The cell membrane is selective — most things cannot cross it freely — and biology textbooks pile up six or seven different transport mechanisms in a single chapter. The cleanest way to organize them is one question: does the process require ATP? If no, it is passive transport, moving substances down their concentration gradient. If yes, it is active transport, moving substances against their gradient. Inside each category there are a few named mechanisms with specific jobs. This article walks you through them in that order.
The Membrane and Why Transport Is Needed
The plasma membrane is a phospholipid bilayer — two layers of phospholipids with their hydrophilic heads pointing out and their hydrophobic tails facing each other in the middle. The hydrophobic interior is the barrier. A few small, nonpolar molecules — oxygen, carbon dioxide, lipid-soluble hormones — slip through freely. Almost everything else needs help: ions are charged, sugars and amino acids are too polar, water moves easier through dedicated channels than through the lipid itself.
So the cell uses membrane proteins to move what cannot cross alone. Two big types matter: channel proteins form open pores for specific ions or water, and carrier proteins bind a substance, change shape, and release it on the other side. Both are involved in passive and active transport — the difference is whether ATP is being burned.
Passive Transport: Down the Gradient, No ATP
Passive transport is "free" — it moves substances from where they are more concentrated to where they are less concentrated, driven by random molecular motion. The cell does not pay for it. Four named mechanisms count as passive.
Simple diffusion is direct movement of small, nonpolar molecules through the lipid bilayer. Oxygen diffuses from a high concentration in the alveolus into a lower concentration in the blood. Carbon dioxide goes the other way for the same reason. No protein required.
Facilitated diffusion is the same downhill movement but through a membrane protein, because the substance cannot cross the lipid directly. Glucose enters most cells through a glucose carrier (GLUT4 in muscle and fat cells). Ions move through specific ion channels. The cell still pays no ATP — the protein is just the door, not the engine.
Osmosis is the movement of water across a selectively permeable membrane, from lower to higher solute concentration. Water passes slowly through the lipid bilayer and quickly through dedicated channels called aquaporins.
The osmotic vocabulary is worth getting right. A solution is hypertonic to a cell if it has more solute (water leaves and the cell shrinks). It is hypotonic if it has less solute (water enters and the cell swells). It is isotonic if concentrations match. A red blood cell in 0.9% saline — the concentration used in IV fluids — sits at isotonic balance.
Filtration uses pressure rather than a concentration gradient to push water and dissolved substances across a permeable barrier. It is what happens at every capillary bed and at the glomerulus in the kidney.
Active Transport: Up the Gradient, ATP Required
When a cell needs to move something against its gradient — from a lower to a higher concentration — it has to spend energy. There are two main flavors of active transport.
Primary active transport uses ATP directly to power a pump. The classic example is the sodium-potassium pump (Na⁺/K⁺ ATPase), which moves 3 Na⁺ out and 2 K⁺ in with each ATP hydrolyzed. Every animal cell runs this pump continuously because the gradients it builds power nerve signals, muscle contraction, and downstream transport. Calcium pumps and proton pumps work the same way for their respective ions.
Secondary active transport does not use ATP directly. It uses an existing ion gradient — usually Na⁺ — as the energy source. A carrier protein lets Na⁺ flow back into the cell down its gradient and drags another molecule along, either in the same direction (a symporter, like the Na⁺/glucose cotransporter in the small intestine) or in the opposite direction (an antiporter, like the Na⁺/Ca²⁺ exchanger in cardiac cells). Since the Na⁺ gradient is built by the sodium-potassium pump, secondary transport is still indirectly powered by ATP.
Bulk Transport: Endocytosis and Exocytosis
Some materials are too big to pass through any channel or carrier — large proteins, droplets of fluid, microbes engulfed by an immune cell. The cell handles these by bulk transport, in which the membrane itself wraps around the cargo.
Endocytosis brings material into the cell. Phagocytosis engulfs large solids — a macrophage swallowing a bacterium. Pinocytosis takes in droplets of extracellular fluid. Receptor-mediated endocytosis uses specific membrane receptors to import target molecules with precision — that is how cells take up cholesterol carried in LDL particles.
Exocytosis sends material out. A vesicle fuses with the plasma membrane and releases its contents. Neurons release neurotransmitters this way; pancreatic cells release insulin this way. Both endocytosis and exocytosis require ATP.
One Cell, All Mechanisms at Once
A single cell uses every mechanism above, often at the same time. Take a small-intestine cell absorbing nutrients. Oxygen diffuses in (simple diffusion). Water moves through aquaporins (osmosis). Glucose enters from the gut lumen by Na⁺/glucose cotransport (secondary active transport), then leaves the cell into the blood by facilitated diffusion through GLUT2. The sodium-potassium pump (primary active transport) keeps pumping Na⁺ back out, maintaining the gradient that powers the glucose import. Newly absorbed proteins may be packaged into vesicles and shipped out by exocytosis. The mechanisms are not isolated — they are layered, each one solving a piece of the same job.
Getting Help
Many of the gradients built by these mechanisms are then used in action potentials explained, where the Na⁺ and K⁺ flows across channels become the electrical signals of the nervous system. For more cell-level walkthroughs, see the full set of Anatomy & Physiology study guides.
Conclusion
Cell membrane transport is mostly the answer to one question: ATP or no ATP? Passive transport — diffusion, facilitated diffusion, osmosis, filtration — moves substances down their gradient without energy. Active transport — primary pumps like the sodium-potassium ATPase, or secondary cotransporters and exchangers — moves substances up a gradient and pays for it. Bulk transport handles anything too large to pass through a channel. Sort each mechanism by direction and by energy source, and the topic compresses into a single page.
