Describe the fluid mosaic model of the cell membrane and explain diffusion, facilitated diffusion, and osmosis.
The cell membrane is built as a phospholipid bilayer: each phospholipid has a water-attracting (hydrophilic) phosphate head and two water-repelling (hydrophobic) fatty acid tails, so in water the molecules spontaneously arrange into two layers with the heads facing outward on both sides and the tails hidden in the middle. Scattered through this bilayer are proteins that act as channels, carriers, and receptors, along with cholesterol molecules that help keep the membrane stable across a range of temperatures. Because the whole structure is a shifting mix of moving lipids and floating proteins rather than a fixed wall, this is called the fluid mosaic model. The membrane is selectively permeable: small, nonpolar molecules like oxygen and carbon dioxide slip through the lipid bilayer directly, while larger or charged molecules need specific help to cross.
Passive transport moves substances without using cellular energy, always down a concentration gradient, from an area of higher concentration to an area of lower concentration. Simple diffusion lets small nonpolar molecules cross directly through the bilayer; facilitated diffusion lets larger or polar molecules, like glucose, cross through a specific channel or carrier protein instead. Osmosis is simply the diffusion of water itself, often through channel proteins called aquaporins, and it depends on comparing solute concentration on the two sides of a membrane: a hypertonic solution has a higher solute concentration than the cell (so water leaves the cell), a hypotonic solution has a lower solute concentration than the cell (so water enters the cell), and an isotonic solution matches the cell's own concentration (so there is no net water movement). Too much water leaving an animal cell in a hypertonic solution causes it to shrivel (crenation); too much water entering an animal cell in a hypotonic solution can cause it to swell and burst (lysis); a plant cell in the same hypertonic situation loses enough water that its membrane pulls away from its rigid cell wall, a condition called plasmolysis.
Mastery looks like: They can describe the fluid mosaic model, distinguish simple diffusion, facilitated diffusion, and osmosis, and correctly predict water movement in hypertonic, hypotonic, and isotonic conditions.
Common stumbles: Mixing up which direction water moves in hypertonic vs. hypotonic solutions; assuming facilitated diffusion requires cellular energy the way active transport does.