AP Biology Unit 2: Comprehensive Guide to Membrane Structure & Permeability

Plasma Membrane

The boundary that separates the living cell from its surroundings.

Fluid Mosaic Model

Concept framing the plasma membrane as a mosaic of protein molecules in a fluid bilayer of phospholipids.

Phospholipid Bilayer

Structural framework of the membrane composed of phospholipids.

Amphipathic

Molecules that possess both a hydrophilic region and a hydrophobic region.

Hydrophilic Head

Part of phospholipids composed of a phosphate group and glycerol, oriented outward.

Hydrophobic Tails

Part of phospholipids made of two fatty acid chains, oriented inward.

Membrane Fluidity

The property of membrane molecules shifting laterally and not being static.

Saturated Tails

Fatty acid tails that pack tightly together, increasing membrane viscosity.

Unsaturated Tails

Fatty acid tails with kinks that prevent tight packing and increase fluidity.

Cholesterol

A wedge-shaped steroid that acts as a temperature buffer for membrane fluidity.

Integral Proteins

Proteins that penetrate the hydrophobic interior of the lipid bilayer.

Peripheral Proteins

Proteins loosely bound to the surface of the membrane.

Glycolipids

Carbohydrates covalently bonded to lipids.

Glycoproteins

Carbohydrates covalently bonded to proteins.

Selective Permeability

The membrane's ability to regulate which substances can cross more easily.

Simple Diffusion

The passage of small, nonpolar molecules across the membrane without aid.

Transport Proteins

Proteins that span the membrane and facilitate the movement of hydrophilic substances.

Channel Proteins

Proteins with a hydrophilic channel for specific molecules or ions to pass through.

Aquaporins

Specialized channel proteins that facilitate the passage of water molecules.

Carrier Proteins

Proteins that bind to specific molecules and change shape to shuttle them across the membrane.

Cell Walls

Extracellular structures in plants, prokaryotes, and fungi that contribute to structural integrity.

Composition of Cell Walls in Plants

Made of cellulose.

Composition of Cell Walls in Fungi

Made of chitin.

Composition of Cell Walls in Bacteria

Made of peptidoglycan.

Water Permeability

Water diffuses slowly across the membrane but moves rapidly via aquaporins.

Mnemonic for Fluid Mosaic

Fluid describes the lateral movement; Mosaic describes the diverse components of the membrane.

Common Mistake: Confusing Cell Wall and Cell Membrane

All cells have membranes; only some have walls; membranes control transport.