Osmosis, by definition, entails the movement of water through a selectively permeable membrane from an area of low solute concentration to an area of high solute concentration. For osmosis to occur, the membrane must be permeable to water but impermeable to the solute, and there must be different concentrations of solute between the two sides of the membrane. If the solutes cannot cross the membrane, then water molecules will naturally migrate from the side with lower solute concentration to the side with higher solute concentration until equilibrium is reached. Water will always migrate to establish equilibrium unless some other external force prevents it (i.e., osmotic pressure, but we’ll let the chemists address that!). Importantly, osmosis is a passive process, requiring no ATP energy expenditure for water movement.

In the body, water movement into or out of cells depends on the solute concentration (osmolarity) of both extracellular and intracellular fluids. When the extracellular fluid's solute concentration is lower than that of the cell, water flows into the cell, causing it to swell. To understand why cells expand or shrink in different solutions, it's essential to differentiate between osmolarity and tonicity.

Osmolarity

Osmolarity quantifies the number of moles of particles per liter of solution, which is different than the more common term of molarity which measures the number of moles of a salt compound (like NaCl) per liter of solution. Why the need for different units? Because the molarity of a solution is not always the same as the solution’s osmolarity. This is due to the nature of substances and how they behave differently in solution: for instance, when NaCl dissolves, it dissociates into Na+ and Cl- ions due to ionic bonding, effectively doubling the particle count. Therefore, a one molar solution of NaCl becomes a two osmolar solution (technically, around 1.8 osmolar due to incomplete dissociation). In contrast, glucose remains intact in water due to covalent bonding, so a one molar solution of glucose is also one osmolar.

The typical solute concentration in body fluids ranges from 285–295 milliosmoles per liter (often rounded to 300 for simplicity), denoted as mOsmoles/liter (mOsM), where the 'm' stands for 'milli' or one-thousandth of an osmole. Using osmolarity allows us to describe solutions with terms like 'isosmotic' (same concentration of solutes in two solutions), 'hyperosmotic' (one solution more concentrated than the other), or 'hypoosmotic' (one solution less concentrated). These prefixes—'iso' meaning 'same,' 'hyper' meaning 'more,' and 'hypo' meaning 'less'—help us describe the solution's nature. Importantly, osmolarity considers all particles in the solution; thus, a liter of solution containing one mole of glucose and one mole of NaCl would be a three osmolar solution.

Tonicity

When discussing solutions and their impact on the body, tonicity emerges as a pivotal concept. The term 'tone' denotes the firmness or stretch of a tissue; hence 'tonicity' describes how a solution influences the firmness or stretching of a cell when immersed in it. This concept delves into the interaction between membranes and particles, helping to explain why cells respond differently to various solutions. The tonicity of a solution predicts the effect of the solution on the cell volume at equilibrium. 

Cells react to different solutions due to the behavior of particles in diffusion. Particles naturally diffuse from regions of high concentration to those of lower concentration until equilibrium is achieved. However, if the membrane isn't permeable to these particles, water will diffuse instead through aquaporins in the cell membrane to reach equilibrium. We call these non-permeable particles osmotically active particles. Thus, to reach equilibrium, water must move instead and if water exits a cell, the cell shrinks, and when it enters, the cell swells.

In an isotonic solution, where the concentrations of osmotically active particles are already balanced, the cell maintains its shape because there is no net movement of water or solutes across the membrane. Isotonic solutions contain the same concentration of osmotically active particles (non-permeable particles) as the cell. If the cell swells, the solution is termed hypotonic, whereas if it shrinks (crenates), the solution is hypertonic.

When considering fluids and ions inside the cell (intracellular fluid) and outside the cell (extracellular fluid), equilibrium is typically reached through the movement of solutes (ions) or water, depending on membrane permeability. However, there are exceptions to this rule in biology. For instance, some solutes can enter the cell and subsequently disappear due to cellular metabolism.

Consider glucose, which can permeate healthy cell membranes. Upon entering the cell, glucose undergoes rapid metabolism, effectively vanishing from the extracellular fluid (ECF). This continual removal of glucose from the ECF sets up a concentration gradient that drives more glucose into the cell. With each glucose molecule transitioning from the ECF to the intracellular fluid (ICF), the ECF's concentration diminishes relative to the ICF's. Consequently, water flows into the cell to maintain equilibrium, resulting in cellular swelling.

To clarify further, let's examine a scenario where a 300 mOsM solution of glucose—matching the concentration found in cells—is considered isosmotic. However, when this solution is combined with the cell's contents, glucose begins moving into the cell along its concentration gradient. This might seem counterintuitive, as the concentrations appear equal. However, the 300 mOsM concentration within cells isn't solely due to glucose but rather a combination of ions like K+, Na+, and other things like proteins. Thus, concerning glucose, the gradient is from the ECF to the ICF.

