Cell membrane facts for kids
The cell membrane is a thin flexible layer around the cells of all living things. It is sometimes called the plasma membrane or cytoplasmic membrane.
Its basic job is to separate the inside of cells from the outside. In all cells, the cell membrane separates the cytoplasm inside the cell from its surroundings. Animal cells are contained in just a membrane. Bacteria, fungi and plants have strong cell walls as well, which support the cell and block the passage of large molecules.
History
Robert Hooke discoved cells in 1665, which led to the proposal of the Cell Theory.
As Hooke observed only plant cells he came to the conclusion that all cells contained a hard cell wall. That was misleading. Microscopists focused on the cell wall for well over 150 years until advances in microscopy were made.
In the early 19th century, after advances in microscopy had been made, it was found that plant cells could be separated. At that time cells were recognized as being separate entities, unconnected, and bound by individual cell walls. This theoryalso began to include animal cells and suggested a universal mechanism for cell protection and development.
By the second half of the 19th century, microscopy was still not advanced enough to make a distinction between cell membranes and cell walls. However, some microscopists correctly identified at this time that cell membranes existed in animal cells and that membranes were not the equivalent of a cell wall to a plant cell. It was also believed that cell membranes were not vital components to all cells. Many refuted the existence of a cell membrane still towards the end of the 19th century. In 1890, an update to the Cell Theory stated that cell membranes existed, but were merely secondary structures. It was not until later studies that cell membranes gained more recognition. In 1895, Ernest Overton proposed that cell membranes were made of lipids.
For many centuries, the scientists cited disagreed with the significance of the structure they were seeing as the cell membrane. For almost two centuries, the membranes were seen but mostly disregarded as an important structure with cellular function. It was not until the 20th century that the significance of the cell membrane as it was acknowledged. Finally, two scientists Gorter and Grendel (1925) made the discovery that the membrane is “lipid-based”. In 1925 the thickness of erythrocyte and yeast cell membranes was determined by Hugo Fricke.
Structure
The membrane is made up of a thin layer called the 'phospholipid bilayer'. This has two layers of phospholipid molecules with phosphate heads on the surfaces and lipid (oil) tails on the inside. The outside heads mix with water, but the tails reject water.
Other proteins and lipids can be added to the cell membrane. By these changes, the cell can adjust what it brings in or puts out. Some proteins are always stuck into it, these are called integral membrane proteins. It also has some which are only sometimes stuck onto it. These are called peripheral membrane proteins. The outer layer protecting the inner cell.
Function
The membrane is selectively permeable. It is active and regulates (adjusts) what comes in and what goes out of the cell. The movement of substances across the membrane can be either passive, occurring without the input of cellular energy, or active, requiring energy.
Proteins in the membrane
Proteins within the membrane are key to its working. These proteins mainly transport chemicals and information across the membrane.
The membrane contains many proteins. The surface proteins can act as gates. They let some chemicals into the cell and let other chemicals leave the cell. It is estimated that up to a third of the human proteome may be membrane proteins. Some of these proteins are linked to the exterior of the cell membrane. An example of this is the CD59 protein, which identifies cells as “self” and thus inhibits their destruction by the immune system.
See also
In Spanish: Membrana plasmática para niños
