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4.6.1Additional lessons

Osmosis — IB Biology HL Revision Notes & Topical Questions

5 min read 5 parts Lesson 51 of 101

📚 IB Biology HL Hub / Topical Revision
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Part 1 of 5Water as a Solvent

4.6.1 Osmosis
Polarity
  • Oxygen attracts the shared electrons more strongly than hydrogen, so water has a slightly negative oxygen end and positive hydrogen ends.
Hydration shells
  • Water molecules cluster around ions and polar molecules, separating them and keeping them dissolved in solution.
Hydrogen bonds
  • Water also forms hydrogen bonds with polar solutes such as glucose, helping them to dissolve.
Non-polar substances
  • Lipids are hydrophobic and do not dissolve, which allows membranes to separate watery compartments.

Part 2 of 5Defining Osmosis

Water diffuses in both directions, but more water moves towards the more concentrated solution.

Osmosis
  • Net movement of water molecules across a partially permeable membrane from a lower solute concentration to a higher solute concentration.
Passive
  • The process needs no ATP, because water moves down its concentration gradient.
Membrane
  • The membrane lets water pass, but restricts larger solutes, so water movement balances the concentrations.
Aquaporins
  • Channel proteins speed up the movement of water across membranes without changing the direction.

Part 3 of 5Comparing Solutions

Hypotonic
  • A solution with a lower solute concentration than the cell, so water enters the cell by osmosis.
Hypertonic
  • A solution with a higher solute concentration than the cell, so water leaves the cell.
Isotonic
  • A solution with the same solute concentration as the cell, so there is no net movement of water.
Relative terms
  • Tonicity always compares two solutions, so a solution is hypertonic or hypotonic only relative to another.
Outside solutionNet water movementCell result
HypotonicInto the cellSwells
IsotonicNoneNo change
HypertonicOut of the cellShrinks

Part 4 of 5Predicting Direction

1
Identify the two solutions on either side of the partially permeable membrane.
2
Decide which solution has the higher total solute concentration.
3
Water moves from the more dilute solution to the more concentrated solution.
4
State the outcome: the volume of the concentrated side rises while the dilute side falls.

Part 5 of 5Exam Checklist & Tips

4.6.1 – Key Points for the Exam

  • Osmosis The net movement of water across a partially permeable membrane towards the higher solute concentration.
  • Hypotonic Having a lower solute concentration than another solution.
  • Hypertonic Having a higher solute concentration than another solution.
  • Isotonic Having equal solute concentrations.

Common Mistakes

  • Saying that solutes move in osmosis; it is the movement of water.
  • Mixing up hypotonic and hypertonic.
  • Stating that osmosis stops at equilibrium; water still moves, but with no net change.
  • Describing water moving from high solute to low solute concentration.

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