01Bulk transport
Big things (proteins, bacteria) move in vesicles. Exocytosis: vesicles fuse with the membrane and dump contents out (neurotransmitters, insulin). Endocytosis: membrane folds in to bring material in.
Three kinds of endocytosis: phagocytosis (cell eating — white blood cells engulfing bacteria), pinocytosis (cell drinking — fluid droplets), receptor-mediated endocytosis (specific molecules bind receptors first, e.g., cholesterol LDL).
02Co-transport
Secondary active transport spends a gradient instead of ATP directly. Example: the sodium-glucose cotransporter lets Na⁺ flow down its gradient into intestinal cells, dragging glucose in AGAINST its gradient.
The Na⁺ gradient itself was built by the ATP-driven Na⁺/K⁺ pump, so ATP is the indirect power source. Plants do the same with H⁺ (proton) pumps.
03Notebook box
Primary active: ATP used directly (Na⁺/K⁺ pump, proton pump). Secondary active: rides another ion's gradient (Na⁺–glucose symport).
Worked example: If the Na⁺/K⁺ pump is poisoned, why does glucose absorption also fail? → No Na⁺ gradient remains to drive the cotransporter.