Lipids, Membranes, and the First Cells

Hypothesis on Biological Evolution: Most biologists hypothesize that biological evolution began with a self-replicating molecule.
Second Great Milestone: The enclosure of these replicators within a membrane.
Importance of Membranes:
- The plasma membrane (or cell membrane) defines life by separating the cell interior from the environment.
- It acts as a selective barrier, preventing damaging substances from entering and allowing necessary substances to pass through.
- By sequestering chemicals in an enclosed area, membranes increase the frequency of reactant collisions, making essential chemical reactions more efficient.
Fundamental Questions: How do membranes form? Which molecules and ions can pass through, and why?

Definition of Lipids: A broad category of carbon-containing compounds characterized by their insolubility in water.
Reason for Insolubility: Due to a high proportion of nonpolar $C—C$ and $C—H$ bonds relative to polar functional groups.
- Solubility: Lipids dissolve in nonpolar organic solvents, such as benzene ( $C<em>6H</em>6$ ).
Hydrocarbons: Molecules composed solely of carbon and hydrogen atoms.
- Examples: Isoprene subunits exhibit this. ( $Figure ext{ }6.1a$ )
- Nonpolar Nature: $C—H$ bonds are nonpolar because carbon and hydrogen have similar electronegativities, leading to equal sharing of electrons and no partial charges.
- Hydrophobicity: Since water is a polar solvent, hydrocarbons (and thus lipids with significant hydrocarbon components) are mostly hydrophobic.
Isoprenoids: Long, branched hydrocarbon chains formed by linking isoprene subunits.
- Functions: Act as pigments, scents, vitamins, and precursors for sex hormones. They are also building blocks for more complex lipids.
Fatty Acids: Simple lipids.
- Structure: Consist of a hydrocarbon chain bonded to a polar carboxyl functional group ( $—COOH$ ) ( $Figure ext{ }6.1b$ ).
- Size: Typically contain $14-20$ carbon atoms, mostly in their long, nonpolar hydrocarbon