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Proteins can move around the membrane. Most proteins do move within the membrane. The membrane is a liquid crystal and has fluid behaviour. Specifically, this is due to the membrane being in a gel-state. This gel state allows phase behaviour which means that the protein is able to move around on the surface. This results in an effect that is often referred ...


16

The membrane bilayer is held together by hydrophobic forces. This is an entropy driven process. When a greasy or hydrophobic molecule is suspended in water, the water molecules form an organized "cage" around the hydrophobic molecule. When two hydrophobic molecules come into contact, they force the water between them out. This increases the entropy because ...


16

I am restricting the answer to only $Na^+$ and $K^+$ channels, assuming similar mechanism for other channels. In these 2 channels, such high level of specificity is achieved because of two main differences between $Na^+$ and $K^+$: difference in size of $Na^+$ and $K^+$ ions. difference in chemical properties of $Na^+$ and $K^+$ ions. Lets discuss these ...


14

What should be the correct reason for bilayer arrangement? I'll answer your second question first, but there is an almost identical question on this site already: Why do cells have a bilayer? There is water on the extracellular and intracellular side of the membrane. What's actually happening at a molecular dynamics level is the self-association of the ...


11

We should first understand what happens when a substance dissolves. During dissolution water interacts with the solute molecule; if the strength of interaction between the molecule and water is higher than the strength of interaction among the solute molecules then the solute dissolves. (Also have a look at this post). Phospholipid is an amphipathic ...


9

Why use membranes? Compartmentalising the cell has lots of advantages and purposes. In Koshland's 2002 essay, compartmentalisation was described as one of the seven fundamental pillars of life. Broadly speaking, membranes create different sets of conditions (chemical and biological) inside the cells. This allows more efficient functioning and advanced ...


9

That quasi-travesty is the Nernst equation in $\log_{10}$ for a positive monovalent ion at physiological temperatures (37 degrees celsius), but they've hidden all that from you. Shame on them. The canonical form of the Nernst equation, for an ion $S$ is $$ E_{S} = \frac{RT}{z_{S}F}\ln{\frac{[S]_{out}}{[S]_{in}}} $$ where $R$ is the gas constant, $T$ is ...


9

This 1969 Steensland paper seems to suggest that the membranes of halophiles are stabilized by sodium ions and they rapidly denature at lower-salt conditions (2.2 vs. 4.3 M). The protein composition of the membrane was generally acidic, stabilized by all the Na+. As far as what the role of the halophile membrane is in sheltering the cell from the high ...


9

I couldn’t find a value for this but I have calculated an efficiency of 84 %. Caution: this seems too high to me, so I show below my calculation, fully dissected, in case anyone can spot an error. First of all the parameters. Ion concentrations are: internal Na+ = 12 mM; external Na+ = 140 mM; internal K+ = 140 mM; external K+ = 5 mM; and membrane ...


9

I think this question has more to do with kinetics / transport phenomenons than biology, but that's okay, everything is connected especially my computer to the internet. ;-) The basic idea behind transport phenomenons is that there will always be a flux of quantitative properties (e.g. charges, particle number, entropy, volume, etc...) where the qualitative ...


9

Yes, various intracellular membranes do have potential differences, but as you can imagine they are more difficult to measure experimentally, so in general data on this is scarce. Summary Mitochondrial membrane: 150mV-180mV with negativity on the matrix side. Seth et al 2011 Endoplasmic reticulum membrane: 75-95mV with negativity in the ER. Qin et al 2011, ...


8

Transmembrane proteins are inserted into the membrane in the ER in a rather complicated system, there is a whole chapter about translocation of proteins in "Molecular Biology of the Cell". The proteins are moved through an aqueous pore in the Sec61 complex, which explains how charged parts of a protein are moved across the membrane. The parts of a ...


7

The System intracellular/membrane/extracellular space is well described by the model of a Concentration cell (see more on Wikipedia). The equation you mentioned is also called the Nernst equation. $$ E_{ion}= 62mV \biggl(\log\frac{[ion]_{outside}}{[ion]_{inside}}\biggr)= \frac{k_\mathrm{B} T}{z \mathrm{e}} \biggl(\ln\frac{[ion]_{outside}}{[ion]_{inside}}\...


7

The original figure that Danielli and Davson proposed looks like this (from the original publication): It shows the phospholipid bilayer of the membrane (which is correct) embedded between two layers of globular proteins. The hydrophobic tails of the lipids are orientated towards each other, while the hydrophilic heads are oriented to the outside. Although ...


