One of my greatest reading experiences as a young student is the chapter on biological membranes in Stryer’s Biochemistry. The text was about how the cell perimeter is built up. The solution is as ingenious as simple: Soap! I was so thrilled that I gave away a copy of the book to a friend. Stryer was almost as exciting as a mystery novel! But what does this have to do with the immune system?
The body consists of billions of cells. The cells in the body can be thought of as a community, where the various tasks are distributed among members. An important function is to defend the community against invaders and enemies. This is the task of the immune cells. These cells operate largely independently, and I find it difficult to explain the immune system without talking about what individual cells are able to do. For example, immune cells talk to each other, touch each other, eat each other, kill each other and move out of the blood vessels and in between all the other cells in the body to check that all is well. So how does a cell look like? What does its utmost border consist of?
The outer limit must be simple, stable and allow communication from the outside world to the cell interior. Both on the inside and the outside of the cell there is much water. The water molecule H2O is slightly electrically charged. This means that the materials that should be stable when mixed with water, must have a small electric charge. Fat has no electric charge. To wash away fat we therefore need soap. Soap molecules are fatty at one end, and electrically charged in the other, and can be mixed in both water and fat.
And now I come to the point: The cell perimeter is made of a double layer of soap. Soap molecules are arranged so that the fat-like portions face towards the center of the double soap layer, while the electrically charged ends stick out toward the water on the inside and outside of the cell. This scheme provides a stable film or membrane constituting the cells’ outer boundary.
The soap double-layer allows proteins to be attached to the membrane, and secure information transfer between the outside and inside of the cell. The soap membrane is in addition “floating” so that proteins attached to the cell membrane in principle move freely in the plane of the soap-membrane . The solution is so ingenious that the same kind of soap film is also used to make a number of small organs inside the cell.
And it is so simple that even children can create something similar: Soap bubbles.
Blog by Anne Spurkland, published 28th November 2014
Originally published in Norwegian 10/09/12