1. Transport Across Membranes: Overcoming the Permeability Barrier
Chapter 8:
Transport Across Membranes: Overcoming the Permeability Barrier

2. Movement Across a Membrane
With concentration gradient
Simple Diffusion
Facilitated Diffusion – transport proteins required
Against concentration gradient
Active transport – transport proteins required

3. Simple Diffusion

4. Osmosis

5. Limits of Simple Diffusion
Size
Polarity
Charge
Rate is affected by concentration gradient

6. Rate of Diffusion

7. Facilitated Diffusion
Large molecules need help crossing the membrane
Channel proteins
Carrier proteins

8. Channel Proteins Form hydrophilic channels through the membrane
No major conformational change
Examples: Ion Channels and Aquaporins (AQPs)

9. Ion Channels Very selective Mechanism not well understood
Most are gated
Voltage
Ligand
Mechanosensitive

10. Aquaporins Help water rapidly cross some membranes
Just large enough for water molecules to pass through one at a time

11. Facilitated Diffusion (cont.)
Large molecules need help crossing the membrane
Channel proteins
Carrier proteins

12. Carrier Proteins Bind solutes on one side of membrane
Undergo conformational change
Deposit solutes on other side of membrane
Alternating Conformation Model
Analogous to enzymes (but no catalytic function!)
Can carry one or two types of solutes
Examples: The Glucose Transporter and The Erythrocyte Anion Exchange Protein

13. One vs. Two Types of Solutes

14. The Glucose Transporter

15. The Erythrocyte Anion Exchange Protein

16. Active Transport
Sometimes molecules need to move up their concentration gradient
Requires energy
Has directionality
Three major functions
Cellular uptake of nutrients
Allow products to exit cell
Maintain non-equilibrium concentrations of ions
Direct vs. Indirect

17. Direct Active Transport
AKA primary active transport
Transport is tied directly to an exergonic reaction
Usually involves hydrolysis of ATP (ATPases)

18. Indirect Active Transport
AKA secondary active transport
Simultaneous transport of two solutes
Energy to move molecule “A” up its concentration gradient tied to moving molecule “B” down its concentration gradient

19. Direct vs. Indirect Active Transport

20. Direct Active Transport (cont)
4 types of ATPases
1. P type = phosphorylation (e.g. Na/K pump)
2. V-type = vacuole
3. F-type = factor (AKA ATP synthases)
4. ABC-type = ATP-binding cassette

21. Na+/K+ Pump Structure

23. Indirect Active Transport (cont)
Simultaneous transport of two solutes
Energy to move molecule “A” up its concentration gradient tied to moving molecule “B” down its concentration gradient
Usually coupled to an ion
Example: Na/Glucose symporter