ACT Membranes key terms

A double layer of lipid molecules arranged with water-fearing tails facing inward and water-loving heads facing outward, forming the basic sheet that separates the cell interior from its surroundings.

Describes a molecule that has both a water-attracting region and a water-repelling region, causing it to arrange itself at interfaces between water and nonpolar environments.

The accepted description of membranes as a dynamic, two-dimensional liquid of lipids in which many different proteins are embedded and drift about laterally.

The interior region of a lipid bilayer formed by fatty acid tails, which excludes water and blocks the free passage of most polar and charged molecules.

A rigid steroid molecule inserted between phospholipids that buffers membrane fluidity, keeping the bilayer from becoming too stiff in the cold or too runny when warm.

The degree to which lipids and proteins move laterally within a membrane, influenced by temperature, tail saturation, and cholesterol content.

A protein firmly anchored in the bilayer, often spanning it completely, that can be removed only by agents that disrupt the lipid environment.

An integral protein that crosses the entire bilayer, typically with one or more hydrophobic segments, linking the inside and outside compartments.

A protein attached to the membrane surface through electrostatic or lipid-linked interactions rather than by embedding in the hydrophobic core, so it is easily released.

The carbohydrate-rich coat of sugars attached to membrane proteins and lipids on the outer surface, involved in cell recognition, adhesion, and protection.

A protein carrying covalently attached sugar chains, commonly displayed on the outside of the plasma membrane where the sugars serve in signaling and identification.

A lipid molecule with attached carbohydrate groups, found mainly on the outer leaflet of the plasma membrane and important for cell recognition and immune interactions.

A small, relatively ordered patch of membrane enriched in cholesterol and certain lipids that clusters signaling proteins together to speed up and localize communication.

The property of a membrane that lets some substances cross readily while restricting others, allowing the cell to control its internal composition.

The net movement of molecules from a region of higher concentration to lower concentration directly through the lipid bilayer, without any protein help or energy input.

Passive movement of a substance down its concentration gradient through a membrane protein, requiring no energy but still limited by the number of available carriers or channels.

A difference in the amount of a dissolved substance between two regions, which stores potential energy that drives passive movement toward equal distribution.

The diffusion of water across a selectively permeable membrane from a region of lower solute concentration to one of higher solute concentration.

A specialized membrane channel that allows water molecules to pass rapidly while blocking ions and other solutes, greatly speeding up osmosis in tissues that need quick water flow.

The ability of an external solution to cause water to enter or leave a cell, described as hypertonic, hypotonic, or isotonic relative to the cell interior.

The pressure that must be applied to a solution to stop water from moving into it across a membrane, reflecting how strongly the solution draws water.

A membrane protein forming a water-filled pore through which specific ions or small molecules pass rapidly down their gradients, often gated open or closed.

A membrane protein that binds a specific solute and changes shape to move it across the bilayer, working more slowly than a channel and often able to pump against a gradient.

Movement of a substance across a membrane against its concentration gradient, driven by energy from ATP hydrolysis or from another ion's gradient.

A membrane protein that uses ATP to move three sodium ions out of the cell and two potassium ions in, maintaining the ion gradients that underlie resting potential and secondary transport.

Secondary active transport in which the downhill movement of one ion powers the uphill movement of a second substance through the same carrier protein.

The voltage difference across a membrane caused by unequal distribution of charged ions, used by cells for signaling and for powering transport.

A gated pore protein that permits specific ions to flow rapidly across the membrane, changing the membrane potential when it opens.

A membrane protein that binds a specific signaling molecule and converts that binding into a change inside the cell, such as an enzyme activation or an ion flow.

A molecule that binds specifically to a receptor, such as a hormone or neurotransmitter, and triggers or blocks the receptor's response.

The process by which an external signal is relayed through a series of molecular steps inside the cell to produce a specific response.

A receptor that crosses the membrane seven times and, when activated by a ligand, switches on an associated G protein that then triggers downstream signaling.

A membrane-associated protein that binds guanine nucleotides and toggles between active and inactive states, relaying signals from receptors to effector enzymes or channels.

A small intracellular molecule produced or released in response to receptor activation that spreads and amplifies the signal to many targets inside the cell.

A receptor that is itself an enzyme; ligand binding causes it to pair up and add phosphate groups to itself, creating docking sites that launch intracellular signaling cascades.

A molecule that binds a receptor and activates it, mimicking the natural signaling molecule's effect.

A molecule that binds a receptor without activating it, occupying the site so that the natural ligand cannot bind and thereby blocking the response.

The use of a proton gradient across a membrane to drive the synthesis of ATP as protons flow back through an enzyme complex.

A difference in hydrogen ion concentration across a membrane that stores energy, used to power ATP production and to drive secondary transport.

A membrane enzyme that lets protons flow down their gradient and uses the released energy to join ADP and phosphate into ATP.

A series of membrane-embedded protein complexes that pass electrons along, using the released energy to pump protons and build a gradient.

The process by which a cell takes in material by folding its membrane inward to form a vesicle that pinches off into the cytoplasm.

The process by which vesicles fuse with the plasma membrane to release their contents outside the cell, also delivering new lipids and proteins to the surface.

A small membrane-bound sac that carries molecules between compartments or to the cell surface, budding from one membrane and fusing with another.

A form of endocytosis in which the cell engulfs large particles or whole cells, wrapping them in membrane to form a vesicle that is then digested.

A selective uptake process in which surface receptors bind specific molecules and concentrate them into coated pits that then pinch off as vesicles.

A membrane protein that helps vesicles recognize and fuse with the correct target membrane by forming tight complexes that pull the two bilayers together.

The merging of two separate lipid bilayers into one continuous membrane, an essential step in vesicle release, fertilization, and compartment mixing.

A connection between membrane proteins and the internal protein scaffold of the cell that holds membrane components in place and shapes the cell surface.