Diffusion

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration due to their random kinetic movement.

Fundamental Principles

What is Diffusion?

  • Net Movement: Particles move down a concentration gradient (high → low)
  • Random Motion: Driven by kinetic energy - particles possess thermal energy causing constant random movement
  • Equilibrium: Occurs when particles are evenly distributed (concentration gradient eliminated)
  • Passive Process: Does NOT require energy (ATP) from respiration

Diagram showing random particle movement and concentration gradient

Key Properties

Property Description
Direction Always down concentration gradient (high → low)
Energy Requirement None - uses existing kinetic energy
Speed Factors Temperature, surface area, concentration gradient, diffusion distance
Barrier Can occur through cell membranes and air/water

Significance in Living Systems

  1. Gas Exchange: Oxygen diffuses from lungs (high concentration) into bloodstream (lower concentration)
  2. Carbon Dioxide Removal: CO2 diffuses from blood (high) to lungs (low) for exhalation
  3. Small Solutes: Water-soluble substances like urea, glucose move by diffusion

Factors Affecting Rate of Diffusion

1. Concentration Gradient

  • Steeper gradient (difference between high and low sides) = faster diffusion
  • As equilibrium approaches, rate decreases because gradient decreases
  • Maximum rate occurs when concentration difference is greatest

2. Surface Area

  • Larger surface area = more particles can cross simultaneously
  • Example: Alveoli in lungs have ~70 m² total surface area for rapid gas exchange
  • Villi and microvilli in small intestine increase surface area for nutrient absorption

3. Temperature

  • Higher temperature = faster kinetic energy = faster particle movement
  • More collisions per second = faster net movement
  • Important for enzyme activity and cellular metabolism

4. Diffusion Distance

  • Shorter distance = faster diffusion
  • Gas exchange surfaces are thin (one cell thick) to minimize distance
  • Example: Capillaries and alveoli walls are ~0.5 μm thick

Example: Gas Exchange by Diffusion

Location Process Direction
Lungs (Alveoli) O2 absorption Blood (low O2) ← Alveoli (high O2)
Lungs (Alveoli) CO2 removal Blood (high CO2) → Alveoli (low CO2)
Intestinal Villi Nutrient absorption Gut lumen (high) → Blood (low)

Osmosis

Osmosis is the net movement of water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane.

Understanding Osmosis

Water Potential Concept

  • Water Potential (Ψ): Measure of tendency of water to move
  • Pure water has highest water potential (Ψ = 0)
  • Adding solutes lowers water potential (more negative value)
  • Water moves from higher (less negative) to lower (more negative) water potential

Partially Permeable Membrane

  • Allows water molecules to pass freely
  • Blocks or restricts movement of solute molecules (ions, sugars, proteins)
  • Cell membranes are partially permeable

Osmosis diagram showing water movement through partially permeable membrane

Types of Solutions and Effects on Cells

Hypertonic Solution

  • Higher solute concentration (lower water potential) outside cell
  • Water moves OUT of cell by osmosis
  • Animal Cell: Shrivels (plasmolysis in plant terminology)
  • Plant Cell: Becomes flaccid, may undergo plasmolysis (membrane pulls away from cell wall)

Hypotonic Solution

  • Lower solute concentration (higher water potential) outside cell
  • Water moves INTO cell by osmosis
  • Animal Cell: Swells and may burst (lysis)
  • Plant Cell: Becomes turgid (turgor pressure builds up against cell wall)

Isotonic Solution

  • Equal solute concentration inside and outside cell
  • No net water movement
  • Animal Cell: Maintains normal shape
  • Plant Cell: Becomes flaccid (limp)

Comparison diagram showing animal and plant cells in hypertonic, hypotonic, isotonic solutions

Turgor Pressure in Plants

Importance of Turgor Pressure

  • Water entering root hair cells creates turgor pressure
  • Pressure pushes against cell wall
  • Rigid cell wall prevents bursting
  • Provides structural support for plant stems and leaves

Plant Support Mechanism

  • Turgid cells = firm stems, upright leaves
  • Loss of turgor = wilting
  • Essential for non-woody plant support

Investigation Methods

Dialysis Tubing Experiment

  • Semi-permeable tubing simulates cell membrane
  • Fill with concentrated solution, place in distilled water
  • Observe volume change over time
  • Demonstrates osmosis in controlled environment

Plant Tissue in Different Solutions

  • Place potato cylinders in solutions of varying sucrose concentrations
  • Measure change in length/weight
  • Determine isotonic point where no change occurs
  • Plot results to find concentration of plant cell sap

Active Transport

Active transport is the movement of molecules or ions across a cell membrane from a region of lower concentration to a region of higher concentration using energy from respiration.

Core Principles

Movement Against Gradient

  • Particles moved FROM low concentration → TO high concentration
  • Opposite direction to diffusion
  • Requires input of metabolic energy

Energy Source

  • ATP produced during respiration
  • Energy used by carrier proteins to pump substances

Carrier Proteins

  • Specific proteins embedded in cell membrane
  • Bind to specific molecules/ions
  • Change shape to transport substance across membrane
  • Reusable after each transport cycle

Active transport diagram showing carrier proteins pumping ions against gradient using ATP

Why Active Transport is Necessary

Examples Where Active Transport Occurs:

  1. Root Hair Cells (Plants)

    • Absorb mineral ions from soil
    • Soil ions often at lower concentration than root cell cytoplasm
    • Active transport pumps ions INTO root cells even against concentration gradient
    • Essential for nutrient uptake

  2. Intestinal Absorption (Animals)

    • Absorption of glucose and amino acids from small intestine
    • Against concentration gradient into bloodstream
    • Requires ATP from intestinal cells

Comparison: Diffusion vs Osmosis vs Active Transport

Feature Diffusion Osmosis Active Transport
Substance Moved Any particles Water only Specific ions/molecules
Direction High → Low concentration High water potential → Low water potential Low → High concentration
Energy Required No No Yes (ATP from respiration)
Membrane Can be through membrane Partially permeable membrane Requires carrier proteins
Example O2 diffusing into blood Water entering plant cells Mineral ions absorbed by roots

Glossary

Term Definition
Concentration Gradient Difference in concentration between two regions
Water Potential (Ψ) Measure of water’s tendency to move; pure water = 0
Partially Permeable Membrane Allows some substances (water) through, blocks others (solutes)
Turgid Swollen with water, pressing against cell wall
Flaccid Limp, not turgid; no pressure against cell wall
Plasmolysis Plant cell membrane pulls away from cell wall
Hypertonic Higher solute concentration outside cell
Hypotonic Lower solute concentration outside cell
Active Transport Movement against gradient using energy