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4.2.24 · Cell Membranes and Transport

Osmosis — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

5 min read 8 parts Lesson 9 of 20 1 quiz

📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision
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Lesson summary

4.2.2 Osmosis — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Define osmosis using water potential and predict the direction of water movement between solutions and cells.

  • Water potential (ψ)
  • Pure water
  • Features of a full definition
  • Special case of diffusion
  • Routes across the membrane
  • What the symbols mean

Section 1 of 8Water potential

Water potential (ψ)
  • Water potential (ψ) a measure of the tendency of water molecules to move from one place to another, measured in kilopascals (kPa).
Pure water
  • Pure water it has the highest water potential, defined as 0 kPa.
Solutions
  • Solutions adding solute lowers water potential, so every solution has a negative ψ, and a more concentrated solution is more negative.
Direction
  • Direction water moves from higher (less negative) to lower (more negative) water potential.
  • The process is passive and continues until water potentials are equal or an opposing pressure balances it.

Section 2 of 8Definition of osmosis

Definition of osmosis
Osmosis
Osmosis the net movement of water molecules from a region of higher water potential to a region of lower water potential, down a water potential gradient, across a partially permeable membrane.
Features of a full definition
  • Features of a full definition it concerns water only, it is net movement, it is down a water potential gradient and it needs a partially permeable membrane.
Special case of diffusion
  • Special case of diffusion osmosis is the diffusion of water across a membrane.
Routes across the membrane
  • Routes across the membrane water crosses directly between phospholipids,
    • which is slow because water is polar, though its small size allows it through.
  • Aquaporins are channel proteins that greatly increase the rate.

Section 3 of 8Solute potential and pressure potential

Equation
  • ψ = ψs + ψp.
    What the symbols mean: Ψ = water potential (kPa) · Ψs = solute potential · Ψp = pressure potential (kPa); water moves from higher to lower Ψ
    • (zero or negative)
Solute potential (ψs)
  • Solute potential (ψs) the effect of dissolved solutes on water potential, always negative.
    • (zero for pure water)
Pressure potential (ψp)
  • Pressure potential (ψp) the pressure exerted by the cell wall on the cell contents as the cell swells.
  • It is usually positive in a turgid cell and zero in a flaccid or plasmolysed cell.
Plant cell in pure water
  • Plant cell in pure water water enters, the vacuole swells and pushes the cytoplasm against the wall, ψp rises and ψ rises to 0,
    • so the cell is fully turgid.
  • There is then no net water entry because the pressure potential balances the negative solute potential.
Animal cell
  • Animal cell it has no wall, so ψp is effectively zero and its water potential equals its solute potential.

Section 4 of 8Cells in different solutions

TermSurrounding solutionNet water movement
HypotonicHigher ψ than the cellInto the cell
IsotonicSame ψ as the cellNone
HypertonicLower ψ than the cellOut of the cell
Red blood cells
  • Red blood cells in pure water the cell takes in water, swells and bursts (lysis), and in a strong solution it loses water and shrinks (crenation).
Plant cells
  • Plant cells they survive in dilute solutions because the wall resists expansion.

Section 5 of 8Where osmosis applies

Root hair cells
  • Root hair cells the cell sap has a lower water potential than soil water, so water enters.
Guard cells
  • Guard cells osmosis changes their turgor, which opens and closes stomata.
Support
  • Support turgor supports non-woody stems and leaves.
Animal cells
  • Animal cells they swell or shrink in different body fluids, so body fluids must keep a constant water potential.
Practical work
  • Practical work percentage change in mass of potato cylinders in sucrose solutions is used to estimate the water potential of tissue.

Section 6 of 8More from the specification

Surface Area to Volume Ratio and Diffusion Rate
  • As an organism or cell increases in size, its surface area to volume ratio decreases,
  • which limits the rate of diffusion across its surface;
    • smaller cells or agar cubes placed in solution achieve faster diffusion penetration per unit volume.
Principles of Osmosis Across Selectively Permeable Membranes
  • Osmosis is the net movement of water molecules from a region of higher water potential to a region of lower water potential (more negative Ψ) down a water potential gradient across a selectively permeable membrane.
    • (less negative Ψ)
Active Transport Mechanisms and Carrier Proteins
  • Active transport is the movement of ions or molecules across a cellular membrane against their concentration gradient via specific transmembrane carrier proteins;
    • this process requires metabolic energy provided by ATP hydrolysis.
Volume
  • The amount of cytoplasm needing supplies and producing waste.
Carrier
  • A female with one recessive sex-linked allele who is unaffected.
Indicator
  • A chemical that changes colour depending on pH.
Endocytosis
  • Bulk transport into a cell using a membrane vesicle.
Bilayer
  • A double layer of phospholipids with tails facing inwards.
Selectively permeable
  • Allowing only some substances through.
Active transport
  • Movement of particles against a concentration gradient using energy from respiration.
Facilitated diffusion
  • Passive movement through transport proteins.

Section 7 of 8Key terms

TermMeaning
Water potentialTendency of water to move from one region to another, measured in kPa; pure water is 0 kPa.
OsmosisNet movement of water from higher to lower water potential across a partially permeable membrane.
Solute potentialComponent of water potential due to dissolved solutes; always negative.
Pressure potentialComponent of water potential due to pressure exerted by the cell wall.
Partially permeable membraneA membrane that allows some substances, such as water, to pass but not others.
AquaporinChannel protein that lets water cross a membrane rapidly.
HypotonicA solution with higher water potential than the cell.
TurgidFully swollen plant cell with high pressure potential.

Section 8 of 8Exam technique

1
Describing osmosis as movement from high to low water concentration without using water potential.
2
Saying a solution of -200 kPa is more concentrated than -400 kPa — -200 kPa is the higher water potential and the more dilute solution.
3
Forgetting pressure potential — a turgid plant cell can have ψ = 0 even though it contains solutes.
4
Stating that water moves across a fully permeable membrane — osmosis needs a partially permeable membrane.

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