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Regulatory Mechanisms and Behavior

Regulatory Mechanisms and Behavior

The teacher understands regulatory mechanisms and behavior.

Behavior and homeostatic regulation are vital to an organism’s health and survival. In this video and slideshow, Introduction to Animal Behavior, Deanne Erdmann, MS, discusses the types of behaviors which occur in animals and gives examples of them in the real world.

Dr. Wade Haaland defines homeostasis and uses blood glucose regulation and diabetes as an example of a feedback mechanism that maintains homeostasis in the human body.

Explanations of the science content behind each of the Descriptive Statements related to this competency are given in each of the following sections.


Responses to Internal and External Stimuli

The beginning teacher describes how organisms respond to internal and external stimuli.

Key Concepts:

  • Nearly all organisms have responses to stimuli, which are correlated to signal molecules and receptors. Hormones, such as auxin in plants or adrenaline in animals, are proteins that bind to cell membrane receptors to elicit a response in a target cell.
  • Single-celled organisms, such as bacteria, are able to detect and respond to changes int their environments, such as the presence of nutrients and changes in pH, temperature or light.
  • Plants often respond to external stimuli through growth. Plant growth responses to a stimulus are called tropisms. Some factors that stimulate tropic responses are light (phototropism), gravity (gravitropism), and physical contact (thigmotropism). Plants also respond to day length, temperature and other environmental factors.
  • Multicellular animals respond to stimuli through the nervous system, which contains many kinds of cells, including specialized cells called neurons. Neurons conduct messages throughout the body by using waves of positive ion influx and efflux to quickly carry signal charges.
  • Complex animals typically have billions of neurons, which can be organized into categories.
    • Central Nervous System: The command center of the body, which processes and responds to extremely complex signals. The CNS typically consists of an animal’s brain (along with the spinal chord in the case of vertebrates).
    • Peripheral Nervous System: All the neurons in the rest of the body. These can be categorized as: sensory neurons (carry signals from the body to the central nervous system) and motor neurons, which carry signals from the central nervous system to elicit a response. Motor neurons can be either: somatic, which are under the voluntary control of an animal (like skeletal muscle control); or autonomic, which act autonomously without an organism controlling them (like the heart beating).
  • Behavior encompasses anything that an organism does, including voluntary actions and responses to stimulation (internal or external). Instinctive behaviors in animals are inherited, and are organized and guided by circuits in the nervous system. Learned behaviors are acquired or modified by experience.


How do Organisms Respond to External and Internal Stimuli? This introductory article by Jacob Sigren explains and illustrates how organisms respond to stimuli at the cellular level.

Introduction to Animal Behavior. This video and accompanying slides with notes from BioEd Online provide an overview of instinctive and learned animal behaviors.

Plant Responses. See this article for more information about environmental factors and plant growth responses.

Bacterial Sensing. This article form Cornell describes sensing mechanisms in bacteria.

Processes that Maintain Stable Internal Conditions (Homeostasis)

The beginning teacher applies knowledge of structures and physiological processes that maintain stable internal conditions.

Key Concepts:

  • Homeostasis is the maintenance of a steady internal environment within a single cell or a multicellular organism.
  • Regulation of internal conditions through homeostasis allows an organism to function in a broad range of external environmental conditions.
  • Homeostasis can be maintained by hormones, the nervous system, or even behaviorally, as is the case with thermoregulation in some ectothermic animals.
  • Examples of homeostasis include blood glucose regulation in animals using hormones (insulin and glucagon) and water regulation in plants through pores in the leaves (stomata).
  • Many diseases, such as diabetes, dehydration or hypoglycemia, are the result of a disturbance of homeostasis.


Cell Homeostasis. This short article from eHow explains how single cells are able to maintain stable internal conditions by regulating transport of water and molecules across the cell membrane.

Plant Homeostasis. Plants also must maintain homeostatic conditions. The background information within this activity gives insight on how plants prevent water loss through pores in leaves known as stomata.

Regulation. Biology4Kids provides a brief introduction to homeostasis.

Feedback Mechanisms to Maintain Stable Internal Conditions

The beginning teacher demonstrates an understanding of feedback mechanisms that allow organisms to maintain stable internal conditions.

Key Concepts:

  • Animals with many specialized cells and organ systems need to maintain a relatively constant extracellular environment. Temperature, pH, concentrations of glucose and oxygen, for example, need to be held relatively constant for cells to function efficiently. Homeostasis is the maintenance of “dynamic constancy” of the internal environment. In other words, conditions continuously are regulated to remain within narrow limits.
  • Stable internal conditions in many organisms are maintained by negative and positive feedback loops.
  • Negative feedback loops act like a thermostat. Changes in the internal environment of an organism trigger a response to counteract the alteration and restore the system back to its set “normal” state. For example, if body temperature begins to rise, sensors in the brain detect the change and act via an integrating center in the brain to stimulate “effectors” (muscles or glands that can change the value of a condition), which in this case are sweat glands. Once a normal temperature is reached, a signal is sent to stop sweating.
  • Negative feedback mechanisms responsible for maintaining homeostasis frequently oppose each other in a “push-pull” fashion, in which the increasing activity of one effector is accompanied by decreasing activity in another effector.
  • Positive feedback mechanisms accentuate or amplify the body’s response to a disturbance. Blood clotting is an example of positive feedback, in which one clotting factor activiates another as part of a cascade of responses that lead to the formation of a blood clot.


Homeostasis in Vertebrate Animals. This article from Biology Mad thoroughly explains negative feedback and how it maintains homeostasis in a variety of ways in the human body.

Homeostasis and Hormones. The BBC produced this interactive tutorial on how conditions in the body are controlled to provide a constant internal environment.

How Evolutionary History Affects Behavior

The beginning teacher understands how evolutionary history affects behavior.

Key Concepts:

  • Behavior can be defined as the way an organisms responds to stimuli in its environment.
  • Animal behavior can be categorized into two groups. An innate behavior typically is carried out by all members of a species and is inherited from the organism’s parents. A learned behavior is acquired by an organism during its lifetime. Examples of learned behaviors include habituation (learning to not respond to a stimulus that is unimportant) and association (correlating one stimulus with another).
  • All innate behavior can be seen to have a proximate and an ultimate cause. Proximate causes involve hormonal secretions or neural impulses. For example, the heart pumps faster during a frightening situation because the adrenal glands secret adrenaline. However, ultimate causation involves the evolutionary history of an organism. In the case of adrenaline, it ultimately causes the heart to pump faster because this gives an animal more oxygen to move, react, and fight, allowing it to survive its frightening situation.
  • Animal behaviors are linked to survival in many ways, and contribute to fitness (the genetic contribution of an individual to succeeding generations). Social organization, altruism, communication, courtship and reproductive strategies, and selection of foods and habitats all are shaped by evolutionary forces.


The Diversity of Behavior. Behavior, which is fundamental to all organisms, is explored in this short article from Nature Education.

Introduction to Animal Behavior. This video and accompanying slides with notes from BioEd Online provide an overview of instinctive and learned animal behaviors.

Animal Behavior. This excerpt from McGraw-Hill provides details on animal behavior, with numerous examples of how behaviors are linked to genetic fitness and reproduction.

Genes and Environment Shape Behavior. This article from Nature Education explains how environment and genes interact to shape animal behavior.

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