Osmotic Pressure and Osmolarity Quiz

Objectives

• Define osmotic pressure
• Calculate osmolarity and osmolality
• Understand terms hyposmotic, hyperosmotic, isosmotic
• Understand the difference between osmolarity and tonicity
• Understand the effects of osmosis at the level of the cell membrane and the capillary wall

Osmolarity (Osmotic Concentration)

• The concentration of all solutes in a solution
• Units of osmolarity: Osm/L (Osmoles per Litre)
• Osmolar (same as molar concentration, M)
• Units of osmolality: Osm/kg
• Usually similar values, variation takes into account only solutes contributing to osmotic pressure
• Some substances dissociate in solution
  • 1 mol/L glucose = 1 Osm/L
  • 1 mol/L NaCl = 2 Osm/L
  • 1 mol/L CaCl₂ = 3 Osm/L

Cell Membranes

• Selectively semipermeable
  • Allow hydrophobic (lipid-soluble) substances to cross easily
  • Allow some small hydrophilic (polar) substances to cross easily
  • Prevent the free passage of large polar substances (e.g., glucose, ions)

Movement of Water Across Cell Membranes

• Water is a polar molecule (+/-)
• Small amounts of water pass through the lipid bilayer by simple passive diffusion
• Membrane permeability to water can be increased by the presence of aquaporins
  • Specialized water channels

Osmosis

• Passive transport mechanism
  • Diffusion of water from an area of high water concentration (high water potential) to an area of low water concentration (low water potential) across a partially permeable membrane
  • High solute concentration = low water concentration
  • Low solute concentration = high water concentration
• Diffusion of water
  • Must be across a semipermeable membrane
    • Permeable to water
    • Impermeable to at least one solute
  • Along a concentration gradient
  • Driving force for osmosis is osmotic pressure (i.e., dilute to concentrated)

Osmosis Example

• Solution in beaker contains 180 g/L sucrose
• Solution in "cell" contains 360 g/L sucrose
• "Cell" membrane permeable to water but not sucrose
• Water moves into the cell, causing the cell to swell.
• Water continues to move until concentration equalizes on both sides (~270 g/L sucrose)

Osmosis and Osmotic Pressure

• Osmotic pressure = pressure required to prevent osmosis
• Pressure exerted on the box = osmotic pressure
• The greater the solute concentration, the greater the osmotic pressure

Body Fluid Compartments

• In the body, all compartments are normally in osmotic equilibrium (~300 mOsm)
• Changes in concentration of ICF or ECF result in fluid shifts between compartments

Comparing Osmolarity

• Comparing osmolarity of 2 solutions using the terms isosmotic, hyposmotic, or hyperosmotic
  • A cell placed in an isosmotic sucrose solution: no net water movement, no change in cell volume
  • A cell placed in a hypoosmotic sucrose solution: water moves into the cell, causing cell swelling (oedema)
  • A cell placed in a hyperosmotic sucrose solution: water moves out of the cell, causing cell shrinkage

Tonicity

• Describes the behavior of cells in solutions
  • Do cells swell, shrink, or stay the same size?
• Osmolarity is affected by all solutes, while tonicity is only affected by solutes that cannot cross the membrane
  • Hypotonic solution: causes cells to swell (less solutes outside cell, water enters the cell)
  • Isotonic solution: causes no change in cell volume (equal solutes on both sides of the membrane)
  • Hypertonic solution: causes cells to shrink (more solutes outside cell, water exits the cell)

Comparing Tonicity Example

• "Cell" in 300 mmol/L urea solution (isosmotic). Urea enters cell followed by water. Cell swells (oedema).

Osmotic Behavior of Cells

• RBCs placed in isotonic solution: maintained shape and volume
• RBCs placed in hypotonic solution: swell, may burst (haemolysis)
• RBCs placed in hypertonic solution: shrink

How Does This Relate to Cells in the Body?

• Cell membranes are permeable to water, water passes in and out by osmosis
• Movement depends on solute concentration in ECF and ICF
• Electrolytes contribute to osmolarity of body fluids
• ECF and ICF must have the same osmolarity
• Osmosis occurs if water is lost from one fluid compartment
• Body fluid osmolarity is approximately 270-300 mOsm/L

Oedema

• Plasma proteins provide osmotic pressure to keep fluid in blood vessels
• If plasma proteins are low, fluid leaves the plasma and moves into tissues
• Hypoproteinaemia (low plasma proteins) is caused by liver disease, kidney disease, malabsorption, or malnutrition
• Oedema (swelling) due to fluid moving from intravascular space to interstitial fluid

Cerebral Oedema

• Stroke, tumours, trauma cause fluid accumulation in the brain
• Increased intracranial pressure, hypoxia, and damage can lead to death
• Treatment often involves hypertonic solutions (e.g., mannitol) intravenously to decrease swelling

Terms Referring to Osmolarity and Tonicity of Solutions

• Isotonic: solution does not change cell volume (containing 300 mOsm/L of non-penetrating solutes, regardless of the concentration of membrane-penetrating solutes)
• Hypertonic: causes cells to shrink (containing greater than 300 mOsm/L of non-penetrating solutes)
• Hypotonic: causes cells to swell (containing less than 300 mOsm/L of non-penetrating solutes)
• Isoosmotic: containing 300 mOsm/L solute, (regardless of its composition of membrane-penetrating and nonpenetrating solutes)
• Hyperosmotic: containing greater than 300 mOsm/L of solutes
• Hypoosmotic: containing less than 300 mOsm/L of solutes

Test your understanding of osmotic pressure, osmolarity, and osmolality with this quiz. You will explore key concepts such as hyposmotic, hyperosmotic, and isosmotic solutions, as well as the effects of osmosis on cell membranes and capillary walls.