The Phospholipid Bilayer
- Cell membranes are asymmetrical bcuz the two sides of a cell membrane face different environments and carry out different functions
- The polar phosphate regions are oriented towards the aqueous external or internal environments
- The nonpolar hydrocarbon fatty acid regions face each other inside the membrane
- Proteins may be loosely attached to inner/outer membrane or extend into the membrane
- Some are transmembrane
- Phospholipids (and some proteins) are amphipathic
- Have polar (hydrophilic) and nonpolar (hydrophobic) regions
Fluid Mosaic Model
- Membrane is made up of of amphipathic proteins embedded in fluid bilayer of phospholipids”
- Membrane is fluid bcuz of weak hydrophobic interactions in interior of membrane that allow proteins to move vertically
- Proteins and lipids move in membrane which stays fluid until temp decreases
- Fluidity of membrane depends on # of saturated and unsaturated fatty acids
- Fluidity affects permeability and protein transport
- Unsaturated hydrocarbons of some phospholipids help keep membranes fluid at lower temp.
- Double bonds stop carbons and fatty acids from packing together
- Cholesterol helps keep membrane fluid at low temp and rigid at high temp
- Lipids get excited (more energy) at high temp → increased fluidity & permeability
- Functional & structural: lipids provide structure but proteins determine function
- Fluidity of membrane depends on # of saturated and unsaturated fatty acids
Permeability
- The cell membrane is amphipathic with a hydrophobic interior, so only nonpolar molecules can pass through → selective permeability
What Can Pass Through?
- Ability of molecule to pass through depends on size, charge, and SP of membrane
- Small, uncharged, nonpolar molecules can freely pass among the membrane bcuz repel water like hydrophobic inside layer
- Do not need channel or carrier proteins
- N2, O2, CO2, H2
- Large, polar molecules and all ions (hydrophilic) are impermeable and can only move across the membrane through EMBEDDED CHANNEL AND TRANSPORT PROTEINS
- Ex: ions, monosaccharides (glucose), amino acids
- Some uncharged, polar molecules (H2O) can pass thru the membrane in small amounts
Plasma Membrane Proteins
- Channel proteins: have hydrophilic channels like a tunnel for certain molecules
- Carrier proteins: bind to molecules (ex: glucose) and change shape to shuttle them across,
- Transport Proteins: use energy (ATP) to transport materials across membrane against gradient
- Recognition proteins: give each cell unique identification → distinction between self/foreign cells & normal/infected cells
- Ex: glycoproteins
- Receptor proteins: binding sites for hormones or other trigger molecules
- Enzymes:
- Anchor proteins: attach cells to other cells or provide anchors for internal filaments
Types of Transport Proteins 
- Integral proteins: amphipathic, penetrate hydrophobic interior, some transmembrane
- Transmembrane proteins: all across membrane
- Peripheral protein: not in lipid bilayer: bound to membrane surface → hydrophilic
Cell Walls (found in plants)
- Cell walls provide a semipermeable barrier that regulates the movement of molecules into the cell
- Cell walls stop plants, fungi, some protists, and prokaryotes from bursting in hypotonic solution