Membrane Proteins - Bumpy Surfaces

We have a page on the basic structure of the cell membrane and other membranes within the cell. They are basic bilayers made of lipids that surround the cell and organelles. The lipid bilayer is not smooth because there are a variety of proteins attached to the surface and embedded in the membrane. You will find millions of embedded protein molecules when you look at the cell membrane. Each type of protein has a specific purpose. Examples of membrane proteins include ion channels, receptor proteins, and proteins that allow cells to connect to each other.

A Tale of Two Types

You will learn about two types of membrane proteins: peripheral proteins and integral proteins. Peripheral proteins have weaker and temporary connections to the membrane. Some just sit on the surface, anchored with a few ionic bonds while others might have small sections that dip into the hydrophobic section of the bilayer. When you look at the entire membrane, there are more peripheral proteins when compared to the number of integral proteins.

As you can guess from the name, integral proteins are permanently connected to the cell membrane. They are hard workers and have large sections embedded in the hydrophobic (middle) layer of the membrane.

Transmembrane proteins are integral proteins that cross the membrane and can act as pathways for ions and molecules. Polytopic transmembrane proteins cross the membrane several times. Some are receptor proteins while others form channels. Ion movement that does not require work is called passive transport while active transport systems use work to move molecules. Active transport is regularly used when membrane proteins pump ions against the concentration gradient.

Discovering Structures

This structure of the membrane with embedded proteins and a lipid bilayer was discovered in the early 1970's. Two scientists, Singer and Nicolson, first developed the theory of the "Fluid Mosaic Model." They used several different methods, such as the freeze-fracture technique and electron micrographs, to look closely at the cell membrane and its structure. They went on to identify the proteins that sat on the surface, were sunk into the membrane, and the others that crossed the membrane.

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Biology4Kids: Passive Transport
Biology4Kids: Active Transport
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