Try to conceptualize osmolarity as a concentration (the number of particles per unit volume). In the image below, we can see red particles (or solutes) being poored into a beaker of water. Some of the particles are clumbed and maybe even bonded together, but as they hit the water, they begin to dissolve. The dissolved solutes become osmoles and the total number of osmoles dissolved in the water give us a number that we visualizing here as 300 mosm/L (or 200 Mosm). 

image here of 4 picts one red and one pink showing 300 and 150 mosm.  

As we continue to explore concepts of osmosis, we will use some simplistic sketches of beakers and water with the various hues of red to represent levels of solute concentration. 

images here that are sketches. 

you is simplistic analogy, if four dots represent 300 mOsM, the cell contains four dots (composed of K+, Na+, proteins, etc.), and the solution contains four dots (consisting solely of glucose). Consequently, both solutions are considered isosmotic, with four dots (300 mOsM). However, this term alone doesn't predict the outcome when they are mixed. Therefore, we rely on the concept of tonicity to understand the practical effect of such combinations. As each molecule of glucose moves into the cell, the outside concentration will begin to decrease from 300 to 299 to 298 etc. The inside osmolarity won’t necessarily increase because the glucose entering is being immediately metabolized, but the 300 (ICF) compared to the 298 (ECF) is not at equilibrium, and as glucose keeps moving that number will eventually drop to zero. Thus, water will start to move into the cell to try and equilibrate the two concentrations (ICF to ECF). Since the ICF is 300 and the ECF essentially zero, water will move until the equilibrium of both solutions is 150 mOsM (300/2 compartments). This scenario illustrates how a solution which is initially isosmotic with body fluids can act as a hypotonic solution due to the cell's metabolic activity. The result is a swollen cell with a reduced mOsM (300/2 = 150). 
On the other hand, permeable solutes like urea that enter the cell but are not metabolized can also affect cell equilibrium. When a cell is placed in an isosmotic urea solution, urea initially moves into the cell, reducing its concentration outside the cell while increasing it inside. As a result, water moves into the cell to restore equilibrium, leading to cell swelling. Thus, any isosmotic solution containing a permeable solute behaves as a hypotonic solution to the cell. The end effect is a swollen cell (hypotonic) but with the same mOsm (300 + 300 = 600 then 600/2 = 300). We could also add a cell into a hyperosmotic solution of urea (600 mOsm) and see that it will always be hypotonic (cell swells) but end with a higher mOsm (600 + 300 = 900 then 900/2 = 450).
Osmolarity and tonicity offer different perspectives on solution effects. Osmolarity compares solute concentrations between solutions or between a solution and the cell before equilibrium. Tonicity, however, describes the solution's effect on the cell and depends on the concentration of non-permeable solutes. While osmolarity disregards solute nature, tonicity considers it, focusing on non-permeable solutes. *Note: while it is true that some Na+ can leak through cells, the Na/K ATPase pump removes the sodium at about the same rate, making Na+ “functionally” non permeable.
The figure below shows what happens to red blood cells when they are placed into hypertonic, isotonic, or hypotonic solutions.
 
Osmotic Pressure on Blood CellsTitle: File: Osmotic pressure 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 placed in a hypertonic solution, red blood cells will shrink or crenate. When placed in an isotonic solution, there will be no change in volume, and when placed in a hypotonic solution, red blood cells will swell. If the concentration of the solution is great enough inside the cell, the cells will swell and even burst (lyse). 
The link below shows what happens to a wilted plant when it is placed into a hypotonic solution.
    Watch on YouTube

Let’s try some more examples. Consider a solution that is composed of a 0.9% NaCl solution (300 mOsM; non permeable) mixed with a 5% dextrose (glucose) solution (300mOsm; permeable). Both solutions are considered isosmotic to the cell, but when added together, they become double the concentration of the cell (600 mOsM). Thus, this solution would be considered hyperosmotic compared to the cell. If we place a cell into this solution, what will happen to the cell and what will be the final mOsM concentration? To answer this question, we need to consider the nature of the particles. NaCl is considered to be non-permeable, while dextrose is considered permeable. Once a cell is added to the solution, the dextrose will immediately move down its concentration gradient into the cell and “disappear” until all that is left will be the 0.9% NaCl. Thus, even though this solution was hyperosmotic to begin with, it becomes isotonic with respect to its interactions with the cell. The final mOsM will be 300 (300 + 300 = 600 then 600/2 = 300). At equilibrium, the cell will not change shape, although it will lose water initially (see graph).
To check understanding, complete the table below by filling in the missing column items with regard to osmolarity and tonicity. Use the terms iso, hypo, and hyper to complete the table.

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