Cellular Membrane

What is the cell membrane

The cell membrane is the structure that cells have that separates them from the external environment. It is also known as the plasma membrane and resembles a balloon filled with jelly. The membrane is responsible for containing and protecting the interior of the cell.

In prokaryotic cells, the cell membrane houses the cytoplasm and genetic material. In eukaryotic cells, the plasma membrane surrounds the cytoplasm, the nucleus, and the organelles.

Initially, it was believed that the cell membrane was a rigid structure, whose mission was simply to keep the components inside the cell enclosed. We now know that it is flexible, fluid and dynamic.

cell membrane schematic

The cell membrane is made up of two layers or sheets of phospholipids (yellow circles with two black lines) with proteins (blue) embedded in the lipid bilayer.

The plasma membrane is characterized by its flexibility. This is due to its lipid and protein composition, which makes the membrane not rigid. In addition, it is semi-permeable, which means that certain compounds, such as glucose and water, can pass through the membrane, while preventing the entry of others, such as galactose.

The composition of the cell membrane also determines its fluidity, that is, the elements that make up the membrane are in constant movement within it, they are not fixed or permanent.

Cell membrane functions

The cell membrane performs the following functions:

  • selective barrier: the cell membrane allows the entry of certain elements and compounds necessary for the cell to fulfill its functions. For example, the glucose that the cell needs for energy enters through transporter channels through the membrane.
  • Locomotion: Some cells, such as amoebas and white blood cells, move thanks to the flexibility of the cell membrane. Other cells, such as paramecia and spermatozoa, move by means of flagella and cilia, specialized structures in the cell membrane.
  • cell transport: the plasma membrane has proteins that function as channels and transporters that allow ions and compounds to pass through.
  • Signaling: cells receive external stimuli that are detected by the cell membrane.
  • communication between cells: on the cell membrane, the cell displays markers or “flags” with which it can contact other cells.
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Cell membrane structure

cellular membrane

Diagram of the structure of a cell membrane, showing the components that form it.

The structure of the cell membrane defines its functions and characteristics. According to the “fluid mosaic” model defined in 1972 by the authors Singer and Nicholson, the plasma membrane is composed of:

  • phospholipids: are lipids whose head is composed of a phosphate group and its two tails are chains of fatty acids. Phospholipids in membranes form two layers of each other, known as a phospholipid bilayer, which makes up a quarter of the membrane.
  • Proteins: They make up 55% of the plasma membrane. There are proteins that cross the membrane from one side to the other (integral proteins) and proteins that just sit on the inner or outer surface of the membrane (peripheral proteins).
  • sterols: are a group of lipids that can be found in cell membranes. In animal cells the main membrane sterol is cholesterol, while in plant cells phytosterol predominates.
  • carbohydrates: heCarbohydrates or sugars are found on the outer surface of the cell membrane, attached to proteins (glycoproteins) or lipids (glycolipids).

See also:

  • Cell parts.
  • Eukaryotic cell
  • Plant cell
  • Animal and plant cell: differences and similarities
  • Cytoplasm

References

Alberts, B. et al. (2008) Molecular biology of the cell. 5th ed. Garland Science. UK.

Nelson, DL, Cox, MM, Hoskins, AA (2021) Lehninger Principles of Biochemistry. 8th ed. Macmillan Learning. Boston