Osmosis is about water. Yep, just water. More specifically, osmosis is the diffusion of water across a selectively permeable membrane or how water passively moves through cell membranes. Let me explain via a scene in my biology class when I was a sophomore in high school.
INT. CLASSROOM – DAY
The teacher answers the students’ questions about the topic of osmosis.
STUDENT #1
So, osmosis is the movement of saltwater?
TEACHER
No, it’s only about the movement of water, not the solutes in the water. However, the salt concentration in the water affects the direction of osmosis.
STUDENT #1
How do the solutes affect the movement of water?
TEACHER
Good question. Let’s say salt cannot diffuse through the cell membrane and that the inside has a higher salt concentration than the fluid outside the cell. Therefore, the inside of the cell has a higher salt-to-water ratio than the outside. Water will leave the cell until the salt concentration on both sides of the membrane is the same.
STUDENT #1
So, if the cell membrane were permeable to salt and water, then the movement of salt and water would be osmosis?
TEACHER
No. Osmosis is only the diffusion of water across a cell membrane. The movement of salt is facilitated diffusion. Both osmosis and facilitated diffusion are types of diffusion. Osmosis only refers to the movement of water through a semipermeable membrane, while facilitated diffusion refers to the movement of ions or polar molecules across a cell membrane.
STUDENT #1
That makes a lot of sense. What happens if the extracellular fluid has more. . . Rain!!!!!
TEACHER
What?
All the students turn their heads toward the windows and stare at the sudden downpour of rain.
STUDENT #1
Look at all that osmosis.
TEACHER
That’s not osmosis.
STUDENT #2
Great, I will have to walk home in all that osmosis.
TEACHER
Rain is not an example of osmosis.
STUDENT #3
According to my weather app, we’ll get an inch and a half of osmosis.
A tear runs down the teacher’s face
STUDENT #4
Hey, look!!! The teacher’s eyes are doing osmosis.
And, scene.
What is Osmosis?
First, yes, I am student #2, and there were no apps in 1990. Second, cell phones were just phones and the size of a Dodge Dart. Memories are imperfect records of past events, and I like to confabulate them from time to time. Second, by “time to time,” I mean daily. It’s bad. I need my memory checked. It’s become so bad that don’t be surprised if this chapter ends with a rant on why green M&Ms taste the best.
So, the movement of water is only osmosis if
- there is a semipermeable membrane separating two aqueous solutions, and
- the water moves passively through said membrane.
If there is no membrane or if another force is moving the water, such as active transport or hydrostatic pressure, then water movement is not osmosis.
Water is the main ingredient in our body fluids – both intracellular and extracellular – and comprises 60% of our body’s mass. The water molecule also has unique properties that make it the perfect essential chemical that all life needs. However, we’ll only focus on how water moves in and out of cells or, simply, osmosis.
‘Cause Brawndo’s Got Electrolytes
Your bodily fluids are a mixture of water, electrolytes, and nonelectrolytes. Simply, electrolytes are substances that conduct electricity, while nonelectrolytes cannot. The more complex distinction is that electrolytes are chemical compounds that dissociate (break apart) into ions when dissolved in water, and nonelectrolytes remain intact in aqueous solutions.
Sodium chloride (NaCl) is an electrolyte because it dissociates into Na+ and Cl– when placed in water. For example, the taste buds on the tongue contain chemoreceptors, which only work when a substance dissolves in water. The taste buds are receptive to Na+ but not to NaCl. Therefore, you can only taste salt when it dissolves in your saliva.
Really?
Don’t believe me? Use a towel to remove all of the moisture from your tongue. When your tongue is dry, put salt on it. There will be no taste until the NaCl disassociates into Na+ in your saliva.
Glucose does dissolve in water but remains intact; therefore, it is not an electrolyte. The taste buds are responsive to glucose, but only if glucose dissolves in water. Taste buds are not glucose-specific and will be stimulated by other molecules structurally similar to glucose, such as fructose and aspartame.
Osmosis is a Passive Process
Water will move to the side of a cell membrane with a higher solute concentration – this does not mean that the side with the higher solute concentration has more water than the side with fewer solutes. However, the side with a higher solute concentration will have a higher solute-to-water ratio than the side with fewer solutes. Therefore, water will move to the side with the higher solute concentration until the solute-to-water ratio is the same on both sides of the membrane (equilibrium).
For example, assume you have a box separated into equal parts by a membrane that only allows water to move through it. Each side of the box has one liter of water, but Side A has 10 grams of sugar, and Side B has 20 grams of sugar. Both sides have the same volume of water but different concentrations of sugar. Side B has twice as much sugar as Side A, so Side B has a higher sugar-to-water ratio than Side A. Remember, only water can diffuse through the membrane, so the amount of sugar on both sides of the membrane remains constant. Therefore, water will move from Side A (lower sugar concentration) to Side B (higher sugar concentration) until both sides have the same water-to-sugar ratio, called equilibrium.
