Notes on How Substances Move Across Lipid Bilayers: Diffusion and Osmosis

Movement of small, uncharged polar and nonpolar molecules across lipid bilayers, spontaneous and without energy input.
Driven by random motion of molecules due to thermal energy, leading to an increase in entropy.
Net movement of solutes occurs from regions of high concentration to regions of low concentration (down a concentration gradient).
This net movement is a spontaneous process, increasing entropy until equilibrium is reached.
At equilibrium, molecular movement continues, but there is no net change in distribution.
This process is known as passive transport.

Specialized diffusion of water across a selectively permeable membrane (permits water, restricts some or all solutes).
Water moves from areas of lower solute concentration to areas of higher solute concentration.
Only unbound water molecules diffuse across the membrane.
Spontaneous process, increases entropy by diluting the higher solute concentration side.
Results in changes in volume and solute concentration on both sides of the membrane.

Describes the effect of external solution solute concentration on cell/vesicle volume.
Hypertonic: Outside solution has higher solute concentration; water moves out, vesicle shrinks.
Hypotonic: Outside solution has lower solute concentration; water moves in, vesicle swells or bursts.
Isotonic: Solute concentrations are equal; no net water movement, vesicle maintains its size and shape.

First lipid bilayers likely served as containers for early replicating molecules, such as RNA.
Early fatty acid bilayers were permeable to ions and ribonucleotides, allowing essential monomers to enter protocells.
Protocells: Simple vesicle-like structures harboring nucleic acids, thought to be intermediates in cellular evolution.
Evolution towards true cells required membranes to selectively import necessary solutes and exclude harmful ones, eventually involving proteins.