B.5 Feedback Systems

• Metabolic pathways allow cells to conduct reactions in small steps that do not disrupt other cellular activities.

• -  Not only are the energy gains and releases managed but so are the byproducts and accessory molecules such as cofactors.

• -  Another significant advantage of metabolic pathways is that the reaction rate of these processes can be regulated.

• -  A cell would not be advantaged by making substances faster than they can be utilized or trying to use a substrate faster than it be utilized or trying to use a substrate faster than it can be supplied.

  Cruise control

• -  This regulatory feature can be compared to cruise control in vehicles. If a vehicle is not going fast enough, it is sped up; if it is going too fast, it is slowed down.

• -  As part of homeostasis, if a metabolic pathway is not needed in a cell, it can be temporarily stopped, or if another one is needed, it can be prompted.

  CO2 in Plasma

• -  A physiological example of this type of maintenance is the regulation of carbon dioxide levels in plasma.

• -  CO2 is a metabolic waste from cellular respiration. As it gets produced by cells, it di0uses into the blood and travels throughout the body.

• -  A portion of the brain known as the medulla oblangata is sensitive to the concentration of CO2 in plasma. When the level is elevated, the medulla initiates nerve impulses that promote breathing and the subsequent excretion of CO2 when it di0uses into the lungs for exhalation.

• -  This effectively reduces the carbon dioxide levels in plasma and through repeated breathing it is maintained at a level that permits survival.

Blood pH

• Blood pH is also maintained by homeostatic mechanisms.

• -  Blood is normally slightly alkaline with a pH of 7.35-7.38 depending on the activity of the

  surrounding cells.

o For example, pH will drop during muscle activity when the tissues are generating

lactic acid and it gets restored when the lactic acid is metabolized.
o A persistent lowered pH is called acidosis .The overall maintenance of pH

is done by managing the hydrogen ion concentration in plasma, which is a function

of the kidneys. They excrete excess hydrogen into urine.
o If this mechanism fails, a lethal condition known as acidemia can result. A

higher than normal pH is known as alkalosis, which may be a sign of kidney failure.

Negative and Positive Feedback

• -  Maintaining homeostasis within cells is equally as important.

• -  The rates of metabolic reactions are controlled through feedback mechanisms that

  function at the molecular level. Negative Feedback (or feedback inhibition) occurs when the concentration of the product of a metabolic pathway increases enough to begin to inhibit the first step in the sequence of reactions that is dedicated to its own production.

• -  Positive feedback is also part of metabolism.

o In this instance, however, the result of an activity reinforces further change, rather

than diminishing it.
o A good example of positive feedback in nature is the co-evolution of a predator and

its prey. The predators that survive best are those that can capture prey; the prey

that survive best are those that can escape.
o Overtime, members of both species become more efficient in their struggle for

survival and both evolve together.

• -  Positive feedback is not regulatory in this sense as it promotes change rather than

  maintaining homeostasis.

Childbirth

• It takes place in organisms to bring change that will allow homeostasis to be restored.

• -  This is what happens during childbirth where the hormone oxytocin causes the

  muscles of the uterus to contract.
  o These muscle contractions, known as labour promote further oxytocin release,

  therefore causing further muscle contractions.

• o This cycle eventually results in birth.
  - Afterwards, the mother’s system returns to its normal metabolic pathway.