The cell membrane, commonly known as the plasma membrane, is a double layer of proteins and lipids surrounding a cell. The function of a cell membrane is to separate the cytoplasm, the cell’s contents, from the outer environment. Both prokaryotic and eukaryotic cells have these features.
The cell membrane encloses the cell constituents such as nucleic acids, proteins, substances and carbohydrates that are part of cellular metabolism. These constituents are generally large water-soluble and highly charged molecules; for cellular respiration, cells need small molecules such as oxygen and carbon dioxide as a byproduct of metabolic functions. These can enter and exit through the Membrane. Water can also cross the Membrane but at a slower rate.
Functions of a cell membrane
The cell membrane has two main functions as follows:
- Work as a barrier to keep the constituents of the cell in and pass the unwanted substances out.
- It works as a gate that allows the transportation of important nutrients into the cell and the movement of the waste product from the cell.
The cell membrane only allows certain molecules to enter or exit like a drawbridge. Thus, it helps in the survival of the cell.
The structure of a cell membrane
Cell membranes are made of lipids and proteins. These membrane lipids are of 2 types as follows.
- Phospholipids
- Sterols
Though the membrane lipids are similar to lipids, they both have a reason that they are attracted to and soluble in water. The amphiphilic property is basic to the role of lipids that are building blocks of a cell membrane. Amphiphilic means that they have a dual attraction of lipid-soluble and water-soluble. There are two types of membrane proteins.
- Calcium bridges or ionic bonds attach the extrinsic protein to the electrically charged phosphoryl surface. They can also get attached to the second type of protein known as intrinsic protein.
- Intrinsic protein: This protein is firmly embedded within the phospholipid by layer. Membranes are involved in active metabolism that contains a proportion of a protein.
Crossing the Membrane
The flexibility of a cell membrane is due to its chemical structure, which has the ideal boundary for growing and dividing cells. The Membrane allows some dissolved substances to pass while blocking others. Lipid soluble molecules can enter the Membrane to some extent, but the lipid by layer repels many large and water-soluble molecules. Besides this, it also manages electrically charged ions that are important for the cell’s survival.
During exocytosis, vesicles form in the cytoplasm and reach the surface of the cell membrane. Further, they merge with the Membrane and release their contents to the outside of the cell. Exocytosis also removes the cell’s waste products, like molecules not used by the cell and old organelles. Endocytosis also includes phagocytosis, commonly known as cell eating, and pinocytosis, known as cell drinking. The cell membranes form a depression during these processes. Later on, it forms a small sphere of Membrane called a vesicle that contains a molecule. It is transported wherever it is the requirement in the cell.
These movements involve a fusion between membrane surfaces, and later on, the reformation of the intact Membrane takes place.
Cell signalling and cell membrane
Besides the functions mentioned above, the cell membrane also plays a vital role in cell signalling and communication. Several proteins are embedded in a cell membrane, and these proteins can bind the molecules found outside of a cell. Also, it passes on the messages to the inside of the cell. Cell signalling is one of the processes that occur in a cell.
Substances produced by the other areas of the body, such as hormones, can also get bound by these receptor proteins on the cell membrane. When a molecule binds to its target receptor on the Membrane, it also creates a signal transduction pathway inside the cell. This pathway further transmits the signal to the designated molecules. Because of this complex signalling pathway system, a cell can perform the action given by the signalling molecule. Making or stopping the production of a protein is an example of such activity.
Conclusion
The cell membrane, a double layer of lipids (20 to 80 % of the Membrane) and proteins, acts as a barrier between the cytoplasm and extracellular space, thus assisting biochemical reactions. Proteins maintain the cell’s climate while lipids provide flexibility. It also replenishes the damaged cell. Cell membranes also contribute to cell signalling and communication with the help of embedded proteins.
The cell membrane also regulates cell growth by striking the balance of endocytosis and exocytosis. It is the base of the cytoskeleton and cell wall in many organisms. The cell membrane works as a security guard and controls what goes in and out of a gate.