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Transport Across Membranes

Subject: Biology
Topic: 2
Cambridge Code: 0610 / 0970 / 5090

Simple Diffusion​

Diffusion - Movement of particles from high to low concentration

Features​

  • Random movement of molecules
  • Net movement: high → low concentration
  • DOES NOT require energy (passive)
  • Down concentration gradient
  • Continues until equilibrium reached

Factors Affecting Rate​

  1. Concentration gradient - Steeper = faster
  2. Temperature - Higher temperature = faster
  3. Surface area - Larger area = faster
  4. Distance - Shorter distance = faster

Examples​

  • Oxygen diffusing into cells
  • Gases exchanging in alveoli
  • Iodine diffusing through agar

Facilitated Diffusion​

Facilitated Diffusion - Diffusion through protein channels

Features​

  • Molecules pass through channel/carrier proteins
  • Still passive (no energy required)
  • Down concentration gradient
  • Specific proteins for specific molecules
  • Faster than simple diffusion

Examples​

  • Glucose entering red blood cells
  • Ions moving through ion channels
  • Movement of polar molecules

Osmosis​

Osmosis - Movement of water across semi-permeable membrane

Definition​

Water moves from region of high water potential (high water concentration) to low water potential (low water concentration)

Water Potential​

Ψ=Ψs+Ψp\Psi = \Psi_s + \Psi_p

where:

  • Ψ\Psi = water potential
  • Ψs\Psi_s = solute potential (always negative/zero)
  • Ψp\Psi_p = pressure potential (turgor pressure)

Solution Types​

Hypertonic - Lower water potential (high solute)

  • Water leaves cell → plasmolysis

Hypotonic - Higher water potential (low solute)

  • Water enters cell → turgid

Isotonic - Same water potential

  • No net movement

Plant Cell Responses​

  • Hypertonic: Plasmolysis (membrane pulls from wall)
  • Hypotonic: Turgor (pressure on wall maintains shape)

Active Transport​

Active Transport - Movement requiring energy (ATP)

Features​

  • Energy required (from ATP)
  • Against concentration gradient (low → high)
  • Carrier proteins involved
  • Can accumulate molecules
  • Requires respiration

Examples​

  • Glucose absorption in small intestine
  • Uptake of mineral ions by roots
  • Sodium-potassium pump in neurons

Sodium-Potassium Pump​

  • Pumps 3 Na⁺ OUT
  • Pumps 2 K⁺ IN
  • Requires ATP
  • Critical for nerve function

Bulk Transport​

Exocytosis​

  • Vesicle fuses with membrane
  • Contents released outside
  • Requires energy
  • Secretion of proteins, hormones

Endocytosis​

  • Membrane surrounds matter
  • Forms vesicle
  • Brings matter into cell
  • Requires energy

Types:

  • Phagocytosis - "Cell eating" (large particles)
  • Pinocytosis - "Cell drinking" (liquids)

Key Points​

  1. Diffusion: high → low, no energy needed
  2. Osmosis: water follows water potential
  3. Active transport: energy required, against gradient
  4. Hypertonic/hypotonic affect turgor
  5. Bulk transport for large molecules

Practice Questions​

  1. Explain why oxygen diffuses into cells faster at higher temperatures
  2. Compare facilitated diffusion and active transport
  3. What happens to plant cells in hypertonic solution?
  4. How does active transport differ from diffusion?
  5. Define water potential and turgor pressure

Revision Tips​

  • Understand gradient direction
  • Learn when energy is required
  • Know osmosis consequences for plant/animal cells
  • Practice calculations of water potential
  • Memorize Na-K pump details