Question about plasma membranes

[marc32123]

Question about plasma membranes...

I read this online and am wondering if this is true. It said that -

"When you think about a membrane, imagine it is like a big plastic bag with some tiny holes. That bag holds all of the cell pieces and fluids inside the cell and keeps any nasty things outside the cell. The holes are there to let some things move in and out of the cell."

How can a cell membrane hold it's contents inside if it has holes in it like a big plastic bag, even if they were tiny? Does a cell membrane really have holes?

 

[Yanick]

Yes, in a way.

See here for more details: http://www.ncbi.nlm.nih.gov/books/NBK21140/

You may want to go back a bit in the book if you want some more background about membranes and such.

Edit: I just remembered, after checking, that the online text is unintuitive to use. You can't just browse the book, instead you need to search for the chapters or sections. Just search for something like "Stryer 13.x" and it will come up. It is an undergraduate biochem text for free so don't be too upset for a bit of work to get the material

 

[Choppy]

Like all analogies in science, there are limitations.

The "plastic bag with holes" is an oversimplification, but it's probably given to generate a clear image to someone who has no initial concept to start off with. As you learn more, you add more complexity to the model.

A cell membrane is actually a rather complex organelle. You start out with a phospholipid bilayer. The phospholipids have one "hydrophilic" end (mixes will with water) and one "hydrophobic" end (does not mix well with water). In water, these come together so that their hydrophobic ends attach - so you have two layers of these molecules all lined up and closed in on itself - like a pair of balloons. Some molecules like oxygen and carbon dioxide can diffuse through this double-layer membrane. But others like water have to be transported through special protein channels (the holes in the bag) that poke through the membrane. These channels give the cell a certain amount of control over what gets transported in and out. There are other proteins that don't even transport anything though, but they have ends on either side of the membrane where an interaction on one side generates an effect or a signal on the other.

 

[Ygggdrasil]

If by "holes" your source is referring to channels, holes is a bad analogy. Although certain molecules can freely pass through the membrane (e.g. lipophillic molecules like steroids), most important biomolecules (e.g. proteins, carbohydrates, ions) cannot. Channel proteins let certain substances to diffuse across the membrane, allowing the cell to exchange material with its surroundings. Unlike a hole in the membrane, however, channels are usually very specific about the types of molecules they allow through. For example, aquaporins only let water pass through the membrane and do not let any other types of molecules pass through. Other channels are selective for other substances (e.g. sodium channels, chloride channels, potassium channels, etc.). Thus, cells can control which substances it wants to be able to freely enter and exit the cell, and which substances it wants to keep out/in. Furthermore, many channels can be opened or closed (a good example are the voltage-gated ion channels in neurons), allowing a cell to better control the flow of molecules across the plasma membrane.

Cells also contain pumps in their membranes which are completely unlike holes. These proteins are like channels in that they allow substances to cross the membrane, but they allow passage in only one direction. These pumps, which are powered by the metabolism of the cell, are important for things like scavenging nutrients from the cell's surroundings and establishing an electrical potential across the membrane of cells.

 

[LudusRex]

I can confirm that the description of a cell membrane as a "big plastic bag with tiny holes" is a simplified analogy. While it is true that the cell membrane is a selectively permeable barrier that allows some substances to pass through while keeping others out, it is not accurate to say that it has physical holes like a plastic bag.

The cell membrane is made up of a phospholipid bilayer, with hydrophobic (water-repelling) tails facing inwards and hydrophilic (water-attracting) heads facing outwards. This structure allows for the membrane to be both flexible and impermeable to most substances.

There are also various proteins embedded in the membrane that serve as channels, pumps, and receptors, allowing for specific molecules to enter and exit the cell. These proteins are not holes, but rather specialized structures that facilitate the movement of substances.

So while the analogy of a plastic bag with holes can help visualize the function of a cell membrane, it is important to understand that the actual structure and mechanisms at work are much more complex.