Transport Mechanisms
Summary of Cellular Transport Mechanisms
| Transport Type | Energy Required? | Direction of Movement | Key Mechanism / Proteins Involved | High-Yield MCQ Examples & Locations |
| Simple Diffusion | ❌ No | Down gradient | Passes directly through the lipid bilayer; no protein needed. | * Gases (O2,CO2) in lung alveoli. * Steroid hormones. * Lipid-soluble drugs. |
| Facilitated Diffusion | ❌ No | Down gradient | Uses a channel or carrier protein to cross. | * GLUT4 in skeletal muscle/fat (insulin-dependent). * GLUT1/3 in RBCs/brain (insulin-independent). * Voltage-gated Na/K channels in axons. |
| Primary Active Transport | Yes (ATP) | Against gradient | Directly hydrolyzes (burns) ATP at the pump. | * Na/K ATPase pump (all cells). * H/K ATPase pump (gastric parietal cells). * SERCA pump Ca reuptake in muscle cells). |
| Secondary Active Transport | Yes (Indirect) | Against gradient | Driven by an ion gradient (usually Na) built by a primary pump. | *Na Glucose cotransport (SGLT1) in intestines. * Na-Amino acid cotransport in kidneys/intestines NaCa exchanger (NCX) in cardiac muscle. |
| Endocytosis | Yes (ATP) | Into the cell | Membrane invaginates to engulf large cargo or fluids. | * Phagocytosis: Macrophages engulfing bacteria. * Receptor-mediated: Clathrin-coated pits taking up LDL or Iron. |
| Exocytosis | Yes (ATP) | Out of the cell | Intracellular vesicles fuse with the membrane to release cargo. | * Neurotransmitter release (e.g., Acetycholine) at nerve terminals. * Secretion of digestive enzymes from the pancreas. |