Membranes L3 Flashcards
What drives transmembrane transport?
Dissipating a concentration gradient and increase entropy
How can you write out the Gibbs energy change
G = Go + RTln[X]i/[X]o
For uncharged species Keq= ?
1 because conc on both sides same at equilibrium
uncharged species Go=?
0
If conc inside is lower than conc outside of an uncharged species… (in terms of delta G)
delta G negative, spontaneous, exergonic transport occurs
What is a typical membrane potential for an animal cell?
-80mV
What is Gibbs change for CHARGED solute?
G = RTln[X]i/[X]o + zFEm
How can you predict the conc gradient that forms passively at Em?
Rearrange gibbs change for delta G =0
What is the full Nernst equation and the one at 20 degrees?
Em = RT/zF.ln[X]o/[X]i Em = 58/z.log[X]o/[X]i
What is the lipid partition coefficient?
amount dissolving in test lipid/ amount dissolving in water
Draw permeability coefficient vs lipid partition coefficient
Straight line with urea then water as outliers above curve
What is Fick’s first law?
Rate of diffusion of small uncharged solute = permeability coefficient x conc gradient
How is lipid permeability of cells good?
In anearobic resp in fungi and plants ethanol leaves cell down conc gradient
What is bad about lipid permeability?
Renders cell vulnerable to pollutants from industrial activities
What characteristics do protein-mediated transport reactions show? (4)
- Highly specific
- Rate can be inhibited by structurally similar solutes
- Rate higher than could be mediated by diffusion (proteins lower activation energy)
- Shows saturation kinetics
Is protein-medicated transport intrinsically vectoral?
No, if conc gradient reversed transport direction reverses
What is a typical carrier?
12 alpha-helices spanning membrane, especially useful for glucose and amino acid transport
Draw “ping-pong” model
see notes
Why do fungi and RBC phosphorylate glucose as soon as it reaches cytoplasm?
so it no longer fits binding site of carrier to prevent its release
What is a typical channel and what is its speed of catalysis?
Usually 24 alpha helices, fastest enzymes known catalysing ~10^7 molecules per second, make small conformational changes
What is patch clamp electrophysiology?
Glass microelectrode with 1 micrometer aperture blunt end is pushed against membrane, electrically isolating patch of membrane in electrode tip, tiny currents that form when ion chennel open can be measured
What gets observed in patch clamp?
Channel openings observed on step increases of 1pA lasting less than 1 msec, represents passage of ~6000 ions
Link in to ion channels…
Calcium signalling
What are ionophores?
Bacterial peptides secreted to kill competing bacteria
How does the ionophore valinomysin work?
see notes
What are 3 modes of water transport?
via aquaporins, bulk flow, osmosis
phi (overall water potential)=?
Solute potential (dissolving solutes reduces this) plus pressure potential (negative pressure reduces this) plus gravitational potential
What are aquaporins?
can be regulated by phosphorylation to control water flow, made up of 4 subunits with many alpha-helices
Why does ATP hydrolysis release energy for primary active transport?
At physiological pH, ATP can be extensively dissociated, as negative charges repel electrons in P=O bonds making O d- and P d+ therefore ATP holds energy to keep P atoms together, which gets released upon hydrolysis
What does the end bit of ATP look like?
see notes
