The construction and function of feedback mechanisms at various levels of the biological hierarchy are the subjects of this portion of the AP Biology curriculum. We’ll start by defining feedback mechanisms and seeing how they affect individual processes inside an organism or bigger system. Then we’ll distinguish between positive and negative feedback systems.
We’ll look at how positive feedback mechanisms help a process come to a conclusion, while negative feedback mechanisms help systems return to a set point or preserve homeostasis. In order to better comprehend various sorts of feedback systems, we’ll look at numerous instances (both biological and non-biological). For example, we’ll look at how two different negative feedback mechanisms maintain blood glucose levels and how positive feedback mechanisms govern lactase enzyme synthesis in E. coli bacteria.
Cells employ feedback systems to adjust to changing internal and external situations, in the same way that a company gets feedback on its services. Positive feedback says “do more of it,” but negative feedback indicates “stop what you’re doing.” A cell’s feedback mechanisms for maintaining homeostasis are quite similar.
Cells use a variety of feedback systems. In reality, cells are constantly receiving both positive and negative signals. Certain feedback systems are in place to encourage good responses, while others are in place to prevent crucial molecules from being overproduced by certain cellular pathways. You must have a deep understanding of these ideas in order to pass the AP Test. So read on to find out all there is to know about feedback mechanisms!
Let’s begin with the definition of feedback mechanisms. When the final result of a system or process operates to control the behaviour of that same process by suppressing or activating different components, this is known as a feedback mechanism. Positive and negative feedback systems are the two kinds of feedback mechanisms. These phrases are not the same as “good” and “bad,” so don’t get them mixed up.
Types of feedback mechanisms.
Positive feedback mechanisms: the final result of a biological process is used to trigger or restart a cellular process in positive feedback systems.
Negative feedback mechanisms: a negative feedback system, on the other hand, employs a process’s end products to shut it down.
All feedback systems strive to keep the cell in a state of homeostasis. To understand these two sorts of feedback systems, let’s look at a basic, non-biological process.
Take a look at a pressure cooker. To prepare your food, this basic kitchen appliance employs both positive and negative feedback systems. A heating element, a sealed cooking pot, and a pressure sensor in the lid are all used in pressure cookers. The liquid inside the pot rises to a boil as soon as the heating element is turned on. It begins to build up pressure within the cooker since it is sealed. While this method speeds up the cooking process, it is also risky. If the heat is kept on the whole time, the gadget may explode. This is why feedback systems are required in pressure cookers.
When the pressure sensor detects low pressure, it sends a signal to the computer, which causes the heating element to produce more heat. This is a kind of positive feedback because it instructs the heating process to keep going. The pressure sensor, on the other hand, provides a fresh signal when the pressure within the pot begins to rise. This shuts off the heat, preventing the pot from boiling and allowing the pressure cooker to keep its pressure without bursting. Negative feedback prevents the process from continuing, allowing the pressure cooker to maintain equilibrium.
Feedback loops at all levels of biology
Consider this… At all levels of life, feedback loops exist, and some are more valuable than others. Have you ever encountered someone who is uncomfortable, flushes, and then becomes even more humiliated as a result of their blushing? This is a form of psychological positive feedback loop that has no obvious function and may be rather unpleasant if you’re the one blushing!
On the other hand, when your blood glucose levels begin to fall, the pancreas is stimulated to produce the hormone glucagon. Glucagon instructs the liver to convert glycogen, a storage molecule, back to glucose and release it into the bloodstream. This is also a negative feedback process, since the glucagon release ultimately blocks the low blood sugar signal that triggered the glucagon release in the first place.
It’s not uncommon for two negative feedback mechanisms to operate on the same system that needs to stay at a certain set point. One negative feedback mechanism kicks in when the level is too high, while the other negative feedback mechanism kicks in when the level is too low.
Positive feedback mechanisms, on the other hand, are used when a process has to reach a specific threshold in order to accomplish a larger activity. Childbirth is a fantastic illustration of this. When a baby’s head reaches a specific size in the uterus, it presses on the cervix. This is a trigger that causes the mother’s circulation to produce the hormone oxytocin. As a result, oxytocin triggers uterine contractions, which continue to press the baby’s head into the cervix. Because the release of oxytocin stimulates the production of additional oxytocin, this is a positive feedback process. The cervix is completely dilated, and the contractions are powerful enough to force the baby out of the birth canal!
While positive and negative feedback systems are extremely diverse, they both assist an organism in performing specified tasks by coordinating the timing and coordination of distinct sections of the body.
In biological organisms, positive feedback systems accelerate reactions and processes. The variable that triggers the response is shifted farther away from the original set point until the operation is finished.
Examples of positive feedback mechanism
Consider the coagulation of blood, which is required when an organism is injured. When cells in the wall of a blood vessel are harmed, chemical signals are released. As a result, platelets in the circulation become activated and recruited. As a result, these platelets emit even more chemical signals, causing even more platelets to be recruited. This process continues until a complete clog of platelets has developed, and the blood artery is no longer open. Until the process of producing a platelet plug is completed, this positive feedback mechanism efficiently releases signals.
Consider the following scenario, which occurs at the molecular genetics level: The method in which E. coli bacteria manufacture enzymes to break down lactose is based on a positive feedback loop. When a cell lacks lactose, a repressor molecule remains connected to the lactase genes, preventing RNA polymerase from producing an mRNA transcript.
In practise, this implies that the bacterial cell is unable to produce the three proteins required for the lactose sugar to be broken down and used. This is advantageous to the cell since the cost of producing superfluous proteins is high, leaving less energy for growth and reproduction.
When lactose is present, however, a small percentage of it spontaneously breaks down into the isomer allolactose. Allolactose functions as an inducer, preventing the repressor from attaching to the DNA by binding to it. This implies that RNA polymerase can bind to DNA, transcribe genes, and make the proteins required for lactose sugar processing.
This is when the positive feedback becomes useful. Lactose is broken down by the beta-galactosidase enzyme (produced by the lacZ gene). However, it also transforms some lactose into allolactose, which binds to the gene repressor! As long as this enzyme is still breaking down lactose, these genes will continue to be expressed.
Examples of Negative feedback mechanism
Negative feedback systems regulate physiological processes to maintain homeostasis in certain circumstances. Negative feedback methods restore a system to its goal set point when it is disrupted.
Feedback inhibition is a relatively prevalent negative feedback mechanism. This is an ingenious molecular feedback mechanism seen in a variety of biological systems. An enzyme catalyses a process that initiates a metabolic pathway in this kind of negative feedback. The metabolic pathway’s final product serves as an inhibitor of the initial enzyme. The enzyme is prevented from catalysing the original substrate by an inhibitor, essentially terminating the metabolic process.
On the organismal level, however, there are several excellent instances of negative feedback. Consider how most animals regulate their body temperatures. This process, like blood sugar management, is governed by a number of negative feedback systems. Two systems kick in when the body becomes too heated. Capillaries dilate, allowing heat to pass through the skin. Furthermore, sweat glands open and perspiration evaporates from the skin, allowing heat to escape into the atmosphere. Both of these feedback processes result in the elimination of the heat stimulus that triggered them in the first place.
On the other hand, when the body becomes cold, the opposing feedback systems kick in. To limit heat loss to a minimum, capillaries constrict near the skin. Sweat glands constrict and hairs rise, trapping warm air near the skin. These processes, like the negative feedback mechanisms on the flip side of thermoregulation, lead to the elimination of the stimulus that triggered them in the first place!