The Effects of Permeable and Nonpermeable Solutes on Osmosis
Permeable solutes are substances that can move through a cell membrane because they are nonpolar or can move through an open channel protein. Nonpermeable solutes are substances that cannot diffuse through a membrane because they are too big, all the channel proteins are, or their polarity or charge. Permeable and nonpermeable solutes affect how water diffuses across a membrane, but each affects the movement of water differently.
(In some textbooks and videos, a permeable solute is referred to as a penetrating solute, and a nonpermeable solute is referred to as a nonpenetrating solute.)
What is the Osmotic Gradient?
In NGSS Biology, you learned that water moves towards the side with a higher solute concentration; however, this is not entirely accurate.
Wait. Why?
The osmotic gradient is only affected by the concentration of nonpermeable solutes on both sides of a cell’s membrane.
What happens to the permeable solutes?
They are still present, but they do not affect the osmotic gradient; therefore, they have no direct effect on the movement of water.
You’re losing me.
Assume a cell membrane is permeable to glucose but not sucrose (table sugar). The solution surrounding a cell (extracellular fluid) contains 50 mmoles/L of glucose and 10 mmoles/L of sucrose, and the intracellular fluid comprises a solution of 20 mmol/L of glucose and 20 mmole/L of sucrose. Will water move into or out of the cell?
Out of?
Why?
Because there are fewer solutes in the solution compared to inside the cell.
Your answer would be correct if the cell’s membrane were impermeable to BOTH glucose and sucrose. However, the nonpermeable solute (sucrose) only affects the osmotic gradient, and we ignore the permeable solute (glucose). Therefore, water will move into the cell because of a higher sucrose concentration (nonpermeable solute).
Wait, I’m still confused.
Ok. Let’s discuss the differences between osmolarity and tonicity, which should help clarify the osmotic gradient.
What is a Mole?
When discussing anatomy and physiology, a mole could refer to a pigmented growth on the skin or a quantity of something. This chapter will refer to the mole used in chemistry to determine the number of small particles (atoms and molecules) in a solution. A mole has a value of 6.022 x 1023, which is a huge number. Avogadro’s number is so large that if you had one mole of dollar bills in your savings account and spent a billion dollars per second, it would take 19 million years to empty your account. One mole of alpacas is almost seven times more massive than Earth (yes, I took time out of my day to calculate the molar mass of alpacas). However, the number of atoms in a 150-pound human is about 7.0×1027. So, when dealing with tiny things like atoms and molecules, Avogadro’s number becomes useful. (The good news for those who hated calculating molarity in Chemistry is that you will not have to calculate molarity. The bad news for those who enjoyed calculating molarity in Chemistry is that you will not calculate molarity.)
One last thing, osmolarity units are in Osmol/L instead of moles/L. The conversion of moles to osmoles is quite simple:
\frac{\bcancel{moles}}{liters} \times \frac{Osmol}{\bcancel{moles}}= Osmol/L
What is Osmolarity?
The number of different solutes – electrolytes and nonelectrolytes – in a liter of water refers to osmolarity. Solutions with more solutes have a higher osmolarity than solutions with fewer solutes. However, the size and shape of a solute do not affect the solution’s osmolarity. For example, glucose is a larger compound than NaCl, but 1 mole/L of glucose has an osmolarity of 1 osmole per liter (Osmol/L), while 1 mole/liter of NaCl is 2 Osmol/L. Salt has twice the osmolarity of glucose because NaCl dissociates into two different water solutes (Na+ and CL-), and glucose remains intact. Remember, a solution’s osmolarity depends on the number of different solutes, not the nature of the substance. Glucose is a larger compound than NaCl, but size does not matter; the number of particles a solute dissociates into determines its osmolarity.
Let’s do a little math to help clarify osmolarity:
Human blood contains 0.140 moles/L of Na+, 0.006 moles/L of glucose, and a BUN (blood urea nitrogen) of 0.007 moles. What is the osmolarity (Osmol/L) of human blood?
\frac{\bcancel{moles}}{liters} \times \frac{Osmol}{\bcancel{moles}}= Osmol/LNa^+\:Osmolarity = \frac{0.140\:\bcancel{moles}}{1\:L} \times \frac{2\:Osmol}{1\:\bcancel{mole}}= 0.280\:Osmol/LGlucose\:Osmolarity = \frac{0.006\:\bcancel{moles}}{1\:L} \times \frac{1\:Osmol}{1\:\bcancel{mole}}= 0.006\:Osmol/LBUN\:Osmolarity = \frac{0.007\:\bcancel{moles}}{1\:L} \times \frac{1\:Osmol}{1\:\bcancel{mole}}= 0.007\:Osmol/LBlood\:Osmolarity = 0.280\:Osmol/L\:+\:0.006\:Osmol/L\:+\:0.007\:Osmol/L= \colorbox{yellow}{0.293\:Osmol/L}The osmolarity of a solution refers to all its contents, both permeable and impermeable. Therefore, a solution with more solutes inside a cell will have a higher osmolarity than the cell’s intracellular fluid. An important thing to note is that osmolarity is a reference in the concentrations of the solutes on both sides of a cell membrane and does not directly affect osmosis.
