When was the cell membrane first discovered

Because they could measure the actual size surface area of a red blood cell and knew approximately how many of those cells they had in their sample , they could calculate the total surface area that would have to be covered by membrane.

When the two numbers were compared, it was clear that the amount of lipid they had extracted could cover twice the area needed to enclose all the cells. Why would there be so much?

Additional experiments showed that lipids could spontaneously form a bilayer when mixed with water Figure 1. Together, these observations suggested that there may be a simple explanation for the results with the red blood cells. The plasma membrane of these cells likely consists of a double layer of lipid surrounding each cell.

As it happens, Gortner and Grendel made some errors in their experiment. They failed to completely extract all the lipids from the cells, and they also underestimated the total surface area of the individual red blood cells.

However, because these two errors canceled each other out, their final conclusions turned out to be correct, regardless of their miscalculations. Thereafter, the idea of a lipid bilayer became the basis for future models of membrane structure. Sadava When the use of electron microscopy started to allow examination of the plasma membrane at high resolution, people noticed that the image clearly showed three layers, not two.

In a key paper, Stoeckenius provided clear pictures of the three-layer structure. He then described in both words and diagrams how the lipid bilayer results in a three-layer image. As it turns out, the inner and outer edges of the bilayer have a different composition than the interior.

Under the view of the electron microscope, the outsides of the lipid bilayer show up as two darker layers, whereas the hydrophobic interior stains less densely, thus showing three apparent "layers" outside layers are represented as blue in Figure 1C.

The first clues to lipid bilayer structure came from results with red blood cell membranes. The ultimate discovery that the plasma membrane is a lipid bilayer with hydrophobic and hydrophilic properties changed the way this structure was viewed. Its semipermeable and liquid nature provided the groundwork for understanding both its physical and biological properties. Edidin, M. Lipids on the frontier: a century of cell-membrane lipids Nature Reviews : Molecular Cell Biology 4 : — Gortner, E.

On bimolecular layers of lipoids on the chromacytes of blood. Journal of Experimental Medicine 41 , — Langmuir, I. The constitution and fundamental properties of solids and liquids II: Liquids. Journal of the American Chemical Society 39 , — Overton, E. The probable origin and physiological significance of cellular osmotic properties.

Vierteljahrschrift der Naturforschende gesselschaft 44 , 88— In Biological Membrane Structure , trans. Park, R. Boston: Little Brown, Sadava, D.

Cell Biology, Organelle Structure and Function. Boston: Jones and Bartlett, Stoeckenius, W. Structure of the plasma membrane: An electron-microscope study. Circulation 26 , — Cell Membranes. Microtubules and Filaments. Endoplasmic Reticulum, Golgi Apparatus, and Lysosomes.

Plant Cells, Chloroplasts, and Cell Walls. Cytokinesis Mechanisms in Yeast. How Viruses Hijack Endocytic Machinery. Discovering the Lipid Bilayer. Discovery of the Giant Mimivirus. Endosomes in Plants. Mitochondria and the Immune Response. Yes, it does restrict many molecules from entering or leaving the cell, but it is also designed so that some molecules can very quickly move through the membrane, and thus enter or leave the cell with ease. Our scientific understanding of membranes began with the American statesman Benjamin Franklin.

I fetched out a cruet of oil and dropped a little of it on the water. I saw it spread itself with surprising swiftness upon the surface… Though not more than a teaspoonful, produced an instant calm over a space several yards square which spread amazingly and extended itself gradually till it reached the [other] side, making all that quarter of the pond, perhaps half an acre, as smooth as a looking glass. He and other scientists developed tools and mathematical methods for calculating the surface area covered by the oil film.

It was this insight — that oil and water repel each other — that led scientists to wonder if the cell membrane might somehow be made of a substance that repels water.

This way, it could keep fluids outside the cell from passing through, while also preventing the fluids inside the cell from leaking out. The fact that, when viewed under a microscope, animal cells look similar to spheres of oil helped to popularize the view that cells were somehow surrounded by an oily film. It took several more decades before scientists came to understand the structural features of the membrane that allow it to repel water.

This understanding came in three major steps. First, chemists observed that all known types of cells contain molecules called lipids that are hydrophobic , or water-insoluble. If cells are mostly water, how do they also contain water-insoluble things? Scientists then imagined that maybe a water-insoluble outer surrounding might be the answer.

If the outer membrane was made of water-insoluble lipids, the membrane would restrict water and water-soluble molecules from passing through, while hydrophobic molecules water-insoluble could pass through the membrane. They had further evidence to back up this idea — oxygen gas is hydrophobic but can pass through cell membranes easily. The second major advance came in with the invention of the electron microscope, which resolved a six-year debate in the scientific community.

