This section of the AP Biology curriculum focuses on the structure and function of feedback mechanisms within different levels of the biological hierarchy. First, we will define feedback mechanisms and learn how they modulate specific processes within an organism or larger system. Then, we’ll separate positive feedback mechanisms from negative feedback mechanisms. We’ll see how positive feedback mechanisms accelerate a process to some sort of conclusion, whereas negative feedback mechanisms tend to return systems to a setpoint or maintain homeostasis. We’ll look at several examples (both biological and non-biological) in order to better understand different types of feedback mechanisms. For example, we’ll see how blood glucose levels are maintained by two separate negative feedback mechanisms and how positive feedback mechanisms regulate the lactase enzyme production in E. coli bacteria.

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ENDURING UNDERSTANDING
ENE-3
Timing and coordination of biological mechanisms involved in growth, reproduction, and homeostasis depend on organisms responding to environmental cues.

LEARNING OBJECTIVE
ENE-3.A
Describe positive and/or negative feedback mechanisms.

ENE-3.B
Explain how negative feedback helps to maintain homeostasis.

ENE-3.C
Explain how positive feedback affects homeostasis

ESSENTIAL KNOWLEDGE
ENE-3.A.1
Organisms use feedback mechanisms to maintain their internal environments and respond to internal and external environmental changes.

ENE-3.B.1
Negative feedback mechanisms maintain homeostasis for a particular condition by regulating physiological processes. If a system is perturbed, negative feedback mechanisms return the system back to its target set point. These processes operate at the molecular and cellular levels.

ENE-3.C.1
Positive feedback mechanisms amplify responses and processes in biological organisms. The variable initiating the response is moved farther away from the initial set point. Amplification occurs when the stimulus is further activated, which, in turn, initiates an additional response that produces system change.

4.5 Feedback Overview

The feedback mechanisms cells use to adapt to changing internal and external conditions are exactly like the feedback a business receives on its services. Negative feedback means “stop what you are doing,” whereas positive feedback means “do more of that.” The feedback mechanisms a cell uses to maintain homeostasis are very similar.

Cells use many different feedback mechanisms. In fact, cells receive both positive and negative feedback on a regular basis. Certain feedback mechanisms are there to promote beneficial responses, while other feedback mechanisms are in place to stop certain cellular pathways from overproducing important substances. In order to pass the AP Test, you will need to have a thorough understanding of these concepts. So, keep reading to learn everything you need to know about feedback mechanisms!

Let’s start by defining feedback mechanisms. Feedback mechanisms happen when the end product of a system or process acts to modulate the behavior of that same process by inhibiting or activating various components. There are two types of feedback mechanisms, positive and negative. Don’t get confused by these terms – they are not the same as “good” and “bad.”

Positive feedback mechanisms use the end product of a biological process to stimulate or restart a cellular process. By contrast, a negative feedback mechanism uses the end products of a process to shut down that process. The ultimate goal of all feedback mechanisms is to maintain the cell in homeostasis. Let’s take a look at a simple, non-biological process to explain these two types of feedback mechanisms.

Consider a pressure cooker. This simple kitchen device uses both positive and negative feedback mechanisms to cook your food. Pressure cookers rely on a heating element, a cooking pot that is sealed within the device, and a pressure sensor in the lid. As the heating element kicks on, the liquid inside the pot comes to a boil. Since it is sealed inside the cooker, it starts to build up pressure. While this pressure cooks your food faster, it is also dangerous. The whole device could explode if the heat was left on the entire time. This is why a pressure cooker needs feedback mechanisms.

When the pressure sensor senses low pressure, it sends off one type of signal to the computer that increases the heat from the heating element. This is a form of positive feedback because it tells the heating process to continue. However, when the pressure starts to build up within the pot, the pressure sensor sends a new signal. This turns the heat off, which stops the pot from boiling and allows the pressure cooker to maintain pressure without exploding. This is negative feedback because it stops the process from continuing, which allows the pressure cooker to maintain homeostasis.

Think about this… there are feedback loops at all levels of biology, and some of them are more useful than others. Have you ever met someone who gets embarrassed, blushes, then gets more embarrassed because they are blushing? This is a type of psychological positive feedback loop that doesn’t serve a clear purpose and can be very uncomfortable if you are the one blushing!

To understand the purpose of feedback mechanisms in biological systems, let’s consider how positive and negative feedback mechanisms actually work in biology. Let’s take a look at how the body keeps glucose levels in a specific range using two negative feedback loops. When you eat a meal, the glucose levels in your blood rise. The rise in glucose levels is the stimulus for the release of insulin. Insulin travels from the pancreas to the liver and other cells throughout the body. In turn, these cells take up large amounts of glucose. This lowers the glucose levels in the bloodstream, which stops the release of insulin. In other words, the release of insulin causes the release of insulin to stop, in a clear negative feedback mechanism.

