osmolarity and tonicity

Question

can someone please explain to me how to answer these questions (including working out) i am so lost

1 - What is the Tonicity of a solution of 0.28 M Urea compared to the tonicity of a Red Blood Cell? (Assume the tonicity of a Red Blood Cell to be 280 mOsm and the Molecular Weight of Urea=60)

2 - What is the Osmolarity of a solution of 180 mM of urea + 100mM of sucrose compared to 100% Physiological Saline? (Assume Osmolarity of 100% physiological Saline to be 300 mOsm and the Molecular Weight of Urea = 60 and Molecular weight of Sucrose=340)

3- What is the Tonicity of a solution of 280 mM sucrose (Cane Sugar) + 280mM urea compared to the tonicity of a Red Blood Cell assuming that RBC is 280 mosm

4-What is the Tonicity of a solution of 280 mM sucrose (Cane Sugar) + 280mM urea compared to the tonicity of a Red Blood Cell? (Assume the tonicity of a Red Blood Cell to be 280 mOsm and the Molecular Weight of Sucrose =340 and the Molecular Weight of Urea=60)

Answer 1

1. Tonicity of 0.28 M Urea vs. RBC: Hypotonic (Effective tonicity = 0 mOsm)

2. Osmolarity of 180 mM Urea + 100 mM Sucrose vs. Physiological Saline: Hypotonic (Total = 280 mOsm)

3. Tonicity of 280 mM Sucrose + 280 mM Urea vs. RBC: Isotonic (Effective tonicity = 280 mOsm)

4. Tonicity of 280 mM Sucrose + 280 mM Urea vs. RBC: Isotonic (Same as question 3)

1. Tonicity of 0.28 M Urea Solution vs. Red Blood Cell

Osmolarity Calculation:

• Urea is a small molecule that freely permeates cell membranes, thus does not contribute to the tonicity (effective osmolarity) that would cause water movement across biological membranes. Therefore, its tonicity relative to a red blood cell is effectively zero because it does not create any osmotic pressure across the red blood cell membrane.

Conclusion:

• Tonicity of 0.28 M Urea relative to RBC: Hypotonic

2. Osmolarity of a Solution of 180 mM Urea + 100 mM Sucrose vs. 100% Physiological Saline

Osmolarity Calculation:

• Urea: 180 mM180 mM (Since urea is permeable, its contribution to osmotic pressure depends on the context, but here we consider its osmolarity.)

• Sucrose: 100 mM100 mM (Sucrose is not permeable to cell membranes and thus contributes fully to osmolarity.)

• Total Osmolarity: 180 mM+100 mM=280 mOsm180 mM+100 mM=280 mOsm

Conclusion:

• Osmolarity of Solution vs. Physiological Saline (300 mOsm): Hypotonic

3 & 4. Tonicity of a Solution of 280 mM Sucrose + 280 mM Urea vs. Red Blood Cell

Both questions 3 and 4 are essentially the same, asking for the tonicity of a solution with the same concentrations against RBCs with a tonicity of 280 mOsm.

Tonicity Calculation:

• Sucrose (280 mM) contributes to the tonicity since it cannot cross the membrane: 280 mOsm280 mOsm

• Urea (280 mM) does not contribute effectively to tonicity due to its permeability: 0 mOsm0 mOsm

• Total Effective Tonicity: 280 mOsm280 mOsm