Membranes, Lipids, and Membrane Proteins — Key Concepts

A comprehensive set of practice flashcards covering membrane origins, lipid types, leaflet topology, cholesterol, membrane proteins, transport mechanisms, and membrane dynamics.

Which part of the endoplasmic reticulum is the main site of lipid biosynthesis?

The smooth endoplasmic reticulum.

What are the primary roles of the rough and smooth regions of the endoplasmic reticulum?

Rough ER is for protein synthesis; smooth ER is the main site of lipid biosynthesis.

What is the Golgi apparatus responsible for in relation to ER-generated cargo?

Sorting and modifying proteins and lipids, and directing them to their destinations such as the cell membrane or other organelles.

Where are glycolipids glycosylated, and why is this timing important?

In the Golgi lumen; sugar head groups are added there, so glycolipids face the exterior leaflet after trafficking.

What structural feature of mitochondria illustrates membrane diversity, and what are its two membranes like?

Mitochondria have a double membrane; the outer membrane is smooth, the inner membrane is highly folded with distinct lipid/protein composition.

What is the central barrier function of cellular membranes?

To separate different aqueous environments and regulate what enters and exits, forming compartments.

What are the two leaflets of a membrane bilayer called in relation to the cytosol?

The cytosolic (cytosolic) leaflet and the noncytosolic (lumen/extracellular) leaflet.

Why do lipid bilayers prefer to form closed structures rather than open sheets?

Exposed hydrophobic interior to water is energetically unfavorable; bilayers seal into closed vesicles or structures.

What are scramblases and flipases, and what roles do they play in lipid distribution?

Scramblases move lipids non-specifically between leaflets; flipases actively move specific lipids to generate and maintain asymmetry.

Why are phospholipids key components of membranes and what makes them amphipathic?

They have hydrophobic tails and a hydrophilic head, enabling bilayer formation via hydrophobic interactions.

How does cholesterol affect membrane properties?

Cholesterol intercalates between phospholipids, modulating rigidity, fluidity, and thickness; its distribution differs among membranes.

What factors determine membrane thickness and lipid packing?

Fatty acid chain length and saturation: longer chains and saturated tails pack more tightly, increasing thickness and decreasing fluidity.

What is cardiolipin, and where is it predominantly found?

A conical phospholipid with four fatty acid chains; enriched in the inner mitochondrial membrane and contributes to curvature.

What is an integral membrane protein, and what characterizes a transmembrane domain?

An integral protein that spans the membrane; a transmembrane domain is typically a hydrophobic segment (often an alpha helix) crossing the bilayer.

Describe the difference between peripheral and integral membrane proteins.

Peripheral proteins bind to membrane surfaces or other membrane proteins (do not span the bilayer); integral proteins span the membrane and may have lipid anchors.

What is a GPI-anchored membrane protein?

A protein tethered to the membrane by a glycosylphosphatidylinositol (GPI) anchor, usually in the noncytosolic leaflet.

What is a beta barrel, and where are beta-barrel proteins commonly found?

A pore formed by arranged beta strands; common in outer mitochondrial membranes and some other organelle membranes.

What is the most common transmembrane structural motif?

The alpha helix, typically 20–25 amino acids long, spanning the membrane.

Describe the seven-transmembrane-domain protein example and its orientation.

Proteins like rhodopsin with seven transmembrane helices; N-terminus outside, C-terminus inside.

How do channels differ from transporters in mediating passive transport?

Channels form pores allowing diffusion of solutes; transporters bind solutes and undergo conformational changes to move them down their gradient.

What is the difference between ATP-driven pumps and gradient-driven pumps?

ATP-driven pumps use ATP hydrolysis to move substances against their gradient; gradient-driven pumps couple movement of one molecule down its gradient to drive another against its gradient (antiporters and symporters).

What is FRAP and what does it reveal about membrane dynamics?

Fluorescence recovery after photobleaching; measures lateral mobility of lipids or proteins in membranes.

What function do tight junctions serve in epithelial cells?

They seal the paracellular space to prevent leakage and restrict diffusion of membrane components between apical and basolateral domains.

Why must transmembrane domains be hydrophobic, and how are transmembrane proteins arranged?

To interact with the lipid bilayer; hydrophilic regions face aqueous environments while hydrophobic regions span the membrane; the overall topology is fixed during insertion.

How are new membrane components typically added during growth, and what ensures proper leaflet distribution?

Membranes grow by adding lipids to the cytosolic leaflet of existing membranes; scramblases move lipids to the opposite leaflet, while flipases help establish lipid asymmetry by moving specific lipids to the other leaflet.