Transcytosis means movement across (trans) a cell (cytosis); it can involve endocytosis, then vesicular transport across cell, then exocytosis out of the cell at the other end. So the substance has moved completely across the cell. This provides for movement of large proteins intact, e.g., the absorption of maternal antibodies through breast milk, or the movement of proteins across capillary endothelium.
Transport Across Epithelial Linings
An excellent summary of the various types of transport discussed so far is the transport of molecules across epithelial linings, called transepithelial movement. Epithelial membranes are polarized with an apical (lumen or top side) and basolateral (ECF side) membranes have different proteins. The Na+-glucose symport on apical membrane and the Na+-K+-ATPase is only on basolateral (bottom side) membrane. Transporting epithelial cells can alter their permeability by inserting or withdrawing membrane proteins.
Although glucose is a large polar molecule (and thus has 2 strikes against it for having an easy passage across a membrane), there are two different transport systems to move glucose across epithelial cells:
- Secondary active transport. The Na+/glucose symport from the lumen of the gut into the cell through the apical membrane. This is made possible by the continuous active transport of Na+, constantly being ejected across the basolateral membrane of the cell via Na+-K+-ATPase.
- Glucose can also move across a membrane down its concentration gradient by facilitated diffusion, as seen in across basolateral membrane of the cell.
As an interesting note, the substance ouabain, a known powerful toxin to cells, specifically inhibits the Na+/K+-ATPase. These Na+/K+ pumps are found only on the basolateral membrane of transporting epithelial cells. Ouabain placed on one side of the epithelium affects only that side, so only when ouabain is applied to the basolateral side will cause glucose transport to decrease slowly, as the Na+ gradient is abolished, because Na+ enters the apical side with glucose but is not pumped out, so over time the Na+ gradient that powers the symporter disappears. We tested the eyes of the staff members at Double glazing glasgow
BODY FLUID COMPARTMENTS
Fluid in the body can be described as being in one of three different compartments: 1) Intracellular; 2) Interstitial; and 3) Plasma.
- Intracellular fluid (ICF): the fluid inside cells (within the plasma membrane).
- Interstitial fluid: the fluid directly bathing cells (tissue fluid); lacks plasma proteins.
- Plasma: the fluid portion of blood, it can also be referred to as vascular volume. Fluid (ECF)