Summary of Biomembrane Structure, Function, and Transport

## Introduction Biomembranes are dynamic, predominantly lipid-based structures that define cellular boundaries and compartmentalize intracellular spaces. They regulate interactions between the cell and its environment, organize biochemical processes, and provide mechanical and signaling functions essential for life. > **Definition:** A biomembrane is a primarily lipid-based barrier formed by one or more leaflets of amphipathic lipids and associated proteins that separates aqueous compartments and organizes cellular processes. ## Structure of Biomembranes ### Lipid bilayer architecture - The basic structural unit is the **lipid bilayer**, formed by amphipathic lipids with hydrophilic headgroups facing aqueous phases and hydrophobic tails packed inward. - Typical thickness: nanometers scale (a few nm), with embedded proteins and associated carbohydrates. > **Definition:** An amphipathic lipid is a molecule that contains both a hydrophilic (polar) head and a hydrophobic (nonpolar) tail. ### Real-life shapes and curvature - In cells, bilayers are not flat. They form closed and curved surfaces (spheres, tubes, vesicles) to avoid exposing hydrophobic edges. - Curvature is stabilized by lipid composition, membrane proteins, and cytoskeletal interactions. ### Membrane asymmetry (functional asymmetry) - The two leaflets (inner and outer) have **different lipid compositions** and **different proteins**. - Functional consequences: - Outer leaflet often enriched in glycolipids and certain phospholipids that interact with the extracellular environment. - Inner leaflet enriched in phosphatidylserine and phosphatidylinositol derivatives involved in signaling. - Asymmetry enables directional signaling, selective recognition, and compartment-specific activities. > **Definition:** Membrane asymmetry means the two sides (leaflets) of the bilayer differ in lipid species, charge, and associated proteins, producing distinct functional surfaces. ## Lipid monolayers in the body - Monolayers occur where a hydrophobic core interfaces directly with aqueous or gaseous phases, so a second polar leaflet is unnecessary. - Examples: - **Lipid droplets:** Neutral lipid core (triacylglycerols) is surrounded by a single phospholipid monolayer facing the cytosol; the monolayer stabilizes the hydrophobic core and provides docking sites for proteins involved in lipid metabolism. - **Pulmonary surfactant:** At the air–liquid interface in alveoli, a lipid monolayer (rich in phospholipids like dipalmitoylphosphatidylcholine) reduces surface tension and stabilizes alveoli during breathing. > **Definition:** A lipid monolayer is a single layer of amphipathic lipids with hydrophilic heads oriented to the aqueous phase and hydrophobic tails facing the hydrophobic interior or air. Fun fact: Lipid droplets are not inert fat blobs; their monolayer surface hosts enzymes that dynamically control lipid storage and release. ## Do all organelles have a single lipid bilayer? - No. Organelle membranes vary: - **Single bilayer**: Endoplasmic reticulum, Golgi apparatus, endosomes, lysosomes, plasma membrane. - **Double membrane**: Nucleus (nuclear envelope), mitochondria (inner and outer membranes), chloroplasts (in plants). - **No limiting lipid membrane**: Ribosomes, cytoskeleton filaments, many protein complexes. - Membrane permeability and composition differ between organelles to suit their functions. ## Membrane permeability and selectivity - Membranes are **selective barriers**. Permeability depends on: - Lipid composition (saturation, cholesterol content) - Presence of specific transport proteins (channels, carriers, pumps) - Membrane thickness and packing - Passive crossing (simple diffusion) applies to: - Small nonpolar molecules (e.g., O$_2$, CO$_2$) - Very small uncharged polar molecules (e.g., H$_2$O to some extent) - Large polar or charged molecules, and ions, require proteins to cross. > **Definition:** Passive transport (diffusion)