Choosing What Gets In
A membrane is a fluid mosaic: a lipid bilayer with proteins floating in and through it. It is selectively permeable: small nonpolar molecules such as oxygen and carbon dioxide slip through the lipid core unaided, while ions and large polar molecules cannot and need a protein.
Transport divides by whether the cell pays:
| Type | Direction | Energy | Example |
|---|---|---|---|
| Simple diffusion | Down the gradient | None | Oxygen, carbon dioxide |
| Facilitated diffusion | Down the gradient | None | Glucose via GLUT |
| Osmosis | Water, down its gradient | None | Water via aquaporins |
| Active transport | Against the gradient | ATP | The sodium-potassium pump |
Osmosis is the movement of water across a semipermeable membrane from lower to higher solute concentration. A cell in a hypotonic solution gains water and swells; in a hypertonic one it loses water and shrinks; in an isotonic one there is no net movement. Plant cells resist bursting because their wall builds up turgor pressure, animal cells have no such protection, which is why intravenous fluids must be isotonic.
ATP is the cell's energy currency. Hydrolysing its terminal phosphate releases about 30.5 kJ/mol under standard conditions, and that release is coupled to work the cell needs done. Cells hold only seconds' worth of ATP and continually regenerate it, a human turns over roughly their own body mass in ATP each day, not by holding it but by cycling it.
Energy is extracted through redox reactions, where oxidation is the loss of electrons and reduction the gain. Electron carriers such as NAD⁺ and FAD shuttle electrons to the chain that ultimately makes ATP. The mnemonic worth keeping is OIL RIG: oxidation is loss, reduction is gain.
Common pitfall: describing osmosis as water moving "to where there is less water". True but unhelpful, the useful statement is that water moves toward the higher solute concentration, because that is the quantity you are given in a problem.