Unit 1.4 : membrane transport

How do cell membranes control what enters and exits the membrane?

Cell membranes control what enters and exits membranes through selective permeability.

·       Increased size = decreased permeability

·       Increased charge=decreased permeability

-does NOT relate to speed, just if it can or cannot pass

-small np molecules diffuse readily, lipophilic

-do not dissolve well in aqueous environments

-small polar molecules pass through membrane

- more polar = less permeable

- large np molecules pass, but not at great rate ( moderate permeability)

- large polar molecules are impermeable ( ex: glucose)

passive transport

·       down electrochemical gradient

·       does not require energy

·       Diffusion, osmosis (can use protein, does not have to) , facilitated diffusion

·       No protein, channel protein, uniporter

active transport

·       up/against electrochemical gradient

·       does require energy

·       Require energy ( ATP for primary active transport or  potential energy for secondary transport- one substance that is moving down gradient release energy to power substance moving up electrochemical gradient)

·       Active transport or bulk transport

·       Cotransporter (secondary) or pumps ( primary)

Transport proteins

•        channel proteins may be gated or ungated.  May allow materials in or out, but always down concentration gradient (facilitated diffusion).

•        carrier protein include uniporters (facilitated diffusion) and cotransporters (secondary active transporters).  Symporters move both materials in the same direction.  Antiporters move materials in opposite directions.

Pumps use ATP to move materials against their concentration gradient (primary active transport

channel proteins

•        Hydrophilic “pore” to allow polar or charged substances to pass through

•        Specific

•        Voltage-(change in membrane potential), mechanical- (pressure applied to membrane allow it to open or close) or ligand-gated,-( any substance bound to gate : ion, hormone, nucleotide, etc. will not open unless ligand bound ) if gated at all

•        Ex: Cystic Fibrosis Transmembrane Regulator (CFTR) – gated by ATP ligand; ATP NOT used to power process

•        Allow chloride ( Cl) ions to diffuse into lung space; this draws sodium ions ( Na+) because of the electrical gradient and water because of osmosis into lung space

•        Defaults in the protein prevent chloride ions dorm leaving cells, which means sodium and water are not drawn into lung spaces ( cystic fibrosis)

•        Gated by ATP ( ligand)

•        Different aquaporins have different affinities for water

•        Allow water + water-like molecules through

uniporters

•        Conformational changes allow binding, moving, and releasing of one ion or molecule at a time.

•        Maximum transport rate across membrane (V_max) depends on number of uniporters and concentration gradient.

•        Michaelis constant (K_m) measures the binding affinity of uniporter for substrate.

•        Calculated as 1/2V_max

•        Smaller K_m = faster transport

•        Ex: Glucose Transporter (GLUT) family of proteins

·       GLUT1 expressed in red blood cells ( high demand because rely on glucose and anaerobic respiration – good at moving glucose into RBCs

·       GLUT2 expressed in liver cells ( low demand because store glucose for body as glycogen)-not as good at moving glucose into RBCs

·       Difference between two is calculated (Km= faster at transporting glucose)

·       Defaults in GLUT proteins in brain can cause seizures, neurological problems

pumps

•        Move 1 or more molecules or ions against their gradients by hydrolyzing ATP to ADP, P_i

•        P-class are composed of two identical a-domains and sometimes a b-domain; one is phosphorylated for transport

•        Pi attached to alpha chain covalently ; triggers change in shape to let go

•        Ex: Sodium-Potassium Pump

-        Quabain causes cardiac arrest by inhibiting Na+/K+ pumps

-        Found everywhere

 

 

·       Ex: Calcium Pump

-moves 2 Ca+ ions per ATP

-muscle cells

 •        All known V-class and F-class pumps operate solely on protons.

•        V-class and F-class pumps are not phosphorylated; couple ATP hydrolysis with proton transport.

V - class pumps

•        V-class pumps are structurally complex, with several rotational features like cogs on a wheel.

•        Ex: V-ATPase uses ATP to move H+ ions into lysosomes

•        Not phosphorylated; use power of phosphorylation to spin cog in structure

•        Use chemical energy into kinetic energy to rotate apparatus to allow H+ to move around and exit opposite side of cell membrane

F-class pumps

•        F-class pumps are structurally similar to V-class pumps, but operate in reverse: using the potential energy of proton (H⁺) facilitated diffusion to create ATP from ADP, P_i

•        Ex: ATP synthase  in mitochondria

•        Using H+ moving down its concentration gradient to generate ATP

cotransporters

•        Use secondary active transport—energy released from one substance down its gradient allows another substance up its gradient.

•        Still uses energy, just not ATP

•        Symporters move both substances in same direction across the membrane.

•        Antiporters move substances in opposite directions across the membrane.

-        Both are secondary transporters; move in different directions

•        Ex: Sodium Glucose Transporter (SGLT)

-symporter in kidney tubule; lumen side where urine is collected and concentrated

-recover glucose from environment ( couple transport of glucose in cell with diffusion of sodium)

Sodium ( more concentrated in cell)  move down gradient to give glucose power to move up concentration gradient

-sodium is pumped out of the cell using Na/K pump on the opposite cell surface

-this keeps sodium concentration low in the cell, maintaining the concentration gradient from preurine