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4.4Cell Communication and Cell Cycle

Feedback — AP BIOLOGY Revision Notes & Topical Questions

5 min read 12 parts Lesson 4 of 6 2 quizzes

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4.4 Feedback — AP Biology notes: Explain how positive and negative feedback help maintain homeostasis.Homeostasis = maintenance of a relatively stable internal environment around a set point.Feedback mechanisms — organisms use them to maintain internal conditions in response to i…

  • Control of signalling
  • Blood glucose
  • Thermoregulation
  • Cellular level
  • Amplification
  • Other examples

Section 1 of 12Homeostasis and feedback

Homeostasis
Homeostasis maintenance of a relatively stable internal environment around a set point.
Feedback mechanisms
  • Feedback mechanisms organisms use them to maintain internal conditions in response to internal and external changes.
Levels
  • Levels they operate at the molecular, cellular and organism levels.
Balance
  • Balance most homeostatic systems use negative feedback, and positive feedback drives processes to completion.
Feedback
  • Feedback the output of a system influences its own input
Control of signalling
  • Control of signalling feedback also controls how strong and how long a signalling response lasts

Section 2 of 12Negative feedback

Principle
  • Principle a change from the set point triggers a response that reduces the initial stimulus and returns the variable to the set point.
Loop
  • Loop stimulus, receptor/sensor, control centre, effector, response that reduces the stimulus, back to the set point.
Blood glucose
  • Blood glucose high glucose makes pancreatic beta cells secrete insulin, cells take up glucose and the liver stores glycogen, so glucose falls.
  • Low glucose makes alpha cells secrete glucagon, the liver breaks down glycogen and glucose rises.
Thermoregulation
  • Thermoregulation high body temperature triggers sweating and vasodilation;
    • low temperature triggers shivering and vasoconstriction.
Cellular level
  • Cellular level end-product (feedback) inhibition of metabolic pathways.

Section 3 of 12Positive feedback

Principle
  • Principle the response amplifies the change, moving the variable further from its starting point until an endpoint is reached.
Amplification
  • Amplification the stimulus is further intensified, which starts an additional response that produces change.
Labor
  • Labor contractions cause more oxytocin release, which gives stronger contractions.
Other examples
  • Other examples blood clotting, fruit ripening by ethylene and lactation.

Section 4 of 12Negative feedback loop in order

1
A variable changes from its set point (stimulus).
2
A receptor detects the change.
3
The control centre compares it with the set point and sends a signal.
4
An effector produces a response.
5
The response opposes the change, returning the variable to the set point, and the signal decreases.

Section 5 of 12Disrupted feedback

Disrupted feedback
Type 1 diabetes
  • Type 1 diabetes no insulin is made.
Type 2 diabetes
  • Type 2 diabetes cells are unresponsive to insulin.
Graves' disease
  • Graves' disease overproduction of thyroid hormone because feedback fails.
Dehydration
  • Dehydration water balance feedback is disrupted.
Hyperthyroidism (Graves' disease)
  • Hyperthyroidism (Graves' disease) high thyroid hormone does not properly shut down TSH

Section 6 of 12Negative and positive feedback compared

FeatureNegative feedbackPositive feedback
ResponseOpposes the changeAmplifies the change
ResultStabilityCompletion of a process
FrequencyCommonRare
ExampleBlood glucose (insulin, glucagon)Oxytocin in labor
Ends whenSet point is restoredEndpoint is reached

Section 7 of 12More examples of negative feedback

1Blood glucose
  • Insulin binds target-cell receptors, cells take up glucose, the liver converts glucose to glycogen and glucose metabolism increases.
  • Glucagon converts glycogen to glucose and other molecules into glucose.
2Antagonistic hormones
  • Antagonistic hormones pairs such as insulin and glucagon give control in both directions, so the variable oscillates around the set point
3Body temperature
  • Body temperature hair erection occurs in the cold
4Metabolic pathways
  • Metabolic pathways end-product inhibition of enzymes, and thyroxine inhibiting release of TSH
5Osmoregulation
  • ADH secretion is adjusted to the water potential of the blood
6ATP
  • ATP it inhibits phosphofructokinase in glycolysis, so respiration slows when ATP is plentiful

Section 8 of 12More examples of positive feedback

Limit
  • Limit it is unstable unless an end point stops it, so it is used for rapid, complete events
Childbirth
  • Childbirth oxytocin from the pituitary makes uterine muscle contract, and stretch receptors in the cervix signal for more oxytocin,
  • so contractions intensify until birth
Fruit ripening
  • Fruit ripening ethylene from ripening fruit diffuses to neighbouring fruit and triggers further ethylene release
Lactation
  • Lactation suckling causes prolactin release, which gives milk production and more suckling
Blood clotting
  • Blood clotting activated platelets attract and activate more platelets
Action potentials
  • Action potentials depolarisation opens voltage-gated Na⁺ channels, letting in more Na⁺ until threshold is passed

Section 9 of 12Graphs and describing a loop

Negative-feedback graph
  • Negative-feedback graph a wave around a mean, with effector changes lagging behind the variable
Positive-feedback graph
  • Positive-feedback graph a steep rise with no levelling until the end point
Describing a loop
  • Describing a loop name the stimulus, sensor, effector and response, and say whether the response reduces or increases the stimulus

Section 10 of 12More from the specification

1
Signal transduction activating target genes
  • Signal transduction activating target genes ligand binding stimulates intracellular kinase cascades that phosphorylate transcription factors to regulate the expression of specific target genes.
2
Adenylyl cyclase
  • The enzyme that converts ATP to cyclic AMP.
3
Direct contact
  • Spread of pathogens by blood or other body fluids from one person to another.
4
Pituitary gland
  • The master gland whose hormones control other glands.
5
Transduction
  • Transfer of DNA between bacteria by bacteriophages
6
Ligand
  • A signalling molecule that binds a specific receptor.
7
Specific
  • Able to act on only one type of substrate.
8
Development
  • The changes as an organism becomes more complex and mature.
9
Cascade
  • A series of reactions in which each step activates many molecules in the next, amplifying the signal.
10
Signal transduction
  • Converting and relaying a signal inside the cell to a response.
11
Population density
  • Number of individuals per unit area or volume.

Section 11 of 12Key terms

TermMeaning
Homeostasismaintaining stable internal conditions
Negative feedbackresponse opposes the change
Positive feedbackresponse amplifies the change
Set pointtarget value
Sensordetects the change
Effectorcarries out the response

Section 12 of 12Exam technique

1
Describing negative feedback as making a change bigger — it reverses the change.
2
Giving childbirth as negative feedback — it is positive feedback ending at birth.
3
Missing out the sensor or effector when describing a loop.
4
Saying positive feedback maintains homeostasis — it amplifies change and needs an end point.
5
Calling negative feedback harmful — it means the response opposes the change.

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