Active transport is the movement of molecules across a membrane against their concentration gradient (from low to high concentration), which requires the direct input of energy, usually ATP.
Active transport is how a cell moves stuff the "wrong way" across its membrane, pushing molecules from where they're scarce to where they're already crowded. Because that fights the natural drift of diffusion, it costs energy, typically ATP. That's the one line you can't forget: active = against the gradient = energy required (EK 2.5.A.3).
Compare that to passive transport, which is the cell coasting downhill. In passive transport, molecules move from high to low concentration with no energy input (EK 2.5.A.2). Active transport is the cell pedaling uphill on purpose. It also includes the bulk-transport processes: endocytosis (folding the membrane inward to swallow large material into a vesicle) and exocytosis (vesicles fusing with the membrane to dump material out). Both move huge amounts of material and both require energy (EK 2.5.B.1).
This lives in Unit 2: Cells, and it's the engine behind a cell maintaining its internal balance. Learning objective AP Bio 2.5.A asks you to describe how organisms keep solute and water balance, and active transport is the tool that lets a cell hold concentrations that diffusion alone would erase. Objective AP Bio 2.5.B covers the big-molecule version through endocytosis and exocytosis. Without active transport, a cell couldn't build the steep ion gradients (like the Na⁺/K⁺ difference) that power nerve signals, nutrient uptake, and osmoregulation. It ties directly into the course theme that cells use energy to maintain order against the constant pull toward equilibrium.
Keep studying AP Biology Unit 2
Passive Transport and Facilitated Diffusion (Unit 2)
These are the downhill counterpart to active transport. Facilitated diffusion uses channel and transport proteins too, but molecules ride the gradient for free (EK 2.6.A.2), so the protein doesn't make it active. The single thing that flips a transport process to "active" is needing energy to go against the gradient.
ATP (Adenosine Triphosphate) (Unit 3)
Active transport is where you cash in the energy currency you study later in cellular energetics. When a pump hydrolyzes ATP to ADP, the released energy is what physically drags ions uphill. This is the link between metabolism and membrane function.
Na⁺/K⁺ Pump (Unit 2)
This is the textbook example of active transport in action. The pump burns ATP to push 3 sodium ions out and 2 potassium ions in, both against their gradients, building the electrochemical gradient that nerves and muscles depend on.
Cell Compartmentalization (Unit 2)
Internal membranes and organelles use active transport to keep different chemical environments separate (EK 2.9). Pumping ions or molecules into an organelle creates a distinct compartment, which is how a cell runs competing reactions without them interfering.
On multiple choice, expect to be asked to classify a process: is energy needed, and is movement with or against the gradient? A classic trap is glucose-uptake data. The 2024 SRFRQ Q3 (and related practice questions) show red blood cells taking in glucose at a rate that climbs then plateaus at high concentrations. That plateau means the transport proteins are saturated, a signature of protein-mediated transport, not just simple diffusion across the bilayer. You'll also see endocytosis and exocytosis tested as the energy-requiring way cells handle large molecules. Be ready to explain WHY active transport needs ATP (it opposes the gradient) and to connect a disrupted energy supply to a cell's failure to maintain its solute balance.
Both use membrane proteins, so students assume both are "active." They're not. Facilitated diffusion moves molecules DOWN the gradient with no energy input (EK 2.6.A.2), so it's passive. Active transport moves molecules UP the gradient and burns energy like ATP. The protein isn't what makes it active; the direction and the energy cost are.
Active transport moves molecules against the concentration gradient, from low to high concentration, and always requires energy (usually ATP).
Passive transport, including facilitated diffusion, moves molecules down the gradient for free, so the presence of a protein alone does not make a process active.
Endocytosis and exocytosis are forms of active transport because moving large materials in bulk requires energy (EK 2.5.B.1).
The Na⁺/K⁺ pump is the go-to example: it spends ATP to build the steep ion gradient that powers cell signaling.
Active transport lets cells maintain solute and water balance (homeostasis) that simple diffusion would otherwise wipe out.
A transport rate that plateaus at high solute concentration signals protein-mediated transport, because the proteins become saturated.
Active transport is the movement of molecules across a membrane against their concentration gradient, from low to high concentration, using energy such as ATP. It's how cells maintain gradients and solute balance that passive diffusion can't sustain (EK 2.5.A.3).
No. Both use membrane proteins, but facilitated diffusion moves molecules down the gradient with zero energy input, making it passive. Active transport moves molecules up the gradient and requires energy, so the difference is direction and energy cost, not the protein.
Most active transport uses ATP directly, like the Na⁺/K⁺ pump. The key requirement for the AP exam is simply the direct input of energy to move molecules against the gradient (EK 2.5.A.3); the energy source is usually ATP.
They're active. Both require energy to move large molecules or large amounts of material, with endocytosis folding the membrane inward to take material in and exocytosis fusing vesicles to release material out (EK 2.5.B.1).
Diffusion goes with the gradient, the natural downhill direction, so it costs nothing. Active transport pushes molecules uphill against the gradient, which is not spontaneous, so the cell must spend energy to make it happen.