4.5: Feedback

In topic 4.4, we explored the factors that activate or inhibit signal transduction pathways. Topic 4.5 explains how cells use signal boosting or inhibition to accomplish their specific functions.

Vocab List

• Homeostasis
  • Negative feedback
  • Feedback inhibition
  • Insulin
    • Glucagon
    • Diabetes
• Positive feedback

Written Explanation

Homeostasis:

Homeostasis is a state of balance within a cell or organism's internal environment. When in homeostasis, an organism or cell is at its optimal functioning point. Organisms constantly resist environmental changes in order to reach homeostasis, trying to keep their internal environment independent of external factors like temperature and humidity.

Negative Feedback:

An organism's most important tool for maintaining homeostasis is the negative feedback loop. These loops act to counteract or reset changes in the current environment. Generally speaking, this means that the outcome of a process has the opposite effect as the trigger for that system. For example, when your body overheats, it begins to sweat. Sweating, in turn, causes the body to cool back down, removing the initial trigger for sweating. The name negative feedback comes from the idea that the result of the process negatively affects the rate of reaction of that process. Feedback inhibition is a type of negative feedback loop, in which the product of an enzymatic pathway disables the operation of that pathway.

Furthermore, negative feedback loops often come in pairs. In the case of the temperature feedback loops, this means that there is one pathway that resists the overheating of the body, and one pathway that resists the over-cooling of the body. When in homeostasis, these processes are both shut down (due to a lack of environmental stimuli).

Another important example of negative feedback (for this AP class) is the body's regulation of blood sugar. Insulin (peptide hormone) causes a decrease in blood sugar, meaning that glucose is absorbed out of the bloodstream. Insulin does this by stimulating the formation of glycogen from glucose, which results in glycogen being stored in the liver, and by encouraging other cells to take in sugar.

Conversely, when blood sugar is too low, glucagon (another peptide hormone) causes an increase in blood sugar, meaning that glucose enters into the bloodstream. Glucagon does this by stimulating the breakdown of that stored glycogen (in the liver) into usable glucose.

In combination, insulin and glucagon act as a pair of negative feedback loops that trigger when there is a chemical imbalance in the body and act to return it back to homeostasis. Individuals with diabetes have a low production of insulin, meaning that their cells don't take up enough glucose to operate (remember that insulin encourages cells to take up glucose from the bloodstream)

Positive Feedback:

As may make sense, positive feedback loops have the opposite goal as negative feedback loops. A positive feedback loop has the output amplify the initial signal received. For example, ripe apples produce the gas ethylene. This chemical then signals to other nearby apples that it's time to ripen, causing a cascade of ripening plants. In other words, the outcome of a process positively affects the rate of that reaction, and the stimulus is amplified.