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

This study guide covers 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 for both SL and HL. C3.1.17 to C3.1.23 (plant growth regulation) are additional higher level, so they are HL only and are marked that way below. The nervous system, brain, spinal cord and reflex statements of C3.1 are taught on other pages.

Use it with the revision notes and the practice set. See also the IB DP Biology hub and the printable checklist. Not sure what to revise first? Try a free 10-minute diagnostic.

What this unit covers

Statement	What you must be able to do	Level
C3.1.11	Describe melatonin secretion by the pineal gland and its role in the sleep-wake cycle	SL and HL
C3.1.12	Explain how epinephrine prepares the body for vigorous activity	SL and HL
C3.1.13	Outline how the hypothalamus and pituitary gland regulate hormone secretion	SL and HL
C3.1.14	Explain feedback control of heart rate using baroreceptors and chemoreceptors	SL and HL
C3.1.15	Explain feedback control of ventilation rate using chemoreceptors	SL and HL
C3.1.16	Distinguish CNS and ENS control of peristalsis	SL and HL
C3.1.17	Record and evaluate observations of tropic responses in seedlings	HL only
C3.1.18	Explain positive phototropism in shoots	HL only
C3.1.19	Describe phytohormones as plant signalling chemicals	HL only
C3.1.20	Explain how auxin efflux carriers maintain concentration gradients	HL only
C3.1.21	Explain how auxin promotes cell growth	HL only
C3.1.22	Explain auxin and cytokinin interactions in root and shoot growth	HL only
C3.1.23	Explain the positive feedback between ethylene and fruit ripening	HL only

Feedback in one paragraph

Most of this unit is about control loops. A receptor detects a stimulus and sends information to a control centre. The control centre signals an effector, and the effector produces a response. In negative feedback the response reduces the original change, so the variable returns towards its set point. In positive feedback the response increases the change. Heart rate, ventilation and most hormone systems here use negative feedback. Ethylene in ripening fruit (HL only) is the positive example.

C3.1.11 Melatonin and circadian rhythms

The pineal gland, a small endocrine gland in the brain, secretes the hormone melatonin. Secretion follows a diurnal (daily) pattern. It is low during the day, begins to rise in the evening as light fades, peaks in the middle of the night and falls towards morning. Light detected by the eyes is relayed to the hypothalamus, which controls the pineal gland, so bright light suppresses melatonin.

Melatonin is not a switch. Rising levels signal night and promote sleepiness, and falling levels near dawn help waking. The result is a roughly 24-hour circadian rhythm of sleeping and waking, reset each day by light. Jet lag occurs because the melatonin cycle takes days to shift to new local daylight.

C3.1.12 Epinephrine and vigorous activity

The adrenal glands, one above each kidney, secrete epinephrine (adrenaline) into the blood when the body must act quickly. Because it travels in the blood, it reaches many target cells at once, and every effect supports intense muscle contraction:

• heart rate and stroke volume rise, so cardiac output rises
• blood vessels to skeletal muscle widen and those to the gut and skin narrow, diverting blood to muscle
• bronchioles widen, so ventilation is easier
• the liver converts glycogen to glucose, raising blood glucose for respiration
• pupils dilate

Worked example. Cardiac output = heart rate × stroke volume. At rest a person has a heart rate of 64 beats per minute and a stroke volume of 75 cm³, so cardiac output is 64 × 75 = 4800 cm³ min⁻¹. During a sprint, heart rate is 150 and stroke volume 112 cm³, giving 16 800 cm³ min⁻¹. That is 3.5 times the resting value, a rise of (16 800 − 4800) / 4800 × 100 = 250%.

C3.1.13 Hypothalamus and pituitary

The hypothalamus links the nervous and endocrine systems. It receives nervous input and monitors blood, then controls the pituitary gland hanging below it. You need a general understanding only, not the differences between the anterior and posterior lobes.

