Transport Across Epithelium

Epithelium separates internal environment of our body from external world.
Transport across epithelium occurs to:
- Take external material inside the body i.e. absorption of nutrients from lumen of gastrointestinal tract, or
- Remove internal substances out of the body i.e. secretion of waste products in renal tubules.

Epithelial cells make a continuous sheet that separates two solutions:
- One solution is from internal environment ⟶ in contact with blood. E.g. extracellular fluid
- The other solution is from external environment ⟶ in contact with external world. E.g. filtrate in renal tubule
Cells in the sheet are connected by tight junctions.
Tight junctions separate two sides of the cell membrane:
- Basolateral membrane: Faces internal environment of the body.
- Lumenal membrane: Faces lumen (which is continuous with the external environment)

Movement through the cell.
Occur in two parts:
- Movement through basolateral membrane.
- Movement through luminal membrane.
For this, both sides of the membrane contain different sets of transport proteins.
Example: Reabsorption of Na in collecting duct of renal tubule:
- Na channels are present only on luminal membrane ⟶ allows Na entry from lumen into the cell.
- Na-K ATPase pumps are present only on basolateral membrane ⟶ moves Na from cell into the interstitium
- Thus there is net movement of sodium from lumen into the interstitium.
Tight junctions prevent mixing of transport proteins on both sides of the membrane ⟶ Allows movement of a substance in desired direction.
Without tight junctions, the transport proteins on luminal and basolateral membrane would mix up and there won't be net movement in any direction. In the above example, without tight junctions ⟶ Na channels and Na-K ATPase pumps would distribute on both the sides of the membrane ⟶ Na would enter the cell from both the sides through Na channel, and would also be extruded on both sides by Na-K ATPase pump ⟶ No net movement across the cell.

Next >>