什么是膜？

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.