Membrane Transport Part 1

Selective Permeability

•  cell determines when they do and don’t want to let molecules in

• impermeable to charged molecules (ions) no matter how small: the charge and high degree of hydration of such molecules prevent them from entering the hydrocarbon phase of the bilayer

What factors determine the rate of diffusion

polarity and charge

What molecules are more/less permeable

Most
non polar

polar
Larger molecules

charged molecules

Least

What are the two classes of transport proteisn

Transporters and carriers

Transporters

• Also known as carriers or permeases

• Like substrate enzyme binding, the solute binds to the binding site and then causes a conformational change in the transporter

Carrier

• Weaker interaction with the solute

• Let the channels cross the membrane passively “downhill”

• Forms pores instead of using solute binding sites

Concentration gradient

• Concentration Gradient - the difference in concentration on the two sides of the membrane 

  • Drives passive transport and determines its direction

Electrochemical Gradient

 difference in membrane potential across the membrane; concentration + change = electrochemical gradient

Uniporter

type of transporter where only one solute is moved

Symporters

Simultaneous movement of more than one solute in the same direction

Antiports

simultaneous movement of more than 1 solute in opposite directions

Multispanning transmembrane

•  function by undergoing reversible conformational changes that alternately expose the solute binding site to one side of the membrane and then the other

Simple Diffusion

• passive

• Solute crosses the membrane without a transport protein (no energy required)

• movement of solute down concentration gradient

Examples of simple diffusion

• Rate determined by hydrophobicity and size:

• Fastest:

  • Nonpolar gases (Oxygen and CO2)

  • Small steroid hormones

  • Small uncharged polar (small amounts of water), ammonia

  • Larger, uncharged polar: Glucose (very slow, needs facilitated transport)

  • Ions: relatively impermeable

Facilitated Diffusion (PASSIVE)

• Transport protein moves solutes down their concentration gradient (Energy not required)

• DOWN concentration gradient

Examples of facilitated diffsuion

• Aquaporins

• Glucose transporter

• Voltage gated na+ channel

• VOltage gated K+ channels

• K+ leaky channelschannelropsins

Aquaporins

• type of facilitated diffusion

• water channel

• Blocks passage of ions

Why can’t aquaporin pass protons through

Two asparagine in the center of the pore prevent protons from passing through the channel

Na+ Glucose transporter

• type of facilitated difussion

• Alternates between conformations

• Solute binding site is sequentially accessible to one side of bilayer, then the other

• All channels (passive)

• Na+ drives the movement of glucose against it’s concentration gradient

• binding of na+ and glucose is cooperative

Voltage gated Na+ channel

type of membrane diffusion

(3 conformations ), open in response to depolarization , Na+ rushes into cell

Voltage gated K+ channels

• type of facilitated diffsuion

• efflux of K+ to restore resting membrane potential

K+ leaky channels

• type of faciliatetd diffusion

• Do not need stimulus to move K+ down concentration gradient

• (opens in resting cell): Maintains resting membrane potential

Primary Active Transport

Requires ATP

against the concentration gradient

ex. Na+ K+ pump, calcium pump , ABC transporter, CTFR protein, V type pump

Na+ K+ pump (P-type):

• primary active transport

• antiport direction of solutes, both move against concentration gradient. ATP hydrolysis required.

• self phosphorylate

• 3 Na+ out, 2 K+ in.

• Na+ concentration is high outside cell, K+ high inside cell

(pumping cycle)

• The concentration of K+ ions are higher inside than outside , and vice versa for Na+ ions

• Na - gradient drives transport of nutrients into animal cells

• Pump is electrogenic (produces a change in the elcyrical potential of the cell)

Calcium pump

• primary active transport

• (P-type) Detailed description of pumping cycle

• maintain Ca 2+ cell gradient

Ca 2+ in the Sarcoplasmic Reticulum

• Sarcoplasmic Reticulum: specialized endoplasmic reticulum in muscle cell cytoplasm, intracellular storage  of calcium ions

•  When the action potential depolarizes the muscle cell plasma membrane, Ca 2+ is released into cytosol from the sarcoplasmic reticulum through Ca 2+ release channels, which stimulates muscle to contract

• The Ca2+ pump moves Ca2+ from cytosol back to sarcoplasmic reticulum, end of muscle contraction

Sarcoplasmic Reticulum

 specialized endoplasmic reticulum in muscle cell cytoplasm, intracellular storage  of calcium ions

ABC transporter

• primary active trasnport

• aka ATP-binding cassette transporters

• ATP binding and hydrolysis influences conformational changes to expose binding sites.