On the flip side, when your blood glucose levels start to drop this acts as a stimulus to the pancreas to release the hormone glucagon. Glucagon tells the liver to convert the storage molecule glycogen back into glucose and to release that glucose into the blood. This is also a negative feedback mechanism since the release of glucagon eventually stops the low blood sugar signal that started the release of glucagon.

In fact, it is very common for two negative feedback mechanisms to operate on the same system that needs to be maintained at a specific setpoint. One negative feedback mechanism kicks in when the level is too high, while the other negative feedback mechanism kicks in when the level gets too low.

By contrast, positive feedback mechanisms are employed when a certain process needs to proceed to a certain threshold so that a greater action can be completed. A great example of this is childbirth. When a baby grows to a certain size in the uterus, its head will start to press on the cervix. This is a stimulus that releases the hormone oxytocin into the mother’s bloodstream. In turn, oxytocin causes contractions in the uterus, which continue to push the baby’s head into the cervix. This is a positive feedback mechanism because the release of oxytocin causes more oxytocin to be released. This process continues to build and build until the cervix is fully dilated and the contractions are strong enough to push the baby out of the birth canal!

While positive and negative feedback mechanisms are very different, they both contribute to the timing and coordination of different parts of an organism that help it complete specific tasks.

Positive feedback mechanisms amplify responses and processes in biological organisms. The variable initiating the response is moved farther away from the initial setpoint until the process reaches a point of completion.

Consider the process of blood clotting, necessary when an organism sustains an injury. When cells in the wall of a blood vessel become injured, they begin releasing chemical signals. In turn, this activates and recruits platelets within the bloodstream. In turn, these platelets release even more chemical signals to recruit even more platelets. This process continues until an entire plug of platelets is formed and there is no longer a hole in the blood vessel. This positive feedback mechanism effectively releases signals until the process of forming a platelet plug is completed.

Let’s consider an example that takes place on the level of molecular genetics. The system that produces enzymes to breakdown lactose in E. coli bacteria functions on a system of positive feedback. When there is no lactose in a cell, a repressor molecule stays attached to the lactase genes and prohibits RNA polymerase from creating an mRNA transcript. In effect, this means that the bacterial cell cannot create the three proteins it needs to successfully break down and utilize the lactose sugar. This is beneficial to the cell because creating unnecessary proteins is energetically expensive and leaves less energy for growth and reproduction.

However, when there is lactose present, a small amount of the lactose spontaneously breaks down into the isomer allolactose. Allolactose acts as an inducer, binding to the repressor molecule and preventing it from binding to the DNA. This means that RNA polymerase can bind to the DNA, transcribe the genes, and produce the proteins necessary to process lactose sugar. Now, here’s where the positive feedback comes into play. The beta-galactosidase enzyme (produced by the lacZ gene) breaks down lactose. But, it also converts some of the lactose into allolactose – the molecule that binds to the gene repressor! This ensures that these genes will continue to be expressed as long as this enzyme is still breaking down lactose.

Negative feedback mechanisms maintain homeostasis for a particular condition by regulating physiological processes. If a system is perturbed, negative feedback mechanisms return the system back to its target set point.

A very common negative feedback mechanism is known as feedback inhibition. This is a very clever molecular feedback mechanism that is present in many biological systems. In this form of negative feedback, an enzyme catalyzes a reaction that starts a metabolic pathway. The end product of this metabolic pathway acts as an inhibitor of the original enzyme. This inhibitor blocks the enzyme from catalyzing the original substrate, effectively stopping the entire metabolic pathway.

However, there are also good examples of negative feedback on the organismal level. Consider the process of thermoregulation in most mammals. Like blood sugar regulation, this process is regulated by multiple negative feedback mechanisms. If the body gets too hot, two mechanisms kick in. The capillaries dilate, spreading heat through the skin. Plus, the sweat glands open up and sweat evaporates off the skin, causing heat to escape into the air. Both of these feedback mechanisms lead to the end of the heat stimulus that caused them.

By contrast, when the body gets cold the opposite feedback mechanisms kick in. Capillaries constrict near the skin, to keep heat loss to a minimum. Sweat glands close and hairs stand up, trapping warm air next to the skin. Like the negative feedback mechanisms on the other side of thermoregulation, these processes lead to an end of the stimulus that caused them in the first place!