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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 ...


14

Proteins can move around the membrane. The protein does move: 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 by a similar process. This is often referred to as the fluid mosaic ...


10

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 ...


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

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 ...


8

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 ...


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 ...


8

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 ...


8

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 with negativity on the matrix side. Endoplasmic reticulum membrane: 75-95mV with negativity in the ER. Golgi: No notable membrane potential....


7

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 ...


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. ...


6

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. $$ E_{ion}= 62mV \biggl(\log\frac{[ion]_{outside}}{[ion]_{inside}}\biggr)= \frac{k_B T}{z e} \biggl(\ln\frac{[ion]_{outside}}{[ion]_{inside}}\biggr) $$ where $...


6

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⁺ ...


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

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....


5

Early histochemical work indicated that the internal surface of the lysosomal membrane has a glycocalyx - a layer of polysaccharide, presumed to have a protective role. Neiss, W. F. (1984) A coat of glycoconjugates on the inner surface of the lysosomal membrane in the rat kidney. Histochemistry 80, 603–608 Subsequently it was found that major membrane ...


5

For an organism (think single-celled) living in just the right kind of environment, it might just be possible to survive using only facilitated diffusion, at least as far as small molecules are concerned. This kind of organism would have to maintain exactly the right concentration of the molecules it wants to keep or get rid of, based on the external ...


5

There are various mechanisms through which membrane proteins can remain localized in the membrane. See the below figure from MBOTC (book):                                     &...


5

There must be charge considerations in the movement of molecules in the lipid membrane. There is also a consideration that some species of phospholipids will migrate to portions of the membrane with sharper or smoother curvature. Waves of electrical potential can propagate along a lipid bilayer as well, which is very important to nerve axons and extended ...


5

Short answer The distinction between Gram positive (Gram+) and negative bacteria (Gram-) has absolutely nothing to do with membrane potentials; it is all about the Gram staining procedure. Background The Gram staining was named after the Danish bacteriologist Hans Christian Gram, who originally devised it in 1882 (Gram, 1884). Gram staining is a common ...


5

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.)


5

I think you have misunderstood the "inside" part of the "positive-inside rule". Perhaps because "inside" is indeed an imprecise term (but now it is history and cannot be changed ;) ). In order to understand it a bit better it helps to think about the topology of the membrane. During synthesis most membrane proteins (ignoring peroxisomal and mitochondrial ...


5

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 ...


5

Under the right conditions, emulsions of lipids (fatty compounds) and water can cross cell membranes. If the emulsion is prepared correctly, a structure known as a liposome forms, which is essentially a "bubble" with a layer of fatty molecules on the outside and water at the center. This has been studied intensely in biotechnology, because such liposomes can ...


4

Conductance is the inverse of resistance, and measures how much of a given substance flows throught a channel. In this context, it means how many calcium ions enter the cell in a period of time. There are at least two ways potassium channels may prevent the calcium to enter in the cell. 1) Potassium intake by ion channels decrease the membrane potential, ...


4

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 ...


4

It depends on the concentration, but at higher concentration the detergent molecules build so called micelles, where the hydrophobic "tail" is orientation into the inner part and the hydrophilic "head" is orientated to the outside. This allows the micelle also to fuse with the membrane and then to desintegrate it. This illustration from the Wikipedia shows ...


4

Your confusion is caused by the assumption that Na+ always leaves the cell and K+ always enters. The Na+/K+ pump is there to maintain membrane potential and relative Na+ and K+ ion concentrations stable inside. When an action potential (AP) is generated, sodium channels open and sodium rushes inside to depolarize the cell( 1st phase of AP). Next, the sodium ...


4

Although the Wikipedia page on dermatographism refers to "weak membranes" it cites no source for this, and like Chris, I haven't found any mention of this in a quick look at the literature. Histamine is usually released from mast cells in response to specific stimuli by a degranulation reaction (internal vesicles fusing with the surface membrane to release ...



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