Cell membrane transport Flashcards
Semester 1 Year 1
What are 2 types of passive transport?
Simple + facilitated diffusion
What are 2 types of active transport?
Primary (direct) + secondary (indirect)
What is the purpose of an electrochemical gradient?
Drives passive transport
What is an electrochemical gradient dependent on?
Always dependent on conc. grad. of the solute. For charged molecules, also dependent on any difference in voltage between ICF and ECF
What is simple diffusion?
Movement of an uncharged solute through lipid bilayer
What does the speed of simple diffusion depend on?
- permeability of membrane to solute
- difference in con. between ECF and ICF
What type of solute does simple diffusion allow to move?
Hydrophobic solutes
What type of solutes do transmembrane proteins allow to move?
Hydrophilic solutes
What are transmembrane proteins composed of?
Membrane-spanning alpha-helical domains
What is a proteins membrane topology?
How the protein moves in + out of the membrane
What are the types of transmembrane protein?
Pore, channel, carrier, pump
How does a pore act as a transmembrane protein?
Form a channel but has not gate on it
How does a channel act as a transmembrane protein?
Is a gated pore - can open and close for substances to move through
How does a carrier act as a transmembrane protein?
It doesn’t have a clear opening, so substances bind to it
How does a pump act as a transmembrane protein?
Requires ATP to change shape for transport
How do transmembrane proteins allow substances to move across?
Create hydrophilic permeation pathways through the membrane
What is special about the amino acid sequence of transmembrane proteins?
Amphipathic helices - alternating hydrophobic and hydrophilic amino acids as determined by their variable regions
Which way do the hydrophobic + hydrophilic surfaces of the transmembrane protein face?
Hydrophobic-lipid membrane (outside)
Hydrophilic-creates central pore (inside)
Describe pores
-allow for facilitated diffusion
-driving force for movement=electrochemical gradient
-always open
-made up of multiple subunits
Describe channels
-allow for facilitated diffusion
-driving force for movement=electrochemical gradient
-gated ion channels
-made up of multiple subunits
What does each channel have?
-a moveable gate
-a sensor
-a selectivity filter
-an open channel pore
What are the types of sensors found in a channel?
-voltage
-ligand
-mechanical (membrane stretches so channel opens)
Describe carriers
-allow facilitated diffusion
-driving force for movement=electrochemical gradient
-never has a continuous transmembrane path
How do carriers work?
-the molecule enters the carrier through the outer gate + binds at the binding site
-outer gate closes + inner gate opens
-molecule exits + enters cell
-inner gate closes
Is carrier mediated facilitated diffusion faster or slower?
Slower
What limits carrier mediated facilitated diffusion?
-can become saturated
-limited by number of carriers in membrane
-limited by the speed by which carrier cycles through steps
Describe active transport
-moves solutes against its electrochemical gradient
-requires metabolic energy
-uses ATP
What are the 2 types of active transport?
Primary and secondary
Describe primary active transport
-uses a pump
-driving force is a chemical reaction e.g ATP hydrolysis into ADP
Describe secondary active transport
-uses cotransporters + exchangers
-uses electrochemical gradient from primary step
-driving force=coupling the uphill movement of 1 solute with the downhill movement of another
What else can carriers mediate?
-pumps
-cotransporters
-exchangers
Describe the sodium potassium ATPase pump
-pump moves sodium out of cell + potassium into it
-breakdown of ATP drives their movement in opposite directions
-pumps can become saturated
Describe cotransporters (symporters)
-move solutes in the same direction
-secondary active transport
-requires a driving solute whose electrochemical gradient provides energy (usually inward Na+)
Describe exchangers (antiporters)
-move solutes in opposite directions
-requires a driving solute whose electrochemical gradient provides energy (usually inward Na+)
Which transmembrane protein has the slowest rate of transport?
Carrier
