Reversible and irreversible steps

• Metabolic processes can have reversible, irreversible steps and alternative pathways which keep the process under strict control.
• Pathways can be modified and contain alternative routes, so steps can be bypassed.
• Intermediate 1 -

  • Glucose diffuses from a high concentration outside the

  cell to a low concentration inside the cell, then

  converted to intermediate 1.

  • This reaction is irreversible

  • It is of benefit to the cell as it maintains a low

  concentration of glucose inside the cell and therefore

  allows glucose to diffuse constantly into the cell.
• Intermediate 2-

  • The conversion of Intermediate 1 to Intermediate

  2 is reversible.

  • If more intermediate 2 is formed than the cell

  needs for the next step then some can be changed

  back into intermediate 1 and used in an alternative

  pathway, e.g to build glycogen in animal cells or

  starch in plant cells.