some transport types simple diffusion (down a chemical or electrical potential gradient) facilitated...

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)