Basic design of the plasma membrane
Consider the idea of a cell. A cell is an enclosed compartment, full of fluid and other stuff, surrounded by fluid. So there is water inside a cell and water outside a cell. The only way to separate one watery compartment from another is to use something that won't mix with water-- a lipid. But, in addition, since this lipid has to touch water on each side, it has to be partly hydrophilic.
That is what we have with phospholipids. You can see that simply by creating a bilayer of phospholipids, we have the hydrophilic head regions facing the water, and the hydrophobic tail regions away from water, causing a hydrophobic core to the membrane. In this manner, we have successfully blocked one watery compartment from another (outside the cell to inside the cell). This structure is called the lipid bilayer. My drawings are only in two dimensions... the ones in your book show you the third dimension. Take a look. You will see in your book that cholesterol, a steroid and thus a different type of lipid, can also take part in forming the lipid bilayer.
This is all well and good as a start for making membranes. However, consider yet another problem-- we need to get some things across the membranes. For example, we need to get nutrients into the cell. If we have created a hydrophobic barrier with the lipid bilayer, how do we get things we want to cross the membrane? Certainly, there has to be something else there in the membrane to help us with that. That is where proteins (and carbohydrates) will come in. I bring this up now because I want you to understand that the lipid bilayer is NOT the membrane-- it is just the basis for the membrane. It is the main structure of the membrane, but not the entire thing.
Cellular membranes are called plasma membranes. The word "plasma" means a thick fluid. Think about what a phospholipid bilayer would be like... Lipids are greasy, oily substances. A bilayer of lipids would also be oily, like a liquid, oily film. Try to picture it like a soap bubble. The soap bubble is not solid, it is liquid. And if you watch it, you'll see the colors in the soap moving around because it is liquid. Well, the plasma membrane is like that. It is quite fluid. The phospholipids drift around all the time.
We are about to continue to understand the structure of plasma membranes. When you see a real image or description of a plasma membrane online, it should be called a "fluid-mosaic" membrane. This says that the lipid bilayer is the basis for the plasma membrane (the "fluid" portion), and that a real plasma membrane MUST also contain proteins and carbohydrates in order to carry out all of its functions. It is called a "mosaic" because of all the proteins and carbohydrates floating around in it.
Therefore, the general structure of a plasma membrane consists of a lipid bilayer, sporadically studded with proteins (and carbohydrates) all over it.
Proteins are Found in the Plasma Membrane:
What are the proteins doing in the membrane? They give the membrane the special functions like selective permeability, recognition and response, cell adhesion, and cytoskeletal attachment. Basically, they are involved in everything except for separating and maintaining the membrane. We're going to look closely at the proteins that allow for selective permeability and recognition of things in the world. For selective permeability we will be studying channels, and for recognition we will be studying receptors. To see how proteins lie in the membrane, check out the green structures in Figure 4-13.
Some proteins are large enough to run all the way through the lipid bilayer as a tube-like structure. This tube or tunnel is called a channel protein. They are highlighted in Figure 4-16. Think back to why we need channels... we have to be able to get certain things across the membrane. Anything that is hydrophilic cannot simply cross the hydrophobic membrane.
Think about these channels for a moment... imagine a very tiny channel, that lets only tiny molecules through. Now imagine an enormous channel that can let other macromolecules through. Can both of these sizes for channels exist? What do you think, knowing what you know about membrane function?
Here's an update to that drawing from last week's material. I have added a protein channel to show you how it might sit embedded within the lipid bilayer. Notice that it has a pore through its middle. This pore could let something pass across the membrane.
The word extracellular means outside the cell, and the word intracellular means inside (within) the cell.
Only very small molecules, from small charged atoms (like Na+, K+, Ca++, or Cl-) to no larger than the building blocks for macromolecules can fit through these channels. And the channels are also VERY specific. Sometimes a channel lets only one particular item through. For example, a channel that is designed to let Na+ ions through will not also let Cl- ions through! Another channel will only allow glucose to pass through. Sometimes a channel will allow a couple of specific things to pass through (like both Na+ and K+). Can you see how this provides our cells with selective permeability?
Consider another question. If a hydrophobic molecule needs to cross the membrane, do we use a channel for it, too?
There are also other proteins in the plasma membrane. Receptors are proteins that face out into the extracellular world in order to recognize material out there. After they recognize material, they then signal other proteins to cause the cell to respond to that material.
How can proteins act as receptors? Easy! Keep in mind that protein shape is very important. A protein can act like a channel if it is shaped like a tube. It can act as a receptor if it has a shape that can link up with a particular item in the world. Every time the receptor and the item it recognizes come in contact, the receptor binds to that item. This binding is what will lead to a signal for the cell.
I have updated the drawing one more time to include a receptor (in blue). Note that it faces the outside world and has a very particular shape so that it could only interact with a chemical with a complimentary shape.
Once a receptor binds to something in the world outside the cell, then it has to trigger the cell to respond. Remember-- the cell has to both recognize AND respond to things in the world. How can a cell respond? Cells are made up of chemicals. The way chemicals act is to undergo chemical reactions. So, let's say the signal in the world was growth hormone and the cell was a cell in bone. When the bone cell recognizes the growth hormone with its growth hormone receptor, the cell can then undergo a response to that hormone-- like undergo cell division to make more bone cells (so that the bone will grow longer). Cell division occurs because of a lot of chemical reactions. They all get started when the hormone binds to the receptor. How do chemical reactions occur? That is beyond the scope of our class, but includes involvement of another protein, the enzyme.
To read more about these proteins, go to the Biology Hypertextbook introduction to membranes. Just so you know, at this site they use the word "transporters" instead of channels.
Carbohydrates are also found in the plasma membrane
Yes. They are. Carbohydrates are found here as small polysaccharides. So few monosaccharides are involved in making these polysaccharides that they are actually called oligosaccharides here. "Oligo-" means "few."
What are they doing here? Carbohydrates (sugars) are sticky. So they are on the outside of the plasma membrane to help membranes recognize material or be recognized in their world, and also to stick to other membranes. In fact, the oligosaccharides are really important in providing an ID marker for the cell (a function mentioned in the membrane functions page).
The oligosaccharides can either be attached to the outside of a phospholipid or of a protein. Once they attach, you now have a macromolecule that is half phospholipid & half carbohydrate, or you have a macromolecule that is half protein & half carbohydrate. These macromolecules thus receive new names:
Is the membrane really fluid?
Yes, the membrane is really fluid. A thick, movable fluid. If it were rigid, then anything that banged into it could cause the cell to break; and if the cytoskeleton of the cell tried to push or pull on it to cause movement, it would shatter. So instead, the membrane is bendable. You can think of it sort of like a balloon material, but even more pliable.
© 2006 STCC Foundation Press, content by Dawn A. Tamarkin, Ph.D.
Last changed: January 21, 2007