2.3 Membrane Transport and Membrane Lipids

Practice questions covering membrane structure, lipid types and distribution, membrane fluidity, osmotic concepts, diffusion, osmosis, and core transport mechanisms (facilitated diffusion, active transport, and related proteins).

What is the Fluid-Mosaic Model of the plasma membrane?

A dynamic structure where phospholipids and proteins diffuse laterally within a phospholipid bilayer, containing components such as phospholipids, cholesterol, proteins, glycoproteins, and glycolipids.

Are phospholipids amphipathic, and what does this mean?

Yes. They have a hydrophilic (polar) head and a hydrophobic (nonpolar) tail, enabling bilayer formation.

Name the three basic membrane structures discussed?

Micelle (single-layer sphere), phospholipid bilayer (two-layer sheet), and liposome (closed bilayer vesicle).

List the five common phospholipids and their head groups.

Phosphatidic acid (PA) – no head group; Phosphatidylcholine (PC) – choline; Phosphatidylethanolamine (PE) – ethanolamine; Phosphatidylserine (PS) – serine; Phosphatidylinositol (PI) – inositol.

What are sphingolipids and give examples?

Lipids with a sphingosine backbone; examples include ceramide, sphingomyelin (SM), cerebroside, and ganglioside; they are longer and more saturated and less abundant than phospholipids.

What is membrane lipid asymmetry and why is it important?

Lipids are distributed unevenly between the two leaflets of the bilayer, affecting signaling and membrane function; some lipids are enriched on the outer vs. inner leaflet and can flip under certain conditions (e.g., during apoptosis).

What is the role of cholesterol in membranes?

Cholesterol helps maintain membrane fluidity and decreases permeability by increasing lipid packing density; it orients with a polar head near the surface and a rigid ring system spanning the bilayer.

What are lipid rafts and their function?

Microdomains rich in sphingolipids and cholesterol that are highly ordered; they organize signaling, endocytosis, exocytosis, and other cellular processes.

What factors influence membrane fluidity?

Temperature, fatty acid saturation (saturated vs. unsaturated), and cholesterol content all affect how fluid the membrane is.

How do saturated and unsaturated fatty acids affect membrane fluidity?

Saturated fatty acids pack tightly and decrease fluidity; unsaturated fatty acids have kinks from double bonds that prevent tight packing and increase fluidity.

What is the transition temperature in membranes?

The temperature at which a membrane changes from a fluid (liquid-crystalline) state to a gel (solid) state.

What is osmosis?

The diffusion of water across a semipermeable membrane from areas of lower solute concentration to higher solute concentration.

Define isotonic, hypotonic, and hypertonic solutions in relation to cells.

Isotonic: equal solute concentrations; Hypotonic: lower external solute causing water influx; Hypertonic: higher external solute causing water efflux (animal cells shrink; plant cells plasmolyze under extreme hypertonicity).

What is diffusion across membranes and its two main forms?

Passive movement down a concentration gradient; Simple diffusion through the lipid bilayer and Facilitated diffusion via transporter proteins or channels.

What are the characteristics of facilitated diffusion?

Specificity for transported molecules, saturable kinetics, and regulatory control of transporter activity.

What are GLUT transporters and how are they regulated by insulin?

GLUT1–GLUT5 are glucose transporters with tissue-specific expression; insulin increases their presence at the cell surface, enhancing glucose uptake.

What is active transport and its two main types?

Movement against a gradient requiring energy; Primary active transport uses direct ATP hydrolysis, while Secondary active transport uses energy stored in a gradient (often via cotransport).

Describe P-type, V-type, and F-type ATPases.

P-type: ATP-driven phosphorylated pumps (e.g., Na+/K+-ATPase, Ca2+-ATPase); V-type: acidify organelles by pumping H+; F-type: ATP synthase, can also run in reverse to make ATP using a proton gradient.

What are ABC transporters and why are they important?

ATP-binding cassette transporters are ATP-dependent pumps that move a wide range of substrates out of cells; they can contribute to multidrug resistance (e.g., MDR1).

What is secondary active transport and give an example?

Cotransport where the downhill movement of one solute drives the uphill transport of another (e.g., Na+/glucose cotransporter in intestinal epithelial cells).

What is the role of ion channels and their gating mechanisms?

Ion channels provide selective diffusion pathways; gating can be voltage-gated, ligand-gated, or mechano-gated, controlling when ions pass through the channel.

Which molecules diffuse readily through the lipid bilayer without transporters?

Very small, uncharged molecules such as O2, CO2, NO, and water (though water can also use aquaporins for faster transport).