Cell Biology
Active Transport
Moving molecules against their gradient — costs energy, controls cell composition
Active transport moves molecules across cell membranes against their concentration gradient — requires energy. Two types: (1) Primary active — directly uses ATP (e.g., Na⁺/K⁺ pump, Ca²⁺ pump, H⁺ pump). (2) Secondary active — uses gradients established by primary transport (e.g., Na⁺-glucose symporter). Critical for: maintaining cellular ion concentrations (Na⁺ low inside, K⁺ high; opposite outside), nutrient uptake against gradients, neuronal signaling, muscle contraction. About 20% of cellular ATP used for Na⁺/K⁺ pump alone.
- DefinitionTransport against concentration gradient; uses energy
- Primary activeDirect ATP use (Na⁺/K⁺, Ca²⁺ pumps)
- Secondary activeUses ion gradients (symporters, antiporters)
- Na⁺/K⁺ pump3 Na⁺ out, 2 K⁺ in per ATP
- Energy cost~20% of cellular ATP for Na/K pump
- DiscoverySkou (Nobel 1997) for Na⁺/K⁺ pump
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Why active transport matters
- Cell function. Maintains ion balance.
- Neurons. Action potentials require pumps.
- Nutrient uptake. Against gradients.
- Drugs. Many target pumps.
- Disease. Cystic fibrosis, cancer drug resistance.
- Energy. Major cellular ATP consumer.
- Pharmacology. Drug absorption.
Common misconceptions
- Active = fast. Specifically uphill (against gradient).
- Diffusion = always passive. Yes — passive uses gradients only.
- Secondary active free. Indirectly uses ATP.
- One direction always. Some pumps reverse if gradient extreme.
- Bilayer prevents transport. Restricts; transporters enable.
- Active transport rare. Most ions, many nutrients.
Frequently asked questions
How does primary active transport work?
Direct use of ATP. Pump protein binds substrate; ATP hydrolysis (ATP → ADP + Pi) provides energy; conformational change moves substrate against gradient. Examples: Na⁺/K⁺ ATPase (3 Na out, 2 K in per ATP), Ca²⁺ ATPase (Ca out of cytoplasm), H⁺/K⁺ ATPase (stomach acid), F-ATPase (mitochondria, ATP synthesis).
How does secondary active transport work?
Uses pre-existing ion gradient (often Na⁺) to drive transport. Energy from gradient. Two types: symporter (both transported same direction, e.g., Na⁺-glucose in intestine), antiporter (opposite directions, e.g., Na⁺-Ca²⁺ exchange). Doesn't directly use ATP — but: gradient established by primary transport. So indirectly ATP-dependent.
What's the Na⁺/K⁺ pump?
Most abundant transport protein. Active in all animal cells. 3 Na⁺ pumped out + 2 K⁺ pumped in per ATP. Establishes resting membrane potential, drives many secondary transports, regulates cell volume. Total ~20% of cellular ATP. Discovered Skou (1957; Nobel 1997). Inhibited by ouabain, digoxin (cardiac drugs).
How is glucose taken up?
Intestinal cells: SGLT1 (Na⁺-glucose symporter). Na⁺ flows down its gradient (high outside); pulls glucose along, even against glucose gradient (uphill for glucose). Net: glucose accumulates inside cell. Exits across basal membrane via GLUT2 (passive). Coupled gradients enable concentrative uptake.
Why so much energy spent?
Maintaining ion gradients critical. Without pumps: gradients dissipate; cells equilibrate with surroundings; can't function. Specific needs: nervous system (action potentials), muscle (contraction), kidneys (concentrated urine), digestion (acidity). Ion gradients also store potential energy — used for many secondary functions.
What happens when active transport fails?
Cell function compromised. Examples: (1) Cardiac glycosides (digoxin) inhibit Na⁺/K⁺ pump → less Ca extrusion → stronger heart contractions (used for heart failure). (2) Cystic fibrosis: chloride channel mutation → defective Cl transport → thick mucus. (3) Kidney disease: reduced active transport → fluid imbalance.
What about ABC transporters?
ATP-Binding Cassette transporters. Family of ATP-using transporters. Move many substrates: drugs, lipids, metals, peptides. Examples: P-glycoprotein (drug efflux from cells; cause of multidrug resistance in cancer), CFTR (cystic fibrosis transmembrane conductance regulator). Important: drug pharmacokinetics, cancer chemoresistance.