Osmosis - VCE Unit 1 Biology Notes
Osmosis - VCE Unit 1 Biology Notes
Definition
Osmosis: The net movement of water molecules from an area of higher water concentration (lower solute concentration) to an area of lower water concentration (higher solute concentration) through a selectively permeable membrane.
Key Phrase: "Net movement" indicates that water moves both ways but one direction dominates.
Driving Force Behind Osmosis
Osmosis occurs because water moves to equalize concentration on both sides of the membrane.
Water moves towards the side with MORE solute as that side has less free water available.
Key Terms You MUST Know
Selective (Partially) Permeable Membrane: A membrane that allows some substances to pass through but not others.
Water can pass easily (often through aquaporins).
Large molecules (e.g., starch) usually cannot pass through.
Solute: A dissolved substance in a solution (e.g., salt, glucose).
Solution: Composed of solute + solvent (water).
Concentration Gradient: A difference in concentration between two areas. Osmosis occurs down a water concentration gradient.
Water Potential (VCE-level)
Water moves from:
High water potential (more free water) to
Low water potential (less free water).
The side with more solute has lower water potential.
Types of Solutions (VERY EXAM IMPORTANT)
Hypotonic Solution:
Definition: A solution with a lower solute concentration than the cell.
Effect: Water moves into the cell causing it to swell.
📌 Implication: Cell swells.
Hypertonic Solution:
Definition: A solution with a higher solute concentration than the cell.
Effect: Water moves out of the cell causing it to shrink.
📌 Implication: Cell shrinks.
Isotonic Solution:
Definition: A solution with the same solute concentration as the cell.
Effect: Water moves in and out equally.
📌 Implication: No net change in cell size.
Osmosis in Animal vs Plant Cells
Animal Cells
Characteristics: Animal cells do not have a cell wall and are therefore more vulnerable to osmotic pressure.
In Hypotonic Solution:
Water enters the cell,
Cell swells,
May burst (lysis).
In Hypertonic Solution:
Water leaves the cell,
Cell shrinks (crenation).
In Isotonic Solution:
Cell maintains normal size.
Plant Cells
Characteristics: Plant cells have a rigid cell wall, which prevents bursting under osmotic pressure.
In Hypotonic Solution:
Water enters the cell,
Vacuole swells,
Cell becomes turgid (firm).
✅ Best state for plants.
In Hypertonic Solution:
Water leaves the cell,
Cytoplasm shrinks away from the cell wall,
Cell becomes plasmolysed.
❌ Result: Plant wilts.
In Isotonic Solution:
Cell becomes flaccid (not firm, not turgid).
Key Plant Terms
Turgid: A plant cell full of water, pushing against the cell wall, creating turgor pressure which helps keep plants upright.
Plasmolysis: Occurs when a plant cell loses water and the membrane pulls away from the cell wall.
Flaccid: A plant cell that has lost turgor pressure and is limp.
Practical Implications of Osmosis
Importance of Osmosis in Living Organisms
In Plants:
Water uptake occurs through roots.
Maintains turgor pressure necessary for structure and support.
Assists in transporting water through xylem.
In Animals:
Maintains water balance in cells.
Involved in osmoregulation (kidney function).
Prevents dehydration or excessive cell swelling.
Practical Example: Potato Osmosis Experiment
Experiment setup: Potato pieces placed in various sucrose solutions.
If potato gains mass:
Water entered the potato,
External solution was hypotonic.
If potato loses mass:
Water left the potato,
External solution was hypertonic.
If no mass change:
Solution was isotonic.
Exam-Style Sentence Starters (use these for marks)
✅ "Water moved by osmosis from an area of higher water concentration to lower water concentration… "
✅ "The membrane is selectively permeable, allowing water molecules to pass but not the solute… "
✅ "The solution was hypertonic to the cell, so water moved out causing the cell to shrink… "
Common Mistakes (that lose marks)
❌ Saying "salt moved into the cell" (this is diffusion, not osmosis).
❌ Forgetting to mention “through a selectively permeable membrane.”
❌ Mixing up hypotonic vs hypertonic solutions.
❌ Incorrectly stating that osmosis is the movement of solute (NO — water only).
Super Quick Summary (perfect for a flashcard)
Osmosis = net movement of water across a selectively permeable membrane from high water concentration → low water concentration.
Hypotonic: Water in.
Hypertonic: Water out.
Isotonic: No net movement.
Plants: Turgid / Flaccid / Plasmolysed.
Animals: Burst / Shrink / Normal.