1. LEQ: Why is homeostasis essential to living things?
Assignment: Reading
Focus on: hypotheses, molecular genetics, biotechnology
Review: characteristics of life, themes of bio
Activator: Thermostats
Discuss how a thermostat works. Add your explanation to your journal
Key terms: homeostasis, metabolism

2. The thermostat in your home operates on the principle of a feedback loop: information on temperature is gathered at the thermostat and when the temperature deviates away from a set point, a message is sent to the furnace/AC (target) to turn it on. The furnace/AC is shut off only when the temperature returns to the thermostat’s set point, creating a loop of information and action between the two parts of this system.

3. Organisms must maintain homeostasis to survive.
Homeostasis maintains a unique internal environment
Manages metabolism – internal chemistry
Coordinates cellular behavior
Set points, e.g. body temperature
Homeostasis involves a cell coordinating its internal chemistry with the environment to maintain conditions suitable for life. A cell’s metabolism must be carefully coordinated with other cells in multicellular organisms to distribute resources and manage changes away from physiological set points.
For example, when core temperature rises in humans, we release sweat from glands to eliminate excess heat. This process may continue indefinitely, sometimes at the expense of one’s own endurance in an effort to reduce adverse effects to cellular metabolism.
This process operates in much the same way that a thermostat does.

4. Organisms are regulated by messages from cells
System: skeleton
Chemical: serotonin
Message: “don’t panic”
~ 95% made in the gut
Regulates mood
Bones are a target!
Homeostasis is a principle that has very broad applications in biology. For example, the skeleton is an organ system that operates in a way similar to thermostat: microcracking will require repair that involves coordinating between osteoclasts and osteoblasts to repair damage in much the same way that a thermostat talks to furnace. Organ systems can also behave dysfunctionally when inappropriate communication occurs between parts.
Recently, a common biological message transmitted in bodies called serotonin has been described as having a role in causing dysfunctional regulation in the skeleton’s of mice. 95% of the molecules of serotonin released in the body are made by the gut and its roles in mood stabilization are well described.

5. Serotonin promotes osteoclast activity and bone resorption:
releases calcium
bone loss
Osteoporosis?
Recently, bones have been experimentally verified as a potential target of this message. This led to progressive bone loss in subjects in ways that suggest a potential role for this message in osteoporosis. The homeostatic message promotes osteoclast activity and bone resorbtion without promoting the classic mechanism of bone formation causing the skeleton to demineralize.
This research demonstrates the potential for a message to exist that promotes changes away from desired biological set points in the skeleton.

6. What are the characteristics of frogs?
How does homeostasis work in a frog?
Wood frogs are unique in that they are well-adapted to different biological set points for different environments. When winter arrives, the metabolism of wood frogs changes dramatically to enable them to overwinter in a cold environment. The arrest of organ function that they undergo is a major departure from the idea of maintaining constant internal conditions at all times in biological systems.
Many types of organisms have evolved ways of maintaining homeostasis that are flexible to deal with environmental extremes.

7. How does the action of serotonin disrupt homeostasis?
Summary:
Homeostasis is the maintenance of constant internal conditions. Why is it important to life?
How does the action of serotonin disrupt homeostasis?
Why are the set points of body temperature in wood frogs different than humans?
Reflection: “No organ is an island” ~ Gerard Karsenty. In what way or for what reason would a bone need to be attuned to the rest of the body?
Homeostasis is essential to life because constant internal conditions inside cells are very conducive to survival. See the thermostat analogy or sweating. However, messages also exist that promote changes in cells that move away from set points that are normally associated with constancy such as the actions of serotonin recently described. Frogs are also a wonderful example of how flexible the idea of homeostasis can be when organisms evolve unique solutions to deal with their environment – two distinct ways of surviving in seasonally changing environments of Pennsylvania.