Plasma Membrane Structure and Membrane Transport

Vocabulary flashcards covering the components, permeability, thermodynamics, and transport mechanisms of the plasma membrane.

Plasma membrane

A selectively permeable barrier consisting of proteins and other molecules embedded in a phospholipid bilayer.

Phospholipids

Membrane lipids that form a bilayer with hydrophilic heads facing water and hydrophobic tails facing inward.

Cholesterol

A lipid component embedded in the plasma membrane that regulates membrane fluidity and stability.

Transmembrane proteins

Integral proteins embedded in the phospholipid bilayer that typically contain alpha helices of approximately $20$ amino acids that span the membrane.

Glycoproteins

Membrane proteins with attached carbohydrates that function in cell recognition.

Aquaporins

Integral membrane proteins forming narrow hydrophilic channels of approximately $0.2\,\text{nm}$ diameter that selectively transport up to $10^9$ water molecules per second per channel while excluding ions.

Gated channels

Membrane channel proteins that open or close in response to voltage or chemical signals.

Connexins

Proteins that form gap junctions for intercellular communication.

Porins

Proteins that allow small molecules and ions to cross certain outer membranes.

Proton pumps

Transport proteins that move $\text{H}^+$ to establish an electrochemical gradient.

Calcium channels

Transport proteins that control $\text{Ca}^{2+}$ entry for processes such as muscle contraction and neurotransmitter release.

Selective permeability

The property of the plasma membrane that allows certain substances to cross more easily than others, influenced by molecular size, polarity, charge, and lipid solubility.

Passive transport

The spontaneous movement of substances down a concentration or electrochemical gradient without requiring direct energy input.

Active transport

Transport that requires an energy source and membrane proteins to move substances across a membrane against their electrochemical gradient.

Simple diffusion

A form of passive transport where molecules move from high to low concentration directly across the lipid bilayer without requiring ATP or membrane transport proteins.

Facilitated diffusion

A passive transport process where selective channel or carrier proteins allow specific molecules to move down their electrochemical gradient without ATP.

Osmosis

The net movement of water across a selectively permeable membrane from a region of low solute concentration (higher water potential) to high solute concentration (lower water potential).

First law of thermodynamics

A principle of thermodynamics stating that energy cannot be created or destroyed, only transferred or transformed.

Second law of thermodynamics

A physical law stating that the total entropy of an isolated system tends to increase during spontaneous processes.

Entropy ($S$)

A thermodynamic measure related to the number of possible arrangements of a system, often described as disorder.

ATP hydrolysis

The reaction $\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_i$ that releases usable free energy under cellular conditions to power cellular work.

Primary active transport

A mechanism of active transport that directly uses energy derived from ATP hydrolysis to move substances against their electrochemical gradients.

Secondary active transport

A transport mechanism that couples the thermodynamically favorable movement of an ion down its electrochemical gradient to drive another solute against its gradient.

Light-driven transport

Active transport that utilizes light energy (photons) to move ions against their electrochemical gradients.

Na+/K+ ATPase

An electrogenic transmembrane protein of $280\,\text{kDa}$ that hydrolyzes $1\,\text{ATP}$ to pump $3\,\text{Na}^+$ out of the cell and $2\,\text{K}^+$ into the cell.

Electrogenic pump

A transport protein that generates a net electrical charge movement across the membrane, such as moving three positive charges out for every two entering.

$\alpha$ subunit of $\text{Na}^+/\text{K}^+$ ATPase

The catalytic subunit of the sodium-potassium pump responsible for ATP hydrolysis and ion transport.

$\beta$ subunit of $\text{Na}^+/\text{K}^+$ ATPase

A subunit that helps stabilize the sodium-potassium pump and supports its proper membrane localization.

FXYD subunit

A regulatory component of the $\text{Na}^+/\text{K}^+$ ATPase that modifies its pumping activity.

Symport

A coupled transport process in which two substances move across the membrane in the same direction.

Antiport

A coupled transport process in which two substances move across the membrane in opposite directions.

Sodium-glucose cotransporter

A symport carrier protein that uses energy from the inward movement of $\text{Na}^+$ down its gradient to transport glucose into the cell against its gradient.

Bacteriorhodopsin

A light-sensitive membrane protein found in certain archaea that absorbs light energy to pump $\text{H}^+$ out of the cell, establishing an electrochemical proton gradient.