The basolateral membrane is a specialized part of the cell membrane found in epithelial cells. This surface faces away from the lumen of an organ and interacts with the underlying connective tissues and blood vessels. It combines both the basal surface, which rests on the basement membrane, and the lateral surfaces, which connect to adjacent cells. This positioning allows it to transport substances within the body.
Location Within a Polarized Cell
Epithelial cells exhibit cell polarity, meaning their different surfaces have distinct structures and functions. The basolateral membrane represents one of these specialized domains, positioned opposite to the apical membrane. The apical membrane faces the lumen or external environment, like a public-facing front door. In contrast, the basolateral membrane acts like a private back door, orienting towards the internal environment and blood supply.
This clear separation between the apical and basolateral domains is enforced by tight junctions. These junctions form a seal around the cell near its apical surface, preventing proteins and lipids from freely diffusing between the two membrane regions. This barrier ensures the unique protein and lipid compositions of each membrane domain are maintained, allowing them to perform their specialized tasks.
Primary Transport Functions
The primary role of the basolateral membrane involves transporting substances out of the epithelial cell and into the surrounding interstitial fluid, from where they can enter the bloodstream. This process represents a final step in transepithelial transport, completing either absorption or secretion. For instance, after nutrients like glucose and amino acids are absorbed from the lumen across the apical membrane, they are moved across the basolateral membrane.
Similarly, ions such as sodium, potassium, and chloride, along with water, are regulated in their movement across this membrane. This outward movement helps maintain the balance of fluids and solutes within the body. The basolateral membrane thus acts as a gateway, facilitating the controlled exit of molecules into the body’s circulation.
Integral Membrane Proteins and Pumps
The specific functions of the basolateral membrane are carried out by various integral membrane proteins and pumps. The Sodium-Potassium (Na+/K+) pump, also known as Na+/K+-ATPase, is a defining feature of this membrane. This protein actively transports three sodium ions out of the cell and two potassium ions into the cell for each molecule of ATP consumed.
This continuous pumping action creates and maintains an electrochemical gradient across the cell membrane, with a higher concentration of sodium outside the cell and a higher concentration of potassium inside. This sodium gradient provides the driving force for other secondary active transporters located on the basolateral membrane. These transporters utilize the energy stored in the sodium gradient to move other molecules, such as amino acids or glucose, out of the cell and into the interstitial fluid.
Significance in Organ Systems
The specialized functions of the basolateral membrane are involved in the operation of several organ systems, particularly those involved in absorption and filtration. In the small intestine, for example, the basolateral membrane plays a direct role in nutrient absorption. After glucose and amino acids are taken up by intestinal epithelial cells from digested food, specific transporters on the basolateral membrane move these absorbed nutrients from the cell’s cytoplasm into the capillaries within the villi, allowing them to enter the bloodstream.
In the kidneys, the basolateral membrane is equally important in the renal tubules, such as the proximal tubule and collecting ducts. Here, it facilitates the reabsorption of substances like salts, water, and glucose from the filtered fluid back into the blood. This reabsorption prevents these substances from being lost in the urine, demonstrating how the basolateral membrane’s transport capabilities are important for maintaining the body’s fluid and electrolyte balance.