Lipids and Biological Membranes (5D) - Chem & Physics

What is a glycolipid (in membranes) defined by?

A lipid with one or more carbohydrate groups on its head. Carbohydrate groups add hydrophilicity and specificity for cell recognition and signaling in membranes.
What is the key structural difference between saturated and unsaturated fatty acid chains?

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.
Which option best describes simple diffusion across a lipid bilayer?

Direct movement of small nonpolar molecules down their concentration gradient. Nonpolar molecules can dissolve in the hydrophobic bilayer interior, moving spontaneously without protein assistance.
What happens to membrane fluidity when fatty acid chain length increases (all else equal)?

Membrane fluidity decreases. Longer chains enhance van der Waals forces, promoting tighter packing and reducing membrane flexibility.
Which type of membrane transport moves solute down its gradient without ATP use?

Passive transport. It relies on concentration gradients as the driving force, conserving energy for thermodynamically favorable processes.
What is cholesterol's primary effect on membrane fluidity at low temperature?

It increases fluidity by preventing tight packing. Cholesterol disrupts ordered packing of fatty acids, maintaining fluidity and preventing gel phase formation.
What happens to membrane fluidity when fatty acid unsaturation increases?

Membrane fluidity increases. Higher unsaturation introduces more kinks, reducing chain interactions and allowing greater membrane flexibility.
What is the defining structural feature of a fatty acid?

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.
What is a triacylglycerol (triglyceride) composed of?

Glycerol esterified to three fatty acids. Esterification of glycerol's three hydroxyl groups with fatty acids forms a nonpolar molecule ideal for energy storage.
What is sphingomyelin composed of (core components)?

Ceramide plus a phosphocholine (or phosphoethanolamine) head group. Ceramide provides the hydrophobic portion, while the phosphate head confers amphipathicity for myelin sheath function.
Which geometric configuration is typical for natural unsaturated fatty acids?

Cis double bonds. Cis configuration is biosynthetically favored and introduces bends that prevent tight packing in membranes.
What is the defining structural feature of a phospholipid used in membranes?

Two hydrophobic tails and a phosphate-containing hydrophilic head. This amphipathic design enables phospholipids to form bilayers with hydrophobic interiors and hydrophilic surfaces.
Which membrane leaflet typically contains most glycolipids in animal cells?

The extracellular (outer) leaflet. Asymmetric distribution orients carbohydrates outward for cell-cell interactions and protection.
What structural feature makes a fatty acid unsaturated?

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.
Which lipid class is the primary long-term energy storage form in humans?

Triacylglycerols (triglycerides). Their high energy density and hydrophobic nature make triacylglycerols efficient for storing energy in adipose tissue.
Which property of phospholipids drives spontaneous bilayer formation in water?

Amphipathic structure (hydrophilic head, hydrophobic tails). The dual nature minimizes unfavorable water-hydrophobe interactions, favoring self-assembly into bilayers.
Identify the direction of water movement in osmosis across a semipermeable membrane.

From lower solute concentration to higher solute concentration. Water moves to equalize osmotic pressure, diluting the side with higher solute to achieve equilibrium.
What is facilitated diffusion across a membrane?

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.
What is the fluid mosaic model's core claim about membrane structure?

Proteins float in or on a fluid lipid bilayer. This model explains membrane dynamism, allowing lateral diffusion of components for function and adaptability.
What is the main structural difference between a glycerophospholipid and a sphingolipid?

Glycerophospholipid: glycerol backbone; sphingolipid: sphingosine backbone. The backbone difference affects membrane properties, with sphingolipids often forming more rigid structures.
What is cholesterol's primary effect on membrane fluidity at high temperature?

It decreases fluidity by restraining phospholipid movement. Cholesterol intercalates between phospholipids, reducing chain mobility and stabilizing the membrane against thermal disruption.
What happens to the melting temperature $T_m$ as fatty acid chain length increases?

$T_m$ increases. Longer chains strengthen intermolecular forces, raising the temperature needed to melt the lipid structure.
What effect do cis double bonds have on fatty acid chain packing in membranes?

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.
What happens to the melting temperature $T_m$ as fatty acid unsaturation increases?

$T_m$ decreases. Increased unsaturation disrupts chain packing, lowering the temperature required for phase transition to liquid state.
What is the basic structural motif of a steroid lipid?

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

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