What are two functions of the cell membrane

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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.​

• The cell membrane is a multifaceted membrane that envelopes a cell's cytoplasm. It protects the integrity of the cell along with supporting the cell and helping to maintain the cell's shape.
• Proteins and lipids are the major components of the cell membrane. The exact mix or ratio of proteins and lipids can vary depending on the function of a specific cell.
• Phospholipids are important components of cell membranes. They spontaneously arrange to form a lipid bilayer that is semi-permeable such that only certain substances can diffuse through the membrane to the cell's interior.
• Similar to the cell membrane, some cell organelles are surrounded by membranes. The nucleus and mitochondria are two examples.

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.

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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.

Microscopic view of phospholipids.

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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.

Lipoproteins and PCSK9 bound to receptors.

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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.

Artwork of chromosomes.

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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.

• Reece, Jane B., and Neil A. Campbell. Campbell Biology. Benjamin Cummings, 2011.

With few exceptions, cellular membranes — including plasma membranes and internal membranes — are made of glycerophospholipids, molecules composed of glycerol, a phosphate group, and two fatty acid chains. Glycerol is a three-carbon molecule that functions as the backbone of these membrane lipids. Within an individual glycerophospholipid, fatty acids are attached to the first and second carbons, and the phosphate group is attached to the third carbon of the glycerol backbone. Variable head groups are attached to the phosphate. Space-filling models of these molecules reveal their cylindrical shape, a geometry that allows glycerophospholipids to align side-by-side to form broad sheets (Figure 1).

Glycerophospholipids are by far the most abundant lipids in cell membranes. Like all lipids, they are insoluble in water, but their unique geometry causes them to aggregate into bilayers without any energy input. This is because they are two-faced molecules, with hydrophilic (water-loving) phosphate heads and hydrophobic (water-fearing) hydrocarbon tails of fatty acids. In water, these molecules spontaneously align — with their heads facing outward and their tails lining up in the bilayer's interior. Thus, the hydrophilic heads of the glycerophospholipids in a cell's plasma membrane face both the water-based cytoplasm and the exterior of the cell.

Altogether, lipids account for about half the mass of cell membranes. Cholesterol molecules, although less abundant than glycerophospholipids, account for about 20 percent of the lipids in animal cell plasma membranes. However, cholesterol is not present in bacterial membranes or mitochondrial membranes. Also, cholesterol helps regulate the stiffness of membranes, while other less prominent lipids play roles in cell signaling and cell recognition.

At physiological temperatures, cell membranes are fluid; at cooler temperatures, they become gel-like. Scientists who model membrane structure and dynamics describe the membrane as a fluid mosaic in which transmembrane proteins can move laterally in the lipid bilayer. Therefore, the collection of lipids and proteins that make up a cellular membrane relies on natural biophysical properties to form and function. In living cells, however, many proteins are not free to move. They are often anchored in place within the membrane by tethers to proteins outside the cell, cytoskeletal elements inside the cell, or both.