Cell Membrane Structure and Transport Mechanisms

Plasma membrane

The membrane surrounding all cells, defining the cell's boundary and separating its internal contents from the external environment.

Fluid mosaic model

A model describing the plasma membrane as a fluid structure within which molecules move laterally, composed of a mixture of lipids and proteins.

Lipid bilayer

A structure formed of two layers of lipids in which the hydrophilic heads face the outside surfaces and the hydrophobic tails are sandwiched in between, isolated from the aqueous environment.

Phospholipid

A major type of lipid found in cell membranes, composed of a glycerol backbone attached to a phosphate group (hydrophilic head) and two fatty acids (hydrophobic tails), making the molecule amphipathic.

Amphipathic

A molecule with both hydrophilic ('water-loving') and hydrophobic ('water-fearing') regions.

Hydrophilic

Having an affinity for water; polar.

Hydrophobic

Lacking an affinity for water; nonpolar.

Micelle

A spherical structure formed by wedge-shaped lipids with bulky heads and a single hydrophobic tail.

Liposome

Spherical bilayer structures formed spontaneously by phospholipids in an aqueous environment that surround a central space.

Lipid raft

Defined patches within the lipid bilayer where specific types of lipids and proteins assemble.

Lipid flip-flop

The spontaneous transfer of a lipid between the two layers of the lipid bilayer, which is very rare.

Integral membrane protein

Proteins permanently associated with cell membranes that cannot be separated without destroying the membrane.

Transmembrane protein

A type of integral membrane protein that spans the entire lipid bilayer.

Peripheral membrane protein

Proteins temporarily associated with the lipid bilayer or integral membrane proteins through weak noncovalent interactions.

Fluorescence recovery after photobleaching (FRAP)

An experimental technique used to demonstrate the mobility of proteins in the cell membrane.

Homeostasis

The active maintenance of a constant internal environment within a cell or organism.

Selectively permeable (semipermeable)

A property of the plasma membrane that allows some molecules to pass through freely, others only under certain conditions, and prevents others from passing at all.

Passive transport

Movement of molecules across the plasma membrane that does not require energy input from the cell; driven by diffusion.

Diffusion

The random movement of molecules, leading to a net movement from an area of higher concentration to an area of lower concentration.

Concentration gradient

A difference in the concentration of a molecule between two regions.

Simple diffusion

Movement of molecules directly across the lipid bilayer down their concentration gradient.

Facilitated diffusion

Movement of molecules across the plasma membrane down their concentration gradient through protein transporters.

Channel

A type of membrane transporter that provides an opening through the membrane for specific molecules to pass.

Gated channel

A membrane channel that opens in response to a signal (chemical or electrical).

Carrier

A type of membrane transporter that binds to specific molecules and undergoes a conformational change to transport them across the membrane.

Osmosis

The net movement of a solvent (such as water) across a selectively permeable membrane from a region of higher solvent concentration (lower solute concentration) to a region of lower solvent concentration (higher solute concentration).

Aquaporin

Specific protein channels in cell membranes that facilitate the transport of water molecules by facilitated diffusion.

Active transport

Movement of substances across the plasma membrane against a concentration gradient, requiring energy input from the cell.

Pump

A type of transport protein used in active transport that uses energy directly to move a substance.

Primary active transport

Active transport that uses energy directly, typically from ATP hydrolysis.

Antiporter

A protein transporter that moves two molecules in opposite directions across the membrane during active transport.

Symporter (cotransporter)

A protein transporter that moves two molecules in the same direction across the membrane during active transport.

Secondary active transport

Active transport driven by the potential energy stored in an electrochemical gradient, rather than directly by ATP.

Electrochemical gradient

A gradient that has both a charge component (electrical gradient) and a chemical component (concentration gradient), particularly for ions.

Hypertonic solution

A solution with a higher solute concentration than the cell.

Hypotonic solution

A solution with a lower solute concentration than the cell.

Isotonic solution

A solution with the same solute concentration as the cell.

Lysis

The bursting of a cell, often due to excessive water intake in a hypotonic environment.

Contractile vacuole

A compartment found in some single-celled organisms that takes up excess water from inside the cell and expels it into the external environment.

Cell wall

A structural layer external to the plasma membrane in many organisms (plants, fungi, bacteria) that provides structural support and protection.

Hydrostatic pressure (turgor pressure)

The force exerted by water pressing against an object, such as the pressure of water inside a plant cell against the cell wall.

Vacuole

A large organelle found in plant cells that can absorb water and contribute to turgor pressure, also storing nutrients, ions, and wastes.