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7.3.1Topic 7 · Run for your Life

Homeostasis- An Overview — Edexcel A Level Biology A (SNAB) 9BI0 Revision Notes & Topical Questions

5 min read 9 parts Lesson 15 of 25 1 quiz

📚 Edexcel A Level Biology A (SNAB) 9BI0 Hub / Topical Revision
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7.3.1 Homeostasis- An Overview — Edexcel A Level Biology A (SNAB) 9BI0 notes: Explain what homeostasis is and how negative and positive feedback loops help to keep internal conditions within narrow limits.Homeostasis is the maintenance of a stable internal environment within narr…

  • Coordinator
  • Communication system
  • Negative feedback
  • antagonistic control
  • Exam-style question 1
  • Exam-style question 2

Part 1 of 7What is homeostasis?

Homeostasis is the maintenance of a stable internal environment within narrow limits, despite changes in the external environment or in the organism's activity. The internal environment is the tissue fluid that bathes cells, and conditions that are controlled in mammals include core body temperature (about 37 °C), blood glucose concentration, blood pH, blood water potential and carbon dioxide concentration, and blood pressure.

Control matters because most cell processes depend on enzymes.

Denatures
A change in temperature or pH alters the rate of reaction and, if extreme, denatures enzymes by breaking hydrogen and ionic bonds in the tertiary structure.
Changes in water potential cause cells to gain or lose water by osmosis, and a lack of glucose starves cells of the substrate for respiration.
Homeostasis therefore keeps conditions close to the optimum so that metabolic reactions run efficiently, without making the organism dependent on the environment.

Part 2 of 7Components of a control system

Every homeostatic system has the same basic parts:

Components of a control system
Set point
(or norm): the optimum value of the variable.
Receptor
Receptor detects a change in the variable (such as thermoreceptors in the hypothalamus and skin, osmoreceptors, chemoreceptors or baroreceptors).
Coordinator
(control centre): processes the information and decides on a response, e.g. the hypothalamus or the medulla oblongata.
Effector
Effector the muscle or gland that carries out the response.
Communication system
Communication system nerves (nerve impulses) or the endocrine system (hormones in blood) that connect them.

The system detects a change from the set point, the effector acts to reverse it, and the receptors then detect that the change has been corrected, completing a loop.

Part 3 of 7Negative feedback

Negative feedback reverses a change. A rise above the set point triggers responses that lower the variable; a fall triggers responses that raise it. This keeps the variable fluctuating around the set point, and it is the main mechanism of homeostasis.

Having two opposing mechanisms gives antagonistic control, which allows a faster and finer response than one mechanism alone.

Negative feedback
In temperature control, if the core temperature rises, the hypothalamus triggers vasodilation of skin arterioles, sweating and relaxation of hair erector muscles; if it falls, it triggers vasoconstriction, shivering, and release of adrenaline and thyroxine to raise metabolic rate.
Other examples are insulin and glucagon in blood glucose control, and ADH in water balance.
The time lag between detecting a change and effects occurring is why variables oscillate slightly around the set point rather than being perfectly constant.

Part 4 of 7Positive feedback

In positive feedback a change causes responses that increase that change, moving the system further from the starting level.

Positive feedback
It is usually not a method of homeostasis but is useful when a rapid, complete outcome is needed.
Examples include the rising oxytocin during childbirth: stretching of the cervix triggers more oxytocin, producing stronger contractions and more stretching until the baby is born; the opening of sodium channels during an action potential; and the platelet plug in blood clotting.
Positive feedback can also be harmful: in hypothermia a low temperature slows metabolism, which lowers temperature still more, and the body can fail to recover.
FeatureNegative feedbackPositive feedback
Effect on the changeReverses itAmplifies it
ResultStable around the set pointRapid move away from set point
ExamplesTemperature, glucose, heart rateChildbirth, action potential, clotting

Part 5 of 7Links forward

The control of heart rate and ventilation during exercise (later lessons) is a negative feedback loop with chemoreceptors and baroreceptors as sensors, the medulla as coordinator and the heart and ventilatory muscles as effectors. The same principle applies to the kidney (osmoregulation) and the liver (glucose) elsewhere in the course.

Part 6 of 7Examples of homeostatic control in mammals

Temperature control shows all the components together.

1
Thermoreceptors in the skin detect the surface temperature and those in the hypothalamus detect the temperature of blood flowing through the brain (the core temperature).
2
The hypothalamus compares this with the set point and sends impulses to effectors in the skin, muscles and glands.
3
Heat can be lost by vasodilation, sweating and flattening of hairs, and heat can be gained or retained by vasoconstriction, shivering (involuntary muscle contraction that releases heat from respiration), raising of hairs and increased secretion of thyroxine.
4
Behavioural responses such as moving into shade, or putting on clothes, work alongside these physiological responses.

Blood glucose is a second example

  • after a meal, a rise in glucose is detected by beta cells in the islets of Langerhans in the pancreas, which secrete insulin;
    • insulin causes cells to take up glucose and the liver to convert it into glycogen, so the level falls.

Examples of homeostatic control in mammals
If glucose falls, alpha cells secrete glucagon, which causes glycogenolysis.
Both examples have a sensor, a coordinator, an effector and negative feedback, and you should be able to apply that general template to any unfamiliar example set in an exam.

Part 7 of 7Exam-style questions

  1. Explain why it is important that body temperature stays close to 37 °C. [3]3 marks

  2. Describe how negative feedback maintains body temperature after a rise in core temperature. [4]4 marks

  3. Give one difference between negative and positive feedback and one example of positive feedback. [2]2 marks

Key terms

TermMeaning
HomeostasisMaintenance of a stable internal environment within narrow limits
Negative feedbackResponse that reverses a change from the set point
Positive feedbackResponse that amplifies a change
Set pointThe optimum value of a controlled variable
ReceptorCell or structure that detects a stimulus
EffectorMuscle or gland that carries out a response
CoordinatorControl centre that processes information and sends instructions
Antagonistic controlTwo opposing mechanisms that raise and lower a variable

Avoid these mistakes

1
Saying negative feedback keeps a variable perfectly constant: it keeps it fluctuating around a set point.
2
Saying that 'negative' means harmfulit means the response opposes the change.
3
Mixing up the receptor (detects) and the effector (acts).
4
Saying the hypothalamus is an effectorit is the coordinator in temperature control.
5
Saying that enzymes 'die' at high temperature: they denature.

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