Homeostasis and Negative Feedback

A-Level Biology ยท Response and Homeostasis

Homeostasis

Homeostasis is the maintenance of a stable internal environment despite changes in external conditions. It keeps factors like blood glucose, temperature, water potential and pH within narrow limits, so that cells and enzymes work optimally.

Negative feedback

Homeostasis works by negative feedback: when a factor moves away from its set point, mechanisms act to reverse the change and return it to normal.

stimulus (change) โ†’ receptor detects it โ†’ coordinator โ†’ effector โ†’ response reverses the change

Having separate mechanisms to increase and decrease a factor gives greater control (you can respond to changes in either direction). Positive feedback (which amplifies a change) is rare โ€” e.g. during childbirth or the generation of an action potential.

Control of blood glucose

Controlled by the pancreas and liver using two hormones:

  • Blood glucose too high โ†’ ฮฒ-cells of the pancreas release insulin โ†’ cells take up more glucose; the liver converts glucose to glycogen (glycogenesis) โ†’ blood glucose falls.
  • Blood glucose too low โ†’ ฮฑ-cells release glucagon โ†’ the liver breaks glycogen back to glucose (glycogenolysis) and makes glucose from other molecules (gluconeogenesis) โ†’ blood glucose rises.

Adrenaline also raises blood glucose (preparing for activity). Insulin works via second messengers (e.g. it activates enzymes and increases glucose transporters in cell membranes).

Diabetes

  • Type 1: the immune system destroys ฮฒ-cells โ†’ no insulin produced; treated with insulin injections.
  • Type 2: cells become less responsive to insulin (receptors don't respond); managed by diet, exercise and medication.

Osmoregulation (outline)

The kidney controls water potential. ADH (antidiuretic hormone) increases the permeability of the collecting duct so more water is reabsorbed when the body is dehydrated โ€” another negative feedback loop.

Worked example

Explain how blood glucose returns to normal after a sugary meal.

  • Rising glucose is detected by pancreatic ฮฒ-cells, which release insulin. Insulin makes cells take up glucose and the liver store it as glycogen (glycogenesis), lowering blood glucose back to the set point โ€” negative feedback. โœ“

Common mistakes

  • Confusing insulin (lowers glucose) with glucagon (raises glucose).
  • Mixing up glycogenesis (make glycogen), glycogenolysis (break glycogen) and gluconeogenesis (make new glucose).
  • Calling homeostasis positive feedback โ€” it's mainly negative feedback.

Exam tips

  • Describe negative feedback as a general loop, then apply it to glucose/temperature/water.
  • Learn the insulin/glucagon actions and the liver processes precisely.
  • Distinguish Type 1 (no insulin) from Type 2 (insulin resistance) diabetes.

Key facts to remember

  • Homeostasis keeps the internal environment stable via negative feedback (reverses changes; separate mechanisms give better control).
  • Blood glucose: insulin (ฮฒ-cells) lowers it (glycogenesis); glucagon (ฮฑ-cells) raises it (glycogenolysis/gluconeogenesis).
  • Type 1 diabetes = no insulin (injections); Type 2 = insulin resistance; ADH controls water reabsorption in the kidney.
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More on Response and Homeostasis

Nervous Coordination: Nerve Impulses and Synapses The Kidney and Osmoregulation Muscle Contraction: The Sliding Filament Theory The Nervous System: Synapses and Action Potentials

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