Lecture Notes: Membrane Permeability

Overview

• Focus on AP Biology curriculum Section 2.5
• Understanding semi-permeability vs. selective permeability
• Importance for AP test

Key Concepts

Permeability

• Describes how a molecule passes through a membrane
• Example: Coffee filter analogy

Lipid Bilayer

• Made of phospholipids (hydrophilic head, hydrophobic tail)
• Hydrophilic heads attract water, hydrophobic tails attract non-polar molecules
• Semi-permeable: Blocks charged, large molecules; allows nonpolar gases (O2, CO2)

Factors Affecting Permeability

• Type of phospholipids (saturated = less permeable, unsaturated = more permeable)
• Temperature and pH influence membrane fluidity and permeability

Semi-Permeable vs. Selectively Permeable

• Semi-permeable: Only blocks substances based on chemical and physical properties
• Selectively permeable: Includes protein channels/carriers for specific molecule transport

Biological Importance

• Selective permeability: Essential for creating internal cellular conditions
• Diffusion and Osmosis: Maintain balance of solutes and water within the cell

Fluid Mosaic Model

• Model for cell membrane structure and function
• Proteins in membranes control selectivity
  • Allow or restrict molecule passage

Membrane Proteins

• Various types: water channels, ion gates, larger molecule carriers
• Cholesterol: Affects permeability by interacting with lipid bilayer

Types of Molecules

• Small non-polar molecules (e.g., O2, CO2): Easily pass
• Small polar molecules (e.g., water): Slightly blocked
• Large polar molecules and ions: Generally blocked, require protein aid

Cell Walls and Permeability

• Made from carbohydrates like cellulose, chitin, peptidoglycans
• Provide structural support
• Filter environment; block large molecules

Conclusion

• Membranes combine semi-permeability and selectivity for cell regulation
• Cell walls add filtering layer

Further Learning

• Additional sections (2.6 - 2.9) cover membrane proteins in detail
• Importance of selective permeability in biological processes