Membrane Proteins and Transport Across Membranes (Sec 2 week 2 )

Artificial bilayer

Impermeable to most water solube molecules

Cell membrane

Membrane trasport proteins present to transfer specific molecules (facilitated transport)

Movement across the lipid bilayer

Permeable:

• Simple diffusion (down concentration gradient)

• Hydrophobic or non-polar molecules

Impermeable:

• Large polar molecules and ions

Proteins involved in membrane transport

-Transmembrane transport proteins transport polar and charged molecules, and each protein transports a specific class of molecule (nucleotide, sugar, amino acid, etc)

Two main classes of membrane transport Proteins

1. Channel - selectivity depends on size and charge of solute. Transient interaction, no conformational change needed for open channels)

2. Transporter - Selectivity depends on if solute fits into binding site. Series of conformational changes required for transport

Passive vs active transport

Passive transport is transport that does not need energy, moves solute down concentration gradient

Electrochemical gradient

Sum of force from the concentration gradient and the membrane potential

If positive charged molecule is on positive side of membrane in high concentration, net driving force for it to cross the membrane is very high - pushed by both forces)

Cell membrane potential

Outside is negative, inside is positive

Ion channels

Found in animals, plants, microorganisms

1. non-gated - always open - K+ leak out generating resting potential

2. Gated channel - requires signal to open

Types of gated channels (4)

1. Mechanically gated - signal is mechanical stress

2. Ligand gated (extracellular) - signal is ligand

3. Ligand gated (intracellular) - signal is ligand

4. Voltage gated - Signal is voltage change across membrane

Transporter proteins (2)

1. Bind specific solute

2. Goes through conformational change

Rate of diffusion transporter vs channel

1. As concentration difference of transported molecule increases, rate of simple diffusion through passive transport (channel) increases linearly while transporter-mediated diffusion tapers off

Uniport transporter protein

1. One solute, passive, reversible transport down electrochemical gradient

Gradient Driven Pump (3)

1. Active

2. Symport moves two solutes in same direction, antiport moves two solutes opposite

3. Can use energy gained from one solute moving down gradient to move second against gradient

Symport example

Na+ and glucose

Antiport example

Na+ and H+ antiport controls cytosolic pH by removing H+ protons

ATP driven pumps

1. Active transport

2. uses energy from ATP hydrolysis to transport solutes

P-type pump

1. ATP pump that is phosphorylated from ATP

2. Moves Na and K against gradient so they can be used to transport other nutrients and maintain pH

ABC transporter

Active transporter that uses 2 ATP to pump small molecules

V-type proton pump

Uses ATP to pump H+ into organelles to acidify lumen

F-type ATP synthase

Uses H+ gradient to drive synthesis of ATP - kind of opposite to v-type pump

2 important things regulated by transporters

1. transcellular transport of glucose

2. generation of membrane potential

Transport of glucose from intestine to bloodstream

1. Glucose from gut transported from microvillus into epithelial cell using symport in conjuction with Na+

2. Glucose passively moves through uniport into extracellular fluid on basolateral side of the epithelial cell (bloodstream)

3. ATP pump moves Na+ and K+ across membrane to maintain potential

4. This all requires ASYMMETRIC distribution of membrane proteins

Generators of membrane potential (Animal cell)

1. K+ leak channel builds potential

2. Na+-K+ pump gives 10% of potential - keeps Na low in cytosol, K high in cytosol. 3 Na pumped out per 2 K pumped in, so net +1 ion out

3. More anions inside the cell

4. Causes resting inside membrane potential to be negative, -20mV to -200 mV

Generators of membrane potential (plant cell)

1. Plasma membrane p-type pump pumps H+ out causing similar negative resting potential