Biological Membranes & Transport

What types of lipids are found in cellular membranes?

Phospholipids (phosphoglycerides, sphingomyelin), glycosphingolipids (cerebrosides, globosides, gangliosides), and cholesterol

What is the function of lipids in membranes?

They form a bilayer providing structural integrity and compartmentalization; they also influence membrane fluidity and signaling.

How does the lipid composition differ between the outer and inner leaflet?

Outer leaflet is rich in phosphatidylcholine, sphingomyelin, glycosphingolipids; inner leaflet is rich in phosphatidylserine, phosphatidylethanolamine, phosphatidylinositol.

What is membrane fluidity and what types of lipid motion occur?

Fluidity is the ability of lipids and proteins to move laterally. Motions include lateral diffusion (fast) and flip-flop (slow).

What enzymes assist with phospholipid translocation across bilayers?

Flippases (outer to inner), Floppases (inner to outer), Scramblases (bidirectional, gradient-driven)

What factors affect membrane fluidity?

Ratio of saturated to unsaturated fatty acids and cholesterol content

What is the Fluid Mosaic Model?

Describes membranes as dynamic structures with proteins floating in or on a fluid lipid bilayer

What are intrinsic (integral) membrane proteins?

Proteins embedded in the lipid bilayer, often spanning it, with hydrophobic regions interacting with lipid tails.

What are peripheral (extrinsic) membrane proteins?

Proteins loosely bound to membrane surfaces via polar interactions or to intrinsic proteins.

What are lipid-anchored membrane proteins?

Proteins covalently linked to lipid molecules embedded in the membrane (e.g., myristate, farnesyl, GPI anchors).

What are microfilaments and their function?

Composed of actin, they help with movement (e.g., streaming, contraction).

What are intermediate filaments and their function?

Composed of proteins like spectrin, they provide mechanical strength and support.

What are microtubules and their function?

Composed of α- and β-tubulin, they support vesicle transport, mitosis, and structure

What is the glycocalyx?

A carbohydrate-rich zone on the outer membrane leaflet, composed of glycoproteins, glycolipids, and polysaccharides.

What are functions of the glycocalyx?

Cell recognition, adhesion, immune defense, cancer detection, and migration.

What are integrins?

Heterodimeric proteins involved in cell-ECM interactions, signaling, and adhesion.

What are cadherins?

Calcium-dependent molecules that mediate homophilic cell-cell adhesion

What are immunoglobulin-like CAMs?

CAMs that bind to identical molecules or integrins on adjacent cells

What are selectins?

Lectins that bind carbohydrates on neighboring cells; important in immune responses.

What are major components of the ECM?

Collagen, fibronectin, laminin, elastin, fibrin, proteoglycans

What are proteoglycans and their function?

Glycoproteins with glycosaminoglycans; function in support, signaling, and molecule trapping

How do CAMs interact with ECM proteins?

Through integrins and glycosaminoglycans like heparan sulfate on proteins like syndecan

How do nonpolar and polar molecules cross membranes?

Nonpolar (O₂, CO₂) diffuse freely; polar require transport proteins (channels, carriers).

Define osmosis, simple diffusion, and facilitated diffusion

Osmosis: water diffusion; simple diffusion: passive movement; facilitated: via carrier proteins

What molecules move via simple diffusion?

Small, nonpolar molecules like O₂, CO₂.

What drives simple diffusion?

Concentration and electrical gradients

What moves via channels or pores?

Ions and small polar molecules like water (via aquaporins).

What are gating mechanisms in channels?

Ligand-gated, signal-gated, voltage-gated, and mechanosensitive

What defines passive transport?

Protein-facilitated diffusion requiring no energy, with saturation kinetics.

What is primary active transport?

Direct ATP use (e.g., Na⁺/K⁺-ATPase) to move molecules against gradients.

What is secondary active transport?

Uses ion gradients (e.g., Na⁺/glucose symport) created by primary transport

Which transport types act against concentration gradients?

Active transport (primary and secondary).

What are uniports, symports, and antiports?

Uniport: 1 molecule, 1 direction; Symport: 2 molecules, same direction; Antiport: 2 molecules, opposite directions.