A special form of passive transport is the movement of water throughout a membrane, or osmosis. By definition, osmosis is the diffusion that water with a selectively permeable membrane from an area of high water potential (low solute concentration) to and also area of low water potential (high solute concentration). Therefore, for osmosis to occur, the membrane must be permeable come water, yet impermeable to the solute, and also the concentration that the solute should be different on the two sides the the membrane. Water will relocate from the side with lower solute concentration to the side with greater solute concentration until the concentrations space equal, or until some exterior force stays clear of further activity of water. This is a passive process, in the no energy expenditure is compelled for the motion of water. In an fabricated system such as the one depicted in the figure below, water will certainly attempt to move from room B to chamber A. Due to the fact that chamber A is a strict chamber, push will develop. The pressure that is just sufficient to prevent water from moving throughout the membrane is referred to as osmotic pressure.

You are watching: Which of the following is necessary for osmosis to occur?


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

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

In the body, water will move right into or out of cells, depending on the solute concentration (osmolarity) that the extracellular fluids vs. The intracellular fluids. If the solute concentration in the extracellular fluid is lower than the solute concentration in the cell, water moves right into the cell and also the cell will certainly swell. Prior to we can explain why cells shrink or expand when placed in a particular kind that solution, we very first need to discuss the difference in between osmolarity and tonicity.

Earlier this semester we disputed how the concentration of particles in a systems is expressed as osmolarity. Recall that osmlarity represents the number of moles the particles every liter the solution, while molarity to represent the number of moles that molecules per liter that solution. Why carry out we have these different ways of to express concentration? We have to adjust because various substances behave in different way in solution. Because that example, when NaCl is dissolved in water, it division apart right into Na+ and also Cl- ions (this is a characteristic of substances organized together by ionic bonds). Thus, over there are now twice as many particles 보다 there were when the substance was dry. Subsequently a 1 molar equipment of NaCl would certainly be a 2 osmolar solution. Glucose is different. Glucose doesn"t rest apart in water due to the fact that the atoms space covalently bonded. Therefore, a 1 molar systems of glucose will likewise be a 1 osmolar solution. The concentration the solutes in body fluids is 285-295 mOsmoles/liter (for simplicity we often round this number to 300). We ar the small m, which represents milli or one-housandth, in front of osmole because we are taking care of very small amounts, 1000 times much less than an osmol. Osmolarity is a advantageous term because now we have the right to use indigenous to explain solutions choose isosmotic, which means two solutions have the same number of particles, or hyperosmotic which means one solution is more concentrated 보다 the other, or hyposmotic which method that one equipment is less concentrated than the other. (Note: Osmolarity takes right into account all of the particles in the solution. Therefore, if you have actually a liter of equipment containing one mole the glucose and also one mole that NaCl girlfriend would have actually a three osmolar solution.)

Perhaps the many 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 provided to define how a systems affects a cell as soon as it is put into the solution. One important characteristic is that currently we begin to resolve membranes and also particles. Why room cells affected by different solutions? The prize lies in the habits of particles v regard to diffusion. Corpuscle will often tend to diffusive from areas of high concentration to areas of reduced concentration to with equilibrium. However, if the membrane is not permeable come the particles, then rather of particles moving or diffusing, water will relocate or diffuse with the cabinet membrane to reach equilibrium. Additionally, in ~ equilibrium, the osmolarities that the two solutions will it is in the same. We call the movement or diffusion the water osmosis, together was discussed above. Once water moves out of a cell, the cell shrinks; likewise, when water moves right into a cell, the cell swells. Thus, if we location a cell right into an isotonic solution, the cell form won"t change because the remedies are currently in equilibrium, therefore there will certainly be no net activity of water or solutes across the membrane. In various other words, isotonic solutions have the exact same concentration the osmotically energetic particles (osmotically particles space non-permeable particles) together are discovered in the cell. If the cabinet swells, us say the the systems was hypotonic, and also if the cell shrinks (crenates) us say the systems was hypertonic.

It seems an easy enough. Water moves as soon as the particles space impermeable to the membrane; however, as soon as particles room permeable, the particles will diffuse through the cabinet membranes, instead of water. Mental the intracellular and also extracellular fluids in the body always move to equilibrium, either by motion of solutes, if they have the right to cross the membrane, or by the movement of water, if the solutes cannot cross. Let"s look at one more example. 5 percent dextrose (dextrose is an additional name because that glucose) is isosmotic with regard to body fluids since it has the same variety of particles together blood; however, glucose is permeable to cabinet membranes. Thus, a 5% dextrose equipment (D5W) may be isosmotic come the cells, but it behaves together a hypotonic solution—the solute moves right into the cell accompanied by water, leading to the cell to swell. Right here is another way to think that osmolarity and also tonicity; osmolarity can be supplied to to compare the concentration of solutes in two solutions. The can likewise be provided to compare the concentration of the solutes in a equipment with those in the cell, before equilibrium is achieved. Tonicity is supplied to describe what impact the solution has actually on the cell. Osmolarity doesn"t take into account the nature of the solutes if tonicity is dependent upon the concentration of the nonpermeable solutes.

The figure listed below shows what wake up to red blood cells once they are inserted into hypertonic, isotonic or hypotonic solutions.


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

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

The link below shows what happens to a wilted plant as soon as it is placed into a hypotonic solution.


To examine our understanding, complete the table below by pour it until it is full in the absent column items through regard come osmolarity and tonicity. Usage the terms iso, hypo, and also hyper to complete the table.




0.9 % Saline

5% dextrose

5% dextrose + 0.9% Saline

0.45% Saline

5% dextrose + 0.45% Saline

You have actually been offered the answers for the table above. Be sure you recognize why the answers room what castle are.

See more: What Spell Did Molly Weasley Use To Kill Bellatrix / Headscratchers

**Note: other texts, also hospitals top top occasion, often tend to use less rigorous interpretations of tonicity. In various other words, definitions are loosely offered to define all hyperosmolar solutions as hypertonic. This is based upon the observation that water have the right to cross the membrane much faster than the permeable solute can cross. It may likewise be based on the incorrect assumption that tonicity and osmolarity are the same thing. Thus, the initial effect of an abrupt readjust in extracellular osmolality may be temporarily various from the guess tonicity change.

In various other words, a 5% dextrose systems in saline would certainly be taken into consideration hypertonic due to the fact that there are much more than 300 mosm that solutes in this solution. The problem with this definition is the it go not differentiate tonicity from osmolarity, together it makes no referral of whether the solutes are permeant or non-permeant, through respect come a specific membrane. We think that loose definitions produce confusion and also therefore have identified tonicity by the an ext rigorous definition of "effective osmolarity."

**You might use the buttons below to walk to the following or previous analysis in this Module**
print this web page