Cellular Transport and Tonicity

Biology 2121

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)

Water Movement and Solute Concentration

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

Vesicular Transport 1. Exocytosis– “ejects” substances

from cells – Hormone and mucous

secretion

2. Endocytosis – Substance moves into

cell and vesicle formed

– transcytosis, vesicular trafficking

– Phagocytosis– Pinocytosis