What exactly do you know about positive and negative feedback loops? You might have heard something about biological systems and homeostasis. In fact, if you’re studying for the AP Biology exam, you probably hear these terms frequently. Still, what do they mean, and why do you need to understand how they work?
First, these definitions might sound a bit challenging. Or you might have the answers in your mind but find it difficult to explain them. If you relate to this, you are in the perfect place to refresh or complement your knowledge about positive and negative feedback loops in biology with straightforward explanations and real-life examples.
Feedback Loops on the AP Biology Exam
Feedback loops are the topic you can find in Unit 4 of your AP Biology course, also known as “Cell Communication and Cell Cycle.” Overall, questions from this section can make up to 15% of your total score.
You can face questions related to this topic in both the multiple-choice section and the free-response parts of the exam.
What topics does this unit include?
- Cell Communication
- Signal Transduction
- Changes in Signal Transduction Pathways
- Positive and Negative Feedback Loops
- Cell Cycle
- Regulation of Cell Cycle
What Are Feedback Loops and Homeostasis?
Source: GeeksForGeeks
Let’s start with the basics. Feedback loops and homeostasis are essential processes for all living organisms. What is the difference?
A feedback loop is a regulatory mechanism in biological systems. The essence of feedback loops is their ability to regulate and maintain stability within a system. How does it work? The output of a process influences the inputs, either positively or negatively. As a result, they affect the next outputs. Feedback loops are tightly connected to homeostasis. What is this?
Homeostasis refers to the ability of an organism or a system to maintain internal stability and balance despite external changes. It occurs in all living organisms (plants, animals, fungi, and bacteria).
What are the functions of these processes? Overall, they affect everything: metabolism, cell growth, photosynthesis, enzyme regulation, DNA replication, gene expression, natural selection, and even more.
While the concept of feedback loops is ancient and dates back to early biological studies, people started using this exact term in the late 19th and early 20th centuries. And this had nothing to do with biology. This idea was greatly developed in engineering and cybernetics. How is it related? In engineering, feedback loops are mechanisms that control a system by using information from its output to adjust its input.
Positive Feedback in Biology
Positive feedback loops are regulatory mechanisms that amplify a physiological response. They create a cascade of events that magnify the original signal. These loops are crucial for facilitating specific biological processes efficiently and swiftly. To better understand this phenomenon, let’s check several examples.
Example #1. Blood Clotting Cascade
Source: JPABS
When blood vessel damage occurs, platelets rush to this site. They release chemicals that attract more platelets. The new ones release extra chemicals, which further attract more platelets and initiate the clotting process. This positive feedback loop continues until the clotting process is complete. As a result, they seal the wound and prevent excessive bleeding.
Example #2. Action Potential in Neurons
Source: SlideServe
When a neuron receives a signal, it depolarizes and generates an action potential, which travels down the neuron’s length. This action results in the release of neurotransmitters at the synaptic terminals. These molecules bind to receptors on neighboring cells, which triggers them and generates new action potentials. Finally, it amplifies the initial signal and allows rapid communication between nerve cells. This process represents a positive feedback loop.
Example #3. Childbirth
Source: StoryMD
During labor, the mother’s body releases the hormone called oxytocin. It causes the muscles of the uterus to contract. They push the baby toward the birth canal, which stimulates the release of more oxytocin. With more oxytocin, the contractions become stronger and more frequent. This leads to further advancement of labor. This positive feedback loop continues until the mother delivers the baby.
Example #4. Lactation
Source: StoryMD
When a baby starts nursing, suckling stimulates nipple nerve endings, sending signals to the mother’s hypothalamus. In response, pituitary glands release oxytocin. This hormone contracts smooth muscles around milk-producing glands, which forces milk into the ducts, making it available for the baby to drink.
As a kid continues nursing and removes more milk from the breast, they stimulate the secretion of new oxytocin. This causes stronger and more frequent contractions of the mammary glands. The cycle continues until the baby stops nursing.
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Negative Feedback in Biology
Negative feedback loops maintain homeostasis by counteracting changes within biological systems. When a parameter deviates from its set point, biological systems activate responses that oppose the initial change. This counteraction aims to return the parameter to its optimal level, ensuring stability and balance. Let’s check several examples that describe a negative feedback loop in a different environment.
Example #1. Thermoregulation
Source: LibreTexts
Body temperature can rise above a set point. It could happen during exercise or exposure to heat. In this scenario, thermoreceptors in the skin and brain detect the change and signal the hypothalamus. It triggers responses such as vasodilation (expansion of the blood vessels) and sweating. These responses are necessary to release heat and cool the body down.
When the body temperature returns to the set point, the thermoreceptors signal the hypothalamus. It decreases these responses, maintaining homeostasis.
Example #2. Blood Glucose Regulation
Source: LetsTalkScience
After meals, blood glucose levels rise, triggering the release of insulin from the pancreas. It promotes the uptake of glucose by cells for energy or storage. As cells take up glucose, their blood levels decrease. When they drop below a certain threshold, the pancreas releases glucagon, which stimulates the release of glucose from storage sites like the liver. This negative feedback loop helps to keep blood glucose levels within a narrow, optimal range.
Example #3. Hormone Levels
Source: YourHormones
Negative feedback loops regulate the secretion of many hormones like cortisol. For example, when stressors trigger its release from the adrenal glands, its levels rise in the bloodstream. This inhibits the excretions of corticotropin-releasing hormone (CRH) and adrenocorticotropic hormone (ACTH) from the hypothalamus and pituitary gland, respectively. This process further reduces cortisol levels. So, it helps to maintain hormone levels within a balanced range.
Example #4. Calcium Concentration
Source: SlidePlayer
When blood calcium levels rise above a certain set point, C cells in the thyroid gland release calcitonin into the bloodstream. It inhibits the activity of osteoclasts. These cells break down bone tissue and release calcium into the blood. As a result, blood calcium levels decrease.
Conversely, when blood calcium levels drop below the set point, the parathyroid glands release parathyroid hormone (PTH). Once again, it stimulates osteoclasts’ activity, leading to bone tissue breakdown (bone resorption). This releases calcium into the blood.
Conclusion
Well, nature is fantastic, isn’t it? Now you know that the feedback loop is a fundamental mechanism for living creatures. Every single organism on Earth needs it to maintain homeostasis. We hope that our overview of the feedback loops was useful and informative. A good understanding of this topic will definitely make you feel more confident on the exam. Good luck!
