Simple Diffusion

Simple diffusion is a form of passive transport in which small, nonpolar molecules move directly through the phospholipid bilayer from high to low concentration, down their concentration gradient, without using energy or transport proteins.

What is Simple Diffusion?

Simple diffusion is the no-frills way molecules cross the cell membrane. Small, nonpolar molecules like O₂ and CO₂ slip straight through the phospholipid bilayer on their own, moving from where they're crowded (high concentration) to where they're sparse (low concentration). No proteins, no ATP, no help required. The molecules just keep spreading until both sides are even, a point called equilibrium.

The key is structure. The inside of the membrane is hydrophobic (water-fearing), so nonpolar molecules feel right at home passing through. That's why simple diffusion is restricted to small, uncharged, nonpolar stuff. Anything large, polar, or charged gets blocked and needs facilitated diffusion (channel or transport proteins) instead. So when you see a molecule crossing the membrane, the first question is always: can it pass through the lipid itself, or does it need a protein door?

Why Simple Diffusion matters in AP Biology

Simple diffusion lives in Unit 2: Cells, right alongside topic 2.6 Facilitated Diffusion. It anchors learning objective AP Bio 2.6.A, which asks you to explain how the structure of a molecule affects its ability to pass through the plasma membrane. EK 2.6.A.2 contrasts simple diffusion with facilitated diffusion, where large polar molecules need proteins, so understanding simple diffusion is what makes that contrast click. The big theme here is structure determining function. The chemical structure of a molecule (size, polarity, charge) literally decides whether it can cross a membrane on its own, which connects straight to how cells maintain homeostasis by controlling what gets in and out.

How Simple Diffusion connects across the course

Facilitated Diffusion (Unit 2)

These two are siblings. Both are passive (down the gradient, no energy), but simple diffusion is for small nonpolar molecules going straight through the lipid, while facilitated diffusion uses channel or transport proteins for charged ions and large polar molecules that can't sneak through alone.

Concentration Gradient (Unit 2)

The concentration gradient is the engine of simple diffusion. Molecules move from high to low concentration, and the steeper the difference between the two sides, the faster they diffuse. No gradient means no net movement.

Osmosis (Unit 2)

Osmosis is basically simple diffusion's water-specific cousin. Water moves down its own gradient across the membrane, though in cells aquaporins (EK 2.6.A.3) speed up large-volume water transport rather than relying on slow passage through the lipid alone.

Active Transport (Unit 2)

Active transport is the opposite of simple diffusion. It pushes molecules against the gradient (low to high) and burns ATP to do it. If a process needs energy, it is not simple diffusion.

Is Simple Diffusion on the AP Biology exam?

Expect simple diffusion in MCQ stems about how substances cross membranes. A classic stem asks how small nonpolar molecules typically cross the cell membrane, and the answer is direct passage through the lipid bilayer by simple diffusion. Gas exchange questions (O₂ and CO₂ across alveolar membranes in the lungs) are also simple diffusion, driven by concentration gradients. You'll often need to distinguish it from facilitated diffusion (glucose uptake, which needs a transport protein) and active transport (moving ions against their gradient, which needs energy). On FRQs, you may have to justify why a given molecule uses simple diffusion based on its size and polarity, or trace water movement across membranes, as in the 2021 PKD long FRQ tying water movement to ion movement.

Simple Diffusion vs Facilitated Diffusion

Both are passive transport moving molecules down their concentration gradient with no energy, so they're easy to mix up. The difference is the path. Simple diffusion goes straight through the phospholipid bilayer and only works for small nonpolar molecules. Facilitated diffusion requires channel or transport proteins because the molecules (charged ions like Na⁺ and K⁺, or large polar molecules) can't cross the lipid on their own.

Key things to remember about Simple Diffusion

• Simple diffusion is passive transport: molecules move from high to low concentration with no energy and no proteins.

• Only small, nonpolar, uncharged molecules like O₂ and CO₂ can use simple diffusion, because the membrane's interior is hydrophobic.

• If a molecule is large, polar, or charged, it can't use simple diffusion and instead needs facilitated diffusion through a protein.

• Net movement stops at equilibrium, when concentrations on both sides of the membrane are equal.

• The bigger the concentration gradient, the faster simple diffusion happens.

• Simple diffusion supports AP Bio 2.6.A, the idea that a molecule's structure determines whether it can cross the membrane.

Frequently asked questions about Simple Diffusion

What is simple diffusion in AP Bio?

Simple diffusion is a type of passive transport where small nonpolar molecules move directly through the phospholipid bilayer from an area of high concentration to low concentration, without using energy or transport proteins. It stops when both sides reach equilibrium.

Does simple diffusion require energy?

No. Simple diffusion is passive, meaning it never uses ATP. Molecules move down their concentration gradient (high to low) on their own. If a process needs energy, it's active transport instead.

How is simple diffusion different from facilitated diffusion?

Both are passive and move molecules down the gradient, but simple diffusion goes straight through the lipid bilayer and only works for small nonpolar molecules. Facilitated diffusion uses channel or transport proteins because charged ions (like Na⁺ and K⁺) and large polar molecules can't cross the membrane alone.

Why can't glucose use simple diffusion?

Glucose is too large and too polar to slip through the hydrophobic core of the membrane. It uses facilitated diffusion through a transport protein, which is a common MCQ answer for glucose uptake.

Is gas exchange in the lungs simple diffusion?

Yes. O₂ and CO₂ are small nonpolar molecules, so they cross alveolar membranes by simple diffusion, driven by their concentration gradients. This is a frequent exam example of the process in action.