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3.6.4Inquiry Process

Epinephrine and Melatonin — IB Biology HL Revision Notes & Topical Questions

5 min read 5 parts Lesson 144 of 160

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Part 1 of 5Epinephrine in the Stress Response

3.6.4 Epinephrine and Melatonin
Source
  • The adrenal medulla releases epinephrine when the sympathetic nervous system signals danger or stress.
Cardiovascular effect
  • Heart rate and the strength of contraction increase, and blood is diverted to muscles from the gut and skin.
Metabolic effect
  • Glycogen in the liver is broken down so glucose enters the blood and fuels respiration.
Breathing
  • Airways widen to allow more air in, supplying extra oxygen for the increased energy demand.
Receptors
  • Epinephrine binds to G-protein-linked receptors on target cells, so a second messenger inside the cell carries out the response.

Part 2 of 5Melatonin and Circadian Rhythms

1
Light enters the eye and is detected by specialised cells in the retina, which send signals to the suprachiasmatic nucleus in the hypothalamus.
2
In daylight the signals prevent the pineal gland from secreting melatonin, so melatonin levels are low.
3
In darkness these signals fall, and the pineal gland releases melatonin into the blood.
4
Melatonin acts on the brain to promote sleep and reduce body temperature, helping to set a 24 hour rhythm.
5
Melatonin levels fall again before dawn, and light the next morning keeps the rhythm matched to the day.

Part 3 of 5Comparison and Applications

Comparison and Applications
Stimulus
  • Epinephrine release is triggered by stress, whereas melatonin release is controlled by the light-dark cycle.
Jet lag
  • Rapid travel across time zones leaves the melatonin rhythm out of step with local time until it gradually resets.
Day length
  • In some mammals, longer nights give a longer melatonin signal, which triggers seasonal breeding.
Duration
  • Epinephrine is cleared in minutes, but melatonin follows a daily cycle across hours.
Treatment
  • Synthetic melatonin is sometimes taken to help reset the body clock, showing how hormone knowledge is used in medicine.
HormoneSourceMain effect
EpinephrineAdrenal medullaPrepares for fight or flight
MelatoninPineal glandSleep timing, circadian rhythm

Part 4 of 5Worked Example

Question

  • Melatonin concentration is 5 units at midday and 55 units at 02:00.
  • Calculate the percentage increase and explain the pattern.

  1. Increase = 55 − 5 = 50 units.
  2. Percentage increase = 50 ÷ 5 × 100 = 1000%.
  3. Midday light inhibits the pineal gland, while darkness at night removes this inhibition.

Answer A 1000% increase, because melatonin is released in darkness.

Part 5 of 5Exam Checklist & Tips

3.6.4 – Key Points for the Exam

  • Epinephrine Hormone from the adrenal medulla that prepares the body for action.
  • Melatonin Hormone from the pineal gland that helps control sleep rhythms.
  • Circadian rhythm Biological cycle of about 24 hours.
  • Pineal gland Small gland in the brain that releases melatonin in darkness.

Common Mistakes

  • Saying melatonin is released in daylight; darkness stimulates it.
  • Mixing up the adrenal medulla (epinephrine) with the cortex (steroid hormones).
  • Treating jet lag as tiredness only; it reflects an out-of-phase body clock.

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