It is the random motion of the molecules that causes them to move from an area of high concentration to an area with a lower concentration. Diffusion will continue until the concentration gradient has been eliminated. Since diffusion moves materials from an area of higher concentration to the lower, it is described as moving solutes "down the concentration gradient".
The end result is an equal concentration, or equilibrium , of molecules on both sides of the membrane. At equilibrium, movement of molecules does not stop. At equilibrium, there is equal movement of materials in both directions. Not everything can make it into your cells. Your cells have a plasma membrane that helps to guard your cells from unwanted intruders. If the outside environment of a cell is water-based, and the inside of the cell is also mostly water, something has to make sure the cell stays intact in this environment.
What would happen if a cell dissolved in water, like sugar does? Obviously, the cell could not survive in such an environment. So something must protect the cell and allow it to survive in its water-based environment. All cells have a barrier around them that separates them from the environment and from other cells. This barrier is called the plasma membrane , or cell membrane. The plasma membrane see figure below is made of a double layer of special lipids, known as phospholipids.
The phospholipid is a lipid molecule with a hydrophilic "water-loving" head and two hydrophobic "water-hating" tails. Because of the hydrophilic and hydrophobic nature of the phospholipid, the molecule must be arranged in a specific pattern as only certain parts of the molecule can physically be in contact with water.
Remember that there is water outside the cell, and the cytoplasm inside the cell is mostly water as well. So the phospholipids are arranged in a double layer a bilayer to keep the cell separate from its environment.
Lipids do not mix with water recall that oil is a lipid , so the phospholipid bilayer of the cell membrane acts as a barrier, keeping water out of the cell, and keeping the cytoplasm inside the cell.
The cell membrane allows the cell to stay structurally intact in its water-based environment. The function of the plasma membrane is to control what goes in and out of the cell.
Some molecules can go through the cell membrane to enter and leave the cell, but some cannot. The cell is therefore not completely permeable. An open door is completely permeable to anything that wants to enter or exit through the door. The plasma membrane is semipermeable , meaning that some things can enter the cell, and some things cannot. Molecules that cannot easily pass through the bilayer include ions and small hydrophilic molecules, such as glucose, and macromolecules, including proteins and RNA.
Examples of molecules that can easily diffuse across the plasma membrane include carbon dioxide and oxygen gas. These molecules diffuse freely in and out of the cell, along their concentration gradient.
Though water is a polar molecule, it can also diffuse through the plasma membrane. The inside of all cells also contain a jelly-like substance called cytosol. An example of this occurs in the kidney, where both forms of channels are found in different parts of the renal tubules. Cells involved in the transmission of electrical impulses, such as nerve and muscle cells, have gated channels for sodium, potassium, and calcium in their membranes.
Opening and closing of these channels changes the relative concentrations on opposing sides of the membrane of these ions, resulting in the facilitation of electrical transmission along membranes in the case of nerve cells or in muscle contraction in the case of muscle cells. Another type of protein embedded in the plasma membrane is a carrier protein.
This aptly named protein binds a substance and, in doing so, triggers a change of its own shape, moving the bound molecule from the outside of the cell to its interior Figure 5. Carrier proteins are typically specific for a single substance. This selectivity adds to the overall selectivity of the plasma membrane. The exact mechanism for the change of shape is poorly understood.
Proteins can change shape when their hydrogen bonds are affected, but this may not fully explain this mechanism. Each carrier protein is specific to one substance, and there are a finite number of these proteins in any membrane. This can cause problems in transporting enough of the material for the cell to function properly. When all of the proteins are bound to their ligands, they are saturated and the rate of transport is at its maximum.
Increasing the concentration gradient at this point will not result in an increased rate of transport. An example of this process occurs in the kidney.
Glucose, water, salts, ions, and amino acids needed by the body are filtered in one part of the kidney. This filtrate, which includes glucose, is then reabsorbed in another part of the kidney. Because there are only a finite number of carrier proteins for glucose, if more glucose is present than the proteins can handle, the excess is not transported and it is excreted from the body in the urine.
Channel and carrier proteins transport material at different rates. Channel proteins transport much more quickly than do carrier proteins. Channel proteins facilitate diffusion at a rate of tens of millions of molecules per second, whereas carrier proteins work at a rate of a thousand to a million molecules per second. 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 of free water molecules to one of low free water molecule concentration. An obvious question is what makes water move at all? Imagine a beaker with a semipermeable membrane separating the two sides or halves Figure 5. 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. The beaker example here occurs in an open system where the volume of fluid can increase and decrease freely. Cells, on the other hand, are composed of proteins and other substances embedded in the aqueous cytoplasm. These substances could be considered solutes for the purposes of predicting osmosis.
The cell membrane keeps most of the proteins and other substances within the cell, causing the cell to have a higher osmolarity than pure water. Suppose you perform an experiment where you placed red blood cells in an environment of pure water. What do you suppose would happen to the cells? Because the concentration of solute is higher in the red blood cell than it is in the beaker, water would rush into the red blood cell. What do you think would happen to the red blood cell, given that its cell membrane is made up of a fixed surface area?
It is likely that the red blood cell will undergo hemolysis, where they swell up with water and burst. It should be noted, however, that most cells have mechanisms to prevent them from taking on too much water. However, red blood cells lack these controls, making them ideal for osmolarity studies.
This is an important consideration for clinicians delivering drugs intravenously. How would the drug have to be formulated, in terms of osmolarity, to prevent red blood cells from undergoing hemolysis? In order to prevent hemolysis of red blood cells in the blood, drugs are typically formulated in an isotonic solution with the blood to maintain osmolarity.
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. Because the cell has a relatively higher concentration of water, water will leave the cell. 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 Figure 5. For a video illustrating the process of diffusion in solutions, visit this site. In a hypotonic environment, water enters a cell, and the cell swells. In an isotonic condition, the relative concentrations of solute and solvent are equal on both sides of the membrane.
There is no net water movement; therefore, there is no change in the size of the cell. In a hypertonic solution, water leaves a cell and the cell shrinks. Remember, the membrane resembles a mosaic, with discrete spaces between the molecules composing it. If the cell swells, and the spaces between the lipids and proteins become too large, the cell will break apart. In contrast, when excessive amounts of water leave a red blood cell, the cell shrinks, or crenates.
This has the effect of concentrating the solutes left in the cell, making the cytosol denser and interfering with diffusion within the cell. Various living things have ways of controlling the effects of osmosis—a mechanism called osmoregulation.
Some organisms, such as plants, fungi, bacteria, and some protists, have cell walls that surround the plasma membrane and prevent cell lysis in a hypotonic solution.
The plasma membrane can only expand to the limit of the cell wall, so the cell will not lyse. In fact, the cytoplasm in plants is always slightly hypertonic to the cellular environment, and water will always enter a cell if water is available. This inflow of water produces turgor pressure, which stiffens the cell walls of the plant Figure 5. Moreover, we demonstrated the use of these membranes for constructing compartmentalized microfluidic cell culture systems to induce physiological tissue differentiation or to replicate interfaces between different tissue types.
Our approach provides a robust platform to produce and engineer biologically active cell culture substrates that serve as promising alternatives to conventional synthetic membrane inserts.
This strategy may contribute to the development of physiologically relevant in vitro cell culture models for a wide range of applications. Mondrinos, Y. Yi, N. Wu, X. Ding and D. To request permission to reproduce material from this article, please go to the Copyright Clearance Center request page. If you are an author contributing to an RSC publication, you do not need to request permission provided correct acknowledgement is given.
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