Cell Biology lecture 23

Electro-osmotic effects in cells

LOs

Electro-osmotic effects in cells.​

Free energy, chemical and electrical work across membranes. ​

Physical basis of the Nernst potential. ​

Membrane potential​

Osmotic pressure and the sodium anomaly. ​

Chemiosmosis and the proton motive force.

why don’t cells don’t just burst?

Plants have strong cell walls to deal with turgor pressure but the imbalance of ions and large charged molecules in animals needs to be dealt with. Use chemiosmosis to help deal with this. 

main components of charge in cell

what allows thermodynamic analysis of mixtures?

The chemical potential 

what is the chemical potential?

= free energy + entropic effects

Potential of these chemicals to move

at equilibrium what happens to the chemical potentials

They must match for each permanent species

For a single permanent species this just means that concentrations balance out atequilibrium​

What happens at a membrane when we have a mixture of permanent and impermanent ions - theory if both were permeable ?​

What happens at a membrane when we have a mixture ofpermanent and impermanent ions? - if just A+ permeable?

What happens at a membrane when we have a mixture ofpermanent and impermanent ions - if b- is impermeable?

what does this electric field do with transfer of permeable and impermeable ions?

The electric field set up by ion imbalance opposes transfer of further ions​

what pushs permable ions in opp directions?

Conc gradient (entropy) and electric potential diff (enthalpy)

Nernst potential

electrical potential difference required to stop the flow of ions arising from a concentration difference​

The potential difference due to charge transfer builds up enough to stop further flow of ions​

No flow = equilibrium ​

• This is the Nernst potential, ΔV = Vnernst​

How do we calculate Nernst?

what does electric potential difference cause across the membrane?

causes a change in Gibbs free energy

change in internal energy = charge x change in potential diff

Nernst equation for conditions of equilibrium across the membrane (just use for understanding of equation)

what is the Nernst potential 

Voltage diff across the membrane created by transferring just enough permanent ions to create an electrostatic barrier against further flow of ions 

Nernst potential and cells

At equilibrium, what is the internal concentration of each ion and the potential difference across the membrane? ​

The external ion concentrations, and the internal macromolecule concentrations are fixed.​

Donnan equilibrium

equilibrium internal conc of each ion and membrane potential diff

Donnan potential

potential difference for Donnan equilibrium

Calculating Donnan potentials

Answer = -10mV

Are cells at Donnan equilibrium?

1. If cells were at Donnan equilibrium, what would their osmotic pressure be?​

2. How does the observed membrane potential match up with each ions Nernst potential?​

What is the osmotic pressure of the cell at Donnan equilibrium?​

osmotic pressure is a stretching force on the cell membrane, caused by an imbalance of stuff on either side. Too much osmotic pressure can burst the cell.​

To be calc in next slides

What is the osmolarity difference for a cell at Donnan equilibrium?​

Answer = 25mOsm

What is the osmotic pressure of the cell at Donnan equilibrium​?

Observed ion concentrations and membrane potential are far from Donnan equilibrium​

Pioneering electrophysiology measurements of membrane potential were performed by inserting electrodes in squid giant axons​

why is cell far from Donnan equilibrium?

This explains why cells don’t burst: the cells are nowhere near electrochemical equilibrium.​

The cell avoids bursting by actively pumping Na+ and K+ ions.​

Also, setting up such a strong imbalance allows the cell to transiently allow Na+ to flow back into the cell by opening ion channels.​

Which is where the action potential comes from.​

why discuss these potentials?

• Basic electrochemistry shows us how membrane potentials arise​

• Cell membrane potentials are a great example of how quantitative analysis gives us the tools to understand how cells must work, ie how physical laws shape biological organisms

The pH of the cytosol is 7.4. The hydrogen ion concentration in mitochondria is 1.6nM.​

What is the pH difference across the mitochondrial membrane ΔpH = pHin - pHout?​

Remember, pH = -log([H+]) ​

Answer = 1.4

Mitochondria as factories

Eukaryotic cells actively maintain a sodium ion imbalance to avoid bursting and, inneurons, to communicate​

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It turns out both mitochondria and bacteria use a similar strategy, but withhydrogen ions, to generate ATP, the energy currency of the cell.

Mitochondrial electrical imbalances and uses

Eukaryotic cells actively maintain a sodium ion imbalance to avoid bursting and, in neurons, to communicate​

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It turns out both mitochondria and bacteria use a similar strategy, but with hydrogen ions, to generate ATP, the energy currency of the cell.

Respiration sets up a large proton concentration (pH) imbalance

The chemiosmotic hypothesis: cellular ATP is generated using the mitochondrial proton motive force​

ATP synthase is a rotary motor powered by proton flux​

This generates ATP​

What is another rotary motor powered by proton flux?

Bacterial flagellar motor 

Summary

• The chemical potential describes the thermodynamic behaviour of mixtures ofspecies​

• Ion gradients across semi-permiable membranes lead to membrane potentialdifference at equilibrium, the Nernst potential​

• Living cells establish membrane potential differences far away from the Nernstpotential by actively pumping ions across their membranes​

• Uses of membrane potential:​

• Prevent excessive osmotic pressure​

• Action potential​

• ATP synthesis​

• Rotation of the bacterial flagella (swimming)