Lipids and Biological Membranes (5D) - MCAT Chemical and Physical Foundations of Biological Systems

Proteins float in or on a fluid lipid bilayer. This model explains membrane dynamism, allowing lateral diffusion of components for function and adaptability.

It increases fluidity by preventing tight packing. Cholesterol disrupts ordered packing of fatty acids, maintaining fluidity and preventing gel phase formation.

Cis double bonds. Cis configuration is biosynthetically favored and introduces bends that prevent tight packing in membranes.

Saturated: no $\text{C=C}$; unsaturated: one or more $\text{C=C}$. Absence of double bonds in saturated chains allows straight, tightly packed structures, while double bonds in unsaturated chains create kinks.

They introduce kinks that decrease packing efficiency. Kinks from cis double bonds disrupt van der Waals interactions, leading to looser packing and increased membrane fluidity.

Membrane fluidity increases. Higher unsaturation introduces more kinks, reducing chain interactions and allowing greater membrane flexibility.

Membrane fluidity decreases. Longer chains enhance van der Waals forces, promoting tighter packing and reducing membrane flexibility.

$T_m$ decreases. Increased unsaturation disrupts chain packing, lowering the temperature required for phase transition to liquid state.

$T_m$ increases. Longer chains strengthen intermolecular forces, raising the temperature needed to melt the lipid structure.

Glycerol esterified to three fatty acids. Esterification of glycerol's three hydroxyl groups with fatty acids forms a nonpolar molecule ideal for energy storage.

Triacylglycerols (triglycerides). Their high energy density and hydrophobic nature make triacylglycerols efficient for storing energy in adipose tissue.

Transport down a gradient via a channel or carrier protein without ATP. Proteins provide specificity and increase rate for polar or charged solutes that cannot easily cross the bilayer.

Two hydrophobic tails and a phosphate-containing hydrophilic head. This amphipathic design enables phospholipids to form bilayers with hydrophobic interiors and hydrophilic surfaces.

Amphipathic structure (hydrophilic head, hydrophobic tails). The dual nature minimizes unfavorable water-hydrophobe interactions, favoring self-assembly into bilayers.

Glycerophospholipid: glycerol backbone; sphingolipid: sphingosine backbone. The backbone difference affects membrane properties, with sphingolipids often forming more rigid structures.

Ceramide plus a phosphocholine (or phosphoethanolamine) head group. Ceramide provides the hydrophobic portion, while the phosphate head confers amphipathicity for myelin sheath function.

It decreases fluidity by restraining phospholipid movement. Cholesterol intercalates between phospholipids, reducing chain mobility and stabilizing the membrane against thermal disruption.

Four fused hydrocarbon rings. The rigid ring system is characteristic of steroids, enabling roles in membrane modulation and hormone synthesis.

The extracellular (outer) leaflet. Asymmetric distribution orients carbohydrates outward for cell-cell interactions and protection.

A lipid with one or more carbohydrate groups on its head. Carbohydrate groups add hydrophilicity and specificity for cell recognition and signaling in membranes.

A long hydrocarbon chain with a terminal carboxylic acid group. This structure allows fatty acids to serve as hydrophobic components in lipids, with the carboxylic acid enabling ester linkages.

At least one carbon–carbon double bond in the hydrocarbon chain. Double bonds reduce hydrogen saturation, introducing flexibility and kinks in the chain that affect physical properties.

From lower solute concentration to higher solute concentration. Water moves to equalize osmotic pressure, diluting the side with higher solute to achieve equilibrium.

Direct movement of small nonpolar molecules down their concentration gradient. Nonpolar molecules can dissolve in the hydrophobic bilayer interior, moving spontaneously without protein assistance.