TRANSPORT ACROSS CELL MEMBRANES

Passive Diffusion

Movement of substances down a concentration gradient due to the kinetic energy of the substance; no ATP required; continues until equilibrium is reached

Simple diffusion

Unassisted net movement of small, nonpolar substances down their concentration gradient across a selectively permeable membrane

Simple diffusion

Exchange of oxygen and carbon dioxide between blood and body tissues; Absorption of alcohol through the gastric mucosa

Facilitated diffusion

Movement of ions and small, polar molecules down their concentration gradient; assisted across a selectively permeable membrane by a transport prote

Channel-mediated

Movement of ion down its concentration gradient through a protein channe

Channel-mediated

Na⁺ entry in neurons during resting phase via leak channels

Carrier-mediated

Movement of small, polar molecule down its concentration gradient by a carrier protein

Carrier-mediated

GLUT1 transporting glucose across the blood-brain barrier

Osmosis

Diffusion of water across a selectively permeable membrane; direction is determined by relative solute concentrations; continues until equilibrium is reached

Osmosis

Solutes in blood in systemic capillaries "pulls" fluid from interstitial space back into the blood; Reabsorption of water in renal collecting ducts via aquaporins

Active diffusion

Movement of substances requires expenditure of cellular energy ( e.g., Nutrient absorption in intestines)

Active transport

Transport of ions or small molecules across the membrane against a concentration gradient by protein pumps

Primary diffusion

Movement of substance up its concentration gradient; powered directly by ATP

Primary diffusion

Ca²⁺ pumps transport Ca²⁺ out of the cell; Na⁺/K⁺ pump moves Na⁺ out and K⁺ in

Secondary diffusion

Movement of a substance up its gradient powered by harnessing the movement of another substance down its gradient

Secondary diffusion

Na⁺/glucose cotransport in intestinal epithelium

Symport

Substance moves in the same direction as Na⁺

Symport

Na⁺/amino acid symporter in renal tubule

Antiport

Substance moves in the opposite direction to Na⁺

Antiport

Na⁺/Ca²⁺ exchanger in cardiac myocytes

Vesicular transport

Endocytosis and exocytosis

Exocytosis

Bulk movement of substance out of the cell by fusion of secretory vesicles with the plasma membrane

Exocytosis

Release of neurotransmitter by nerve cells; Insulin release by pancreatic beta cells

Endocytosis

Bulk movement of substances into the cell by vesicles forming at the plasma membrane

Phagocytosis

Endocytosis in which vesicles engulf particulate materials external to the cell using pseudopo

Phagocytosis

Macrophage engulfing Mycobacterium tuberculosis

Pinocytosis

Endocytosis in which vesicles are formed as interstitial fluid is taken up by the cell

Pinocytosis

Formation of small vesicles in capillary wall to move substances

Receptor-mediated endocytosis

Endocytosis in which membrane receptors bind specific substances for uptake

Receptor-mediated endocytosis

LDL (low-density lipoprotein) uptake into liver cells