L8: Membrane Transport Flashcards
What are the two types of transport across biological membranes?
- Active (needs ATP)
- Passive (doesn’t require ATP)
When is ATP required in transport?
- Against a concentration or electrochemical gradient, ATP is required
- Down a concentration or electrochemical gradient, ATP is not required
What are the two types of passive transport?
- Simple diffusion - no membrane proteins involved and driven by concentration gradients
- Facilitated diffusion - membrane proteins involved and driven by concentration gradients
Describe simple diffusion
- Molecules move stochastically (across the membrane in both directions)
- Molecules move down a concentration gradient (from high to low concentration) until equilibrium is reached and there is no gradient
What factors determine solute movement across a membrane in simple diffusion?
- Concentration gradient
- Size of the molecule
- Hydrophobicity/charge
Can hydrophobic molecules diffuse across the plasma membrane?
- The plasma membrane is permeable to hydrophobic molecules
- Able to pass directly through the hydrophobic core of the lipid bilayer - complementary
- e.g. O2, CO2, N2 and steroid hormones
Can small uncharged polar molecules diffuse through the plasma membrane?
- Can ever so slightly pass through the membrane, but generally not due to their polar nature - cannot pass through the non-polar, hydrophobic centre of the lipid bilayer
- e.g. H20, urea and glycerol
Can large uncharged polar molecules diffuse through the plasma membrane?
- Cannot diffuse through the membrane
- The plasma membrane is highly impermeable to these molecules
- e.g. glucose and sucrose
Can ions diffuse through the plasma membrane?
- Highly impermeable to ions - cannot pass through the non-polar hydrophobic centre of the lipid bilayer as the ions are hydrophilic and repelled by the hydrocarbon fatty acid tails
- e.g. H+, Na+, K+, Ca2+ and Cl-
What are inorganic ions and small molecules required for in cells?
- Regulation of intracellular ion concentrations
- Uptake of nutrients
- Excretion of metabolic waste products
What is facilitated diffusion?
Diffusion down a concentration gradient involving membrane proteins for inorganic ion/smaller molecules
What are the two types of membrane protein?
- Channel proteins - discriminate on what they allow through based on size and charge of the ion
- Uniporter carrier proteins - involve a binding site for solutes
Describe what an electrochemical gradient is and how they function
- An electrochemical gradient combines the concentration gradient and membrane potential
- With a negatively charged membrane potential, this enhances the electrochemical gradient, meaning positively charged ions are more likely to move across the membrane
- With a positively charged membrane potential, this reduces the electrochemical gradient, reducing the movement of the positively charged solute across the membrane
What features do ion channels exhibit?
- Exhibit selectivity
- Driven by a concentration/electrochemical gradient
- Fast - transport up to 10^7 molecules per second
- May be regulated (open and close in response to a stimulus)
Name some different ion channels?
- Voltage gated
- Ligand-gated (extracellular ligand)
- Ligand- gated (intracellular ligand)
- Mechanically gated
What is the most common ion channel?
K+ ion channel
Describe the K+ ion channel
- Continuously open - not gated
- Highly selective for K+
- Moves K+ very quickly from inside the cell where K+ is hydrated
- K+ is dehydrated in the ‘vestibule’ of the channel protein and the carboxyl oxygens of amino acids line up and selectively filter the K+ out of the cell, rehydrating K+
What is an example of a uniporter carrier protein?
Glucose transporter (Glut2) in gut epithelia
How does the glucose uniporter carrier protein work?
- Highly selective to glucose
- Has a binding site that will recognise and bind glucose
- The binding of glucose causes a conformational change in the protein, causing the glucose to move to the other side
- Movement down a concentration gradient
How do uniporter carrier proteins differ to channel proteins?
- Uniporter carrier proteins are much slower than channel proteins - <1000 molecules per second
- Because of the requirement for a conformational change in the protein
Describe the selectivity in a glucose transporter (Gluts)
- Uniporters (only transport glucose)
- Expressed by most cell types
- 12 pass membrane spanning proteins
- Alternate between two conformations
What happens if there is a Glut1 deficiency?
Syndrome characterised by seizures, microcephaly and retarded development
Describe the transport of glucose into a erythrocyte
- Glucose concentration is higher in the blood than the erythrocyte
- Glucose is transported down a concentration gradient into the erythrocyte by the Glut1 uniporter
- Glut1 can work in both directions- leads to equilibrium - so the concentration gradient has to be maintained
- Glucose is converted into glucose-6-phosphate by the addition of phosphate
- Glucose-6-phosphate is not recognised by Glut1 making it a one-directional transport - not able to bind
Why do cells maintain electrochemical gradients?
- Drive transport of molecules across membrane
- Maintain osmotic balance (water balance)
- Electrical forces inside and outside the cell must be balanced (though small localised differences at the plasma membrane are allowed)
Why is active transport so important?
Without active transport (req. ATP) to maintain electrochemical gradients, ions would flow down their gradients through channels, disturbing osmotic balance
What is the essential feature of active transport?
- Moves solutes against their electrochemical gradients
- To achieve this energy is required
What type of active transport is an ATP driven-pump?
Primary active transport
How is the energy for active transport provided?
- ATP-driven pumps - couple the transport of a solute against its gradient to the hydrolysis of ATP
- Coupled transporters - couple the transport of one solute with the gradient to another against the gradient
- Light-driven pumps - couple the transport of a solute against its gradients to the input of energy from light
What type of active transport is a coupled transporter?
Secondary active transport
What is the difference between primary and secondary active transport?
