Illustration by Alison Czinkota. ThoughtCo. Updated on October 07, 2019 The cell membrane (plasma membrane) is a thin semi-permeable membrane that surrounds the
cytoplasm of a cell. Its function is to protect the integrity of the interior of the cell by allowing certain substances into the cell while keeping other
substances out. It also serves as a base of attachment for the cytoskeleton in some organisms and the cell wall in others. Thus the cell membrane also serves to help
support the cell and help maintain its shape. Key Takeaways
Another function of the membrane is to regulate cell growth through the balance of endocytosis and exocytosis. In endocytosis, lipids and proteins are removed from the cell membrane as substances are internalized. In exocytosis, vesicles containing lipids and proteins fuse with the cell membrane increasing cell size. Animal cells, plant cells, prokaryotic cells, and fungal cells have plasma membranes. Internal organelles are also encased by membranes.
Cell Membrane Structure
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The cell membrane is primarily composed of a mix of proteins and lipids. Depending on the membrane’s location and role in the body, lipids can make up anywhere from 20 to 80 percent of the membrane, with the remainder being proteins. While lipids help to give membranes their flexibility, proteins monitor and maintain the cell's chemical climate and assist in the transfer of molecules across the membrane.
Cell Membrane Lipids
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Phospholipids are a major component of cell membranes. Phospholipids form a lipid bilayer in which their hydrophilic (attracted to water) head areas spontaneously arrange to face the aqueous cytosol and the extracellular fluid, while their hydrophobic (repelled by water) tail areas face away from the cytosol and extracellular fluid. The lipid bilayer is semi-permeable, allowing only certain molecules to diffuse across the membrane.
Cholesterol is another lipid component of animal cell membranes. Cholesterol molecules are selectively dispersed between membrane phospholipids. This helps to keep cell membranes from becoming stiff by preventing phospholipids from being too closely packed together. Cholesterol is not found in the membranes of plant cells.
Glycolipids are located on cell membrane surfaces and have a carbohydrate sugar chain attached to them. They help the cell to recognize other cells of the body.
Cell Membrane Proteins
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The cell membrane contains two types of associated proteins. Peripheral membrane proteins are exterior to and connected to the membrane by interactions with other proteins. Integral membrane proteins are inserted into the membrane and most pass through the membrane. Portions of these transmembrane proteins are exposed on both sides of the membrane. Cell membrane proteins have a number of different functions.
Structural proteins help to give the cell support and shape.
Cell membrane receptor proteins help cells communicate with their external environment through the use of hormones, neurotransmitters, and other signaling molecules.
Transport proteins, such as globular proteins, transport molecules across cell membranes through facilitated diffusion.
Glycoproteins have a carbohydrate chain attached to them. They are embedded in the cell membrane and help in cell to cell communications and molecule transport across the membrane.
Eukaryotic Cell Structures
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The cell membrane is only one component of a cell. The following cell structures can also be found in a typical animal eukaryotic cell:
- Centrioles—help to organize the assembly of microtubules.
- Chromosomes—house cellular DNA.
- Cilia and Flagella—aid in cellular locomotion.
- Endoplasmic Reticulum—synthesizes carbohydrates and lipids.
- Golgi Apparatus—manufactures, stores and ships certain cellular products.
- Lysosomes—digest cellular macromolecules.
- Mitochondria—provide energy for the cell.
- Nucleus—controls cell growth and reproduction.
- Peroxisomes—detoxify alcohol, form bile acid, and use oxygen to break down fats.
- Ribosomes—responsible for protein production via translation.
Sources
- Reece, Jane B., and Neil A. Campbell. Campbell Biology. Benjamin Cummings, 2011.