Principles of homeostasis and negative feedback (A-level only)
AQA
A-Level
Homeostasis is the maintenance of a relatively constant internal environment despite changes outside the body and in the body's own activity. The internal environment means the blood and the tissue fluid that bathe the cells: their temperature, pH, water potential and the concentrations of glucose and ions. Every control system has the same parts: a set point, receptors that detect a deviation from it, a coordinator, usually a centre in the brain or an endocrine gland, that receives that information, effectors that carry out a correction, and feedback of the new value to the receptors. Notice the word restricted rather than constant. Levels fluctuate around the set point rather than being held exactly, because a correction can only begin once a deviation has been detected.
18
Objectives
18
Exam Tips
29
Pitfalls
34
Key Terms
31
Mark Points
Subtopics in this area
Homeostasis in mammals involves physiological control systems that maintain the internal environment within restricted limits.
The importance of maintaining a stable core temperature and stable blood pH in relation to enzyme activity.
The importance of maintaining a stable blood glucose concentration in terms of availability of respiratory substrate and of the water potential of blood.
Negative feedback restores systems to their original level.
The possession of separate mechanisms involving negative feedback controls departures in different directions from the original state, giving a greater degree of control.
Students should be able to interpret information relating to examples of negative and positive feedback.
Principles of homeostasis and negative feedback (A-level only) Revision Guide
Learning Objectives
What you need to know and understand
- Define homeostasis and explain what is meant by the internal environment.
- Identify the receptor, coordinator and effector in an unfamiliar physiological control system.
- Explain why a homeostatically controlled variable fluctuates around a set point rather than staying fixed.
- Explain why enzymes are denatured above their optimum temperature.
- Explain why a change from blood pH 7.4 reduces the rate of a metabolic reaction.
- Interpret a graph of reaction rate against temperature.
- Explain why brain cells are the first to be affected by a fall in blood glucose concentration.
- Predict the direction of water movement between blood and cells when blood glucose rises sharply, using water potential.
- Explain why the appearance of glucose in urine leads to dehydration.
- Explain how negative feedback returns a variable to its set point, naming every stage of the loop.
- Explain why a negative feedback system must switch off its own corrective response.
- Distinguish negative from positive feedback using a named physiological example of each.
- Explain why two opposing mechanisms give closer control than a single corrective mechanism.
- Name a pair of mechanisms that correct departures in opposite directions for a named variable.
- Predict the consequence for control if one of a pair of opposing mechanisms fails.
- Decide from an unfamiliar trace whether a system shows negative or positive feedback, and justify the decision with data.
- Identify the receptor and the effector in a control system described only in an exam passage.
- Explain why the response in a positive feedback system must be ended by some separate event.
Marking Points
Key points examiners look for in your answers
- one mark for defining homeostasis as the maintenance of a constant internal environment within narrow or restricted limits
- one mark for identifying the internal environment as the blood and tissue fluid surrounding the cells
- one mark for naming receptors, a coordinator and effectors as the components of a physiological control system
- one mark for stating that the level fluctuates around a set point rather than being held exactly constant
- one mark for linking stability to enzyme activity and therefore to metabolic reactions
- Enzymes are globular proteins; the tertiary structure forms an active site complementary to a specific substrate.
- Raising temperature increases kinetic energy, so more enzyme-substrate complexes form and the rate of reaction rises.
- Above the optimum temperature, hydrogen and ionic bonds break, the tertiary structure and active site change shape, and the enzyme is denatured; this is usually irreversible.
- A change in blood pH alters hydrogen ion concentration, disrupting ionic and hydrogen bonds in the tertiary structure and altering the active site.
- Normal blood pH is approximately 7.4; deviation slows or stops metabolic reactions because fewer enzyme-substrate complexes form.
- Core temperature and blood pH are homeostatic variables maintained within narrow limits by negative feedback.
- one mark for glucose being the main respiratory substrate, respired to release energy for the synthesis of ATP
- one mark for a low blood glucose concentration meaning less ATP is produced, affecting brain cells in particular
- one mark for glucose being a solute that lowers the water potential of the blood
- one mark for a high blood glucose concentration making the water potential of blood more negative, so water leaves cells by osmosis
- one mark for relating cell dehydration, or water loss in the urine, to the need for control in both directions
- one mark for the change being detected by receptors as a deviation from a set point or norm
- one mark for the effector response acting in the opposite direction to the original change
- one mark for the variable being returned to, or towards, its original level
- one mark for the corrective mechanism then being switched off, so the system does not overshoot
- one mark for contrasting this with positive feedback, in which the response increases the original change
- one mark for stating that one mechanism corrects a rise while a separate mechanism corrects a fall
- one mark for naming a valid pair, such as insulin and glucagon, or sympathetic and parasympathetic stimulation
- one mark for explaining that the variable is actively returned from either direction rather than drifting back
- one mark for stating that this gives faster correction and keeps the variable within narrower limits
- one mark for relating this to a greater degree of control overall
- one mark for a classification that carries its justification, naming the feedback as negative or positive together with the reason read from the information, since the name on its own is not the mark point
- one mark for using data from the figure, quoting values or times, to show the direction of the change
- one mark for stating whether the response opposes the change and returns the variable to its original level, or amplifies it
- one mark for naming the receptor, coordinator or effector identified from the information provided
- one mark for explaining the biological benefit or the consequence of the feedback described
Examiner Tips
Expert advice for maximising your marks
- 💡Learn the receptor, coordinator, effector chain as a template; it structures answers on temperature, glucose and water potential alike.
