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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)

Transcellular Movement

  • 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.

Paracellular Movement

  • Movement through the space in between two cells.

  • Depends on how tight the junctions are.

  • Some junctions are leaky: Allows movement. E.g. proximal part of renal tubule

  • Tight junctions: Do not allow movement. E.g. Collecting duct


Next >>

Membrane Potentials and Action Potential > ▶️ Nernst Equation & Goldman Equation


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