Chapter 5 Membranes: The Interface Between Cells and Their Environment

Phospholipids

The primary structural component of membranes which are amphipathic molecules with hydrophobic "tails" facing in and hydrophilic "heads" facing out.

Glycolipids

Carbohydrates attached to membrane lipids, primarily found in the extracellular leaflet.

Glycoproteins

Carbohydrates attached to membrane proteins.

Fluid-mosaic model

Describes the membrane as a mosaic of lipid, protein, and carbohydrate molecules where the lipids and proteins can move relative to each other within the membrane.

Transmembrane proteins

Proteins that span both leaflets of the membrane.

Lipid-anchored proteins

Proteins in which an amino acid of the protein is covalently attached to a lipid.

Peripheral membrane proteins

Proteins noncovalently bound to regions of other proteins or to the polar portions of phospholipids.

Integral membrane proteins

Proteins that have a portion that is integrated into the hydrophobic region of the membrane (includes transmembrane and lipid-anchored proteins).

Membrane fluidity

The ability of lipids and proteins to move in 2 dimensions within the plane of the membrane.

Lipid rafts

Areas where some lipids strongly associate with each other, anchoring certain proteins.

Flippase enzyme

Enzyme that is responsible for the "Flip-flop" of lipids between leaflets, which requires energy input.

Membrane transport

A key function of membranes allowing the passage of some ions and molecules but not others.

Selective permeability

The characteristic of the plasma membrane that allows the passage of some ions and molecules but not others.

Simple diffusion

The unassisted movement of a solute across the membrane which depends on size, polarity and charge of the molecule.

Transmembrane gradient

When the concentration of a solute is higher on one side of a membrane than the other.

Isotonic solution

A solution with the same solute concentration on both sides of the membrane.

Hypertonic solution

A solution with more solute concentration compared to another solution.

Hypotonic solution

A solution with less solute concentration compared to another solution.

Osmosis

The diffusion of water across a membrane from HIGH water concentration to LOW water concentration.

Transport proteins

Transmembrane proteins that provide a passageway for the movement of ions and hydrophilic molecules across membranes.

Channels

Provide an open passageway that can facilitate the diffusion of hydrophilic molecules or ions; most are gated.

Transporters (carriers)

Bind their solutes in a hydrophilic pocket and undergo a conformational change that switches the exposure of the pocket from one side of the membrane to the other.

Uniporter

A transporter that binds and transports a single solute.

Symporter

A transporter that binds two or more solutes and transports them in the same direction.

Antiporter

A transporter that binds two or more solutes and transports them in opposite directions.

Active transport

The movement of a solute across a membrane against its gradient, requiring energy input.

Primary active transport

Directly uses energy (typically released from ATP hydrolysis) to transport a solute against its gradient.

Secondary active transport

Involves the use of energy stored in a pre-existing gradient to drive the active transport of another solute.

Gap junctions

Intercellular channels that connect animal cells, allowing direct movement of substances between adjacent cells.

Plasmodesmata

Intercellular channels that connect plant cells, allowing direct movement of substances between adjacent cells.

Desmotubule

Connects the smooth ER membranes of adjacent cells in plasmodesmata.

Exocytosis

A mechanism of vesicular transport where materials inside the cell are packaged into vesicles and excreted to the extracellular environment.

Endocytosis

A mechanism of vesicular transport where the plasma membrane invaginates (folds inward) to form a vesicle that brings substances into the cell.

Receptor-mediated endocytosis

Uses receptor proteins to bring in specific cargo.

Pinocytosis

Primarily brings in fluid, allowing cells to sample the extracellular environment.

Phagocytosis

Involves bringing in very large particles (e.g., a bacterial cell); only some cells are phagocytes.

Cell adhesion molecules (CAMs)

Integral membrane proteins that participate in forming anchoring junctions.

Cadherins and integrins

Two types of CAMs (Cell adhesion molecules).

Anchoring junctions

Link cells to each other and to the ECM.

Adherens junctions

Connect cells to each other, use cadherins, and bind actin filaments.

Desmosomes

Connect cells to each other, use cad