- 💡Use the phrase within restricted limits in a definition, because it separates a full mark from a partial one.
- 💡Where an unfamiliar variable is given, identify the receptor and effector from the information rather than from memory.
- 💡Use the three-step chain: bonds break, tertiary structure changes, active site no longer complementary to the substrate.
- 💡When discussing human metabolism, state that normal blood pH is approximately 7.4 and explain that deviation alters enzyme active sites.
- 💡Link temperature and pH answers to the same underlying mechanism: disruption of bonds maintaining tertiary structure.
- 💡The statement names two reasons, so a question on it almost always needs both; write a sentence on each.
- 💡Use more negative water potential rather than lower, which is ambiguous.
- 💡Link osmosis to a water potential gradient every time; a bare by osmosis usually loses the explanation mark.
- 💡Draw the loop as a circle with four labels, stimulus, receptor, effector, return, and write from it.
- 💡The word restores is doing work in the specification; say the level returns to normal, not just that it changes.
- 💡For an unfamiliar system, decide first whether the response opposes or amplifies the change, because that judgement usually carries the mark.
- 💡Always give a paired example, one mechanism for each direction, and say what each does to the variable.
- 💡Unpack the phrase greater degree of control as faster return and narrower fluctuation.
- 💡Prepare one worked example, glucose or temperature, that you can deploy in either context.
- 💡The bare word negative or positive is usually worth nothing; the explanation carries the marks.
- 💡Quote at least one pair of figures with units when you justify a conclusion from a graph.
- 💡Check whether the trace returns to its starting value, because that is the clearest signature of negative feedback.
Common Mistakes
Pitfalls to avoid in your exam answers
- defining homeostasis as keeping conditions the same, with no mention of narrow limits or fluctuation around a set point
- confusing the internal environment with the contents of a cell
- listing organs rather than the roles of receptor, coordinator and effector
- saying homeostasis keeps the external environment constant
- forgetting that a deviation must occur before correction can begin, so the value cannot be fixed
- Writing that the active site is on the substrate. Correction: the active site is a region of the enzyme formed by its tertiary structure.
- Claiming denaturation breaks peptide bonds. Correction: denaturation disrupts hydrogen and ionic bonds that maintain tertiary structure; peptide bonds in the primary structure remain intact.
- Explaining pH effects without linking them to bond disruption. Correction: always state that pH changes alter ionic and hydrogen bonding, changing tertiary structure and active site shape.
- Describing denaturation as reversible. Correction: denaturation of an enzyme is usually irreversible because the tertiary structure cannot reform.
- writing that respiration produces energy rather than releasing energy to make ATP
- saying a high glucose concentration gives a higher water potential, when adding solute makes water potential more negative
- answering only about respiration and leaving out the water potential half of the statement
- claiming water moves into cells when blood glucose rises
- stating that brain cells store glycogen and so are unaffected by a low blood glucose concentration
- describing negative feedback as something that stops a change, rather than reversing it back to the set point
- omitting the switch-off step, so the system described would overshoot indefinitely
- taking negative to mean that the value falls, rather than that the response opposes the change
- labelling a homeostatic mechanism as positive feedback simply because the level rises
- writing about hormones or nerves without ever naming receptor, coordinator and effector
- naming a pair that acts in the same direction, such as glucagon and adrenaline, when opposing mechanisms are wanted
- saying the two mechanisms cancel each other out
- stating that two mechanisms give better control without explaining why one alone is worse
- describing insulin as switching glucagon off, rather than both being controlled separately by blood glucose concentration
- treating vasodilation and vasoconstriction as one mechanism in reverse rather than as two separate responses
- naming the type of feedback but giving no data, when the justification carries the marks
- quoting data without saying what it shows about the direction of the response
- assuming that every physiological example must be negative feedback
- reading a variable that stays high as negative feedback, when nothing has returned it to the set point
- treating a correlation in the data as proof of a causal control mechanism