MR S2 Flashcards by Jess Sidney

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

Hydrophobic molecules
Small uncharged polar molecules
Large uncharged polar molecules
Ions

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What is passive transport reliant on?

Membrane permeability

Concentration gradient

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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

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

Intra: ~12mM
Extra: ~145mM

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How does increased concentration gradient affect rate of passive transport?

Rate of passive transport increases linearly with increasing concentration gradient

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

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

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

Intra: ~4.2mM
Extra: ~123mM

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

Intra: ~4mM
Extra: ~155mM

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

Symport

Antiport

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What is a uniport?

A transport protein which moves one molecule per cycle

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What is a co transporter?

A transport protein which simultaneously transports two molecules per cycle

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What is PMCA?

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

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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.

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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

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What is ATP synthetase?

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

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What type of active transport occurs in co transporters?

Secondary

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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
-pH
-Cell volume
-Ca concentration
-resting potential
-nutrient uptake eg glucose uptake in small intestine

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Name two calcium transporters

Ca2+ Mg2+ ATPase

Na+ Ca2+ Exchanger (NCX)

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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)

Antiport
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)

Antiport
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

Symport
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 ~diarrhoea~