32 votes
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Why don't membrane proteins move?

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-...
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17 votes
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Why doesn't the cell membrane just...break apart?

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"...
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17 votes
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How do ion channels transport only specific ions?

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 ...
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14 votes
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Why should phospholipid non-polar tails be "protected" in the membrane bilayer?

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 ...
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14 votes
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Why can't H3O+ ions pass through aquaporins?

This question has been directly addressed by the paper The Mechanism of Proton Exclusion in the Aquaporin-1 Water Channel. I think it's a pretty good one too! I paste the abstract below: Aquaporins ...
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11 votes

Why don't phospholipid bilayers dissolve?

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 ...
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10 votes
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Why don't the heads of phospholipid bilayers repel hydrophobic molecules?

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 ...
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9 votes

Why do cell membranes have a lipid bilayer instead of a monolayer?

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. ...
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9 votes
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Problems understanding membrane potential

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 ...
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9 votes
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How does membrane potential vary between intraceullar membranes and the cellular membrane?

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 ...
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9 votes
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At what point during an action potential are the sodium potassium pumps working?

The Sodium-Potassium Pumps are always at work. One can think of them as a continuous process that maintains the equilibrium potential for the individual ions. They always are grabbing internal sodium ...
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8 votes
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Can general soap kill bacteria?

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 (...
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8 votes
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Why do soap molecules not break down your skin cells when you wash your hands?

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 ...
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8 votes

Is plasma membrane permeable to sucrose

No, but yes. Sucrose is a large polar solute. Because it is polar, it cannot easily pass the hydrophobic core of the membrane. So, if the lipids of the plasma membrane are mostly impermeable to ...
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7 votes

Problems understanding membrane potential

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 ...
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7 votes
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Why was the Davson-Danielli model rejected?

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 ...
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7 votes

What is a membrane potential?

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, ...
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7 votes
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What recovers normal polarisation after hyperpolarisation?

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 ...
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7 votes
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Difference between protein channels, protein carriers and protein pumps?

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 ...
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7 votes

Why don't membrane proteins move?

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 ...
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7 votes
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Difference between going against and going down a concentration gradient

"Concentration" is "how much stuff is there someplace?" "Concentration gradient" is "how much is concentration changing from point A to point B?" Imagine a terrain where concentration is represented ...
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7 votes
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Why does K+ move out of the cell?

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 ...
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6 votes

Why do cell membranes have a lipid bilayer instead of a monolayer?

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 ...
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6 votes
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Why does a "cascade" of events happen during signal transduction?

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 ...
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6 votes

Why don't phospholipid bilayers dissolve?

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 ...
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  • 1,995
6 votes

What is the transmembrane 'Positive-Inside Rule' nowadays? Has the definition changed over time?

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 ...
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6 votes
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Membrane Permeability to Pyruvate

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, ...
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6 votes
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How do lipid-soluble substances diffuse through the cell membrane?

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 ...
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6 votes

At what point during an action potential are the sodium potassium pumps working?

Very good question. Most of your arguments, to the best of my knowledge are accurate. As to answer your questions, I'll provide a basic model of understanding. (Disclaimer:- I'm sorry if the ...
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6 votes

Why don't membrane proteins move?

There are two types of proteins that are present in a membrane, because you have not been specific about which type of protein you are talking about I will consider that you are talking about Integral ...
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