Which Can Move Easily Across the Membrane Without Facilitated Diffusion

Small, nonpolar molecules such as oxygen (O₂), carbon dioxide (CO₂), and steroid hormones can move easily across the cell membrane without facilitated diffusion, because they dissolve directly through the lipid bilayer in a process called simple diffusion. Water and ethanol also cross, though water’s passage is more limited and often aided by channels. The membrane is a phospholipid bilayer—two layers of fat-like molecules with water-loving heads facing outward and water-fearing tails facing inward—and only substances that can slip through that oily interior cross unaided. Anything large, charged, or strongly polar generally needs a transport protein, which is what “facilitated diffusion” means: protein-assisted movement down a concentration gradient.

Simple Diffusion Basics

Simple diffusion is the spontaneous movement of particles from an area of higher concentration to an area of lower concentration, driven entirely by random molecular motion. No energy input from the cell is required, and no protein is involved. The Khan Academy biology treatment of transport makes the key point: the only requirement is a concentration gradient and a membrane the substance can pass through.

Why do some molecules slip through the bilayer? The membrane’s interior is a hydrophobic (water-fearing) zone formed by the fatty-acid tails of phospholipids. A molecule that is itself nonpolar—having no separated electrical charges—blends into that oily region and diffuses across. A molecule that is polar or charged is repelled by it.

Think of it like a drop of oil in water: oil does not mix with water, but it merges easily with other oil. Nonpolar gases behave similarly inside the membrane. This is why the lipid bilayer acts as a selective barrier rather than a solid wall—it is permissive to some substances and restrictive to others.

The rate of simple diffusion depends on the size of the concentration gradient, temperature, and the molecule’s solubility in lipids. A steeper gradient and higher lipid solubility mean faster crossing. Notably, no saturation occurs: a larger concentration difference produces a proportionally faster diffusion rate, because no protein “workers” can be maxed out.

Diffusion is passive—it needs no ATP. The cell does not “pump” these molecules; they simply drift until the concentration is equal on both sides. This is fundamentally different from active transport, which uses energy to push substances against their gradient.

Molecules That Cross Freely

The substances that cross the lipid bilayer most readily share two traits: they are small and nonpolar. Well-established examples include:

• Oxygen (O₂) — a tiny, nonpolar gas that diffuses from the lungs into blood and from blood into cells. It crosses membranes essentially unimpeded.
• Carbon dioxide (CO₂) — also small and nonpolar, and even more lipid-soluble than oxygen. It diffuses out of cells as a waste product.
• Steroid hormones — cortisol, testosterone, estrogen, and aldosterone are built from cholesterol, making them lipid-soluble. They diffuse straight through the plasma membrane to reach receptors inside the cell. The NCBI Bookshelf describes steroid hormones as acting on intracellular receptors precisely because they enter cells this way.
• Small lipids and fatty acids — being lipid-like themselves, they dissolve into the bilayer and pass through.
• Ethanol — a small polar molecule that nevertheless crosses membranes fairly easily because of its small size and partial lipid solubility.
• Water (H₂O) — crosses to a limited degree directly, and much more readily through aquaporin channels.

A useful worked example: in a working muscle, O₂ diffuses down its gradient from the blood into the muscle cell, while CO₂ produced by metabolism diffuses the opposite direction into the blood. Neither step requires a transport protein. A steroid hormone such as cortisol released into the bloodstream can enter nearly any target cell by the same passive route, which is one reason steroid hormones act broadly and relatively slowly compared with water-soluble hormones that must bind surface receptors.

Notably, none of these molecules require a channel, carrier, or energy. They are the “easy passengers” of the membrane.

Why Size and Polarity Matter

Membrane permeability follows a few reliable rules. Small and nonpolar passes; large or charged does not. This is the single most useful way to predict whether facilitated diffusion is needed.

Two properties govern crossing:

• Size — Smaller molecules diffuse faster. O₂ and CO₂ are among the smallest biologically relevant molecules, so they cross quickly. Glucose, by contrast, is a large polar molecule and cannot slip through, despite being uncharged. It requires GLUT carrier proteins.
• Polarity and charge — Nonpolar molecules dissolve in the lipid core. Polar molecules are repelled by it. Ions such as Na⁺, K⁺, Ca²⁺, and Cl⁻ are charged and essentially cannot cross the bare bilayer at any meaningful rate—they need ion channels. The Nature Education Scitable overview of membrane transport notes that the hydrophobic core blocks ions and most polar solutes.

A simple decision rule:

So the answer to whether facilitated diffusion is required comes down to a quick check: if the molecule is small and can dissolve in fat, it goes through on its own; if it is big or carries a charge, it needs help.

Key Takeaways

• Direct answer: Small, nonpolar molecules—O₂, CO₂, steroid hormones, small lipids, and ethanol—move easily across the membrane without facilitated diffusion. Water crosses directly but slowly, and usually uses aquaporins.
• Simple diffusion is passive movement down a concentration gradient; it needs no ATP and no transport protein.
• Two rules predict crossing: small size and nonpolar character. Large or charged molecules need channels or carriers.
• Glucose and ions cannot cross the bare bilayer—glucose needs carriers, ions need channels.
• Water is a partial exception: it crosses directly at a limited rate, with aquaporins handling most of the flow.
• For personal health questions, consult a qualified professional.