01.22.10Lecture 5: Membrane transport

Ion concentrations within the cell are different from those outside

Few molecules cross the membrane by passive diffusion

Each cell membrane transports specific molecules

Solutes cross membranes by passive or active transport

• Passive transport is driven by concentration gradients & electrical forces

• Active transport is requires energy

An electrochemical gradient is driven by 2 forces

• Concentration gradient - ions move across a membrane from high to low concentrations

• Voltage across the membrane

• High for sodium, low for potassium

There are 3 main classes of membrane transport proteins

Passive transport by glucose carrier protein (GLUT2)

• Carrier protein randomly switches between two states• Glucose moves down it’s concentration gradient

Active transport is mainly driven in 3 ways

• 1. Coupled transporters couple uphill transport of one solute to the downhill transport of another

• 2. ATP-driven pumps use hydrolysis of ATP to uphill transport

• 3. Light driven pumps couple transport to light absorbtion

Example: the Na+-K+ pump

• Uses ATP hydrolysis to pump sodium out, potassium in

• Helps to maintain a negative electric potential inside the cell

Example: the Na+-K+ pump

Sodium gradients do work: glucose transport

• Glucose-Na+ symport protein

• Electrochemical Na+ gradient drives import of glucose

Two types of glucose carriers enable epithelial cells to transport glucose in the gut

Ion channels are selective pores in the membrane

• Ion channels have ion selectivity - they only allow passage of specific molecules

• Ion channels are not open continuously, conformational changes open and close

Gated ion channels respond to different kinds of stimuli

The membrane potential is produced by the distribution of ions on either side of the

bilayer

K+ leak channels establish the membrane potential across the plasma membrane

The action potential provides rapid, long-distance communication

• Action potential (nerve impulse): a wave of electrical activity propagated along the length of a neuron

• Very fast (~100 m/sec), dose not weaken over distance

Action potentials are propagated along an axon

Voltage-gated Na+ channels mediate action potentials

• Exist in 3 states: closed opened, and inactivated

Action potentials are propagated along an axon

Conversion of an electrical signal to chemical signal

Conversion of biochemical signal back into electrical