All body solutions are in one of the following osmolarity states:
- A solution is hyperosmotic when it contains more solutes -permeable and nonpermeable – than the solution on the other side of the membrane.
- A solution is hypoosmotic when it contains fewer solutes -permeable and nonpermeable – than the solution on the other side of the membrane.
- A solution is isosmotic when it contains the same concentration of solutes -permeable and nonpermeable – as the solution on the other side of the membrane.
What is Tonicity?
Tonicity and osmolarity are similar in that they both involve solute concentrations on both sides of a cell membrane. However, there is one key difference: tonicity is influenced only by the number of nonpermeable solutes. For example, below is a box separated into equal parts. Both sides contain one liter of water, but Side A has 5 grams of NaCl and 10 grams of glucose, and Side B has 5 grams of glucose and 20 grams of salt. The membrane is permeable to water and NaCl but not to glucose. Which side will the water diffuse towards?
Side B?
No. You would be correct if you only looked at the solutions’ osmolarity because Side B has a higher osmolarity than side A. However, when referring to tonicity, the correct answer is Side A.
Why?
Because Side A has more nonpermeable solutes than Side B. Since NaCl can diffuse through the membrane, the concentration of NaCl will reach equilibrium. Glucose cannot diffuse through the membrane, so the only way to establish equilibrium is for water to move from Side B to Side A until both chambers have the same glucose concentration. Thus, the osmotic gradient is moving towards Side A.
All body solutions are in one of the following states of tonicity:
- A solution is hypertonic when it contains more nonpermeable solutes than the solution on the other side of the membrane. Water will leave the cell, and the cell will shrink.
- A hypotonic solution contains fewer nonpermeable solutes than the solution on the other side of the membrane. Water will enter the cell, and the cell will swell and may lyse (split open).
- A solution is isotonic when it contains the same concentration of nonpermeable solutes as the solution on the other side of the membrane. Water will move back and forth evenly.
It is important to note that the tonicity and osmolarity of intracellular and extracellular fluids are in constant flux; therefore, their hypo/hyper/iso states constantly change. Also, the extracellular fluid can be hyperosmotic and hypotonic simultaneously. But, it cannot be hypertonic and hypotonic or hyperosmotic and hypoosmotic simultaneously (this applies to the intracellular fluid as well).
Quick Summary of the Differences Between Osmolarity and Tonicity
Osmotic Pressure
Below is the same box from earlier in the chapter.
The box has two equal compartments separated by a semipermeable membrane. Side A is hypertonic; it has 10 grams of glucose dissolved in one liter of water. Side B is hypotonic because it has 5 grams of glucose dissolved in one liter of water. The membrane is permeable to water but not sugar; therefore, water will move towards the hypertonic Side A.
As water diffuses towards Side A, Side A builds an opposing resistance to the diffusing water. The pressure on the hypertonic Side A increases until the pressure is equal to the force of osmosis. The force pushing against the influx of water is the osmotic pressure. Osmotic pressure is the force required to stop osmosis. The osmotic pressure builds on the side that water diffuses towards; therefore, the hypertonic Side A.
Let’s say there is a game in which a moveable wall separates two rooms. Each room has a team of players who aim to push the wall toward the opposing team’s side. All the people in this scenario are clones; therefore, they have the same physiological properties (size and strength). In this analogy, the clones represent water molecules, and the wall is a cell membrane. The hypertonic side will have fewer people than the hypotonic side.
A few people from the hypotonic side defect to the hypertonic side. Since the hypertonic has gained players and the hypotonic side has lost players, the hypertonic side will now push the wall with more force (osmotic pressure) because it has more clones (water). The hypotonic side will push with less pressure because it lost clones (osmosis of water). This process will continue until equilibrium, where both sides have an equal number of clones (water), therefore, the same osmotic pressure. Both sides of the membrane are now isotonic.
Why Do I Need to Know All of This Osmosis Stuff?
For those thinking of a career in medicine, you will most likely administer intervenous (IV) fluids, known as crystalloid or colloid fluids. Under normal circumstances, we get water and nutrients by eating and drinking. The digestive system and the kidneys control the water and nutrient transport into and out of the blood, which prevents our internal bodily fluids from becoming too hypertonic or too hypotonic. (You can significantly disrupt the tonicity by drinking seawater, liters of freshwater all at once, or eating copious amounts of sugar, but these are not normal circumstances.) However, when administering a crystalloid or colloid fluid intravenously or rectally, the digestive system’s regulatory functions are bypassed, and change to a patient’s body fluid osmolarity and tonicity is quick. In other words, if a physician gives a patient the wrong crystalloid fluid, it can kill them. Therefore, knowing the osmosis properties is essential to ensure doctors, nurses, and paramedics administer the right concentration of crystalloid fluid.
Wait. Why are IV fluids administered rectally?
IV infusions are not always possible. For example, finding a suitable vein for an infant, the elderly, long-term IV drug users, or a patient suffering from hemorrhagic shock can be difficult. If a vein is not accessible, rectal administration of a crystalloid fluid may be needed. The rectal infusion will diffuse into the blood through the lining of the large intestine.
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