Positive and Negative Feedback Systems Positive and Negative Feedback

Systems A system is a group of parts that work together to perform a function or produce a result Components Input Process Output Feedback

Example System Is this a system? Group of parts? Working together to perform a function? YES! What is the input? What is the output?

Negative Feedback To get back to a specific target point Negative feedback acts like a stabilizer in a system. There is a “set” or “start” point that the system maintains. When you drive by a speed guilt sign, most drivers will utilize the gas pedal to slow down (letting off of gas pedal) or speed up (pressing down on the gas pedal) to try and maintain the speed limit set point (example shows 25 MPH and the driver’s speed hovering just around that 25 MPH).

A Nonliving Example of Negative Feedback: Control of Room Temperature Response No Heat Produced Room Temperature Decreases Heater turned off Set point Too hot Set point Control center: thermostat Increases on cold Heat

Negative Feedback: Maintenance of Glucose Homeostasis by Insulin and Glucagon Beta cells of pancreas are stimulated to release insulin into the blood. Insulin Liver takes up glucose and stores it as glycogen. Body cells take up more glucose. Blood glucose level declines to set point; stimulus for insulin release diminishes. STIMULUS: Rising blood glucose level (for instance, after eating a carbohydrate- rich meal) Homeostasis: (about 90 mg/100 mL) rises to set point; stimulus for glucagon Dropping blood glucose skipping a meal) Alpha cells of pancreas are stimulated to release glucagon into the blood. Liver breaks down glycogen and releases glucose into blood. Glucagon

Positive Feedback Over amplification and moves away from the target point If you decide that you want to be Speed Racer and go beyond the speed limit set point, you are an example of positive feedback. Positive feedback is an amplification (increase) that moves away from the set point, usually disrupting the system. Other examples of PF are speaker or microphone feedback where you hear that ear splitting high pitched squealing or low groans. A great example of positive feedback (moving away from a set point) is at a concert. The sound crew comes in before the concert and adjusts all of the sound settings to a specific set point (volume, treble, bass, etc.) so that the concert sounds great from the fans’ perspective. However, back in the early 1990’s Grunge music often utilized distortion in their music and positive feedback, via speakers, was a great way to do this. (Click on video link of Nirvana: Show students that at the beginning of the video, the music sounds good because the set point is being maintained in the sound system – a negative feedback loop – the sound system is stable. Then fast forward the video to 7:10. This is where the singer starts creating a positive feedback system where he amplifies the sound causing a lot of distortion within the sound system – groaning and squealing from the speakers.)

Positive Feedback (Fruit) Ethylene Gas We have an apple tree and a ripe apple. We also have a gas molecule that called “Ethylene Gas.” When fruits and vegetables ripen, they give off ethylene gas. As this apple ripens, it gives off more and more ethylene gas. The more gas, the faster the apple ripens and the more it ripens the more gas given off. The apple will never go back to its start point of being unripen. In fact, it will ripen until it spoils (rots). The increase in ethylene gas production is an amplification – which is a positive feedback loop. Remember, positive feedback moves away from the set point. What would happen if you threw a ripe or rotten apple into a bag of unripe apples? The ripe apple would give off ethylene gas and that gas triggers the unripe apples to begin ripening. This is where the saying “One bad apple spoils the bunch” originated.

An Example of Positive Feedback: the Induction of Labor Estrogen Oxytocin from ovaries from fetus and mother's posterior pituitary Induces oxytocin receptors on uterus Stimulates uterus to contract Stimulates placenta to make Prostaglandins Stimulate more contractions of uterus Positive feedback

Temperature Regulation Feedback Flow Chart Homeostasis Feedback Alteration Negative Positive Diabetes mellitus Stabilization Amplification Temperature Regulation Ask students what they understand about the concept of homeostasis (we define it next slide). Students should understand that homeostasis is maintained through negative and positive feedback systems (mostly negative). Giving birth is a positive feedback example. An example for each is given – Negative: temperature regulation in mammals Positive: fruit ripening (positive feedback is usually fast and doesn’t return to the set point) Does homeostasis work all of the time? Usually, yes, but sometimes things happen to a system that causes problems. An example, within our bodies, of an alteration would be diabetes. Typically, someone with diabetes must add insulin to their system in order to maintain homeostasis because the body/pancreas is either unable to produce insulin or the system is not sensitive to the insulin hormone. Fruit Ripening

Engineering Practices Two main ideas when designing in engineering Criteria – the rules or directions that must be followed; the requirements that must be met Constraints – restrictions that keep something from being the best it can be; problems that arise or issues that come up. What were the criteria and constraints in our design challenge?