some transport types simple diffusion (down a chemical or electrical potential gradient) facilitated...
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Some Transport Types
• Simple diffusion (Down a chemical or electrical potential gradient)
• Facilitated diffusion (Down a chemical or electrical potential gradient)
• Primary active transport (Against a chemical or electrical potential gradient)
• Secondary active transport (Against a chemical or electrical potential gradient)
• Ion channel (Down a chemical or electrical potential gradient, may be gated)
• Ionophore mediated transport (Down a chemical or electrical potential gradient)
Classes of Transport
• Uniport
• Cotransport– Symport– Antiport
Fig 12-43
Aquaporin
• Six helices per chain
• Four chains form a pore– Single file, 5 108 molecules per second
• Cf. 4 107 s-1 turnover number for catalase
– Water “flows” in the direction of the osmotic gradient
Glucose Transporter
• 12 transmembrane helices per chain
• 5 chains form a pore– GluT1 (erythrocytes) Kt ~ 1.5 mM
– GluT2 (liver and other) Kt ~ 66 mM
– GluT4 (myocytes and adipocytes) insulin stimulates installation in membrane
Figure 12-27
Box 12-2 figure 1
Chloride-Bicarbonate Exchanger
• In respiring tissues– CO2 in by simple diffusion, then HCO3
- out, Cl- in
• In lungs– HCO3
- in, made into CO2, Cl- out
• Antiport
Fig 12-28
Active Transport
• The hydrolysis of ATP or other energy-releasing reaction is required.ATP + H2O ADP + Pi
• The energy is required to move against a chemical (concentration) gradient or an electrical (voltage) gradient (the latter usually means both)
Fig 12-30
ATPases
• P-type– Cation transporters
• V-type– Proton transporters (acidifiers)
• F-type– Proton transporters in bacteria, mitochondria,
and chloroplasts (also ATP synthases)
• Multidrug transporter (tumor cells)
Na+K+ ATPase• P-type
• 3 Na+ for every 2 K+ in
• Fig 12-33
Other P-type systems• Plasma membrane Ca+ pump
– SERCA pump
• Cotransport systems
Fig 12-36
Cotransporter can pump to a 30,000 to 1 glucose level!
Other Ion Channels
• Ligand and Voltage gated– Na+, K+, and Ca++
– On-off operations• Neurons, etc.
Ionophores (Fig 12-37)