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Edexcel International A Level Biology

Revision Notes

Home / International A Level / Biology / Edexcel / Revision Notes / 7. Respiration, Muscles & the Internal Environment / The Internal Environment / 7.16 Negative & Positive Feedback


7.16 Negative & Positive Feedback


Negative & Positive Feedback

Negative feedback

  • The majority of homeostatic control mechanisms in organisms use negative feedback to maintain homeostatic balance, i.e. to keep certain physiological factors, such as internal temperature or blood glucose concentration, within certain limits
  • Negative feedback control loops involve
    • A receptor detects a stimulus that is involved with a physiological factor
      • E.g. a change in temperature or blood glucose level
    • A coordination system transfers information between different parts of the body
      • This could be the nervous system or the hormonal system
    • An effector carries out a response
      • Effectors are muscles or glands
  • The outcome of a negative feedback loop is
    • If there is an increase in the factor the body responds to make the factor decrease
    • If there is a decrease in the factor the body responds to make the factor increase
  • Negative feedback systems work by reversing a change in the body to bring it back within normal limits, e.g.
    • If body temperature rises a negative feedback system will act to lower body temperature, bringing it back to normal
    • If blood glucose levels drop a negative feedback system will act to raise blood glucose, bringing it back to normal

Negative feedback loop

Negative feedback loops involve the monitoring of physiological factors and act to reverse any changes, keeping the factors within normal limits. Information can be transferred via nerve signals, as shown here, or by hormonal signals.

Positive feedback

  • In positive feedback loops the original stimulus produces a response that causes the factor to deviate even more from the normal range
    • They enhance the effect of the original stimulus
  • An example of this is the dilation of the cervix during labour
    • The cervix stretches as baby pushes against it
    • Stretch receptors in the cervix are stimulated and send impulses to the brain
    • The pituitary gland is stimulated to release oxytocin which increases the intensity of uterine contractions
    • This pushes the baby further down the birth canal and stretches the cervix even further
  • Positive feedback loops are useful to quickly activate a process, e.g. blood clotting to close up a wound
    • When the body is injured, platelets become activated
    • They release chemicals which will activate more platelets, which in turn, will release chemicals that will activate even more platelets etc.
    • This ensures that the wound is quickly closed up by a blood clot before too much blood is lost or too many pathogens enter the bloodstream
    • The body will revert to negative feedback mechanisms once the blood clot has formed
  • Positive feedback may also kick in when homeostatic mechanisms break down
    • E.g. during prolonged exposure to extreme cold hypothermia can occur; body temperature drops, resulting in decreased metabolism which in turn causes body temperature to drop further
  • Since these mechanisms do not maintain a constant internal environment, they are not involved in homeostasis

Negative Feedback in Maintaining Conditions

The control of negative feedback

  • Negative feedback loops help maintain a normal range or balance within an organism
    • They reduce the initial effect of the stimulus
  • Receptors detect any deviations in a factor from the normal range; this results in a corrective mechanism to return the factor back to its normal range
  • In negative feedback loop there are usually two corrective mechanisms
    • One for when the factor becomes too low
    • One for when the factor becomes too high
  • The corrective mechanisms may involve the nervous system or the endocrine system
  • The magnitude of the correction required to bring a factor back within its normal range is monitored and regulated by negative feedback
    • As the factor gets closer to its normal value the level of correction reduces

