Question about Biology, Membrane.

[bluesolic]

---

Members do not see this ad.

Hi,

I am studying AP cliff biology now,

and I don't get some information from this book.

here is what they are saying.

" Phospholipid membrane is selectively permeable. Only small, uncharged, polar molecules ( such as h20, co2 ) and hydrophobic molecules


channel protein opens passageways through the membrane for cetrain hydrophilic (water-soluble) substances such as polar and charged molecues.

and also aquaporins also passways for the h2o.

ok..

where the heck is h2o moving through ?


I thought h2o is polar so, it can't go through the phospholipid membrane right ?

 

[Dotoday]

Water can move into the cell or out of the cell by Osmosis. Yes water is polar, but it can move through channels into or out of the cell. The channels are proteins that allow water to move in and out. They are saying that membrane is selectively permeable, but membrane of the cell consists not entirely of phospholipids, there are proteins, sugars and other molecules that are attached to the membrane.

Hope this helps.

 

[indigenoustw]

So I guess what you are asking is how does H2O pass through aquaporin?

Aquaporin is a channel protein that allows the passage of water molecules. If you check out the crystal structure of this protein, you will find several charged residues positioned in just about the right location within the channel to allow ONLY water molecules to enter. Now you also need to understand that channel proteins are embedded within the membrane by having hydrophobic residues outside the the channel just so that it sticks to the membrane (how each of the proteins folds plays a role in its function and location of the membrane - integral, peripheral, ....).

That's about it. Try Wiki "membrane protein" and "aquaporin" to read more about it.

 

[dentalWorks]

Hi,

I am studying AP cliff biology now,

and I don't get some information from this book.

here is what they are saying.

" Phospholipid membrane is selectively permeable. Only small, uncharged, polar molecules ( such as h20, co2 ) and hydrophobic molecules


channel protein opens passageways through the membrane for cetrain hydrophilic (water-soluble) substances such as polar and charged molecues.

and also aquaporins also passways for the h2o.

ok..

where the heck is h2o moving through ?


I thought h2o is polar so, it can't go through the phospholipid membrane right ?

Basically, there are 3 ways things move into/out of the cell:

1) Passive diffusion:
example, most gases (Oxygen, carbon dioxide, etc) along with small hydrophilic molecules (such as water). These will move right through the member, no protein-transfer needed. If your dealing with the small hydrophilic molecules (water, amonia, etc), the rate isn't all that great, cause remember, they are still charged.

2) faciliated diffusion:
similar to #1, but this one uses a protein for transfer. Its a little better cause its more specific (for that particular molecule that its trying to transfer).

3) Active transport:
Uses ATP, blah blah blah, you know this one from high school biology.

Aquaporns, are more of #2 above I believe (not 100% sure tho)

 

[yappy]

OP are you asking why the book says water passes through an aqua porin AND membrane? It's because water does both at different speeds.

To clarify this point - water can move through the membrane through two pathways:

The lipid pathway and the protein pathway. An above poster discussed the facilitated diffusion of water through an aqua porin. This was the pathway through a protein. It is also the fastest method for water traveling across a membrane.

The second method is through the membrane itself - the lipid pathway. This method is determined by the gradient of water on one side of a membrane vs the other, composition of lipids in membrane, and the surface area of the membrane. So water does pass through the membran - just more slowly than if it has a free aqua channel.

 

[FeralisExtremum]

Aquaporns, are more of #2 above I believe (not 100% sure tho)

Correct, aquaporins are facilitated diffusion.

I am however curious as to why, then, a distinction is made between osmosis and facilitated diffusion in terms of types of diffusion. Shouldn't osmosis technically just be a specific form of facilitated diffusion?

"For many years, scientists assumed that water leaked through the cell membrane, and some water does. "But the very rapid movement of water through some cells was not explained by this theory," said Agre."

So I believe dentalWorks is entirely correct here: water can move through the cell membrane as part of the passive diffusion process (osmosis), but this occurs slowly. For rapid changes to the water content of a cell, the facilitated diffusion process (via aquaporins) is used.

 

[amajid08]

Membrane are made up of phospholipids. They are hydrophilic on the outside and hydrophobic on the inside. IF the membrane was totally phospholipid, it would naturally allow some stuff to pass, such as small, uncharged, polar molecules like water, or small hydrophobic molecules like CO2. However, the CO2 would enter at a much faster rate than water because it is hydrophobic and would not experience repulsion as it interacted with the hydrophobic interior like water does. Both of these are examples of simple diffusion: molecules simply passing through a membrane WITHOUT HELP. Osmosis is the diffusion of water, so it is a specific form of simple diffusion, not a specific form of facilitated diffusion.

If simple division were the only way water could enter, cells would have a huge problem because they NEED water to survive. Why? All the biochemical reactions of the cell occur in an aqueous environment and water is generally part of the reaction mechanism in some way, Shapiro, or form for most reactions, either donating protons or picking them up off of other stuff.

To solve this problem, we have facilitated diffusion. This is 'diffusion with help'. Basically there are two types of proteins in the cell, channels and carriers. This post is kind of long already, lol, so I'll just talk about channels. Basically, they are cylindrical, hollow proteins that line holes in the membrane. They allow for things that couldn't diffuse into the membrane, due to size or charge, to quickly enter the cell. This also allows stuff that enters too slowly by simple diffusion, ie water, to enter at a much more rapid rate. These channels are important because the things they allow to enter are usually always needed for reactions to occur. The specific channel that allows water to enter is called an aquaporin. Some channels can be always open, like aquaporins, or gated, ie some thing has to trigger it to open, like a change in electrical charge of the surrounding area or the binding of a ligand. Facilitated diffusion moves things DOWN their gradients and DOES NOT use ATP.

Lol, so I guess to answer your question, water enters cells through simple diffusion. But because that does not 'quench the thirst' of the cell, cells insert aquaporins in their membranes so water can flow directly into the cell, unabated.

Sorry for the long post, but I just wanted to be as helpful as possible. Good Luck!