• differ structurally  from P-type ATPases and primarily pump small organic molecules across cell membranes

• Examples: MDR protein

Clinical Importance of ABC Transporters

• Multi-drug resistance (MDR) proteins also called P-glycoprotein: high levels in cancer cells, makes cells resistant to cytotoxic drugs (chemo)

• Pasmodium falciparum (causes malaria) transporter pumps 

CTFR Protein

• Primary active transport

• (why unique?) Acts like a channel

• balances the cl- ions in the cell

V type pump:

• primary actove transport

• Generate H+ gradient (acidify lysosome, vesicles)

  • to aicdifiy the interior of these organelles

• acidify vacuole compartments (lysosomes, endosomes golgi)

  • Uses the Gibbs free energy of ATP hydrolysis to make an H+ gradient

What do uncouplers do?

collapse the proton gradient by  equalizing proton concentrations on both sides of the membrane 

How is DNP an uncoupler

• Equalize proton concentration on both sides of the membrane

• caused increased oxygen consumption and an increased metabolic rate, but a decline in ATP production

Secondary active trasnport

• Ion gradient (potential energy) required to moves another solute against its concentration gradient.

• Indirect use of ATP (pump that maintains ion gradient)

• AGAINST concentration gradient

ex. Na+/Glucose transorter

Na+/Glucose transporter

Use Na+ gradient (provided by Na+ K+ pump) to drive glucose against its gradient (symporter)

• Na⁺/K⁺ ATPase pumps 3 Na⁺ OUT and 2 K⁺ IN; both move uphill. 

• Na⁺ is higher outside; K⁺ is higher inside -> electrogenic. 

• Na⁺ moves downhill to help glucose move uphill into the cell. 

• Na⁺ and glucose bind cooperatively 

• Occupied symporter becomes occlude 

Transcellular transport of glucose

Asymmetric distribution of transporters in plasma membrane

The Na/+K+pump helps maintain the Na+ gradient

It’s found exclusively in basal and lateral domains

Mechanism of Na+ driven glucose transporter

• secondary active transport

• movement of Na+ (from Na+ K+ pump) down its concentration gradient is used to move glucose AGAINST its concentration gradient

• binding of Na+ and glucose happens through symporter that switches between occluded (closed) and non-occluded states and utilizes the cooperative binding nature of Na+ and glucose

Mechanism fo Na+ Glucose Symporter

Symporter is in an occluded-empty state and opens to the intestinal lumen

Na+ and glucose bind to the symporter

symporter closes and enters an occluded-filled state

symporter opens to the intestinal lumen, allowing the Na+ and the glucose to enter the lumen

lastly, the symporter will return to its original occluded-empty state and the cycle starts again

GLUT

Na+/ Glucose trasnporter

1. Glc fits into the GLUT binding site, 

2. Binding causes a conformational change

3. Low [Glc] causes dissociation

4. GLUT bounces back to it’s original conformation

Ways to use Active transport

1. Coupled transporters harness the energy stored in concentration gradients to couple the uphill transport of one solute across the membrane to the downhill transport of another. 

2. ATP-driven pumps couple uphill transport to the hydrolysis of ATP. 

3. Light- or redox-driven pumps, which are known in bacteria, archaea, mitochondria, and chloroplasts, couple uphill transport to an input of energy from light, as with bacteriorhodopsin and photosystem II (discussed in Chapters 10 and 14, respectively), or from a redox reaction, as with cyto chrome c oxidase (discussed in Chapter 14). 

Classes of ATP driven pumps

• p-type

• ABC

• V-type pumps

MDR protein (ABC transporter)

pump that hydrolyzes two ATP molecules per transport cycle

channel proteins

do not possess a binding site for a specific solute, and instead interact weakly with the solute

can only carry out passive transport

What happens after Ca2+ binds ATP and is phosphorylated?

a conformational change that opens a passageway to the sarcoplasmic reticulum lumen, causing two Ca 2+ ions to exit into the sarcoplasmic reticulum

(not a typo)

Mechanism of Glucose Transporter on basal side of intestinal cell

passive transport through a uniporter

high concentration of glucose through cell → too big to leave w/o a transporter

Glucose transporter open to intestinal cell

allow the glucose to bind to the interior of the transporter → causes conformational change → transporter opens toward extracellular fluid → Since lower conc. i extracellular fluid, glucose will detach and leave → loss of glucose will cause a consequent conformational change and the transporter will open to the intestinal cell

• How would the inhibition of the Na+ K+ pump affect the abortion of glucose in the cell

would inhibit absorption of glucose

if the Na+ K+ pump is inhibited , Na+ wouldn’t be removed from cell → collapses the gradient needed for Na+ glucose symporter