Cell Bio Exam 2 : Membran Transport

what can pass and what cannot?

can pass :

-small , nonpolar molecules (O2, CO2)

-small, uncharged polar molecules (h20-freely in and out and ethanol)

cannot pass:

-large unchared polar molecues (glucose)

-ions (H+, K+, Ca+)

Electrochemical gradient

determines direction charged solute will flow across membrane by passive transport 

driven by charge and concentration

voltage (difference in charge across membrane; outisde- positive, inside- negative)and concentration gradient in same direction : goes from high concentration to low with steep electrochemical gradient

if voltage and electrochemical gradient act in opposite directions: 

goes from high to low concetration with very small gradient 

passive transport

spontaneously flow from high conc. to low (no additional force)

active transport 

substance flows from low conc. to high conc.  (requires additional force- ATPp) 

done by pumps

passive transport proteins

channel mediated- move across through specific channel proteins (creates hydrophilic passageway) from high conc. to low conc. wiithout requiring energy

transporter mediated- from a high to a low concentration with the help of specific membrane proteins, without the cell expending energy (bind to a molecule, shape change to move it across the membrane)

active transport protein

carrier protein aka pumps

specificity of transporters 

Transporters are highly specific; typically only one molecule

passive transporter example

the glucose transporter (moves glucose down conc. gradient)

• in outward open state, binding sites are exposed (glucose binds) when more molecules outside 

• then changed to occluded state (sites not accessible either side)

• then in inward open state to move into cytoplasm 

active transport example

transmembrane pumps (move solutes against their electrochemical gradient)

ATP-driven pumps

use the energy from ATP hydrolysis to move ions and molecules across a cell membrane against their concentration gradient

ex. the sodium-potassium pump (active transport)

-helps maintain membrane potential (charge on each side of membrane)

-transports Na+ out cell and K+ in

coupled pumps 

move two different substances across a cell membrane, using the movement of one substance down its concentration gradient to move the other against its gradient

the glucose-sodium symporter

symporter ( moves both in same direction)

-actively transports glucose

• the pump opens and allows Na+ to bind but has to wait for the rare glucose to bind as well 

• closes

• then opens inwardly allowing the proper electrochemical gradient of Na (from high to low) and active transport of glucose (from low to high) 

gut epithelial

use both active ( has Na driven glucose symport at top of cell that takes glucose from the gut to absorb into epithelial cell)’

creating high conc. intside

passive transport of glucose ( passive glucose uniport on basal membrane that transports glucose down gradient and out to be used by other tisses)io

ion channels

allow the passage of specific ions

(open and close in response to different signals) 

How does membrane potential affect diffusion of ions through ion channels?

an electrical force that works with or against the chemical (concentration) force

positively charged ions - a negative membrane potential pulls them into the cell, while a positive potential pushes them out

negatively charged ions- negative membrane potential (inside) repels them, positive pulls them in

mechanically gated ion channels

pulled open by physcial force

ligand gated ion channel

binding of a molecule (either intracellular or extracellular)

voltage gated channel 

opening controlled by membrane potential 

ion channels using membrane potental in signaling

by opening and closing in response to stimuli, which changes the electrical charge across the cell membrane

-have to maintain a negative inside membrane potential, so it controls the selectivity of ion channels