Active Transport
Active transport is the movement of molecules or ions across a cell membrane against their concentration gradient (from low to high concentration), requiring the expenditure of cellular energy in the form of ATP. Primary active transport uses ATP directly to power transport proteins called pumps, while secondary active transport uses the electrochemical gradient established by primary active transport to drive the movement of another solute. Active transport is essential for maintaining cellular ion balances, nutrient uptake, and nerve impulse transmission.
Comparison of Primary and Secondary Active Transport
| Feature | Primary Active Transport | Secondary Active Transport |
|---|---|---|
| Energy source | ATP directly | Electrochemical gradient (indirect ATP) |
| Protein type | ATPase pump | Symporter or antiporter |
| Direction of transported molecule | Against gradient | Can be against gradient |
| Classic example | Na⁺/K⁺-ATPase pump | Sodium-glucose cotransporter (SGLT1) |
| Where found | All cell membranes | Intestinal epithelium, kidney tubules |
| Net ATP cost | Direct (1 ATP per cycle) | Indirect (Na⁺ gradient consumed) |
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Related Terms
Diffusion (Biology)
Diffusion is the passive net movement of molecules or ions from a region of higher concentration to a region of lower concentration, driven by the concentration gradient until equilibrium is reached. In biology, simple diffusion across cell membranes occurs for small, nonpolar molecules such as oxygen, carbon dioxide, and ethanol, which can pass directly through the phospholipid bilayer. Facilitated diffusion involves channel or carrier proteins to transport polar or charged molecules down their concentration gradient without requiring energy.
Osmosis (Biology)
Osmosis is the passive movement of water molecules across a selectively permeable membrane from a region of higher water potential (lower solute concentration) to a region of lower water potential (higher solute concentration). This net movement continues until equilibrium is reached or until an opposing pressure prevents further flow. Osmosis is fundamental to cell function, driving turgor pressure in plant cells, reabsorption of water in kidney tubules, and the movement of water into roots from soil.
Endocytosis
Endocytosis is the process by which a cell engulfs extracellular material by invaginating its plasma membrane to form an intracellular vesicle, allowing large molecules, particles, or even other cells to be taken up without crossing the membrane directly. The three main types are phagocytosis (engulfment of solid particles), pinocytosis (engulfment of fluids and dissolved solutes), and receptor-mediated endocytosis (selective uptake via surface receptors such as clathrin-coated pits). Endocytosis requires energy (ATP) and plays critical roles in immune defense, nutrient uptake, and signal regulation.
From Latin "activus" (active, effective) and Latin "transportare" (to carry across). The concept of active transport was established in the mid-20th century; the Na⁺/K⁺-ATPase pump was discovered by Jens Skou in 1957, earning him the Nobel Prize in Chemistry in 1997.