7

I think inf3rno's answer is very complete, so I will just be adding some notes that might help OP understanding what's happening. Say that we increase the intracellular concentration of potassium by 10 mM, a +1 valence ion which contributes to POSITIVE membrane potential. Let's say we do that, in an in vitro cell model, using a syringe with only K⁺ ...


7

Remember that the action potential gets more positive in the first place, so increasing positivity is achievable. Net Na+ movement into the cell makes the potential more positive. This occurs as the Na+ gate (right on the image below) are open. The key message is that the membrane can move charge to cause an increase in either positive and negative potential....


7

No, carriers are not the same as pumps. Carriers may or may not carry out active transport and pumps always use energy. Carriers, for example, can make use of the concentration gradient of a certain ion built up by pumps to transport other molecules actively against their gradient. For example, the glucose transporter uses the sodium gradient to transport ...


7

The other answer is a bit misleading. "Another cause is the the intracellular K+ concentration" No, this is exactly the same cause, the differing concentrations is what causes the equilibrium potential. You can think of the equilibrium potential for one ion as "how much voltage does there need to be to prevent this ion from flowing down its concentration ...


7

The outer most layer of the mammalian epidermis (cornified layer or stratum corneum) is composed of 15-20 layers of dead cells called corneocytes, which are basically dead keratinocytes filled with keratin intermediate filament cross-linked other proteins as well as some lipids. As keratinocytes differentiate into corneocytes (a process called cornification),...


7

Your question is rooted in a misundertsanding of the hydrophobic effect. Hydrophillic and hydrophobic molecules do not repel but, rather, attract one another through van der Waals interactions. The tendency of hydrophobic molecules to aggregate in aqueous solution (ie the hydrophobic effect) is, instead of some repulsive force, actually driven entropically. ...


6

Why Bilayer and not a Monolayer Lipid monolayer vesicles are possible as you mentioned (for example micelles). However, you have to understand that the cellular interior i.e. the cytoplasm, is aqueous and therefore a monolayer vesicle like micelles would not work. In micelles, the two compartments - interior and exterior, have to be of opposite nature for ...


6

The existence of membrane potential is just that: as long as they are alive, cells try to keep their cytosol different from the outside soup, mainly by expelling sodium ions. There is a longer story, with cells pumping other ions in or out, or leaking ions due to the imbalance, but resting membrane potential really revolves around sodium. There is an ...


6

There is one main reason: Amplification of the signal. You can start a signal downwards the cascade with relatively few receptors which need to be activated which allows even for weak signals to be translated into the nucleus. This figure shows this for G protein coupled receptors (from here): For example one molecule of cAMP can activate many molecules ...


6

Found the following on wikipedia. Seems pretty self explanatory: The Golgi, ER, and lysosomes are likely to have evolved as a result of the plasma membrane going through invagination. An increase in the overall volume of a cell would require the plasma membrane to fold in order to maintain a constant surface area to volume ratio. These folds may ...


6

The answer to your question is basically: It is a bilayer. There are two layers of phospholipids, thus tucking the hydrophilic ends safely away from any extracellular and intracellular fluids. The whole surface of the bilayer is hydrophilic. (Picture from here.)


6

Pyruvate is negatively charged and quite polar, which makes it unfavourable to diffuse directly through any membrane. The outer mitochondrial membrane contains porins, which allow small molecules, like pyruvate, to passively diffuse through. Specifically, pyruvate uses voltage dependent anion channels. The inner mitochondrial membrane lacks such channels and ...


6

Soap kills nearly all the bacteria it comes into contact with by dissolving the bacterial membrane. Some viruses with protein coats can resist soap, but many viruses have similar membranous coats (like HIV) and are usually disrupted by soap. I'm sure it washes some away too, but to say they don't kill bacteria is misleading. In the end, though, they are ...


6

See this paragraph and image from The Cell: A Molecular Approach. 2nd edition.: During passive diffusion, a molecule simply dissolves in the phospholipid bilayer, diffuses across it, and then dissolves in the aqueous solution at the other side of the membrane...Passive diffusion is thus a nonselective process by which any molecule able to dissolve in the ...


6

No other answer has mentioned this so I created an account just to say this. Some membrane proteins do not move. This is because they are fixed in that position in the membrane due to the cytoskeleton. Erythrocytes are a good example of this. The main protein that is immobilised in erythrocyte membrane is Band 4.1 protein, and its immobilised by Spectrin. ...


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