Movement Across a Membrane
With concentration gradient
Simple Diffusion
Facilitated Diffusion - transport proteins required
Against concentration gradient
Active transport - transport proteins required
Simple Diffusion (permeable)
Movement of solute molecules across the membrane from high to low concentrations
Osmosis (not permeable)
Solute cannot cross membrane, water diffuses from the lower solute concentration (less water) to the higher solute concentration (more water)
Limits of Simple Diffusion
Size
Polarity
Charge
Rate is affected by concentration gradient
Rate of Diffusion
Graph:
Facilitated Diffusion- Hyperbolic
Simple Diffusion- Linear
Facilitated Diffusion
Large molecules need help crossing the membrane
Channel proteins
Carrier proteins
Channel Proteins
Form hydrophilic channels through the membrane
No major conformational change
Examples: Ion Channels and Aquaporins (AQPs)
Ion Channels
Very selective
Mechanism not well understood
Most are gated
Voltage
Ligand
Mechanosensitive
Aquaporins
Help water rapidly cross some membranes
Just large enough for water molecules to pass through one at a time
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 Trans
Uniport
...
Symport
...
Antiport
...
The Erythrocyte Anion Exchange Protein
...
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
Direct Active Transport
AKA primary active transport
Transport is tied directly to an exergonic reaction
Usually involves hydrolysis of ATP (ATPases)
Direct Active Transport
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
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
Indirect 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
Usually coupled to an ion
Example: Na/Glucose symporte