2.1 - Membrane Transport Flashcards
components permeable to the cell membrane
CO2, N2, O2, ethanol
Types of Molecules Crossing Plasma Membrane
Most Permeable → Least Permeable
gases (nonpolar molecules)
small uncharged polar molecules
large uncharged polar molecules
ions
charged polar molecules
water flow in hypotonic and hypertonic solutions
hypotonic: water flows into cell
hypertonic: water flows out of cell into solution
contractile vacuole
- evoluationary adaptation in paramecium for osmoregulation
- can expell fluid from the cell when the vacuole fills up
aquaporin
- a “water channel”
- alpha-helices form central pore
- channel diameter just large enough for water molecule
- hydrophlic amino acids in channel to help attract water molecules through
- no conformation change or energy used
aquaporin frog egg experiment
control:
- natural frog eggs – frog eggs do not normally contain aquaporins and are impermeable to water
experiment:
- mRNA encoding aquaporin protein injected into frog eggs
- frog eggs expressing aquaporin protein placed into water
- frog eggs burst within minutes
conclusion
- aquaporin = water channel protein
selective transport / facilitated transport
- a specific protein embedded in the membrane is the transporter
- solute moves down gradient
- no ATP needed
- example: channel
cotransport proteins
symporters and antiporters
involve the movement of two molecules across the membrane simultaneously
transport rates of transporters: fastest to slowest
Fastest
- channels
- transporters
- ATP-powered pumps
Slowest
uniport transporter protein kinetics
- kinetics of uniporter similar to that of enzyme kinetics demonstrating Michaelis-Menten kinetics
- can use Michaelis-Menten model to describe transporter activity
Michaelis-Menten Constant
Km = conc. of substance at 1/2 Vmax
Types of GLUT transporters
GLUT 1 = erythrocytes
GLUT 2 = liver cells
GLUT 4 = adipose / muscle cells
process of removing integral membrane protein for study
- disrupt membrane
- add detergen to solubilize protein
- mix detergent-covered proteins with phospholipids
- dialyze or dilute to remove detergent molecules
- result: liposome with transport protein in membrane
non-gated potasium channels
- specific to K+
- always open
- channel environment similar in nature and size to when an K+ is surrounded by water molecules
- water molecules stripped from K+ when K+ passes through the channel
membrane electric potential with the membrane is impermeable
generation of a transmembrane electric potential (voltage) depends on the selective movement of ions across a semipermeable membrane – if the membrane is impermeable then there will be no generation of membrane potential
Process of Gate Ion Channel Opening/Closing
- resting state – positively charged alpha-helices atttracted to the negative charges on the cytosolic side of the resting membrane, which keeps gate closed
- depolarization – voltage-sensing helices move through the phospholipid bilayer toward the outer membrane surface ⇒ conformational change and gate opens
- short after depolarization – ball and chain (inactivating segment) moves into open channel and prevents further flow
- repolarization – voltage-sensing helices return to the resting position, channel inactivating segment replaced, and gate closes
Structure of Voltage Gated K+ Ion Channel
- N-terminus is globular domain
- ball of the ball and chain inactivating segment
- cytosolic face
- 6 membrane-spanning alpha helices
Types of ATPases
P-class, V-class, F-class, and ABC
P-class Pump
- has two identical alpha subunits, each with an ATP-binding site
- during transport, at least one of the subunits becomes phosphorylated; transported ions move through
P-Class Pump Examples
- Na+/K+ pump
- H+/K+ pump of apical membrane of stomach
- Ca2+ pumps (i.e. ER, SR)
Sodium-Potassium Pump Process
- 3 Na+ and 1 ATP attach (cyt. side)
- phosphorylation of aspartate
- conformational change
- 2 K+ attach, 3 Na+ release (extra. side)
- dephosphorylation and conformational change
- 2 K+ release (cyt. side)
Typical Intracellular and Extracellular Ion concentrations (high/low)
Intracellular: K+
Extracellular: Na+, Ca2+, Cl-
Calcium ATPase Structure
Cytosolic face has:
- actuator domain
- phosphorylation domain
- nucleotide-binding domain
Ca2+ ATPase in SR: Process
- calcium and ATP-binding on cytosolic side (E1)
- phosphorylation of aspartate (E1)
- conformational change (E1)
- calcium release (E2)
- dephosphorylation (E2)
- conformational change (E2)
V-Class Proton Pumps
- transport only protons (H+) and does so in a process that doesn’t require phosphoprotein intermediate
- often used to lower pH of an environment
Examples of V-Class Proton Pumps
- endosomal and lysosomal membranes in animal cells
- plasma membrane of osteoclasts
getting an acidic environment with V-class pumps
V-class pumps pump H+ into the cell, but this generates an electric potential –> need to pair with import of a (-1) molecule like Cl- to keep the charge neutral so that more protons can get into the cell
F-class proton pumps
- H+ flow through to lead down concentration gradient in order to produce
- ATP synthases
ABC Superfamily
- “ATP-binding casette”
- two transmembrane domains
- two nucleotide binding domains
- can act as a flippase
- known as multi-drug resistant transporter
glucose transporters in hepatocytes
- the sodium-potassion pump in the basolateral surface membrane generates Na+ and K+ concentration gradients
- the outward movement of K+ ions through non gated K+ channels generates an inside-negative membrane potential across the entire plasma membrane. both the Na+ conc. gradient and the membrane potential are used to drive uptake of glucose from the intestinal lumen by the two-Na+/one-glucose symporter located in the apical surface membrane
- glucose leaves the cell via facilitated diffusion catalyzed by GLUT 2, a glucose uniporter located in the basolateral membrane
calcium transport out of a myocardial cell
To relax muscle → get calcium out of the myocardial cell
Calcium Antiporter; gets energy from Na+ gradient which was created by the Sodium Potassium pump (one Ca++ pumped out for every 2 Na+ pumped in)
acidification of the stomach lumen by parietal cells in the gastric lining
Function → Acidifying the Stomach (pH=1) → High H+ Concentration
apical membrane has H+/K+ ATPase (P-class pump); K+ and Cl- channels leading from cytosol to stomach lumen, H+ from H20 — OH- combines with Co2 to give HCO3- that is transported to blood through HCO3-/Cl- antiporter
ways to inhibit calcium antiporter
digoxin and oubin
