Lecture 6- transport across membranes

major component of cellular membranes

phospholipids

hydrophobic transmembrane domains interact with
fatty acid what of membrane lipids

tails

many membrane proteins have # transmembrane domains

multiple

an enzyme that breaks proteins into very small pieces, too small (generally) to see in an SDS-PAGE ge

trypsin

rypsin can or can not get into a cell – thus transmembrane
and internal proteins are protected

can not

some molecules penetrate the lipid
bilayer:
oil-soluble (partition coefficient) or very # molecules (O2)

small

Large uncharged polar molecules like glucose, sucrose, amino acids or IONS are not permeable and must be transported by what

proteins

from area of high concentration to low concentration

Passive Transport

from area of low concentration to high concentration

Active Transport

movement of H2O from an area of low solute
concentration to an area of higher solute concentration

Osmosis

In passive transport H2O can diffuse what through membrane

quickly

H2O enters/exits cells through specialized pores called what

aquaporins

H2O molecules pass through aquaporins how

one by one

Aquaporins’ Channel wall is positively charged and binds to negatively charged what; thereby disrupting H bonds that link H2O molecules together

Oxygen

facilitated diffusion (transport) of what through membranes via transporter

glucose

Most cells contain a glucose transporter that facilitates the diffusion of glucose from the blood stream into the cell to be used for what

energy

hormone produced by endocrine cells of the pancreas -
it maintains blood sugar levels

Insulin

at what insulin levels few transporters are on the cell surface. (limits uptake)

low

Insulin stimulates the what of the glucose transporters

exocytosis

small ions (K+, Na+, Ca2+, Cl-) can or can’t diffuse through lipid bilayers

can’t

most ion channels are what for specific ions

selective

determined by the concentration difference of the substance on the two sides of the membrane

chemical gradient

determined by the charge difference between the two sides of the membrane

electro-potential gradient

pen based on differences in ionic charge between the inside and outside of the cell
(membrane potential)

voltage-gated channels

open based on a “ligand”
binding to the channel (conformational change)

can bind to either inside or outside of membrane

ligand-gated channels

open in response to force or other stimuli

mechanosensory gated channels

K+ channel of bacteria: only permits K+ ions to bind

selectivity filter

channel (alpha helices), selectivity filter, and the gate (M1-M2 helices ~ S5-S6 helices)

Pore domain

Senses the voltage across the membrane

Voltage-sensor domain

resting

negative membrane potential

S4 (+ charges)move from cytoplasmic exposure to extracellular
exposure

positive membrane potential

conformational changes does what the channel

close

genetic diseases linked to ion channels

channelopathies

Transient Receptor Potential Vanilloid (TRPV) is gated by
ligands or by what (& pain)

heat

you discovered a new channel!
• you name your new channel “Ch 303 - HD”
• your observation: it multimerizes and folds in such a way that there is a very small pore lined with amino acids with a positive or partially positive charge

predict what most likely travels through this channel:

anions (negatively charged ions)

you try a “patch clamp” experiment
• measures ion flow by measuring electrical current
• you find that your channels is specific for Cl-
• but, you only observe current flow when you add cAMP into your cell, not when you apply a voltage

what kind of channel is this?

ligand-gated ion channel

more about channel 303…
given that the Cl- concentration is 110 mM outside
the cell, and 10mM within the cell, what direction do
you expect Cl- ions to flow in the presence of ligand?
you make a mutant form of your channel that deletes the
C-terminal region:

How to you expect this to alter ion flow?

makes it constitutive (ie, unregulated and constant)

(pumps) use ATP for energy

primary active transporters

use stored energy (gradients of other molecules) to move a substance against its gradient

secondary active transporters

movement against a concentration gradient occurs by what

active transport

what mediate active transport – drive a given ion in only one
direction

pumps

Pumps are critical:
• maintain what pH inside lysosomes
• maintain what pH inside the stomach
• store ionic energy

low

pumps move substances how a concentration gradient (active transport)

against

This pump creates the big difference in the concentrations of Na+ and K+ inside vs outside the cell

he Na+/K+ ATPase pump

what binds to the transport protein on the inside of the cell with high affinity

Na+

what is hydrolyzed and the released P binds to the transport protein

ATP

binding of P changes the transporter’s
configuration and affinity for what
released to the outside of the membrane

Na+

most of the time proteins are associated with something else in the cell in a complex (C) via which bonds

non-covalent

If the protein and ligand have what affinity, very little ligand is required to get a complex

high

K+ binds to the pump
P dissociates causing the pump resumes its original conformation
this does what to K+ binding affinity

lowers

K+ diffuses into the what

cytoplasm

must have a higher binding affinity for what inside the cell and a lower binding affinity for what outside of the cell

Na+

must have a higher binding affinity for what outside of the cell and a
lower binding affinity for what inside the cell

K+

different affinities are achieved by phosphorylating the what protein

transport

Na+/K+ pump is only in which cells

animal

H+/K+ pump in the what (pumps acid into; pump translocates to plasma membrane after eating)

stomach

H+ proton pump in what is important for import of solutes and
control of pH

plants

superfamily of pumps
• present in bacteria through mammals
• pump ions, sugars, peptides, polysaccharides, proteins!

ATP-binding cassette (ABC)

Defects in pumps, transporters, or channels often lead to
what

disease

tumor cells become resistant to chemotherapy

multi-drug resistance (MDR)

MDR-1 protein is part of a pump (ABC transporter) that pumps
what materials out of cells

toxic

MDR-1 expressed in normal liver and what to export
toxic molecules

kidney

BUT in cancer cells: MDR-1 gene is amplified and over-
expressed so chemotherapy drugs that diffuse through the which cell membrane are pumped out

cancer

what (secondary) transport takes advantage of stored energy

coupled

gradients created by what ion pumping store energy that can be coupled to other transport processes

active