BIOL125 Lecture Notes - Lipid Bilayer, Semipermeable Membrane, Osmosis
Diffusion and Osmosis
• 1.13. Definitions related to movement of molecules
o 1.13.1. semipermeable membrane
• Must be lipid soluble
▪ Only small, uncharged molecules or larger lip-soluble molecules can
diffuse freely across the phospholipid bilayer
• If impermeable no molecule could cross membrane
• If freely permeable any molecule could pass across membrane
• Movement of molecules based on
▪ Lipid solubility
▪ Electrical charge
▪ Size
▪ Molecule (3D) shape
• Movement may be active (requires energy in the form of ATP) or passive (no
energy expenditure)
• Benefits
▪ Small uncharged molecules e.g. O2 and CO2 passively diffuse (down the
concentration gradient) across the phospholipid bilayer
▪ Small charged molecules move down their concentration gradient and
diffuse across membranes through channels with no energy
expenditure
▪ Proteins e.g. enzymes and structural proteins stay in cells as they are
too big to cross
• Disadvantage - cells must expend energy ATP in active transport process e.g.
to pump molecules against their concentration gradient, or export wastes via
exocytosis
o 1.13.2. solute
• Body contains a large percentage of water: interstitial fluid (ISF), blood, lymph
and cytoplasm
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