Diffusion, Osmosis & Active Transport

Diffusion definition

• Net movement of particles (atoms, molecules, ions) from region of higher concentration → region of lower concentration.

• Continues until equilibrium: equal distribution (but particles still move randomly both directions).

• Driven by kinetic energy of particles in gases or liquids.

• Movement is random and collisions slow diffusion.

Factors affecting rate of diffusion

1. Temperature:

   • ↑ temperature → particles have more kinetic energy → move faster → faster diffusion.

2. Concentration difference (gradient):

   • Greater difference = faster rate of diffusion (steeper gradient = faster net movement).

   • Diffusion always down a concentration gradient (from high → low).

3. Diffusion Distance

   • Shorter distance leads to faster diffusion

4. Surface Area

   • Increased surface area leads to faster diffusion

Simple diffusion

• Movement of small/non-polar molecules (e.g. O₂, CO₂) directly through the phospholipid bilayer.

• Barrier: hydrophobic core of bilayer resists charged/polar substances.

🧮 Rate of diffusion and surface area (practical)

• Rate measured in two ways:

  1. Distance travelled ÷ time.

  2. Volume filled ÷ time.

• Agar block experiment:

  • Agar blocks with indicator (phenolphthalein).

  • Immersed in NaOH → diffusion distance measured.

  • Surface area, volume, and SA:V ratio compared.

  • Results: smaller blocks with larger SA:V ratio show faster diffusion.

Key principle:

• Larger SA:V ratio → higher diffusion rate (important for exchange surfaces).

Diffusion across membranes

• Membranes partially permeable.

• Non-polar molecules (O₂, CO₂, lipid-based molecules) diffuse easily.

• Polar molecules (H₂O, ions) diffuse much slower.

• Factors influencing rate across membranes:

  • Surface area (larger = faster).

  • Thickness of membrane (thinner = faster).

Facilitated diffusion (define & properties)

• For polar molecules/ions (can’t diffuse through bilayer directly).

• Uses membrane proteins:

  • Channel proteins → form pores for specific ions/molecules.

  • Carrier proteins → change shape when specific molecule binds, transporting it across.

• Properties:

  • Passive (no ATP).

  • Down concentration gradient.

  • Selectively permeable: proteins only allow specific molecules/ions through.

  • Rate depends on:

    • Temp, gradient, membrane SA, thickness.

    • Number of transport proteins available.

🧪 Investigations into diffusion (model cells)

• Dialysis tubing (partially permeable, with pores):

  • Small molecules (e.g. glucose) pass through.

  • Large molecules (e.g. starch) cannot.

  • Used as model cell membrane.

• Practical setup:

  • Dialysis tubing tied at ends, filled with solution, placed in water.

  • Investigates diffusion across membrane.

  • Can vary concentration, temperature, or molecule size.

• Example:

  • Glucose diffuses out of model cell → tested with Benedict’s solution (quantitative + qualitative).

  • Starch remains (too large to pass).

• This models selective permeability and allows investigation of factors affecting diffusion.

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📊 Key experimental insights

• Diffusion rate ↑ with:

  • Higher temperature.

  • Greater concentration gradient.

  • Larger SA:V ratio.

  • Thinner exchange surface.

  • More transport proteins (facilitated diffusion).

• Some molecules (e.g. glucose) diffuse freely through dialysis tubing, but only cross real cell membranes via facilitated diffusion.

Active Transport

• Definition: Movement of molecules/ions against a concentration gradient (from low → high concentration).

• Requires:

  • Energy (ATP) → hydrolysed to ADP + Pi.

  • Carrier proteins (act as ‘pumps’).

• Used in: absorption, nerve impulse transmission, muscle contraction, root hair cells in plants, etc.

