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Flashcards in MR S2 Deck (45):

What is passive transport reliant on?

Membrane permeability
Concentration gradient


Order these molecule types in order of most to least permeable to phospholipid membranes:
Hydrophobic molecules
Small uncharged polar molecules
Large uncharged polar molecules

Hydrophobic molecules
Small uncharged polar molecules
Large uncharged polar molecules


Describe active transport

Allows transport of ions against an unfavourable concentration gradient and/or electrical gradient
Energy directly or indirectly from ATP
Some cells spend 30-50% of their energy on active transport


What is the approximate free ion concentration of sodium intra and extracellularly?

Intra: ~12mM
Extra: ~145mM


How does increased concentration gradient affect rate of passive transport?

Rate of passive transport increases linearly with increasing concentration gradient


What is the approximate free ion concentration of calcium intra and extracellularly?

Intra: ~10^-7M
Extra: ~1.5mM


What is the approximate free ion concentration of chlorine intra and extracellularly?

Intra: ~4.2mM
Extra: ~123mM


What is the approximate free ion concentration of potassium intra and extracellularly?

Intra: ~4mM
Extra: ~155mM


What are the two types of transporter involved in co transport?



What is a uniport?

A transport protein which moves one molecule per cycle


What is a co transporter?

A transport protein which simultaneously transports two molecules per cycle


What is PMCA?

A primary active transporter
Full name: plasma membrane calcium ATPase
Moves calcium ions across a membrane using ATP


Why do co transporters use secondary active transport?

They use the concentration gradients of one molecule to transport an extra molecule
Aka they don't directly use ATP
BUT still active transport because the concentration gradients that allow co transport are set up by membrane proteins which require ATP.


What is the Na+K+ATPase (aka Na+ pump)?

Moves 2 K+ ions into the cell and 3 Na+ ions out of the cell (usually against their concentration gradients) simultaneously
Requires ATP
A P-type ATPase
~25% of BMR used for this protein


What is ATP synthetase?

Active transport in reverse mode
Uses high proton concentration gradient to produce ATP
Seen in electron transport chain/oxidative phosphorylation


What type of active transport occurs in co transporters?



Why is the Na+ K+ pump important?

Sets up Na and K gradients
Responsible for ~-5mV of the membrane potential
Necessary for electrical excitability
Drives many secondary active transport regulatory processes eg:
-ion homeostasis
-Cell volume
-Ca concentration
-resting potential
-nutrient uptake eg glucose uptake in small intestine


Name two calcium transporters

Ca2+ Mg2+ ATPase
Na+ Ca2+ Exchanger (NCX)


What is the difference between a symport and an antiport?

Symports transport two molecules in the same direction across a membrane
Antiports transport two molecules across a membrane in different directions


Describe the Na+ Ca2+ Exchanger (NCX)

Moves one Ca out and three Na in per cycle
Low affinity, high capacity

Transport of Na ions down concentration gradient provides energy for transport of Ca against concentration gradient.


Describe the Ca2+ Mg2+ ATPase transporter

Moves one Ca or Mg out of the cell
Active transporter
Requires ATP
High affinity, low capacity


Describe the Na H exchanger (NHE)

Exchanges one extracellular Na for one intra cellular H
Electroneutral (1:1 ion exchange)
Regulates pH
Regulates cell volume
Inhibited by amiloride
Activated by growth factors


Describe the Na glucose co transporter

Entry of Na provides enough energy for entrance of glucose into cell agains concentration gradient

Important in small intestine and kidney


What can increase the passive transport of water?

Increased osmotic gradient
Water channels aka aquaporins


Where can aquaporins be found?

The proximal kidney tubules


Why are hydrophilic molecules less likely to cross a membrane?

Has to travel through hydrophobic area
So would require high free energy (ΔG) change
So thermodynamically unlikely


What are some important roles of transport systems?

Maintenance of intracellular pH
Maintenance of ionic composition
Regulation of cell volume
Concentration of metabolic fuels and building blocks
Extrusion of waste products of metabolism and toxic substances
Generation of ionic gradients necessary for electrical excitability of nerve and muscle tissue


What are the models for facilitated transport?

Protein pores (aka channels)
Carrier molecules (ping-pong)
Flip flop (thermodynamically unlikely)


Why is control of calcium so important?

There's a ~20 000 fold difference between intra and extra cellular calcium so VERY tightly controlled
High intracellular calcium is highly toxic to cells
Cells signal by SMALL changes of intracellular calcium


What is SERCA?

Sarco(Endo)plasmic Reticulum Ca2+ ATPase
Transports calcium into and protons out of the endoplasmic/sacro plasmic reticulum
Uses ATP
High affinity, low capacity


What is the mitochondrial Ca uniport and when is it used?

Uses facilitated diffusion
Operates at high intracellular Ca concentrations to buffer potential damaging calcium


What are the types of gated pores?

Ligand gated channels - open or close in response to binding of ligand to receptor site
Voltage gated channels - open or close in response to change in normal membrane potential
Gap junctions - these close when intracellular calcium rises above 10μM or when cell becomes acid


Does NCX always pump ions in the same direction? What effect does this have?

No, in depolarisation NCX's normal function is reversed
This contributes to the Ca influx during the cardiac action potential
Can also contribute to Ca toxicity ischaemian


Name some membrane proteins involved in control of pH

NHE aka Na H exchanger
NBC aka sodium bicarbonate cotransporter aka Na dependent Cl/HCO3 exchanger
AE aka Anion transporter aka Cl/HCO3 exchanger


Name the bicarbonate transporters and their actions

AE acidified the cell
NBC alkalinises the cell

Both are involved with cell volume regulation


What is facilitated diffusion?

The membrane permeability of a substance can be increased by the incorporation of certain membrane proteins into the membrane
Relies on concentration/electrical gradient for substrate movement


Outline bicarbonate reabsorption in the proximal tubule

Na/K pump sets up Na gradient
NHE pumps H out of cell
H "picks up" bicarbonate and brings it back into the cell


Describe the theory of renal hypertensive therapy

The goal is to reduce mineral ion reabsorption from urine
Therefore less water returns via osmosis
Therefore blood volume and so blood pressure falls


Describe aquaporin

Allows water to readily cross the membrane
It's inclusion in membranes is stimulated by ADH


Describe loop diuretics

Block Na reuptake in thick descending limb of kidney


Describe amiloride

Prevents the reuptake of Na
Acts in the distal convoluted tubule on ENaC
Acts on the proximal tubule on NaH


What action does aldosterone have?

Up regulates transporters in the kidney to increase ion and water reabsorption in the kidney


Describe spironolactone

Gluticocorticoid antagonist
Used to treat high aldosterone levels


Describe the role of membrane transporters in cystic fibrosis

Na/K pump sets up gradient
CFTR protein non-functional
Cl builds up in cell
Water moves into cell
Mucus is thick and sticky


What is the role of transport proteins in diarrhoea?

CFTR protein overly active after phosphorylated by ATP
Cl transported into lumen of gut
Water follows