Lecture 7 -Transport Across Cell Membranes

Diffusion

Tendency by which molecules move from areas of high concentration to low concentration

At equilibrium in diffusion, molecules continue to move, but there is no __ _____

net flow

Amphipathic lipids are proteins are constantly moving ______ thus creating opportunities for compatible molecules to join or pass through the membrane

laterally

Membrane potential

voltage difference across the membrane generated by the differential distribution of ions

The diffusion of non-charged solutes is determined solely by its ______ ______

concentration gradient

the net drive force of the concentration gradient and membrane potential is called _____ _____

electrochemical gradient

The electrochemical gradient determines the direction of solute flow in ______ _____

passive transport

3 types of cell membrane transport

• passive transport

• active transport

• vesicular transport

Difference between passive and active transport

passive transport: requires no energy input

active transport: energy input required

3 types of passive transport (diffusion)

• simple diffusion

• facilitated diffusion

• osmosis

2 type of active transport

• primary active transport (requires ATP directly)

• secondary active transport (requires ATP indirectly)

3 types of vesicular transport

• endocytosis

• exocytosis

• transcytosis

Characteristics for molecules to participate in simple diffusion across the cell membrane

small and non polar

What allows ionic and polar molecules to get across cell membrane?

Membrane transport proteins

two types of membrane transport proteins

• transporters (carriers): change conformation

• channels: form tiny hydrophobic pores for diffusion

Draw the difference between a transporter and a channel

(see image)

How many conformations must a transport (carrier) protein have

2

Are carrier proteins ever completely open?

No!

What determines the specificity of a carrier protein

carrier binding site

Is carrier-mediated transport active or passive

can be either

carrier proteins can either move ___ or __ specific molecules at a time

one or two

Two types of coupled transport in carrier proteins

• symport: same direction

• antiport: opposite direction

Active transport requires (2 things)

• energy

• transporter proteins (pumps)

Active transport is ________ (directionality)

unidirectional

2 types of active transport pumps

• ATP driven

• coupled pumps

ATP driven pumps

• use energy from ATP hydrolysis

• primary active transport

Coupled pumps

• linked uphill transport of one solute to downhill of another

• secondary active transport

The Na+/ K+ pump moves Na+ __ of the cell and K+ __ the cell (restores the concentration gradient)

out; inside

Chemical process of Na/K pump

1. 3 Na+ binds inside the cell

2. pump catalyzed by ATP hydrolysis; pump is phosphorylated

3. conformational change occurs → 3 Na+ to the outside

4. 2 K+ accepted from outside

5. Dephosphoylation triggers conformational change → 2 K+ expelled inside

What is osmosis

Passive diffusion of water through membranes

2 ways osmosis can occur

• pass through membrane slowly via simple diffusion

• transported through channel protein (aquaporins) via facilitated diffusion

Water transport is always ______

passive

What determines the specificity of an ion channel

• diameter; shape

• distribution of charged and polar amino acids that line the interior

Protein channels have a ____ ___ at its narrowest part

selectivity filter

How does the selectivity filter work

• water soluble ion enters selectivity filter

• brings ion into contact with channel wall

• only those with appropriate size and charge offer favorable energy to pass through in single file