cell depending on the molecule's size and lipid solubility.
If we graph the rate of diffusion as a function of the concentration gradient, we get a simple linear function.
Interpretation: Notice the rate of diffusion increases as the concentration gradient increases. If the concentration of molecules outside the cell is very high relative to the internal cell concentration, the rate of diffusion will also be high.
0コメント