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