3.1 The Cell Membrane

Cell membrane

The pliable outer plasma barrier that separates the inner contents of a cell from its exterior environment and regulates the passage of materials.

Phospholipid bilayer

The dual-layered foundation of the cell membrane consisting of back-to-back phospholipids arranged tail-to-tail.

Hydrophilic head

The polar, negatively charged phosphate group of a phospholipid that is attracted to water and faces the aqueous intracellular and extracellular fluid.

Hydrophobic tail

The non-polar lipid region of a phospholipid consisting of fatty acids that faces inward to exclude watery fluids from the inner membrane.

Intracellular fluid (ICF)

The watery fluid interior enclosed within the boundaries of a cell's plasma membrane.

Extracellular fluid (ECF)

The fluid environment blanketing the outside of a cell's membrane structure.

Fluid mosaic model

The concept describing the cell membrane as a flexible, dynamic surface containing shifting, laterally mobile lipids, proteins, and carbohydrates.

Cholesterol (Membrane)

A lipid molecule tucked between phospholipids that maintains membrane fluidity at low temperatures and stiffens the barrier at high temperatures.

Integral protein

Any membrane protein that is directly embedded into the hydrophobic core of the phospholipid bilayer, often spanning across the entire membrane.

Transmembrane protein

An integral protein that stretches entirely across the lipid bilayer, protruding from both the intracellular and extracellular sides.

Peripheral protein

A protein temporarily attached to either the inner or outer surface of the phospholipid bilayer rather than embedded within the core.

Channel protein

A specific type of integral transmembrane protein that forms an open pore to selectively allow particular materials or ions to cross the membrane.

Carrier protein

A transport protein that binds to a specific substance and undergoes conformational shape changes to move the material across the membrane.

Cell recognition protein

An integral protein serving to mark a cell's specific molecular identity so it can be distinguished by other cells.

Receptor

A specialized recognition protein that selectively binds a specific extracellular molecule, inducing a downstream chemical cascade within the cell.

Ligand

The specific extracellular molecule (such as a hormone or neurotransmitter) that binds to and activates a membrane receptor protein.

Glycoprotein

An integral membrane protein that features attached carbohydrate molecules extending outward into the extracellular environment.

Glycolipid

A membrane lipid molecule that has a carbohydrate sugar chain directly attached to its polar head, extending into the extracellular fluid.

Glycocalyx

The fuzzy, carbohydrate-rich coating around the cell formed collectively by the sugars branching off membrane proteins and lipids.

Glycocalyx identity function

The physiological role of the glycocalyx to establish a unique genetic identity, allowing immune defense cells to know not to attack body cells.

Selective permeability

The property of a cell membrane that allows only substances meeting specific criteria (like small, nonpolar materials) to pass through it unaided.

Amphipathic

A molecule possessing both a distinct hydrophilic (water-loving) region and a distinct hydrophobic (water-fearing) region, such as a phospholipid.

Passive transport permeability

The feature allowing small, uncharged, nonpolar molecules like oxygen, carbon dioxide, and alcohol to diffuse across the lipid bilayer unaided.

Passive transport barrier

The property of the hydrophobic lipid core that blocks large, polar, or highly charged molecules (like glucose, amino acids, and ions) from passing unaided.