In , two competing scientists came up with opposite conclusions about the structure of the membrane. A Danish-American scientist named Hugo Fricke performed calculations involving the surface area of those cells , and their capacity for electric charge. Based on these calculations, he found that the layer of lipids surrounding the cell is 3.

Although his measurements were dramatically accurate, lack of understanding of the structure of lipids led him and others to the conclusion that the layer of lipids around the cell could only be one layer thick. They extracted all of the lipids from a sample of red blood cells and allowed them to spread out on a watery surface, much like Ben Franklin had done with the oil.

Thus, Gorter and Grendel concluded that the lipid surface surrounding the cells must be two layers. It turns out that the limited technology of the time led to two major errors in their work. First, they did not completely extract all of the lipids from the red blood cells.

Second, they underestimated the surface of the red blood cell because they were unaware of its double-concave shape. However, the two mistakes acted to cancel each other out almost exactly and their conclusions were correct. This was dramatic and convincing evidence that the membrane consists of a double layer of lipids.

Even more dramatically, the electron microscope revealed that the cell membrane also had visible structures embedded in it Figure 1. For this clever experiment, the scientists grew human cells in one dish and mouse cells in another. They used a technique, brand new at the time, to attach a fluorescent labels to some of the proteins on the outside of cells. They labeled some of the proteins in the human cells with a fluorescent blue dye, while labeling the proteins on the mouse cells with a red dye.

Then, they used a virus to trick the cells into fusing together. These hybrid cells that were half human, half mouse did not survive for very long, but they did live just long enough to show us something about membranes.

At first, just after the cells had fused, all of the blue label was segregated on one half of the hybrid cell, while the red label was on the other half. However, very, quickly, the labels began to intermix with each other and within 40 minutes, the blue and red labels were evenly distributed throughout the surface of the hybrid cell Figure 2.

The quick mixing of the fluorescent labels means that the proteins that are on the surface of the cell are not fixed in place — they can and do diffuse rapidly around the exterior of the cell, while still being embedded in the plasma membrane. This realization led to the development of the fluid-mosaic model of membrane structure, which was first fully articulated by S.

Singer and Garth L. Singer and Nicolson explained the plasma membrane as a bilayer, two layers of lipid molecules , with protein molecules embedded in the layers. They compared this to a mosaic of colored tiles that are inlaid to form a design or picture. However, in this case, the tiles are the molecules of lipid and protein, and they are not fixed in place — they move about through diffusion. Another way to imagine the surface of the membrane is to picture the surface of the ocean on a rough and windy day.

See Figure 3 to see an illustration of the concept. Since , we have learned a great deal about the molecular components of biological membranes and our current understanding of the very complex and dynamic nature of membranes is a far cry from the static film that was once imagined. Scientists began deriving embryonic stem cells from mice in the s, and in , James Thomson isolated human embryonic stem cells and developed cell lines.

His work was then published in an article in the journal Science. It was later discovered that adult tissues, usually skin, could be reprogrammed into stem cells and then form other cell types.

These cells are known as induced pluripotent stem cells. The discovery of the cell has had a far greater impact on science than Hooke could have ever dreamed in In addition to giving us a fundamental understanding of the building blocks of all living organisms, the discovery of the cell has led to advances in medical technology and treatment.

Today, scientists are working on personalized medicine, which would allow us to grow stem cells from our very own cells and then use them to understand disease processes. All of this and more grew from a single observation of the cell in a cork. Robert Hook refined the design of the compound microscope around and published a book titled Micrographia which illustrated his findings using the instrument.

The audio, illustrations, photos, and videos are credited beneath the media asset, except for promotional images, which generally link to another page that contains the media credit. The Rights Holder for media is the person or group credited. Tyson Brown, National Geographic Society. National Geographic Society. For information on user permissions, please read our Terms of Service. If you have questions about how to cite anything on our website in your project or classroom presentation, please contact your teacher.

They will best know the preferred format. When you reach out to them, you will need the page title, URL, and the date you accessed the resource. If a media asset is downloadable, a download button appears in the corner of the media viewer. If no button appears, you cannot download or save the media.

Text on this page is printable and can be used according to our Terms of Service. Any interactives on this page can only be played while you are visiting our website. You cannot download interactives. A cell is the smallest unit that is typically considered alive and is a fundamental unit of life. All living organisms are composed of cells, from just one unicellular to many trillions multicellular. Cell biology is the study of cells, their physiology, structure, and life cycle. Teach your students about cell biology using these classroom resources.