E1: Lab 2 Osmolarity and Hemoolysis

describe osmolarity

the total number of solute particles per liter of solution (how many particles are in a solution) ex: glucose (doesnt dissociate)→ 1 mol glucose= 1 mOsm, NaCl dissociates into Na+ and Cl- —> 1 mol NaCl = 2 mOsm

2. Understand how to calculate the osmolarity of solutions. List which molecular properties of the solutes are important to consider when making these calculation

Osmolarity = Molarity x number of particles the solute dissociates into

3. Explain the two mechanisms by which water can move across plasma membranes.

1. simple diffusion through lipid layer

2. facilitated diffusion via aquaporins (major pathways in RBC)

water always moves DOWN its osmotic gradient

Know the typical value of intracellular osmolarity of human cells.

Explain what you need to consider to determine if a

solute is a penetrating or nonpenetrating solute.

Describe the characteristics of iso-osmotic and isotonic solutions.

Explain how you would determine if a solution is hyperosmotic, hypo-osmotic, hypertonic, and hypotonic (relative to

intracellular fluid). Provide an example of a solution that would be described by each of the above terms.

intracellular osmolarity

• 300 mOsm

penetrating vs nonpenetrating solutes

• Penetrating : cross the membrane (eg, urea)

• nonpenetrating : do NOT cross (NaCl)

iso-osmotic v isotonic

• isoosmotic : same total osmolarity as ICF (300 mOsm)

• isotonic : no net change in cell volume

  • depends only on nonpenetrating soluts

solution types

Term	Meaning	Example
Hyperosmotic	>300 mOsm total	400 mOsm urea
Hypo-osmotic	<300 mOsm total	200 mOsm NaCl
Hypertonic	More nonpenetrating solute than ICF	400 mOsm NaCl
Hypotonic	Less nonpenetrating solute than ICF	150 mOsm NaCl

5. Explain how an iso-osmotic solution is not necessarily an isotonic solution of particles that freely cross the membrane

an isoosmotic solution has the same total solute concentration as the cell, but if the solutes can freely cross the membrane , they will potentially cause water to enter or leave the cell ex: 300 mOsm urea (can freely cross the cell membrane)

 Predict what would happen to the volume of red blood cells if they were placed in a 1400 mOSM solution of NaCl.

Predict what would happen to the volume of RBCs if they were placed in a solution containing 700 mOSM of NaCl plus

700 mOSM of urea. Explain your thought processes.

RBCs in 1400 mOsm NaCl

• NaCl=nonpenetrating

• solution is hyperosmotic and hypertonic

• water leaves the cell

• RBCs shrink (crenation)

RBCs in 700 mOsm NaCl + 700 mOsm urea

• Total osmolarity = 1400 mOsm (hyperosmotic)

• NaCl = non-penetrating → determines tonicity

• Urea = penetrating → does not affect tonicity

What happens:

• Urea diffuses into RBC but equilibrates

• Outside effective osmolarity = 700 mOsm NaCl

• Inside effective osmolarity = ~300 mOsm

• Water leaves the cell

➡ RBCs shrink (crenation)

7. Explain how the presence of approximately 150 mM NaCl (saline) can prevent osmotic swelling of red blood cells in a

solution that also contains some penetrating solute, such as urea.

• 150 mM NaCl dissociates → ~300 mOsm

• NaCl is non-penetrating → stays outside → maintains osmotic gradient

• Urea is penetrating → equilibrates across membrane → becomes osmotically irrelevant

• NaCl outside effectively matches 300 mOsm inside → no net water movement

• RBC volume remains stable