Download - Cellular Transport and Tonicity
Selective Permeability of the Plasma Membrane
• Which substances can move freely through the phospholipid bilayer?– Water, gases (oxygen, carbon dioxide); fats– Small- polar; nonpolar molecules
• Which substances are blocked? – Glucose; ions– Larger substances (polar)
• Processes– Passive (no ATP)
• High to lower concentration gradients• Driving Force? KE of the substances
– Active (requires ATP) • Low to higher concentration gradients
Passive Processes• 1. Diffusion vs.
Osmosis • 2. Simple Diffusion
– Gases; nonpolar; lipid soluble substances (fat-soluble vitamins)
• 3. Facilitated Diffusion– Sugars (glucose; amino
acids; ions)– Integral or Transmembrane
proteins • Channel or carrier proteins
• 4. Osmosis– Movement of water
dependent on solute concentration (extracellular/intracellular)
Tonicity • Measure of the ability of a solution to
cause a change in cell shape or tone caused by osmotic flow of water
• Why does osmosis occur?– Water concentration differences – Solute concentration affects water concentration
• Dependent on the number (concentration)
– Osmolarity– Permeability of solute molecules
• Permeable to all solute molecules – equilibrium • If membrane is impermeable (see U-tube)
Tonicity – Conditions 1. Isotonic – Cells have same
concentrations of non-penetrating solutes as found in cells
– Our cellular fluids and IV’s
2. Hypertonic– Solutions have higher
concentration of non-penetrating solutes
– Salty or high saline conditions
3. Hypotonic – Solutions containing a lower
concentration of non-penetrating solutes
– “dilute”
Active Transport • Driving Force? ATP
– Solute “pumps” (ions- K+, Na+, etc.)
• Sodium-Potassium Pump– “primary active
transport” – Neurons, cardiac and
skeletal tissue
• Vesicular Transport– Fluids contain large
particles and macromolecules
– Vesicles