Small nonpolar molecules such as O₂ and CO₂ dissolve in the lipid and slip straight through the bilayer, moving down their concentration gradient. No protein and no ATP are needed.
Every cell is wrapped in a thin border about $7\text{–}8$ nm thick — the cell membrane. It is not a solid wall but an oily film that lets some things through and stops others. Think of it as a nightclub door: tiny, "cool" molecules slip in unnoticed, while big or charged ones need a bouncer to escort them, and a few VIPs are even carried uphill against the crowd.
The film is a phospholipid bilayer. Each phospholipid has a water-loving head and two water-fearing tails. Drop them in water and they self-assemble: heads point outward toward the watery extracellular fluid and cytoplasm, tails hide in the middle. Embedded proteins form channels and pumps. Because the molecules drift sideways (fluid) and the proteins dot the surface (mosaic), this is the fluid mosaic model.
Three routes are passive (no ATP, always down the gradient): simple diffusion sends small nonpolar molecules straight through the lipid; facilitated diffusion sends glucose and ions through a channel protein; osmosis sends water toward the saltier side. One route is active: the Na⁺/K⁺ pump burns ATP to push ions up their gradient, exchanging $3\,\text{Na}^+$ out for $2\,\text{K}^+$ in. The gradient slider sets how steep the concentration difference is, and the temperature slider sets how fast molecules jiggle — both speed up passive flow, but the pump runs on ATP regardless of the gradient.
Start in simple diffusion and push the gradient to zero — net flow nearly stops (equilibrium), even though molecules keep moving. Switch to facilitated diffusion and watch glucose funnel through the blue channel instead of the lipid. In osmosis, follow the water. Finally choose active transport and notice the ions travel the "wrong" way, up their gradient, only because the pump spends ATP.
| Mode | What moves & how | Energy |
|---|---|---|
| Simple diffusion | Small nonpolar molecules (O₂, CO₂) dissolve in and cross the lipid bilayer, down the gradient | Passive — no ATP |
| Facilitated diffusion | Glucose and ions pass through channel or carrier proteins, down the gradient | Passive — no ATP |
| Osmosis | Water diffuses across (through lipid and aquaporins) toward the side with more solute | Passive — no ATP |
| Active transport | Na⁺/K⁺ pump moves 3 Na⁺ out and 2 K⁺ in, up their gradients | Active — uses ATP |
Diffusion is just random molecular motion averaged over huge numbers of particles. When one side is more crowded, more molecules happen to wander away from it than toward it, so there is a net flow from high to low concentration. A steeper gradient means a bigger imbalance and faster net flow; a higher temperature means faster jiggling and again faster flow. At equilibrium the concentrations are equal, molecules still cross both ways, but the two flows cancel, so nothing changes overall.
The pump moves both ions against their gradients, which cannot happen on its own, so it spends one ATP per cycle. Sending out three positive charges for every two it brings in helps make the inside of the cell slightly negative. These steep Na⁺ and K⁺ gradients store energy the cell reuses — for example, to fire nerve impulses, contract muscles, and drag glucose into intestinal cells by coupling it to the Na⁺ flowing back in (secondary active transport).
The cell membrane is a phospholipid bilayer: two sheets of phospholipids arranged tail-to-tail. Each phospholipid has a hydrophilic phosphate head and two hydrophobic fatty-acid tails. The heads face the watery fluid on both sides while the tails huddle in the middle, away from water. Proteins, cholesterol, and attached sugar chains are scattered throughout, which is why it is called the fluid mosaic model.
Key takeaway: a self-assembling oily bilayer studded with proteins separates the inside of the cell from the outside.Passive transport moves substances down their concentration gradient, from more crowded to less crowded, and needs no cell energy. Simple diffusion, facilitated diffusion, and osmosis are all passive. Active transport moves substances up their gradient, from low to high, which is uphill and requires energy, usually ATP. The Na⁺/K⁺ pump is the classic example.
Key takeaway: passive transport coasts downhill for free; active transport spends ATP to push molecules uphill.Both are passive and both move molecules down their gradient, so neither uses ATP. The difference is the route. In simple diffusion small nonpolar molecules such as oxygen and carbon dioxide slip straight through the lipid bilayer. In facilitated diffusion larger or charged particles such as glucose and ions cannot cross the oily middle, so they pass through specific transport proteins (channels or carriers) that span the membrane.
Key takeaway: simple diffusion goes through the lipid, facilitated diffusion goes through a protein, but both flow downhill without energy.Osmosis is the diffusion of water across a selectively permeable membrane. Water moves from a side with more water and less solute (hypotonic) toward a side with less water and more solute (hypertonic), evening out the concentrations. Small amounts of water cross the bilayer directly, and cells with heavy water traffic also use protein channels called aquaporins. Osmosis is why a cell in pure water swells and one in salty water shrinks.
Key takeaway: in osmosis, water diffuses toward the saltier side to balance solute concentrations.The Na⁺/K⁺ pump is a membrane protein that uses one ATP to move three sodium ions out of the cell and two potassium ions in, both against their gradients. Binding of Na⁺ and splitting of ATP change the pump's shape so it flips the ions to the other side, then resets. Because it exports three positive charges for every two it imports, it helps keep the cell interior negatively charged, which is essential for nerve and muscle cells.
Key takeaway: the pump spends ATP to swap 3 Na⁺ out for 2 K⁺ in, building the gradients cells rely on.The middle of the membrane is a layer of fatty-acid tails that is oily and nonpolar. Oxygen is a small, nonpolar molecule, so it dissolves easily in this layer and slips through by simple diffusion. Glucose is much larger and polar, with many atoms that attract water, so it cannot dissolve into the oily core and must travel through a transport protein. Ions face the same problem because they are charged and strongly repelled by the nonpolar interior.
Key takeaway: small nonpolar molecules cross the lipid directly, while large or charged ones need a protein doorway.Individual molecules never stop, but the net movement stops at equilibrium. Diffusion continues as long as there is a concentration gradient, and the steeper the gradient and the higher the temperature, the faster the net flow. Once the concentration is equal on both sides, molecules still cross in both directions but at the same rate, so there is no net change. This balanced state is dynamic equilibrium, not a frozen stop.
Key takeaway: molecules keep jiggling forever, but net diffusion ends when concentrations are equal on both sides.