1. Positive and Negative Feedback
Systems
Positive and Negative Feedback

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

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

4. 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).

5. 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

6. 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

7. 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.)

8. 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.

9. 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

10. 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

11. 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?