Primary And Secondary Active Transporters Flashcards
1
Q
How does primary active transport work ?
A
- Driven directly by energy released by hydrolysis of ATP to ADP.
- ATP hydrolysis catalysed by the transporter as part of the reaction cycle.
2
Q
What are examples of primary active transport?
A
- Plasma membrane Ca2+ATPase (PMCA: Ca2+ - Mg2+- ATPase) - uses energy released from the hydrolysis of ATP to drive calcium out of the cell, it doesn’t transport magnesium but uses but the ATPase requires ATP in the form of magnesium.
- F1F0 -ATPase (ATP synthase) - Primary active transport in reverse mode, ATP synthase found in inner membrane of mitochondria, it uses energy from the proton gradient produced by the electron transport chain and use the energy as the potential energy in the gradient and d allow the re-entry of protons into the matrix and drive the synthesis of ATP.
- Na+ - K+ - ATPase (Na+ pump)
Pg 5-6
3
Q
What is a Co-transport?
A
- More than one type of ion or molecule may be transported on a membrane transporter per reaction cycle
4
Q
What is the meaning of uniport , symport and antiport?
A
- Uniport is if the transported molecule/ion is carried on its own through a channel or transporter.
- Symport is two transported ions or molecules transported in the same direction during the reaction cycle.
- Antiport if the co-transported molecules or ions are transported in opposite directions.
5
Q
How does the Na+ - K+ ATPase (Na+ pump) work?
A
- The ATPase uses the energy from the hydrolysis of ATP to pump 3 sodium ions out of the cell against their concentration gradient and 2 potassium ions against its concentration gradient into the cell.
This is a primary active transporter, which is an antiporter.
Pg 9
6
Q
What are some important information about the sodium pump?
A
- Plasma membrane associated pump
- uses ATP (energy source) to pump ions - Primary active transport
- 25% of the basal metabolic rate used for pump, so 25% of ATP produced by the cell to drive the activity of the pump.
- Called a P-type ATPase - because during the reaction cycle ATP phosphorylates Aspartate, producing a phosphoenzyme intermediate, this phosphorylation drives the initial conformational change to allow the transport of ion.
7
Q
What does the alpha and beta subunit of the sodium pump contain?
A
Alpha subunit (contains binding sites) - K+, Na+, ATP and ouabain (inhibitor of the process) binding sites.
Beta subunit - glycoproteins directs pump to surface, in association with the alpha subunit.
- in absence of the beta subunit, the alpha subunit is not transported to the plasma membrane after synthesis .
Pg 11
8
Q
What are the key concepts of the sodium pump?
A
- 3 NA+ ions expelled for every 2 K+ ions that enter.
- Very important for generating the ion gradient that are used to allow secondary active transport and action potentials
- only a small contribution to resting membrane potential
9
Q
What is the effect of the sodium pump on membrane potential?
A
- Sodium pump creates the high intracellular [K+]
- K+ diffusion outwards through channels is mainly responsible for generating membrane potential (-70mV)
- so the pump is important for generating gradients, but not important in generating the resting membrane potential.
- Sodium pump generates only about 5-10mV through electrochemical pump activity
10
Q
How does secondary active transport work?
A
- Driven indirectly by the energy released by the hydrolysis of ATP to ADP.
- Dissipation of gradients formed by another ion or substances provides the energy for the secondary active transport.
11
Q
What are examples of secondary active transport?
A
- Na+ -Ca2+ - exchanger (antiport) : uses the sodium gradient produced by the sodium pump, to allow the re-entry of 3 sodium down its concentration gradient and removes 2 calcium from the cell into the extracellular space.
- Na+ - H+ - Exchanger (antiport) : uses the sodium gradient produced by the sodium pump to pump hydrogen out and sodium into the cell, used for regulation of pH.
- Na+ - Glucose cotransport : the energy from the dissipation of the sodium gradient to drive glucose and 3 sodium renters into the cell e.g. small intestines and kidney
12
Q
What are the different types of calcium transport?
A
- Ca2+-Mg2+ -ATPase (primary active transport)
- driven by ATP hydrolysis
- driving calcium out of the cell
- High affinity for calcium but low capacity
- has high affinity to bind to calcium and although it has a low capacity, it can remove the last bit of calcium that needs to be removed.
- Na+ -Ca2+ - exchanger
- transport driven by the dissipation of Na+ gradient
- diving calcium out at 3 sodium in (antiport)
- low affinity, high capacity
- due to its high capacity it is important in restoring the calcium concentrations in the cell to low concentration after calcium has been raised.
Pg 15
13
Q
What is the importance of transporters medicine?
A
- Transporters in Cystic fibrosis: The CTFR gene codes for the cystic fibrosis transmembrane conductance regulator which allows for the transport of chloride across the membrane and extruded into the lumen.
- chloride is an osmotic ion so water will follow it and allows the mucus to be mobile.
- mutation of the CTFR means no movement of chloride across the membrane into the lumen so no movement of water and so mucus is thick and immobile.
pg 22 - Transporters in diarrhoea: e.g. vibrio cholera
- In the infected person the vibrio cholera infection activates protein kinase A which phosphorylates the CTFR and increases its activity
- Changing the balance of ion transport in the tissue, chloride is much readily passed across the membrane and extruded into the lumen.
- Water follows it and leads to diarrhoea
pg23
14
Q
What are the functions of the sodium pump?
A
- It form Na+ and K+ gradients
- Necessary for electrical excitability.
- contributes less than -5 mV to resting membrane potential. - Drives secondary active transport
- controls intracellular pH ( sodium- hydrogen exchange)
- regulation of cell volume and intracellular calcium concentration (sodium-calcium exchange)
- absorption of sodium ions in epithelia
- nutrient uptake e.g. glucose from the small intestines ( sodium-glucose co transport )
15
Q
Why does intracellular calcium need to be controlled?
A
- intracellular calcium concentration is low (10-7M or 50-100 nM).
- Extracellular calcium concentration is higher - 2mM
- High intracellular calcium is toxic to the cell, this is because phosphate is one of the key intracellular buffers and calcium phosphate forms an easy precipitate.
- this precipitate is needed for bone formation, so can lead to ossification of cytoplasmic constituents.
- Also cells signal by small changes in intracellular calcium concentration.
