A special form of passive transfer is the activity of water across a membrane, or osmosis. By definition, osmosis is the diffusion of water with a selectively permeable membrane indigenous an area of high water potential (low solute concentration) to and also area of low water potential (high solute concentration). Therefore, because that osmosis to occur, the membrane need to be permeable to water, but impermeable come the solute, and also the concentration the the solute need to be various on the 2 sides of the membrane. Water will move from the next with reduced solute concentration come the next with higher solute concentration till the concentrations are equal, or till some exterior force prevents further movement of water. This is a passive process, in the no power expenditure is forced for the motion of water. In an artificial system such as the one illustrated in the figure below, water will certainly attempt to relocate from chamber B to chamber A. Because chamber A is a strictly chamber, press will develop. The push that is just adequate to prevent water from moving throughout the membrane is referred to as osmotic pressure.

You are watching: Which of the following is not required for osmosis to occur?


Image by BYU-I student, Hannah Crowder, 2013.

Movement of water from room B to chamber A will generate a pressure well-known as osmotic pressure.

In the body, water will certainly move into or out of cells, depending upon the solute concentration (osmolarity) of the extracellular fluids vs. The intracellular fluids. If the solute concentration in the extracellular fluid is reduced than the solute concentration in the cell, water moves into the cell and also the cell will certainly swell. Before we can explain why cells shrink or increase when put in a details kind that solution, we first need to talk about the difference in between osmolarity and also tonicity.

Earlier this semester we debated how the concentration of corpuscle in a equipment is expressed together osmolarity. Recall the osmlarity represents the variety of moles that particles per liter of solution, when molarity to represent the number of moles the molecules per liter that solution. Why execute we have actually these different ways of express concentration? We have to readjust because different substances behave in different way in solution. Because that example, as soon as NaCl is dissolved in water, it division apart right into Na+ and Cl- ion (this is a characteristics of substances hosted together by ionic bonds). Thus, there are now twice as numerous particles than there were once the substance was dry. In turn a 1 molar equipment of NaCl would be a 2 osmolar solution. Glucose is different. Glucose doesn"t rest apart in water since the atoms are covalently bonded. Therefore, a 1 molar solution of glucose will likewise be a 1 osmolar solution. The concentration of solutes in human body fluids is 285-295 mOsmoles/liter (for simplicity we regularly round this number to 300). We place the little m, which means milli or one-housandth, in former of osmole because we are taking care of very small amounts, 1000 times less than an osmol. Osmolarity is a useful term because now we can use native to define solutions like isosmotic, which way two solutions have the same variety of particles, or hyperosmotic which method one solution is more concentrated than the other, or hyposmotic which way that one solution is less focused than the other. (Note: Osmolarity takes right into account every one of the particles in the solution. Therefore, if you have actually a liter of systems containing one mole the glucose and also one mole of NaCl friend would have a three osmolar solution.)

Perhaps the most important principle when talking around solutions and also how they affect the human body is tonicity. Tonicity (G. tonus, ton = firmness or stretch that a tissue) is a term supplied to describe how a systems affects a cell when it is inserted into the solution. One crucial characteristic is that now we start to deal with membranes and also particles. Why space cells influenced by different solutions? The prize lies in the habits of particles through regard to diffusion. Particles will have tendency to diffuse from locations of high concentration to locations of reduced concentration to reach equilibrium. However, if the membrane is no permeable to the particles, then rather of particles moving or diffusing, water will relocate or diffuse with the cell membrane to with equilibrium. Additionally, in ~ equilibrium, the osmolarities of the two services will it is in the same. We contact the movement or diffusion of water osmosis, as was stated above. Once water moves out of a cell, the cell shrinks; likewise, when water moves right into a cell, the cabinet swells. Thus, if we location a cell into an isotonic solution, the cell shape won"t change because the solutions are already in equilibrium, therefore there will be no net movement of water or solutes throughout the membrane. In various other words, isotonic solutions have actually the same concentration that osmotically active particles (osmotically particles are non-permeable particles) together are discovered in the cell. If the cabinet swells, we say the the solution was hypotonic, and if the cabinet shrinks (crenates) we say the equipment was hypertonic.

It seems straightforward enough. Water moves once the particles room impermeable come the membrane; however, as soon as particles space permeable, the particles will diffuse v the cabinet membranes, rather of water. Remember the intracellular and extracellular fluids in the body always move come equilibrium, one of two people by movement of solutes, if they can cross the membrane, or by the activity of water, if the solutes can not cross. Let"s look at an additional example. 5 percent dextrose (dextrose is an additional name for glucose) is isosmotic through regard to human body fluids due to the fact that it has actually the same variety of particles together blood; however, glucose is permeable to cabinet membranes. Thus, a 5% dextrose solution (D5W) might be isosmotic to the cells, yet it behaves as a hypotonic solution—the solute moves into the cell accompanied through water, bring about the cells to swell. Below is another way to think the osmolarity and also tonicity; osmolarity deserve to be supplied to compare the concentration that solutes in 2 solutions. The can additionally be offered to to compare the concentration the the solutes in a systems with those in the cell, before equilibrium is achieved. Tonicity is used to define what impact the solution has actually on the cell. Osmolarity doesn"t take into account the nature of the solutes when tonicity is dependent ~ above the concentration the the nonpermeable solutes.

The figure below shows what happens to red blood cells once they are put into hypertonic, isotonic or hypotonic solutions.


Title: File:Osmotic press on blood cells diagram.svg; Author: LadyofHats; Site: https://commons.wikimedia.org/wiki/File:Osmotic_pressure_on_blood_cells_diagram.svg; License: public Domain

When put in a hypertonic solution, red blood cells will certainly shrink or crenate. When put in an isotonic solution, there will certainly be no adjust in volume, and also when inserted in a hypotonic solution, they will swell. If the concentration of the equipment is an excellent enough, they will certainly burst (lyse).

The link below shows what wake up to a wilted plant once it is placed into a hypotonic solution.


To check our understanding, complete the table listed below by filling in the absent column items through regard to osmolarity and tonicity. Usage the terms iso, hypo, and also hyper to finish the table.




0.9 % Saline

5% dextrose

5% dextrose + 0.9% Saline

0.45% Saline

5% dextrose + 0.45% Saline

You have been given the answers for the table above. Be certain you know why the answers space what they are.

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**Note: various other texts, even hospitals on occasion, often tend to use less rigorous definitions of tonicity. In various other words, meanings are loosely given to define all hyperosmolar services as hypertonic. This is based on the observation that water can cross the membrane faster than the permeable solute can cross. That may additionally be based on the incorrect assumption that tonicity and also osmolarity space the exact same thing. Thus, the initial effect of one abrupt readjust in extracellular osmolality might be temporarily various from the predicted tonicity change.

In other words, a 5% dextrose equipment in saline would certainly be taken into consideration hypertonic due to the fact that there are more than 300 mosm of solutes in this solution. The difficulty with this definition is that it go not differentiate tonicity from osmolarity, as it provides no referral of whether the solutes space permeant or non-permeant, through respect to a particular membrane. We think that loose definitions produce confusion and therefore have characterized tonicity through the more rigorous an interpretation of "effective osmolarity."

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