In AP Bio, passive transport is the movement of molecules across a cell membrane without any energy (ATP) input, driven by the concentration gradient as the system moves toward equilibrium. It includes simple diffusion, facilitated diffusion, and osmosis.
Passive transport is how stuff crosses a cell membrane for free, meaning the cell spends no ATP. The driving force is the concentration gradient. Molecules naturally drift from where they're crowded to where they're sparse until things even out at equilibrium. Think of it like a crowd spreading out across a field on its own, no one pushing.
There are a few flavors. Small, nonpolar molecules (like O₂ and CO₂) slip straight through the lipid bilayer in simple diffusion. Bigger or charged things need help. Facilitated diffusion uses channel or transport proteins so large polar molecules and charged ions can pass down the gradient (EK 2.6.A.1, EK 2.6.A.2). Sodium (Na⁺) and potassium (K⁺), for example, can't cross the hydrophobic interior alone, so they ride through channel proteins. And osmosis is just passive transport for water, often through aquaporins (EK 2.6.A.3), moving from high water potential to low water potential. The common thread: no energy, always down the gradient.
Passive transport sits at the heart of Unit 2: Cells. It directly supports LO 2.6.A (how a molecule's structure affects whether it crosses the membrane) and LO 2.7.A (how concentration gradients drive movement). It's the mechanism behind tonicity, osmosis, and water potential (ψ = ψₚ + ψₛ), which the exam expects you to calculate and interpret. It also ties into LO 2.9.A and 2.9.B, because compartmentalization by internal membranes only works if cells can control what passively moves where. Bigger picture, this connects to the enduring idea that cells maintain homeostasis through the constant, energy-free movement of molecules across membranes.
Keep studying AP Biology Unit 2
Active Transport (Unit 2)
Active transport is the mirror image. It burns ATP to push molecules against their gradient, while passive transport coasts down the gradient for free. If a process stops when you block ATP, it was active, not passive.
Tonicity and Osmoregulation (Unit 2)
Osmosis (passive water movement) is what makes a cell swell in a hypotonic solution or shrivel in a hypertonic one. Organisms like protists use a contractile vacuole to bail out the water that floods in passively, linking passive transport to survival.
Channel Proteins and Facilitated Diffusion (Unit 2)
Channel proteins are the doorways that make facilitated diffusion possible. Charged ions like Na⁺ and K⁺ can't cross the lipid core, so they pass through these proteins, still without energy, still down the gradient.
Cell Compartmentalization (Unit 2)
Internal membranes create separate compartments by limiting what diffuses freely. Passive transport explains why some molecules stay penned inside an organelle while others slip across, which is how a cell keeps competing reactions apart.
On the multiple-choice section, passive transport shows up in scenario stems that ask you to tell it apart from active transport. A classic move: a researcher treats cells with an ATP synthase inhibitor and the transport stops, so it must have been active, not passive. Another common stem describes glucose moving across a membrane and asks which scenario passive transport could NOT explain (answer: when glucose moves into a region where it's already more concentrated). For FRQs, you'll apply the water potential equation, predict the direction of water movement, and reason about liposome or red blood cell experiments. The 2024 free-response, for example, tested whether aging red blood cells lose the ability to take up glucose, which is exactly the kind of facilitated-diffusion thinking this term covers. Your job is to connect membrane structure, gradients, and energy use into one coherent explanation.
Both move molecules across membranes, but the energy and direction differ. Passive transport uses no ATP and goes down the concentration gradient toward equilibrium. Active transport burns ATP to move molecules up the gradient, away from equilibrium. The quick test: if blocking ATP stops the movement, it's active; if it keeps going, it's passive.
Passive transport moves molecules across a membrane with zero energy input, always down the concentration gradient toward equilibrium.
It comes in three forms: simple diffusion (small nonpolar molecules straight through the bilayer), facilitated diffusion (large or charged molecules through channel/transport proteins), and osmosis (water, often via aquaporins).
Charged ions like Na⁺ and K⁺ and large polar molecules need protein help, but as long as no ATP is used and they move down the gradient, it's still passive.
Osmosis follows water potential (ψ = ψₚ + ψₛ): water moves from high water potential to low water potential, or from low solute concentration to high solute concentration.
The fastest way to spot the difference on the exam: if blocking ATP stops the transport, it was active, not passive.
Passive transport is the movement of molecules across a cell membrane without the cell using energy, driven by the concentration gradient as the system moves toward equilibrium. It includes simple diffusion, facilitated diffusion, and osmosis.
No. That's the whole point. Passive transport runs on the energy already stored in the concentration gradient, so it uses no ATP. If a process stops when ATP is blocked, it was active transport, not passive.
Passive transport uses no energy and moves molecules down their concentration gradient toward equilibrium. Active transport uses ATP to pump molecules against their gradient, away from equilibrium.
Facilitated diffusion is passive. It uses channel or transport proteins to help large polar molecules and charged ions cross the membrane, but no ATP is used and substances still move down the concentration gradient (EK 2.6.A.2).
Yes. Osmosis is the passive movement of water across a membrane, often through aquaporins, from regions of high water potential to low water potential. It's exactly what you use the ψ = ψₚ + ψₛ equation to predict on the exam.