Transport Across the Cell Membrane
The outer and inner surfaces of the cell membrane are hydrophilic, whereas the interior of the membrane is hydrophobic; this arrangement means the membrane allows limited penetration by substances and is said to be selectively permeable. The principle of "like dissolves like" explains how molecules are prevented from crossing the membrane. Polar molecules, such as glucose, are hydrophilic and can interact with the cell membrane exterior because the glycerol heads of the phospholipids are also hydrophilic. However, they experience difficulty in crossing the cell membrane because their movement is impeded by the hydrophobic phospholipid tails. Small, nonpolar molecules such as O2 can be dissolved in the lipid bilayer and freely travel across the cell membrane. In general, charged and large molecules cannot pass through the membrane freely, whereas uncharged and small molecules can.
The cell uses a number of mechanisms to transport material across the membrane. Some types of transport do not require the cell to expend energy and are called passive transport. Other types of transport do require the cell to expend energy and are called active transport. Active transport can be used to move a few molecules at a time or to transport many molecules at one time in bulk transport.
Bulk Transport
Sometimes large amounts of material are moved quickly into and out of the cell. This is done through a bulk transport mechanism. As a general overview of this mechanism, the transported substances are enclosed in a membrane bubble, called a vesicle, created by pinching off a portion of the cell membrane. The vesicle then moves through the cell and fuses with an internal or external membrane. Fusion of the vesicle membrane with another membrane releases the contents of the vesicle. Two specific processes are involved in bulk transport: endocytosis and exocytosis.
Endocytosis is a form of bulk transport that moves material into a cell from the environment. This transport can move parts of cells, particles, or even whole cells into a cell. The substance needing entry clusters at a location of the cell membrane, often due to the presence of specific receptors. The cell membrane then forms a pocket around the substances. This pocket, or invagination, pinches off from the internal surface of the cell membrane, forming a vesicle. The vesicle travels inside the cell and fuses with a membrane-bound structure such as a lysosome. The fusion of the vesicle opens the pocket and allows for the release of the material from the vesicle.
Exocytosis is a form of bulk transport used to move large molecules to the outside of the cell. This can be thought of as a reverse process of endocytosis. Exocytosis involves expelling a substance from the cell into the extracellular fluid that surrounds the cell. This is done first by enclosing the material in an intracellular vesicle that buds from a membrane-bound structure such as the Golgi apparatus. The vesicle travels to the cell membrane and fuses with the interior side of the cell membrane. Fusion opens the vesicle by creating an invagination in the exterior side of the cell membrane. The contents of the vesicle are then released to the environment.