A single substance tends to move from an area of high concentration to an area of low concentration until the concentration is equal across a space. You are familiar with diffusion of substances through the air. For example, think about someone opening a bottle of ammonia in a room filled with people.
The ammonia gas is at its highest concentration in the bottle; its lowest concentration is at the edges of the room. The ammonia vapor will diffuse, or spread away, from the bottle; gradually, more and more people will smell the ammonia as it spreads. Diffusion expends no energy. On the contrary, concentration gradients are a form of potential energy, dissipated as the gradient is eliminated. Diffusion : 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.
Each separate substance in a medium, such as the extracellular fluid, has its own concentration gradient independent of the concentration gradients of other materials. In addition, each substance will diffuse according to that gradient. Within a system, there will be different rates of diffusion of the different substances in the medium. Molecules move constantly in a random manner at a rate that depends on their mass, their environment, and the amount of thermal energy they possess, which in turn is a function of temperature.
This movement accounts for the diffusion of molecules through whatever medium in which they are localized. A substance will tend to move into any space available to it until it is evenly distributed throughout it. After a substance has diffused completely through a space removing its concentration gradient, molecules will still move around in the space, but there will be no net movement of the number of molecules from one area to another.
This lack of a concentration gradient in which there is no net movement of a substance is known as dynamic equilibrium. While diffusion will go forward in the presence of a concentration gradient of a substance, several factors affect the rate of diffusion:. A variation of diffusion is the process of filtration.
In filtration, material moves according to its concentration gradient through a membrane; sometimes the rate of diffusion is enhanced by pressure, causing the substances to filter more rapidly. This occurs in the kidney where blood pressure forces large amounts of water and accompanying dissolved substances, or solutes, out of the blood and into the renal tubules.
The rate of diffusion in this instance is almost totally dependent on pressure. Osmosis is the movement of water across a membrane from an area of low solute concentration to an area of high solute concentration.
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. Semipermeable membranes, also termed selectively permeable membranes or partially permeable membranes, allow certain molecules or ions to pass through by diffusion. While diffusion transports materials across membranes and within cells, osmosis transports only water across a membrane.
The semipermeable membrane limits the diffusion of solutes in the water. Not surprisingly, the aquaporin proteins 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.
If there is more solute in one area, then there is less water; if there is less solute in one area, then there must be more water.
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.
Osmosis : In osmosis, water always moves from an area of higher water concentration to one of lower concentration. In the diagram shown, the solute cannot pass through the selectively permeable membrane, but the water can. 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 passing 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.
In the beaker example, this means that the level of fluid in the side with a higher solute concentration will go up. Tonicity, which is directly related to the osmolarity of a solution, affects osmosis by determining the direction of water flow.
Tonicity is the reason why salt water fish cannot live in fresh water and vice versa. If you place a salt water fish in fresh water, which has a low osmolarity, water in the environment will flow into the cells of the fish, eventually causing them to burst and killing the fish.
Tonicity describes how an extracellular solution can change the volume of a cell by affecting osmosis. 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, causing the cell to expand. Changes in Cell Shape Due to Dissolved Solutes : Osmotic pressure changes the shape of red blood cells in hypertonic, isotonic, and hypotonic solutions.
Because the cell has a relatively higher concentration of water, water will leave the cell, and the cell will shrink. In an isotonic solution, the extracellular fluid has the same osmolarity as the cell. If the osmolarity of the cell matches that of the extracellular fluid, there will be no net movement of water into or out of the cell, although water will still move in and out. Blood cells and plant cells in hypertonic, isotonic, and hypotonic solutions take on characteristic appearances.
Cells in an isotonic solution retain their shape. Cells in a hypotonic solution swell as water enters the cell, and may burst if the concentration gradient is large enough between the inside and outside of the cell. Cells in a hypertonic solution shrink as water exits the cell, becoming shriveled. Facilitated diffusion is a process by which molecules are transported across the plasma membrane with the help of membrane proteins.
Channel-mediated facilitated diffusion functions much like a bridge over a river that must raise and lower in order to allow boats to pass. In that sense, it's indirectly using our ATP molecule, or indirectly using our energy that generated this sodium gradient. That's what happens when we have our sodium and our glucose absorbed into our intestine. After it's absorbed into our enterocytes, they actually need to go to our bloodstream, so I'll draw a little blood vessel right here.
That's where our nutrients need to go to to flow elsewhere in our body. In order to get out of our enterocyte and end up in our bloodstream, we actually use the same type of channel to help our glucose molecule that just entered our enterocyte to leave the cell and travel to this blood vessel or this capillary. What's unusual is that, again, sodium is in play here, but this time, it's actually going in the opposite direction, but this completely makes sense.
Remember, there's a higher concentration of sodium outside of our cells, and so it naturally wants to flow down its ion gradient into the cell. Still, this is an example of secondary active transport, but instead of symport, now we have what's called antiport, antiport because our two molecules are moving in the opposite direction of each other. We'll talk again about secondary active transport in a part of the kidney call the nephron. For now, let's talk about bigger molecules trying to cross the membrane.
As you see, we've been getting progressively larger from one ion to two ions to now we're talking about a bigger molecule like glucose. What about if we have a giant protein that wants to enter the cell? What I've drawn here is not the protein. Instead, I'll draw the protein inside of here. I'll just write a. There's a couple of amino acids in here that want to enter our cell.
This guy is a vesicle. A vesicle is just a small pocket of cell membrane that's surrounding some type of cargo or some type of thing we're trying to transport. In this case, it's this protein. Remember, it's the exact same membrane we have around our cell, and once this vesicle gets close enough to our membrane right here, it'll actually fuse, and in doing so, will then create an opening from the inside of the vesicle, connecting it to the inside of our cell, which means that our amino acid can actually enter inside.
Another method of movement across the membrane is osmosis. Osmosis is the movement of water from a region of higher concentration to one of lower concentration. Osmosis occurs across a membrane that is semipermeable. A semipermeable membrane lets only certain molecules pass through while keeping other molecules out. Osmosis is really a type of diffusion involving only water molecules. A third mechanism for movement across the plasma membrane is facilitated diffusion.
Certain proteins in the membrane assist facilitated diffusion by permitting only certain molecules to pass across the membrane. The proteins encourage movement in the direction that diffusion would normally take place, from a region with a higher concentration of molecules to a region of lower concentration. A fourth method for movement across the membrane is active transport.
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.
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