ACT Membranes flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Fluid mosaic model

The model describing membranes as a fluid phospholipid bilayer in which proteins are embedded or attached and can move laterally within the plane of the membrane.

Amphipathic phospholipid

A lipid with a hydrophilic phosphate-containing head and two hydrophobic fatty-acid tails; this dual nature drives bilayer formation in aqueous environments.

Cholesterol in membranes

A steroid lipid that buffers membrane fluidity, decreasing it at high temperatures by restricting tail movement and maintaining it at low temperatures by preventing tight packing.

Integral membrane proteins

Proteins embedded within the lipid bilayer, often spanning it; they require detergents to be extracted because of their hydrophobic interactions with lipid tails.

Peripheral membrane proteins

Proteins attached to the membrane surface by electrostatic or hydrogen-bond interactions with lipid heads or integral proteins, removable by changing salt concentration or pH.

Membrane asymmetry

The unequal distribution of lipids and proteins between the two leaflets; for example, phosphatidylserine faces the cytosol and glycoproteins face the extracellular space.

Unsaturated fatty-acid effect

Cis double bonds create kinks in hydrocarbon tails that prevent tight packing, increasing membrane fluidity compared with saturated tails.

Simple diffusion

Passive movement of small nonpolar molecules such as oxygen and carbon dioxide across a membrane down their concentration gradient without protein help or energy input.

Facilitated diffusion

Protein-mediated movement of solutes down their concentration gradient with no ATP; the transport rate saturates when all transporters are occupied.

Na+/K+ ATPase

A primary active transporter that uses one ATP to move three sodium ions out of the cell and two potassium ions in, maintaining electrochemical gradients.

Secondary active transport

Coupled transport in which uphill movement of one solute is driven by downhill movement of another; symport moves solutes the same way, antiport in opposite directions.

Osmosis

The net diffusion of water across a selectively permeable membrane toward the side with higher total solute concentration, lowering that concentration over time.

Tonicity

The ability of an external solution to cause water movement across a cell membrane; hypertonic solutions shrink cells, hypotonic solutions swell them, and isotonic solutions cause no net change.

Aquaporins

Channel proteins that allow rapid, selective passage of water molecules across membranes, greatly increasing water permeability compared with lipid diffusion alone.

Gated ion channels

Selective membrane channels that open in response to a specific stimulus such as voltage change, ligand binding, or mechanical force, allowing passive ion flow.

GPCR structure

G protein-coupled receptors are proteins with seven transmembrane helices that bind ligands and activate heterotrimeric G proteins inside the cell.

G protein activation

Ligand-bound GPCRs promote exchange of GDP for GTP on the G protein alpha subunit, activating it; intrinsic GTPase activity later turns signaling off.

Receptor tyrosine kinases

Receptors that dimerize and cross-phosphorylate tyrosine residues upon ligand binding, creating docking sites that propagate intracellular signaling cascades.

Ligand-gated ion channel

A membrane receptor that opens an ion pore when a specific ligand binds, converting a chemical signal directly into an ionic or electrical response.

Second messengers

Small intracellular molecules such as cAMP, IP3, DAG, and calcium that are produced after receptor activation and amplify and spread a signal within the cell.

Phospholipase C products

This enzyme cleaves PIP2 into IP3, which releases calcium from the endoplasmic reticulum, and DAG, which activates protein kinase C at the membrane.

Signal amplification

The process by which one activated receptor or enzyme activates many downstream molecules in a cascade, producing a large cellular response from a small initial signal.

Receptor desensitization

Reduced receptor responsiveness after prolonged stimulation, often through phosphorylation by GRK and arrestin binding that blocks further G protein activation.

Chemiosmotic theory

The concept that electron transport pumps protons to create an electrochemical gradient whose energy drives ATP synthesis through ATP synthase.

Proton motive force

The stored energy in a proton gradient across a membrane, comprising a chemical component (pH difference) and an electrical component (membrane potential).

ATP synthase

A membrane enzyme that uses the flow of protons down their gradient to rotate and catalyze ATP synthesis from ADP and inorganic phosphate.

Electron transport chain

A series of membrane protein complexes that pass electrons to a final acceptor while pumping protons across the membrane to build a gradient.

Uncouplers

Molecules such as dinitrophenol that carry protons across a membrane, dissipating the gradient so electron transport continues without ATP synthesis and heat is released.

Oligomycin effect

Inhibition of ATP synthase blocks proton flow through it, causing the proton gradient to build up and electron transport to slow down.

Exocytosis

The process in which vesicles fuse with the plasma membrane and release their contents outside the cell, adding membrane lipids and proteins to the surface.

Receptor-mediated endocytosis

Uptake in which specific ligands bind surface receptors that concentrate in clathrin-coated pits, which invaginate to form internalized vesicles.

SNARE proteins

Proteins on vesicles (v-SNAREs) and target membranes (t-SNAREs) that pair to mediate membrane fusion during exocytosis and vesicle delivery.