ACT Membranes cheat sheet

The fluid mosaic model describes membranes as a fluid phospholipid bilayer with proteins embedded in or attached to it; lipids and proteins diffuse laterally.

Phospholipids are amphipathic: a hydrophilic phosphate head faces the aqueous environment, and two hydrophobic fatty-acyl tails face the interior of the bilayer.

Saturated fatty-acid tails pack tightly and decrease fluidity; unsaturated tails with cis double bonds create kinks that increase fluidity.

Cholesterol buffers fluidity: it restricts movement of phospholipid tails at high temperature and prevents tight packing at low temperature.

Integral (transmembrane) proteins span the hydrophobic core and require detergents for removal; peripheral proteins attach to the surface and are removed by changes in salt concentration or pH.

Membrane asymmetry: phosphatidylserine and phosphatidylethanolamine are concentrated on the cytosolic leaflet; glycolipids and glycoproteins face the extracellular side.

Glycocalyx carbohydrates on the outer leaflet function in cell recognition, adhesion, and protection.

Membrane thickness is roughly 5-8 nm, consistent with a bilayer about two phospholipid molecules thick.

Simple diffusion moves small nonpolar molecules (O2, CO2, steroid hormones) down their concentration gradient without protein or energy input.

Facilitated diffusion uses channels or carriers down a gradient with no ATP; the rate saturates as transporters become fully occupied.

Primary active transport couples uphill movement to ATP hydrolysis, as in the Na+/K+ ATPase (3 Na+ out, 2 K+ in per ATP).

Secondary active transport couples uphill movement of one solute to the downhill movement of another; symport moves both in the same direction, antiport in opposite directions.

Osmosis is the net diffusion of water across a selectively permeable membrane toward the side with higher total solute concentration.

Tonicity: in a hypertonic solution a cell loses water and shrinks; in a hypotonic solution it gains water and may lyse; isotonic solutions cause no net water movement.

Aquaporins are channel proteins that greatly increase the rate of water movement across membranes.

Ion channels are selective and often gated (voltage-gated, ligand-gated, mechanically gated), allowing rapid passive ion flow down electrochemical gradients.

G protein-coupled receptors (GPCRs) have seven transmembrane helices and activate heterotrimeric G proteins by promoting GDP-for-GTP exchange on the alpha subunit.

G protein alpha subunits are active when bound to GTP and inactive when bound to GDP; intrinsic GTPase activity turns signaling off.

Receptor tyrosine kinases dimerize on ligand binding and cross-phosphorylate tyrosine residues, creating docking sites for downstream proteins.

Ligand-gated ion channels open a pore upon binding a ligand, converting a chemical signal directly into an electrical or ionic response.

Second messengers such as cAMP, IP3, DAG, and Ca2+ amplify a signal after receptor activation.

Phospholipase C cleaves PIP2 into IP3 (releases Ca2+ from the ER) and DAG (activates protein kinase C at the membrane).

Signal amplification occurs because each activated receptor or enzyme can act on many downstream molecules, producing a cascade effect.

Receptor desensitization, such as phosphorylation by GRK followed by arrestin binding to a GPCR, attenuates signaling during prolonged stimulation.

The chemiosmotic theory states that electron transport pumps protons to create an electrochemical gradient (proton motive force) that drives ATP synthesis.

Proton motive force has two components: a chemical gradient (delta pH) and an electrical gradient (membrane potential, delta psi).

ATP synthase uses the proton gradient to rotate and drive ATP production from ADP and Pi; it is a rotary motor.

The electron transport chain transfers electrons through complexes to a final electron acceptor (O2 in mitochondria) while pumping protons across the inner membrane.

In mitochondria, the inner membrane is the site of electron transport and ATP synthase; the outer membrane is highly permeable via porins.

In chloroplasts, light-driven electron transport pumps protons into the thylakoid lumen, and ATP synthase faces the stroma.

Uncouplers such as dinitrophenol carry protons across the membrane, dissipating the gradient so electron transport continues but ATP synthesis falls and heat is released.

Oligomycin inhibits ATP synthase directly, blocking proton flow through it; electron transport then slows because the gradient backs up.

Exocytosis fuses vesicles with the plasma membrane to release contents; endocytosis invaginates the plasma membrane to internalize material.

Phagocytosis engulfs large particles; pinocytosis takes in fluid; receptor-mediated endocytosis uses clathrin-coated pits to concentrate specific ligands.

SNARE proteins on vesicles (v-SNAREs) and target membranes (t-SNAREs) mediate membrane fusion during exocytosis.

Rab GTPases act as molecular switches that help target vesicles to the correct destination membrane.

Membrane flow: lipids and proteins internalized by endocytosis pass through endosomes and are recycled to the plasma membrane or routed to lysosomes for degradation.

Lysosomes contain hydrolytic enzymes at acidic pH and degrade material delivered by endosomes and phagosomes.

The Golgi apparatus sorts and modifies proteins and lipids and directs them to the plasma membrane, lysosomes, or secretory vesicles.

Membrane fluidity allows the redistribution of lipids and proteins that underlies vesicle budding, fusion, and the continuous recycling of membrane components.