• Hypothalamic neurosecretory cells make hormones. Some pass down to the pituitary and are released into the blood. Others are releasing hormones that control what the pituitary secretes.
• Pituitary hormones may act directly on a target (antidiuretic hormone on the kidney) or control another gland (thyroid-stimulating hormone on the thyroid).

Example: when thyroxine in the blood falls, the hypothalamus releases more thyrotropin-releasing hormone. The pituitary then releases more thyroid-stimulating hormone, and the thyroid secretes more thyroxine. Rising thyroxine inhibits the hypothalamus and pituitary, which is negative feedback.

C3.1.14 Heart rate: baroreceptors and chemoreceptors

The heart beats without nervous input, but its rate and force are adjusted by the medulla oblongata in the brainstem.

• Baroreceptors monitor blood pressure. They sit in the walls of the carotid arteries and the aortic arch.
• Chemoreceptors detect blood pH, carbon dioxide and oxygen levels. They sit in the carotid and aortic bodies beside those arteries.

Sensory neurons carry impulses to the medulla, which sends impulses along nerves to the heart. Sympathetic nerves increase heart rate and stroke volume. The parasympathetic vagus nerve decreases heart rate.

Change detected	Medulla response	Result
Blood pressure rises	More impulses along the vagus, fewer along sympathetic nerves	Heart rate and stroke volume fall, pressure falls
Blood pressure falls	Reverse	Heart rate and stroke volume rise, pressure rises
Carbon dioxide rises, pH falls	More sympathetic impulses	Faster heart rate, more blood to the lungs

Worked example. An ECG shows 0.80 s between successive R waves at rest. Heart rate = 60 / 0.80 = 75 beats per minute. After a sprint the interval is 0.60 s, so heart rate = 60 / 0.60 = 100 beats per minute, a 33% increase.

C3.1.15 Ventilation rate and chemoreceptors

Cells produce carbon dioxide in respiration. It dissolves in blood plasma and forms carbonic acid, which releases hydrogen ions, so more carbon dioxide means lower blood pH. Anaerobic respiration adds lactate, which also lowers pH.

Chemoreceptors in the brainstem detect the fall in pH and send impulses to the ventilation centre in the medulla. The medulla signals the diaphragm and intercostal muscles to contract faster and harder, so ventilation rate and depth rise. More carbon dioxide leaves in exhaled air, blood pH rises and the stimulus is removed: negative feedback. A fall in carbon dioxide gives the opposite response. Chemoreceptors in arteries also contribute.

Worked example. Ventilation rate (volume per minute) = tidal volume × breaths per minute. At rest, 0.5 dm³ × 14 = 7.0 dm³ min⁻¹. In hard exercise, 2.1 dm³ × 32 = 67.2 dm³ min⁻¹, which is 9.6 times higher.

C3.1.16 Peristalsis

Peristalsis is a wave of muscle contraction that pushes food along the gut. Circular muscle contracts behind the food and longitudinal muscle shortens the segment, while the region ahead relaxes.

• Voluntary (CNS): starting a swallow, and releasing faeces (egestion) through the anal sphincter.
• Involuntary (ENS): between these points the enteric nervous system, a network of neurons in the gut wall, detects stretch and coordinates the waves without conscious thought.

The ENS makes sure contractions occur in the right order so food moves one way, not back and forth.

C3.1.17 Observing tropic responses (HL only)

A tropism is a directional growth response to a directional stimulus. To observe it, place seedlings in a box with a light slit on one side. Record qualitative observations as labelled diagrams: which way each shoot has bent and whether it did so below the tip. Record quantitative data too, by taking the angle of curvature from a photograph with a protractor.

Worked example. Eight seedlings with lateral light had curvatures of 34, 41, 38, 45, 36, 40, 43 and 35 degrees. Mean = 312 / 8 = 39 degrees, range 34 to 45 degrees. Eight seedlings with the tips covered by foil gave a mean of 2.1 degrees. The tip is therefore needed for the response.