🔄 Steps of Active Transport (Figure 1)

1. Molecule/ion binds to receptor site on carrier protein on outside of cell.

2. ATP binds to carrier protein on inside → hydrolysed to ADP + Pi.

3. Phosphate attaches to protein → protein changes shape → molecule released inside cell.

4. Molecule/ion enters cell interior.

5. Phosphate released from protein → recombines with ADP → forms ATP.

6. Carrier protein returns to original shape.

Key point: Active transport is specific – only certain substances transported by each protein.

2. Bulk Transport

• For very large molecules (enzymes, hormones, whole cells e.g. bacteria) too large for carrier proteins or large numbers of molecules released into cell environment

• Requires vesicle formation + ATP.

e.g.

• Exocytosis

• Endocytosis

➡ Endocytosis

• Phagocytosis = solids.

• Pinocytosis = liquids.

• Process:

  • Cell-surface membrane invaginates (bends inwards) around material.

  • Membrane encloses → forms vesicle.

  • Vesicle pinches off into cytoplasm.

  • Vesicles may fuse with lysosomes for digestion (enzymes break down material).

⬅ Exocytosis 

• Reverse of endocytosis.

• Vesicles (usually from Golgi apparatus) move to surface → fuse with cell-surface membrane.

• Contents released outside of cell.

Note: Energy from ATP required to move vesicles along cytoskeleton + change cell shape.

Facilitated diffusion vs Active transport:

• Both use carrier proteins.

• Facilitated diffusion = passive, down concentration gradient, no ATP.

• Active transport = active, against concentration gradient, requires ATP.

What is Osmosis?

• Definition: A type of diffusion – specifically the diffusion of water molecules across a partially permeable membrane (PPM).

• Passive process (no ATP/energy required).

Water Potential (Ψ)

• Water potential: Pressure exerted by water molecules as they collide with a membrane/container due to the tendency of water molecules to move from one place to another

• Units: kPa (kilopascals). Symbol = Ψ.

• Pure water: Ψ = 0 kPa (highest possible water potential).

• All solutions: Ψ negative (more solute = more negative Ψ= less free moving water molecules).

• Rule: Water moves from higher Ψ (less negative) → lower Ψ (more negative).

• Movement continues until equilibrium (Ψ equal on both sides).

Study Tip: Remember all water potentials are negative, except pure water which is 0.

🌟 Animal Cells & Osmosis

• Surrounded by plasma membranes only, no cell wall.

• If placed in:

  • Higher Ψ solution (less negative, e.g. pure water):

    • Water enters by osmosis.

    • Cell swells, hydrostatic pressure builds.

    • Cell bursts = Cytolysis.

  • Equal Ψ (isotonic):

    • No net water movement.

    • Cell stays normal.

  • Lower Ψ (more negative, concentrated solution):

    • Water leaves cell by osmosis.

    • Cell shrinks, membrane puckers = Crenation.

🌿 Plant Cells & Osmosis

• Surrounded by cell wall → prevents bursting.

• If placed in:

  • Higher Ψ solution:

    • Water enters by osmosis.

    • Vacuole enlarges, pushes cytoplasm against cell wall.

    • Wall resists further entry, hydrostatic pressure builds = Turgor pressure.

    • Cell is turgid (important for plant support).

  • Equal Ψ solution:

    • Water moves equally in/out → no net change.

  • Lower Ψ solution:

    • Water leaves cell by osmosis.

    • Cytoplasm and vacuole shrink, membrane pulls away from wall = Plasmolysis.

    • Protoplast shrinks.

4. Investigating Osmosis (Plants & Animals)

Plant cell example: Potato cores in sugar/salt solutions of varying concentration.

• Water enters/leaves depending on relative Ψ.

• Measure mass before and after.

• % change in mass shows direction/extent of osmosis.

• The point where no net change in mass occurs = Ψ of the cells.

Animal cell example:

• Chicken egg with shell removed (leaves single membrane).

• Placed in sugar syrup solutions of varying concentrations.

• Water moves in/out depending on solution Ψ.

• Behaves like an animal cell.