Cell Membranes and Transport
Describe phospholipid bilayer structure and explain diffusion, osmosis, facilitated diffusion, active transport, endocytosis, and exocytosis.
A lesson of Biology, in Cells and Cellular Processes, about 10 minutes.
By the end of this lesson, you will be able to describe the fluid mosaic model, explain how molecules move across membranes (passive vs. active transport), predict the direction of osmosis, and explain how membrane permeability determines cellular regulation.
The Fluid Mosaic Model
The cell membrane is a phospholipid bilayer with embedded proteins:
- Phospholipids: hydrophilic head faces water; hydrophobic tail faces inward → bilayer self-assembles
- Integral proteins: span the membrane; include channels and transporters
- Peripheral proteins: attached to surface; involved in signaling and structure
- Cholesterol: inserted between phospholipids; regulates membrane fluidity (less rigid at low temperature, less fluid at high)
- Carbohydrates (glycoproteins/glycolipids): cell recognition, immune function
Membrane Permeability
Freely permeable: small nonpolar molecules (O₂, CO₂, lipids)
Selectively permeable: small polar molecules (water, via aquaporins)
Requires transport proteins: charged ions (Na⁺, K⁺, Cl⁻), large polar molecules (glucose, amino acids)
Cannot cross directly: macromolecules (use vesicle-based endo/exocytosis)
Passive Transport (No Energy Required)
Simple diffusion: net movement from high → low concentration; continues until equilibrium.
Facilitated diffusion: channel or carrier proteins help molecules move with the concentration gradient (no ATP required).
Osmosis: diffusion of water across a selectively permeable membrane from high water potential (low solute) to low water potential (high solute).
Osmosis and Tonicity
Isotonic solution: same solute concentration as cell → no net water movement
Hypotonic solution: lower solute concentration than cell → water enters cell (animal: swells/lyses; plant: turgor, firm)
Hypertonic solution: higher solute concentration than cell → water leaves cell (animal: crenates; plant: plasmolysis)
Why plants need turgor pressure: cell wall prevents lysis in hypotonic conditions → turgid plants are firm (wilting = plasmolysis when water leaves).
Active Transport (Energy Required)
Moves molecules against their concentration gradient (low → high concentration).