The plasma membrane separates the interior of a cell from its outside environment and is composed of phospholipids, proteins, cholesterol and carbohydrates.
The main fabric of the membrane is composed of an amphiphilic phospholipid molecule. The hydrophilic head contains a charged phosphate group which comes into contact with the aqueous fluid both inside and outside the cell. The tail is comprised of two non-polar lipid tails which are hydrophobic or water-hating molecules. When placed in water, hydrophobic molecules tend to form a ball or cluster. The hydrophilic regions of the phospholipids tend to form hydrogen bonds with water and other polar molecules on both the exterior and interior of the cell. Thus, the membrane surfaces that face the interior and exterior of the cell are hydrophilic. In contrast, the middle of the cell membrane is hydrophobic and will not interact with water. Therefore, phospholipids form an excellent lipid bilayer cell membrane that separates fluid within the cell from the fluid outside of the cell.
Proteins make up the second major component of plasma membranes. Integral proteins are integrated completely into the membrane structure, and their hydrophobic membrane-spanning regions interact with the hydrophobic region of the phospholipid bilayer. Some span only part of the membrane—associating with a single layer—while others stretch from one side of the membrane to the other, and are exposed on either side. Proteins are important for the transport of larger molecules and charged particles over the membrane layer.
Carbohydrates are the third major component of plasma membranes. They are always found on the exterior surface of cells and are bound either to proteins (forming glycoproteins) or to lipids (forming glycolipids). Along with peripheral proteins, carbohydrates form specialized sites on the cell surface that allow cells to recognize each other.
In the study of biology, the cell membrane is often drawn as a solid line but when examined it is fluid and movable. The fluid mosaic model describes the structure of the plasma membrane as a mosaic of components which give the plasma membrane a fluid character.
Multiple factors lead to membrane fluidity. First, the mosaic characteristic of the membrane where the integral proteins and lipids exist in the membrane as separate but loosely-attached molecules means they can flow past one another. The second factor that leads to fluidity is the nature of the phospholipids themselves. In their saturated form, the fatty acids in phospholipid tails are saturated with bound hydrogen atoms; there are no double bonds between adjacent carbon atoms. This allows them to move over a range of temperatures. In animals, the third factor that keeps the membrane fluid is cholesterol. It lies alongside the phospholipids in the membrane and tends to dampen the effects of temperature on the membrane. Thus, cholesterol functions as a buffer, preventing lower temperatures from inhibiting fluidity and preventing higher temperatures from increasing fluidity too much. There are areas of high cholesterol concentration in the membrane where the composition of the proteins, carbohydrates and different lipids differ from the rest of the membrane – these are called lipid rafts and they are thought to be specialized microdomains in the membrane.
In plants instead of cholesterol often waxes are found. They are composed of a long-chain fatty acid bonded to an alcohol group. Waxes contribute to rigidity in the cell membrane and help provide structure.
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Key Points
• The principal components of the plasma membrane are lipids (phospholipids and cholesterol), proteins, and carbohydrates.
• Integral proteins help with structure and transport over the membrane
• Cholesterol and waxes offer support and structure to the membrane
• The phospholipids hydrophobic tail points away from water and the hydrophilic head points towards water meaning it forms a bilayer.
• The fluid mosaic model describes the ever-changing nature of the cell membrane as not just a rigid structure but a fluid model.
Key Terms
fluid mosaic model: describes the plasma membrane as a fluid combination of phospholipids, cholesterol, and proteins
phospholipid: the main fabric of the plasma membrane, has a hydrophilic head and two hydrophobic tails
hydrophilic: water-loving, usually in contact with aqueous environments
hydrophobic: water-hating, usually sequestered away from aqueous environments
phospholipids: a major component of cell membranes consisting of two hydrophobic fatty acid tails and a hydrophilic head consisting of a phosphate group
amphiphilic: a chemical compound possessing both hydrophilic and hydrophobic properties
integral protein: is any protein which has a special functional region for securing its position within the cellular membrane
plasma membrane: the semipermeable barrier that surrounds the cytoplasm (inside contents) of a cell
cholesterol: a type of lipid which is an essential structural component of animal cell membranes
lipid raft: an area in the membrane where there is a high concentration of cholesterol, and a different composition of carbohydrates, proteins and other lipids compared to the rest of the membrane
waxes: they are composed of a long-chain fatty acid bonded to an alcohol group found in plants and contribute to the rigidity of the cell membrane