Control of negative feedback

Two corrective mechanisms are involved in the negative feedback loop



  • 1. Molecules, Transport & Health
    • Biological Molecules
      • 1.1 The Importance of Water
        • 1.2 Saccharides
          • 1.3 Core Practical 1: Estimating the Concentration of Sugars & Starch
            • 1.4 Condensation & Hydrolysis
              • 1.5 Triglycerides & Ester Bonds
              • The Circulatory System
                • 1.6 The Need for a Circulatory System
                  • 1.7 Blood Vessels: Structure & Function
                    • 1.8 The Cardiac Cycle
                      • 1.9 The Role of Haemoglobin
                        • 1.10 Atherosclerosis
                          • 1.11 Blood Clotting
                          • Diet & Health
                            • 1.12 Reducing Risk Factors of CVD
                              • 1.13 Dietary Antioxidants & CVD
                                • 1.14 Core Practical 2: Investigate the Vitamin C Content of Food & Drink
                                  • 1.15 Interpreting Data on Risk Factors
                                    • 1.16 Designing Studies into the Effects of Risk Factors
                                      • 1.17 Perception of Risk vs Actual Risk
                                        • 1.18 Data on Cholesterol & Lipoproteins
                                          • 1.19 Data on Effect of Diet
                                            • 1.20 Treatments for CVD: Benefits & Risks
                                          • 2. Membranes, Proteins, DNA & Gene Expression
                                            • Gas Exchange, Cell Membranes & Transport
                                              • 2.1 Properties of Gas Exchange Surfaces
                                                • 2.2 Cell Membranes
                                                  • 2.3 Core Practical 3: Investigating Membrane Structure & Permeability
                                                    • 2.4 Osmosis
                                                      • 2.5 Diffusion, Facilitated Diffusion & Active Transport
                                                      • Proteins
                                                        • 2.6 Amino Acids, Proteins & Protein Structure
                                                          • 2.7 Enzymes: Roles & Modes of Action
                                                            • 2.8 Core Practical 4: Investigating the Rate of Enzyme Reactions
                                                              • 2.9 Nucleotides, DNA & RNA, Base Pairing
                                                              • DNA & Gene Expression
                                                                • 2.10 DNA Replication
                                                                  • 2.11 The Nature of the Genetic Code
                                                                    • 2.12 How Bases Code for a Polypeptide Chain
                                                                      • 2.13 Transcription & Translation
                                                                      • Inheritance
                                                                        • 2.14 Mutations
                                                                          • 2.15 Patterns of Inheritance & Sex Linkage
                                                                            • 2.16 Cystic Fibrosis
                                                                              • 2.17 Genetic Screening
                                                                                • 2.18 Ethical & Social Issues of Genetic Screening
                                                                              • 3. Cell Structure, Reproduction & Development
                                                                                • Cell Structure & Organisation
                                                                                  • 3.1 Cell Theory
                                                                                    • 3.2 Levels of Organisation of Cells
                                                                                      • 3.3 Eukaryotic Cells
                                                                                        • 3.4 The Rough Endoplasmic Reticulum & Golgi
                                                                                          • 3.5 Prokaryotic Cells
                                                                                            • 3.6 Electron Microscopy of Animal Cells
                                                                                              • 3.7 Microscopy: Magnification & Resolution
                                                                                                • 3.8 Core Practical 5: Light Microscopy
                                                                                                • Reproduction & Inheritance
                                                                                                  • 3.9 Gene Locus
                                                                                                    • 3.10 Meiosis & Variation
                                                                                                      • 3.11 Mammalian Gametes
                                                                                                        • 3.12 Fertilisation: Mammals
                                                                                                          • 3.13 Fertilisation: Flowering Plants
                                                                                                            • 3.14 The Cell Cycle & Mitosis
                                                                                                              • 3.15 Core Practical 6: Observing the Stages of Mitosis
                                                                                                                • 3.16 Calculation of Mitotic Index
                                                                                                                  • 3.17 Stem Cells & Cell Potency
                                                                                                                    • 3.18 Cell Specialisation
                                                                                                                      • 3.19 Post-Transcriptional Changes to mRNA
                                                                                                                        • 3.20 Gene Interaction & Epigenetics
                                                                                                                          • 3.21 Polygenic Inheritance & Continuous Variation
                                                                                                                        • 4. Plant Structure & Function, Biodiversity & Conservation
                                                                                                                          • Plant Structure & Function
                                                                                                                            • 4.1 Plant Cell Structure
                                                                                                                              • 4.2 Electron Microscopy of Plant Cells
                                                                                                                                • 4.3 Starch & Cellulose: Structure & Function
                                                                                                                                  • 4.4 Properties of Cellulose
                                                                                                                                    • 4.5 The Vascular Structure of Plants
                                                                                                                                      • 4.6 Core Practical 7: Identifying Tissue Types Within Stems
                                                                                                                                      • Plants & Conservation
                                                                                                                                        • 4.7 Plant-Based Products for Sustainability
                                                                                                                                          • 4.8 Water & Inorganic Ions in Plants
                                                                                                                                            • 4.9 Core Practical 8: Determining the Tensile Strength of Plant Fibres
                                                                                                                                            • Plants & Bacterial Growth
                                                                                                                                              • 4.10 Bacterial Growth Conditions
                                                                                                                                                • 4.11 Plant Products with Antimicrobial Properties
                                                                                                                                                  • 4.12 Core Practical 9: Antimicrobial Properties of Plants
                                                                                                                                                    • 4.13 Development of Drugs & Drug Testing
                                                                                                                                                    • Classification & Biodiversity
                                                                                                                                                      • 4.14 The Three Domains of Life
                                                                                                                                                        • 4.15 The Variety of Life
                                                                                                                                                          • 4.16 Biodiversity & Endemism
                                                                                                                                                            • 4.17 Species Richness & Heterozygosity Index
                                                                                                                                                              • 4.18 Index of Biodiversity