- Primary involves ATP directly
- Secondary does not involve ATP directly and uses another gradient
What is an example of an ATP-driven pump?
The Na+ and K+ electrochemical gradient/pump
Are there more Na+ ions intracellularly or extracellularly?
Extracellularly
Are there more K+ ions intracellularly or extracellularly?
Intracellularly
Why is the Na+/K+ pump so important?
- Absence of sodium ions pumped out and potassium ions in causes the ions to flow down their gradients
- This affects the osmotic balance and prevents ‘secondary’ active transport
What maintain the concentration difference between Na+ and K+?
Sodium-potassium ATPase
What % of a cell’s total energy consumption if used in the ATPase pump?
30%
How is the sodium-potassium ATPase pump electrochemical gradient maintained?
- Operates continuously to expel Na+ that enters cell through other carrier proteins and channels
- Hydrolyses ATP to ADP and Pi - both an enzyme and carrier protein
- Couples the export of Na+ to the import of K+
Where is the ATPase pump found?
Every plasma membrane of all eukaryotic cells
Describe how the Na+/K+ ATPase mechanism works?
(For Na+)
- Both an alpha and a beta
- The beta chain trafficks the pump to the plasma membrane
- 3x Na+ ions bind to the open pump on the inside
- Pump hydrolyses the ATP into ADP and the terminal phosphate (-vely charged) binds to the pump - phosphorylates the pump
- Na+ dependent phosphorylation causes pump to undergo conformational change
- Sodium ions transported across the membrane and released into the extracellular environment
Describe how the Na+/K+ ATPase mechanism works?
(For K+)
- 2x K+ ions are able to bind to the pump as Na+ is released
This dephosphorylates the pump - K+ dependent phosphorylation causes the pump to return to its original conformation
- K+ is transferred across the membrane and released into the intracellular environment
How does secondary active transport work?
- Moves solutes against the concentration/electrochemical gradient by coupling transport to Na+ gradient created by the Na+/K+ ATPase
- Does not depend on the hydrolysis of ATP
What is a symporter, in coupled transporters? Give an example
- Molecules move in the same direction
- Sodium down its electrochemical gradient with glucose at the same time
What is an antiporter, in coupled transporters? Give an example
- Molecules move in different/opposite directions
- Sodium down its electrochemical gradient and calcium against its electrochemical gradient
Describe how the Na+/glucose symporter works
- High Na+ in the gut and low in the cell
- High glucose in the cell and low in the gut
- Glucose has to move against its concentration gradient into the gut
- Binding on glucose and Na+ is co-operative (binding on glucose is dependent on Na+)
- Na+ concentration is much higher outside of the cell, so glucose binds to the same carrier protein as Na+ and is more likely to bind this way
- Conformational change of the carrier protein
- Releases both Na+ and glucose into the gut epithelial cell
- Net flow into the cell
- Uses energy provided by the sodium electrochemical gradient
Where is the Na+/glucose symporter located?
- Luminal side of the epithelial cell on the GI tract
- Glucose concentration is higher in the epithelial cells than in the lumen
Why are glucose symporters required for the absorption of glucose from the GI tract?
Epithelial cells are very tightly joined together by tight junctions between them, which restricts paracellular movement
- Transcellular movement required for small molecules to transfer between the blood and gut
Describe the overall process for the absorption of glucose
- Na+ gradient is created by the Na+/K+ ATPase (primary active transport) - provides the energy needed to move glucose from low to high concentration in the cell
- Na+/glucose symporter transports glucose into the epithelial cells (against its concentration gradient) by utilising the Na+ electrochemical gradient (secondary
active transport) - Glut2 (uniporter) transports glucose out of the cells (along its concentration gradient) and into the blood supply (facilitated diffusion) - because other processes mean glucose concentration is higher in the epithelial cells than in the blood
Where is the Na+/glucose symporter located?
Apical membrane on the lumen of the GI tract
Where is Glut2 located?
Basolateral membrane
Describe the Na+/Ca2+ antiporter
- Uses the sodium gradient
- 3x Na+ moved out of the cell using its electrochemical gradient
- 1x Ca2+ moved into the cell against its gradient
What is the Na+/Ca2+ antiporter important for?
- Cardiac muscle
- Cardiac muscle cell contraction is triggered by a rise in intracellular calcium
- The antiporter reduces intracellular Ca2+ concentration, which reduces the strength of cardiac muscle contraction - modulate contraction
What drug has an inhibitory effect on the Na+/K+ ATPase?
Ouabain
What effect would ouabain have on cardiac contractility?
- The concentration of Na+ pumped out of the cell is reduced as the ATPase is inhibited
- Means an increase in intracellular Na+
- There is a reduction in the Na+ gradient so the effectiveness of the the Na+/Ca2+ antiporter is reduced, so less Ca2+ is transported out of the cell
- Maintains intracellular Ca2+ concentration, increasing cardiac muscle cell contractility
When is ouabain used in a clinical setting?
Improve cardiac contractility to prevent heart failure
The SGLT2 transporter facilitates which types of transport?
- Secondary active transport
- The glucose is moving against the concentration gradient , but does not use ATP directly
- ATP provided by the sodium electrochemical gradient maintained by the sodium potassium pump
What is the function of SGLT2 transporters?
- Takes glucose out of the urine and transports it back into the blood
- Regulates blood glucose concentration
- Useful for treating diabetes - limits blood glucose concentration by allowing more to be excreted in the urine
Draw a diagram to summarise membrane transport
