Phospholipid membrane question

[hunterpostbacst]

Yes, of course, it's easy to memorize what molecules are passing through or not. But, anyone could tell me in reasoning?
Only small, uncharged, polar molecules and hydrophobic molecules freely pass across the membrane. In contrast, large polar molecules and all ions are impermeable. Why is like that?

Does 'polar' means 'charged'?

how can hydrophobic molecules pass through the phospholipid membrane? Do you remember it's hydrophobic inside of the phospholipid membrane?

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[UCB05]

The PHOSPHOLIPID bilayer is hydrophilic on its outside and hydrophobic inside. Like dissolves like, and hydrophobic molecules like cholesterol have an easier time interacting and passing through the hydrophobic membrane. You said yourself small polar uncharged molecules can pass through. polar =/= charged. ions = charged. small polar molecules can pass through by virtue of their small size. Yes, you should know the bilayer is hydrophilic on the outside, hydrophobic on the inside - it's a basic concept.

Large polar molecules and ions have too much of a problem with the hydrophobic membrane to be able to punch through with any ease. These molecules usually need transport proteins which (*surprise*) are polar or charged on the side their substrates pass through.

 

[pez123]

Remember that the membrane is a double phospholipid membrane with the polar hydrophilic heads facing outward and the nonpolar hydrophobic tails facing inward towards each other. (see attachment, this was drawn in word and looks awful, but you get the idea)
..

Remember back to chemistry about polar molecules...it's based on the dipole moment in the molecule, for example H2O is polar due to the oxygen being more electronegative causing it to "steal" more of the electron charge than hydrogen. It's an unequal sharing of the charge.

Charged molecules are your ions: cations (+ charge) and anions (-charged) for example NaCl when it breaks apart forms Na+ and Cl-.

Relating this back to your membrane, the membrane is selectively permeable, meaning only small, uncharged, polar molecules (ex. water and carbon dioxide), and hydrophobic molecules (O2) can pass easily. Think like dissolves like, with taking size into account.

The charged molecules and large molecules can get into/out of the cell through different types of proteins embedded in the cell membrane.

Hope that helps

 

[ZH1988]

Yes, of course, it's easy to memorize what molecules are passing through or not. But, anyone could tell me in reasoning?
Only small, uncharged, polar molecules and hydrophobic molecules freely pass across the membrane. In contrast, large polar molecules and all ions are impermeable. Why is like that?

Does 'polar' means 'charged'?

how can hydrophobic molecules pass through the phospholipid membrane? Do you remember it's hydrophobic inside of the phospholipid membrane?

yes polar means charged ex sugar, amino acids....etc

Ok think about it this way any anything that has a higher concentration on the outside can diffuse inside. now for small uncharged molecules they will diffuse simple through the lipid just because of the concentration gradient. For polar molecules, those cant just diffuse. They need a channel called Ion gated channel. ex. KcsA channel..its made of two ligand that displace Oxygen from K and then K can go through it. this is also dependent on concentration (high-->low). For hydrophobic molecules, they simply diffuse because the inside of the lipid is hydrophobic. This depends on the partion coefficient, which basically states the more hydrophobic the faster it will diffuse.

So now if you have two molecules with the same partion coefficient, the smaller one will diffuse faster.

As for large polar molecules, they use facilitative diffusion (type of passive transport), meaning the cant just diffuse but they depend on a transporter. For example if a glucose is present, it attaches itself to the facilitative protein and changes its confermation which opens it on the other side and releasing it there.

So if you have any polar molecule you need a type of transportation like the KcsA channel or the facilitative transporter becuase they are charged and are hyrdrophilic so cant just go inside the hydrophobic side of the lipid membrane.


For anything that goes against its concentration gradient it needs an active transport which uses ATP.
hopefully this helps

 

[hunterpostbacst]

Thank you so so so much🙂

yes polar means charged ex sugar, amino acids....etc

Ok think about it this way any anything that has a higher concentration on the outside can diffuse inside. now for small uncharged molecules they will diffuse simple through the lipid just because of the concentration gradient. For polar molecules, those cant just diffuse. They need a channel called Ion gated channel. ex. KcsA channel..its made of two ligand that displace Oxygen from K and then K can go through it. this is also dependent on conentration (high-->low). For hydrophobic molecules, they simply diffuse because the inside of the lipid is hydrophobic. This depends on the partion coefficient, which basically states the more hydrophobic the faster it will diffuse.

So now if you have two molecules with the same partion coefficient, the smaller one will diffuse faster.

As for large polar molecules, they use facilitative diffusion (type of passive transport), meaning the cant just diffuse but they depend on a transporter. For example if a glucose is present, it attaches itself to the facilitative protein and changes its confermation which opens it on the other side and releasing it there.

So if you have any polar molecule you need a type of transportation like the KcsA channel or the facilitative transporter becuase they are charged and are hyrdrophilic so cant just go inside the hydrophobic side of the lipid membrane.


For anything that goes against its concentration gradient it needs an active transport which uses ATP.
hopefully this helps