IB DP Biology: Hormonal and feedback integration in animals, and plant growth regulation (DP Biology) – Revision Notes

These revision notes condense statements C3.1.11 to C3.1.23 of the IB Biology guide, first assessment 2025, for DP Biology. C3.1.11 to C3.1.16 are SL and HL. C3.1.17 to C3.1.23 are additional higher level, so they are HL only. For full explanations and worked examples, read the study guide, then test yourself with the practice set.

Related pages: IB DP Biology hub, printable checklist and the free 10-minute diagnostics.

Feedback loop skeleton

Stimulus, receptor, control centre, effector, response.

Type	Effect of the response	Examples in this unit
Negative feedback	Reduces the change, returns to set point	Heart rate, ventilation rate, thyroxine level
Positive feedback	Increases the change	Ethylene and ripening (HL only)

C3.1.11 to C3.1.13 Hormonal signals

Item	Key facts
Melatonin	Made by the pineal gland. Low in the day, rises in the evening, peaks in the night, falls by morning. Light suppresses it. Sets the sleep-wake cycle (circadian rhythm).
Epinephrine	Made by the adrenal glands. Travels in the blood. Prepares for vigorous activity.
Hypothalamus	Brain region linking nervous and endocrine systems. Makes hormones and releasing hormones.
Pituitary	Gland below the hypothalamus. Releases hormones that act on targets or on other glands.

Epinephrine effects (all aid muscle contraction):

• heart rate and stroke volume up
• blood diverted to skeletal muscle, away from gut and skin
• bronchioles wider
• liver converts glycogen to glucose
• pupils wider

Thyroid example (negative feedback): low thyroxine, hypothalamus releases more releasing hormone, pituitary releases more thyroid-stimulating hormone, thyroid releases more thyroxine, thyroxine rises and inhibits the hypothalamus and pituitary.

C3.1.14 and C3.1.15 Heart and ventilation control

Feature	Baroreceptors	Chemoreceptors
Detect	Blood pressure	pH, carbon dioxide, oxygen
Location	Carotid arteries, aortic arch	Carotid and aortic bodies; brainstem (for ventilation)
Coordinated by	Medulla	Medulla
Output	Nerves to the heart change rate and stroke volume	Heart nerves; diaphragm and intercostal muscles

Method in steps: heart rate response

1. Stimulus changes (pressure or carbon dioxide).
2. Receptors send impulses to the medulla.
3. Medulla sends impulses along sympathetic nerves (speed up) or the vagus (slow down).
4. Heart rate and stroke volume change.
5. The stimulus is reduced, so receptor signals change back.

Method in steps: ventilation response to exercise

1. More respiration, more carbon dioxide, which forms carbonic acid, so blood pH falls.
2. Brainstem chemoreceptors detect the pH fall.
3. Medulla signals the diaphragm and intercostal muscles.
4. Faster, deeper breaths remove carbon dioxide.
5. pH rises back towards normal.

Anaerobic respiration adds lactate, which also lowers pH.

C3.1.16 Peristalsis

Stage	Control
Starting a swallow	CNS, voluntary
Passage through oesophagus, stomach and intestines	ENS, involuntary
Egestion of faeces	CNS, voluntary

The ENS lies in the gut wall and coordinates waves of contraction so material moves in one direction.

C3.1.17 and C3.1.18 Tropisms (HL only)

• A tropism is a directional growth response to a directional stimulus.
• Positive phototropism: shoot grows towards lateral light. Cells on the shaded side elongate more.
• Qualitative data: annotated diagrams. Quantitative data: angle of curvature in degrees.
• Improve reliability: many seedlings, same age, same light and temperature, repeats, camera square to the seedling.
• Improve precision: measure from photographs with a protractor or image software, to a consistent unit.

C3.1.19 to C3.1.23 Plant growth regulation (HL only)

Statement	Key points
Phytohormones	Chemical signals made in small amounts. Auxin, cytokinin, gibberellin, abscisic acid and ethylene are examples.
Auxin efflux carriers	Auxin diffuses in but cannot diffuse out. Carriers on one side of each cell move it out. Aligned carriers create a one-way flow and a gradient.
Auxin and cell growth	Proton pumps put hydrogen ions into the cell wall space. Lower pH loosens cellulose cross links. Water enters, turgor stretches the wall, the cell elongates.
Auxin and cytokinin	Shoot tips make auxin and send it towards roots. Root tips make cytokinin and send it towards shoots. Together they integrate root and shoot growth.
Ethylene	Gas (ethene). Stimulates ripening, and ripening increases ethylene. Positive feedback gives rapid, synchronised ripening.

Phototropism chain: lateral light, auxin moves to the shaded side by efflux carriers, more proton pumping there, faster elongation, bend towards light.