Precision falls if the protractor is read coarsely or a curved shoot is hard to measure. Accuracy falls if the camera is not square to the seedling. Reliability improves with many seedlings of the same age and species, identical light and temperature, and repeats.

C3.1.18 Positive phototropism (HL only)

Positive phototropism is growth of a shoot towards light. With lateral light, cells on the shaded side elongate more than those on the lit side, so the shoot bends towards the source. The bend happens in the growing zone just below the tip.

C3.1.19 Phytohormones (HL only)

Phytohormones are signalling chemicals made in small amounts in plant tissues. They control growth, development and responses to stimuli. Several different chemicals do this:

Phytohormone	Example role
Auxin	Cell elongation, phototropism, apical dominance
Cytokinin	Promotes cell division, made in root tips
Gibberellin	Stem elongation, seed germination
Abscisic acid	Closes stomata in water stress
Ethylene	Fruit ripening

C3.1.20 Auxin efflux carriers (HL only)

Auxin is a weak acid. In the acidic apoplast (cell wall space) it can diffuse into cells, but in the more alkaline cytoplasm it cannot diffuse out. It leaves through efflux carriers (PIN proteins), which are active transport proteins. If the carriers sit on one side of each cell and every cell in a tissue lines them up the same way, auxin is passed cell to cell in one direction. This maintains a concentration gradient. In a shoot exposed to lateral light, carriers move so that auxin collects on the shaded side.

C3.1.21 Auxin and cell growth (HL only)

Auxin stimulates proton pumps in the cell surface membrane. Hydrogen ions are secreted into the apoplast, lowering its pH. The acid loosens cross links between cellulose molecules in the wall. Water enters the cell by osmosis and turgor pressure stretches the weakened wall, so the cell elongates. More auxin on the shaded side means faster elongation there, which explains phototropism.

Worked example. Stem segments of initial length 10.0 mm were placed in solutions for six hours. Final lengths: pH 7 buffer, 10.8 mm; pH 4.5 buffer, 12.6 mm; pH 7 buffer with auxin, 12.4 mm. Percentage increases are 8.0%, 26.0% and 24.0%. Auxin at pH 7 gives almost the same growth as acid alone, which supports the idea that auxin acts by acidifying the wall.

C3.1.22 Auxin and cytokinin (HL only)

Shoot tips make auxin, which is transported down towards the roots. Root tips make cytokinin, which is transported up to the shoots. Each organ therefore receives a chemical signal about the other. Their interaction coordinates the two systems: for example, auxin from the shoot tip inhibits growth of lateral buds, while cytokinin from the roots opposes this and promotes lateral bud growth. A balance between them helps match shoot growth to root growth.

C3.1.23 Ethylene and ripening (HL only)

Ethylene (IUPAC name ethene, C₂H₄) is a gas that stimulates ripening: chlorophyll breaks down, starch becomes sugar, cell walls soften, and colour and aroma develop. Ripening in turn increases ethylene production. This is positive feedback.

Worked example. Ethylene production (µl kg⁻¹ h⁻¹) in one fruit on days 0 to 8 was 0.1, 0.2, 0.5, 1.8, 6.0, 14.0, 22.0, 24.0, 23.0. Production rose 220-fold from day 0 to day 6, then levelled off. The benefit is that ripening becomes fast and synchronised, because ethylene diffuses to neighbouring fruit, so seeds are available to dispersers at the same time.

Common errors

• Saying melatonin “makes you sleep” without linking it to darkness and the circadian rhythm.
• Placing baroreceptors in the heart or lungs. They are in arteries.
• Saying the brain senses oxygen as the main trigger for ventilation. Carbon dioxide and pH are the usual driver.
• Writing that auxin makes cells “grow” with no mechanism: say proton pumping, acid, loosened cross links, osmosis.
• Calling ethylene feedback negative.

Official syllabus

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