                                                                                                                                                                • 4.19 Ecological Niches & Adaptations
                                                                                                                                                                  • 4.20 Hardy-Weinberg Equation
                                                                                                                                                                    • 4.21 Roles of Seed Banks & Zoos in Conservation
                                                                                                                                                                  • 5. Energy Flow, Ecosystems & the Environment
                                                                                                                                                                    • Photosynthesis
                                                                                                                                                                      • 5.1 Photosynthesis: Overview
                                                                                                                                                                        • 5.2 The Role of ATP in Photosynthesis
                                                                                                                                                                          • 5.3 Photosynthesis: Light-Dependent Stage
                                                                                                                                                                            • 5.4 Photosynthesis: Light-Independent Stage
                                                                                                                                                                              • 5.5 Chloroplasts
                                                                                                                                                                                • 5.6 Absorption & Action Spectra
                                                                                                                                                                                  • 5.7 Separation of Photosynthetic Pigments with Chromatography
                                                                                                                                                                                    • 5.8 Core Practical 10: Rate of Photosynthesis
                                                                                                                                                                                    • Ecosystems
                                                                                                                                                                                      • 5.9 Ecological Productivity
                                                                                                                                                                                        • 5.10 Energy & Biomass Transfers
                                                                                                                                                                                          • 5.11 Ecology: Key Terms
                                                                                                                                                                                            • 5.12 Factors Affecting Populations
                                                                                                                                                                                              • 5.13 Niche
                                                                                                                                                                                                • 5.14 Core Practical 11: Quadrat & Transect Study
                                                                                                                                                                                                  • 5.15 Ecological Succession
                                                                                                                                                                                                  • Environmental Biology
                                                                                                                                                                                                    • 5.16 Evidence for Climate Change
                                                                                                                                                                                                      • 5.17 Anthropogenic Climate Change
                                                                                                                                                                                                        • 5.18 Carbon Cycle & Environmental Management
                                                                                                                                                                                                          • 5.19 Models for Predicting Climate Change
                                                                                                                                                                                                            • 5.20 How Climate Change Affects Species
                                                                                                                                                                                                              • 5.21 The Effect of Temperature on Enzyme Reactions
                                                                                                                                                                                                                • 5.22 Core Practical 12: Effect of Temperature on Development
                                                                                                                                                                                                                  • 5.23 Mutation, Natural Selection & Evolution
                                                                                                                                                                                                                    • 5.24 Isolation & Speciation
                                                                                                                                                                                                                      • 5.25 Contentious Issues in Environmental Science
                                                                                                                                                                                                                        • 5.26 Examples of Sustainable Conservation
                                                                                                                                                                                                                      • 6. Microbiology, Immunity & Forensics
                                                                                                                                                                                                                        • Microbiology
                                                                                                                                                                                                                          • 6.1 Culturing Microorganisms
                                                                                                                                                                                                                            • 6.2 Measuring the Growth of Microorganisms
                                                                                                                                                                                                                              • 6.3 The Bacterial Growth Curve
                                                                                                                                                                                                                                • 6.4 Core Practical 13: Rate of Growth of Microorganisms
                                                                                                                                                                                                                                  • 6.5 Comparison of Bacterial & Viral Structure
                                                                                                                                                                                                                                  • Immunity
                                                                                                                                                                                                                                    • 6.6 Tuberculosis & HIV
                                                                                                                                                                                                                                      • 6.7 Pathogens: Routes of Entry
                                                                                                                                                                                                                                        • 6.8 Non-Specific Immune Responses
                                                                                                                                                                                                                                          • 6.9 Specific Immune Responses
                                                                                                                                                                                                                                            • 6.11 Developing Immunity
                                                                                                                                                                                                                                              • 6.12 Pathogens vs Hosts: An Evolutionary Race
