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WHAT IS HOMEOSTASIS? HOMEOSTASIS • the maintenance of a Biology constant internal environment in response to changes in: – the changing conditions of the external environment. – the changing conditions of the internal environment.

HOW IS HOMEOSTASIS HOW IS HOMEOSTASIS ACHIEVED? ACHIEVED?

To maintain cells, tissues and entire structural: organisms within their biological the animal or plant has particular tolerance limits, there are various physical features which help its mechanisms. survival in an otherwise hostile environment.

HOW IS HOMEOSTASIS HOW IS HOMEOSTASIS ACHIEVED? ACHIEVED?

functional: behavioral: the metabolism of the animal or plant is the actions and interactions of the able to adjust to changes in individual, either alone or with others, conditions as they are detected. help it to survive in its particular environment.

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FEEDBACK LOOPS FEEDBACK LOOPS • 4) Effector: The cells or tissue, usually a • Feedback mechanisms have certain essential components. gland or muscles, which cause the response to happen. • 1) Stimulus: The change from ideal or • 5)) Response: An action, at cell, tissue resting conditions. or whole organism level which would not • 2) Receptor: The cells or tissue which have occurred in the absence of the detects the change due to the stimulus. stimulus. • 3) Relay: The transmission of the message, via nerves or hormones or • 6) Feedback: The consequence of the both, to the effector. response on the stimulus. May be positive or negative.

FEEDBACK MECHANISMS Negative Feedback

Feedback mechanisms are the general mechanism of nervous or hormonal regulation in animals.

1. Negative feedback is when the response diminishes the original stimulus.

2. Positive feedback is when the response enhances the original stimulus.

Positive Feedback Negative feedback is most common in biological systems.

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Examples of Negative Feedback Examples of Negative Feedback

Ex. 1: Glucose and Insulin: Ex. 2: Exercise & sweating • Blood glucose concentrations rise after a • Exercise creates metabolic sugary meal (the stimulus), heat which raises the body • the hormone insulin is temperature (the stimulus), released and it speeds up • cooling mechanisms such as the transport of glucose out of the blood and into vasodilatation (flushed skin) selected tissues (the and sweating begin (the response), response), • so blood glucose • body temperature falls (thus concentrations decrease (thus decreasing the original decreasing the original stimulus). stimulus).

FEEDBACK MECHANISMS Positive feedback is less Feedback mechanisms are the general mechanism of nervous or hormonal common, which is regulation in animals. understandable, as most

1. Negative feedback is when the response changes to steady state pose diminishes the original stimulus. a threat, and to enhance them would be most unhelpful. 2. Positive feedback is when the response enhances the original stimulus.

Examples of Positive Feedback Summary

Ex. 1: Bad apples • Summarize homeostasis and feedback • A ripening apple releases the loops. volatile plant hormone • What is the difference between positive ethylene (the stimulus). and negative feedback loops? • Ethylene accelerates the ripening of unripe fruit in its vicinity so nearby fruit also ripens, releasing more ethylene (the response). • All the fruit quickly becomes ripe together.

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