The Cell Membrane
The cell or plasma membrane was once thought to be a simple barrier that
kept the contents of the cell, the cytoplasm, contained. It serves as a
gateway which helps to control materials going in and out of the cell. With
more research, it turns out the cell membrane is very important in a wide
range of cell activities including functions related to cancer and AIDS.
Structurally, the membrane is a lipid bilayer. What this means is that,
under the electron microscope two separate layers can be seen. The layers
are composed of a two part molecule called a phospholipid. The lipids (fatty
acids) are "water fearing" (hydrophobic) molecules. Just try to
mix oil and water to see what that means. The phosphate end is water loving
(hydrophilic). The membrane forms when the phosphate ends point out, attracted
to the watery environment of the cell and the lipid ends point in, trying
to avoid the water. Embedded in this lipid bilayer are several different
kinds of protein molecules. Click here to see a
diagram of a cell membrane.
The membrane does not form a solid sheet like a piece of cellophane. Its
structure is called a "fluid mosaic" because pieces of the membrane
can rip off without creating a hole or merge in with other membranes. The
membranes surrounding the internal organelles, such as the endoplasmic reticulum,
Golgi bodies, lysosomes and vacuoles use this action to help carry out their
functions.
Jobs of the Cell Membrane
I. Transport
The cell membrane is the border surrounding the entire cell. Obviously if
food, oxygen, water and wastes are going to move in and out of the cell
there has to be some way to do that. Are there holes in the membrane? What
if something is too big to go through the membrane? What about things that
don't mix with lipids such as those found inside the membrane? Read
here about the ways that materials can move in and out of cells.
2. The Immune System
If cells are going to fight off germ invaders they have to have a way to
recognize which cells belong to you and which cells don't. Cells identify
themselves by marker molecules on the cell membrane.
Cells that don't belong to you (even if it is in a transplanted heart!)
get attacked and destroyed. Even your own cells, if they change somehow
(like when they get cancer), can be destroyed by your immune system. Click
here to find out more about your immune system.
3. Cell Junctions
What would you look like if your cells didn't stick together? What holds
cells together? How does information pass from cell to cell? Click here
to find out.
Electron microscope
picture of the cell membrane.
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