• A change/mutation in the structure of a receptor protein or signaling molecule affects the activity of the signaling pathway and can alter the cellular response
    • Chemicals that interfere with any component of the signaling pathway may activate or inhibit it: toxins, antibiotics

figure: on the left, the hedgehog would not create a signal transduction pathway because the inhibition prevents it from going any further.

Feedback Mechanisms

Negative Feedback and Homeostasis

  • Organisms use feedback mechanisms to maintain their internal environments and respond to internal and external environmental changes.
  • Negative feedback mechanisms maintain homeostasis
    • Tend to push to stability after abnormality, counteracts change, processes that need to be maintained
      • Processes operate at the molecular and cellular levels.
    • Temperature regulation, blood glucose & calcium levels,
    • Product accumulates, process that makes it slows down and less is made or output inhibits

Positive Feedback and Homeostasis

  • Positive feedback mechanisms amplify responses and processes in organisms.
  • Move system away from starting state; tend to push organisms to extreme behaviors
  • Still act to maintain homeostasis (less common); occur in abnormal situations and increase abnormality so situation becomes normal
    • pregnancy contractions, fruit ripening, blood clotting

Example: Blood Vessel Injury

If there is an injury to a cell, molecules are released and trigger previously inactive clotting factors.

Clotting factors activated → activated more clotting factors → amplification continues until injury healed



Alterations in Loops

Disrupt homeostasis → Diabetes: cells don’t work so when glucose lvl increases, no insulin made to regulate