                                                                                                                                                                                                                                              • Antibiotics
                                                                                                                                                                                                                                                • 6.13 Antibiotics
                                                                                                                                                                                                                                                  • 6.14 Core Practical 14: The Effects of Different Antibiotics
                                                                                                                                                                                                                                                    • 6.15 Current Usage of Antibiotics
                                                                                                                                                                                                                                                    • Decomposition & Decay
                                                                                                                                                                                                                                                      • 6.16 The Role of Microorganisms
                                                                                                                                                                                                                                                      • Forensics
                                                                                                                                                                                                                                                        • 6.17 Polymerase Chain Reaction
                                                                                                                                                                                                                                                          • 6.18 Gel Electrophoresis in Forensics
                                                                                                                                                                                                                                                            • 6.19 DNA Profiling
                                                                                                                                                                                                                                                              • 6.20 Types of Data Provided by Forensic Analysis
                                                                                                                                                                                                                                                            • 7. Respiration, Muscles & the Internal Environment
                                                                                                                                                                                                                                                              • Respiration
                                                                                                                                                                                                                                                                • 7.1 Overview of Respiration
                                                                                                                                                                                                                                                                  • 7.2 Glycolysis
                                                                                                                                                                                                                                                                    • 7.3 Link Reaction & Krebs Cycle
                                                                                                                                                                                                                                                                      • 7.4 The Electron Transport Chain
                                                                                                                                                                                                                                                                        • 7.5 Anaerobic Respiration
                                                                                                                                                                                                                                                                          • 7.6 Respiratory Quotient
                                                                                                                                                                                                                                                                            • 7.7 Core Practical 15: Investigation of Respiration in Yeast
                                                                                                                                                                                                                                                                              • 7.8 Core Practical 16: Respirometer to Calculate RQ
                                                                                                                                                                                                                                                                              • Muscular Movement
                                                                                                                                                                                                                                                                                • 7.9 The Musculoskeletal System
                                                                                                                                                                                                                                                                                  • 7.10 The Muscle Fibre
                                                                                                                                                                                                                                                                                    • 7.11 The Process of Skeletal Muscle Contraction
                                                                                                                                                                                                                                                                                      • 7.12 The Role of Muscle in the Cardiac Cycle
                                                                                                                                                                                                                                                                                        • 7.13 Calculation of Cardiac Output
                                                                                                                                                                                                                                                                                          • 7.14 The Role of Adrenaline
                                                                                                                                                                                                                                                                                            • 7.15 Core Practical 17: The Effects of Exercise
                                                                                                                                                                                                                                                                                            • The Internal Environment
                                                                                                                                                                                                                                                                                              • 7.16 Negative & Positive Feedback
                                                                                                                                                                                                                                                                                                • 7.17 Homeostasis
                                                                                                                                                                                                                                                                                                  • 7.18 The Kidney: Structure
                                                                                                                                                                                                                                                                                                    • 7.19 Urea as a Waste Product
                                                                                                                                                                                                                                                                                                      • 7.20 Selective Reabsorption in the Kidney
                                                                                                                                                                                                                                                                                                        • 7.21 Hormones in Osmoregulation
                                                                                                                                                                                                                                                                                                          • 7.22 Control of Gene Expression
