5.1:
What are Membranes?
Membranes are dynamic layers composed primarily of phospholipids, proteins, and carbohydrates that enclose a cell, forming selectively permeable boundaries and interior compartments. One component, phospholipids, are polar molecules that spontaneously arrange, often as a continuous bilayer about seven nanometers thick. The hydrophilic, or water loving, polar heads face the outside and inside of the cell.
The hydrophobic, or water fearing, non-polar tails line up in the middle to avoid exposure to water. While phospholipids provide the basic structure, it's how the other components, the membrane proteins and carbohydrates, associate with them that mostly contribute to the functional properties. Depending on the cell's needs, some proteins function as receptors to transduce signals in the cell's environment.
For example, a cell surface receptor binds a signal in the extracellular space and generates intracellular signals. Others connect as transporters to provide a path for different types of molecules to cross. Additional proteins may serve as structural links to connect the cytoskeleton to the extracellular matrix or adjacent cells.
Finally, carbohydrates will bind to either the phospholipids forming glycolipids or to proteins on the membranes, creating glycoproteins. Both serve important functions, such as binding hormones or neurotransmitters as well as allowing cells to recognize each other.
5.1:
What are Membranes?
A key characteristic of life is the ability to separate the external environment from the internal space. To do this, cells have evolved semi-permeable membranes that regulate the passage of biological molecules. Additionally, the cell membrane defines a cell’s shape and interactions with the external environment. Eukaryotic cell membranes also serve to compartmentalize the internal space into organelles, including the endomembrane structures of the nucleus, endoplasmic reticulum and Golgi apparatus.
Membranes are primarily composed of phospholipids composed of hydrophilic heads and two hydrophobic tails. These phospholipids self-assemble into bilayers, with tails oriented toward the center of the membrane and heads positioned outward. This arrangement allows polar molecules to interact with the heads of the phospholipids both inside and outside of the membrane but prevents them from moving through the hydrophobic core of the membrane.
Proteins and carbohydrates contribute to the unique properties of a cell’s membrane. Integral proteins are embedded in the membrane, while peripheral proteins are attached to either the internal or external surface of the membrane. Transmembrane proteins are integral proteins that span the entire cell membrane. Transmembrane receptor proteins are important for communicating messages from the outside to the inside of the cell. When bound to an extracellular signaling molecule, transmembrane receptors undergo a conformational change that serves as an intracellular signal. Other proteins, such as ion channels, serve to regulate the passage of large or polar molecules across the hydrophobic membrane core.
Carbohydrates are bound to either lipids or proteins on the exterior face of the cell’s membrane. The unique patterns of glycoproteins and glycolipids present on a cell’s exterior surface allow cellular recognition to take place. Human immune cells are able to distinguish self from non-self by recognizing the carbohydrate modifications on cell surfaces. Together, the proteins, carbohydrates, and lipids present on a membrane create a functional and flexible boundary for cells.
Suggested Reading
Sych, Taras, Yves Mély, and Winfried Römer. "Lipid self-assembly and lectin-induced reorganization of the plasma membrane." Philosophical Transactions of the Royal Society B: Biological Sciences 373, no. 1747 (2018): 20170117. [Source]
Tarbell, John M., and L. M. Cancel. "The glycocalyx and its significance in human medicine." Journal of Internal Medicine 280, no. 1 (2016): 97-113. [Source]