Transport Across Membranes

Why can the lipid bilayer enable the cell or intracellular compartment to maintain a different environment to the outside?

Because it is an impermeable barrier to polar molecules

What are required to transport molecules across membranes?

Certain proteins

What are membrane transport proteins?

Specialised proteins that enable molecules to pass across membranes

What are the two factors that determine transport across the membrane?

• Size of the molecule

• Polarity and charge of molecule

What are the two types of membrane transport proteins?

Carrier (transporter) proteins and channel proteins

What is facilitated diffusion?

• Carriers and channels enable facilitated diffusion (passive transport)

• The concentration gradient determines the direction of flow

• If it is an ion (and has a charge) then concentration and charge determine the direction of flow. This is called the electrochemical gradient (pumping of H+ into the intermembrane space generates an electrochemical gradient, H+ then flows through ATP synthase via facilitated diffusion)

What is active transport?

• Pumping solutes across a membrane against their concentration gradient

• Mediated solely by carriers

• It requires energy: light energy (bacteria), energy release from electron transfer, ATP hydrolysis

• Eukaryotes carry out active transport via two main mechanisms: coupled carriers (secondary active transport) & ATP driven pumps (primary active transport)

What are the different types of passive transport and active transport shown on a diagram?

What are the two types of coupled carriers?

Symporters and antiporters

What are symporters?

They move two molecules in the same direction

What are antiporters?

They move two molecules in the opposite direction

What are channel proteins?

• Channel proteins form pores across a membrane

• Some are large (eg. Porins) and if they were on the plasma membrane then it would allow lots of things in and out (poor regulation)

• Channels on the membrane are very narrow, highly selective pores that open and close are gated

• They are specifically concerned with the transport of ions

• Channels only participate in facilitated diffusion

What are the different types of gated channel proteins?

How do different molecules get transported across the membrane?

• Non-polar = no problem. They dissolve in the bilayer and diffuse across

• Small, no charge but polar = OK. It will be very slow to move across

• Large, uncharged and polar = probably going to need a transporter of some sort

• Ions = charged. Need specialised mechanism (same as charged, large and polar)