Membrane transport Flashcards
Name the two types of passive transport
Simple diffusion
Facilitated diffusion
Define passive diffusion
Requires no energy input
What is a chemical gradient?
Difference in concentrations of an ion inside and outside the cell
What is an electrochemical gradient?
Combines the chemical gradient and the membrane potential
What is the membrane potential?
The difference in charge between the inside and outside of cells
The ability of a solute to cross the membrane depends on what 4 things?
Concentration gradient
Electrochemical gradient
Hydrophobicity
Size
Membranes are highly ________ to ions
Impermeable
Name the two classes of membrane protein involved in facilitated diffusion
Channels
Transporters
Describe ion channels
They exhibit ion selectivity
Driven by concentration or chemical gradient
Work quickly
Regulated
Describe K+ channels
Most common ion channel
Subdivided into leak and gated
10,000 more effective for K+ than Na+
What is present at the entrance of the K+ channel?
Negatively charged amino acids to repel negatively charged ions
What happens to K+ ions at the exit of the K+ channel?
K+ ions in hydrated state are too big to pass through
K+ is dehydrated
Energy lost in dehydration = energy regained when K+ interacts with carbonyl oxygens
K+ is rehydrated
Why doesn’t Na+ go through the K+ channel?
The dehydration of Na+ occurs in the channel
The Na+ is too small to be rehydrated by the carbonyl oxygens
Energetically unfavourable
Describe transporters
Exhibit selectivity
Slow transport
Oscillate between two conformations
Describe glucose transporters
Uniporters (only glucose) Many isoforms 12 pass membrane spanning proteins Present in all cells Glut1 deficiency syndrome = microcephaly, seizures, retarded development
How do we ensure glucose concentration is maintained?
Once transported into the cell, glucose is converted into glucose-6 phosphate
The glucose concentration remains higher outside the cell
What is active transport?
Transport against the concentration or electrochemical gradient that requires ATP
What are the two types of membrane proteins involved in active transport?
ATP-driven pumps
Coupled transporters
What do ATP-driven pumps do?
Move solutes against the concentration/electrochemical gradient by expending energy
Descrive Na+K+ATPase
Expressed by all cell types Some cells have 30 million Hydrolyses ATP Uses 30% of the cells energy (more in nerve cells) Continually expels Na+ and brings K+ in
How does Na+K+ATPase work?
3Na+ bind and the pump is phosphorylated by ATP
Pump undergoes conformational change
Na+ transferred across the membrane and released
2K+ ions bind and the pump is dephosphorylated
K+ transferred across the membrane and released
Describe coupled transporters
Move solutes against their concentration/electrochemical gradient by coupling their transport to the Na+ gradient created by Na+K+ATPase
What makes coupled transporters an example of secondary active transport?
They don’t rely directly on the hydrolysis of ATP
What is a symporter?
Moves both molecules in the same direction
What is an anti porter?
Moves the two molecules in opposite directions
Describe the Na+Glucose symporter
Expressed by epithelial cells in the GI tract
Glucose concentration is higher in epithelial cells than in the lumen
Symporter utilise the Na+ electrochemical gradient to transport glucose into the cells against its concentration gradient
How does the Na+Glucose symporter work?
Oscillates between two conformations
Binding is cooperative
Na+ binding increases the affinity for glucose
Glucose is more likely to bind on the extracellular side where Na+ concentration is high
Glucose more likely to be moved in than out
What prevents paracellular movement?
Tight junctions
What proteins make up tight junctions?
Claudin
Occludin
What does Glut2 do?
Transports glucose out of the cell via facilitated diffusion
What is asymmetric distribution?
The arrangement of transporters in cells so that concentrations are maintained
Describe the Na+Ca2+ anti porter
Expressed by cardiomyocytes in heart muscle
Ca2+ concentration is greater in the interstitial fluid than in cardiomyocytes
Antiporter uses Na+ to move Ca2+ out of the cells
This relaxes muscle contraction
How do you strengthen muscle contraction?
By inhibiting Na+K+ATPase
Reduce Na+ gradient
Ca2+ not moved out of the cell
Reduces muscle relaxation
Describe an aquaporin
A tetramic complex made of four monomers
Each monomer is a six pass membrane spanning protein
Each monomer is a channel itself