Small worked reminders

Quantity	Rule	Example
Cardiac output	heart rate × stroke volume	90 × 85 = 7650 cm³ min⁻¹
Heart rate from ECG	60 / R-R interval in seconds	60 / 0.40 = 150 beats per minute
Ventilation rate	tidal volume × breaths per minute	1.0 × 20 = 20 dm³ min⁻¹
Percentage change	(new − old) / old × 100	(36 − 30) / 30 × 100 = 20%
Mean angle of curvature	sum of angles / number of seedlings	(30 + 35 + 40) / 3 = 35 degrees

Always give the unit and say how many significant figures or decimal places you used if the question asks.

Method in steps: explain a data table

1. Quote the trend with two figures from the table.
2. Name the receptor or gland involved.
3. Give the pathway in order (stimulus, receptor, control centre, effector, response).
4. State whether the feedback is negative or positive.

Extra detail worth knowing

• Melatonin: the retina detects light and the hypothalamus passes the information on to the pineal gland, which is why a bright screen at night can delay the evening rise.
• Epinephrine travels in the blood, so it reaches target cells throughout the body. The nervous system acts faster and more locally, so the two systems work together.
• The hypothalamus makes hormones that are released from the pituitary into the blood, and it also makes releasing hormones that tell the pituitary what to secrete.
• Medulla outputs to the heart travel along sympathetic nerves (speed up) and the vagus nerve (slow down).
• Auxin leaves cells through carriers rather than by simple diffusion, so a gradient can be built and kept across a tissue. (HL only)
• Ripening changes: chlorophyll breakdown, starch converted to sugar, cell walls softened, aroma and colour developed. (HL only)

Must-know distinctions

• CNS and ENS: voluntary start and end of the gut tract; involuntary middle.
• Baroreceptors and chemoreceptors: pressure versus chemistry.
• Negative and positive feedback: return to set point versus runaway change.
• Qualitative and quantitative: description or diagram versus measured numbers.
• Auxin and cytokinin: made at opposite ends of the plant and travelling opposite ways.

Quick self-test

1. Name the gland that secretes melatonin and state when its secretion peaks.
2. State two effects of epinephrine on the circulation.
3. Calculate cardiac output for a heart rate of 78 beats per minute and a stroke volume of 64 cm³.
4. Calculate the heart rate if the R-R interval is 0.50 s.
5. Calculate ventilation rate for a tidal volume of 0.55 dm³ and 12 breaths per minute.
6. Where are baroreceptors found?
7. Which part of the gut control is voluntary?
8. (HL only) Calculate the mean curvature of seedlings with angles of 20, 26 and 31 degrees.
9. (HL only) A stem section grew from 15.0 mm to 18.6 mm. Calculate the percentage increase.
10. (HL only) Explain why one ripe fruit can speed ripening of fruit nearby.
11. State what is changed when the medulla alters the heart’s stroke volume, and name the nerve that slows the heart.
12. (HL only) State where cytokinin is made and the direction it is transported.

Answers

1. The pineal gland; secretion peaks during the night (darkness).
2. Heart rate and stroke volume increase; blood is diverted to skeletal muscle.
3. 78 × 64 = 4992 cm³ min⁻¹.
4. 60 / 0.50 = 120 beats per minute.
5. 0.55 × 12 = 6.6 dm³ min⁻¹.
6. In the walls of the carotid arteries and the aortic arch.
7. Starting the swallow and releasing faeces.
8. (20 + 26 + 31) / 3 = 25.7 degrees (3 significant figures).
9. (18.6 − 15.0) / 15.0 × 100 = 24.0%.
10. The ripe fruit releases ethylene gas, which stimulates ripening in nearby fruit; they then make more ethylene (positive feedback).
11. The volume of blood pumped out by each beat changes; the vagus nerve slows the heart.
12. In the root tips; it is transported up to the shoots.

Where marks are usually lost

• Giving only one effect of epinephrine when asked to explain preparation for activity.
• Stating “the brain” instead of the medulla, or “the heart” instead of the nerves to the heart.
• Writing that exercise lowers oxygen so breathing increases, with no mention of carbon dioxide and pH.
• Leaving out units, such as dm³ min⁻¹ or beats per minute.
• Naming the wrong receptor type: baroreceptors for pressure, chemoreceptors for chemicals.
• Describing feedback without saying whether it is negative or positive.
• Saying auxin “makes cells grow” with no mechanism.
• Confusing where auxin and cytokinin are made.
• Quoting an angle of curvature without a mean or a sample size.
• Treating a hormone effect as instant: epinephrine acts through the blood and nerves act faster, so say which is which when comparing.

Official syllabus

This page follows the IB Biology guide, first assessment 2025, published by the International Baccalaureate Organization.