                                                                                                                                                                                                                                                                                                        • 8. Coordination, Response & Gene Technology
                                                                                                                                                                                                                                                                                                          • The Nervous System
                                                                                                                                                                                                                                                                                                            • 8.1 Neurones: Types and Functions
                                                                                                                                                                                                                                                                                                              • 8.2 How a Response is Generated by Effectors
                                                                                                                                                                                                                                                                                                                • 8.3 The Reflex Arc
                                                                                                                                                                                                                                                                                                                  • 8.4 The Nerve Impulse
                                                                                                                                                                                                                                                                                                                    • 8.5 Myelination & Saltatory Conduction
                                                                                                                                                                                                                                                                                                                      • 8.6 Synapses & Neurotransmitters
                                                                                                                                                                                                                                                                                                                        • 8.7 The Effects of Drugs on Nervous Transmission
                                                                                                                                                                                                                                                                                                                          • 8.8 Detection of Stimuli
                                                                                                                                                                                                                                                                                                                            • 8.9 Habituation
                                                                                                                                                                                                                                                                                                                              • 8.10 Central & Peripheral Nervous System
                                                                                                                                                                                                                                                                                                                              • Plant Hormones
                                                                                                                                                                                                                                                                                                                                • 8.11 Effects of Plant Hormones
                                                                                                                                                                                                                                                                                                                                  • 8.12 Core Practical 18: Amylase in Germinating Cereal Grains
                                                                                                                                                                                                                                                                                                                                    • 8.13 Nervous & Hormonal Coordination
                                                                                                                                                                                                                                                                                                                                    • The Brain
                                                                                                                                                                                                                                                                                                                                      • 8.14 Human Brain Structures & Functions
                                                                                                                                                                                                                                                                                                                                        • 8.15 Techniques to Investigate the Brain
                                                                                                                                                                                                                                                                                                                                          • 8.16 Brain Disease
                                                                                                                                                                                                                                                                                                                                          • Gene Technology
                                                                                                                                                                                                                                                                                                                                            • 8.17 Drug Production from Genetically Modified Organisms
                                                                                                                                                                                                                                                                                                                                              • 8.18 Recombinant DNA
                                                                                                                                                                                                                                                                                                                                                • 8.19 Transfer of Recombinant DNA into Other Cells
                                                                                                                                                                                                                                                                                                                                                  • 8.20 Identification of Active Genes
                                                                                                                                                                                                                                                                                                                                                    • 8.21 Bioinformatics
                                                                                                                                                                                                                                                                                                                                                      • 8.22 Risks & Benefits of Using GMOs


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                                                                                                                                                                                                                                                                                                                                                    Author: Marlene

                                                                                                                                                                                                                                                                                                                                                    Marlene graduated from Stellenbosch University, South Africa, in 2002 with a degree in Biodiversity and Ecology. After completing a PGCE (Postgraduate certificate in education) in 2003 she taught high school Biology for over 10 years at various schools across South Africa before returning to Stellenbosch University in 2014 to obtain an Honours degree in Biological Sciences. With over 16 years of teaching experience, of which the past 3 years were spent teaching IGCSE and A level Biology, Marlene is passionate about Biology and making it more approachable to her students.


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