IB Biology · Theme C: Interaction and interdependence · SL and HL
C3.1 Integration of body systems
A one-page summary of C3.1 Integration of body systems, the key terms to know, and sample exam questions with answers. For the full lesson, open the illustrated revision slides or practise in the app.
Guiding questions
What are the roles of nerves and hormones in integration of body systems?
What are the roles of feedback mechanisms in regulation of body systems?
What C3.1 covers
Principles of system integration
- C3.1.1Coordination makes a system whole
- C3.1.2A cheetah is more than its cells
- C3.1.3Two signals, two very different speeds
- C3.1.3The blood system is a third integrator
The nervous system as an integrator
- C3.1.4One organ, many inputs
- C3.1.5A second integrating centre, working unconsciously
- C3.1.6The nervous system's input side
- C3.1.7The nervous system's output side
- C3.1.8A nerve is a bundle, not a single cell
- C3.1.9A reflex that never reaches the brain
- C3.1.10The brain region behind smooth movement
Hormonal and rhythmic control
- C3.1.11A hormone that tracks day and night
- C3.1.12One hormone, effects everywhere
- C3.1.13The endocrine system's master switch
Feedback control of physiological variables
- C3.1.14Keeping blood pressure on target
- C3.1.15Chemistry drives every breath
- C3.1.16One tract, two kinds of control
- C3.1.16 · Data bookletHow a peristaltic wave moves food
Plant responses and phytohormones · HL
- C3.1.17 · HLTurning a bent stem into data
- C3.1.18 · HLBending toward the light
- C3.1.19 · HLA plant's substitute for nerves
- C3.1.20 · HLOne-way doors for a growth signal
- C3.1.21 · HLAcid unlocks a longer cell
- C3.1.22 · HLRoot and shoot, talking in both directions
- C3.1.23 · HLRipening that feeds on itself
C3.1 Integration of body systems: summary
System integration
- Coordinated via nervous, hormonal and blood transport channels.
- Hierarchy of subsystems produces emergent properties (e.g. a cheetah's sprint).
Nervous integration
- Brain combines sensory inputs plus learning/memory; spinal cord integrates unconscious reflexes independently.
- Cerebellum coordinates muscle contraction and balance.
Nerves in detail
- Sensory neurons carry input to the CNS; motor neurons carry output to effectors.
- A nerve is a mixed bundle of both, plus a protective sheath.
Hormonal & rhythmic control
- Melatonin (circadian sleep cycle) and epinephrine (vigorous-activity response) act via the blood.
- Hypothalamus-pituitary links neural input to hormonal output.
Feedback control
- Baroreceptors/chemoreceptors adjust heart rate; chemoreceptors adjust ventilation rate — both negative feedback.
- CNS + ENS split control of peristalsis.
HL · Plant integration
- Auxin efflux carriers create the concentration gradient driving phototropism via the acid growth mechanism.
- Auxin-cytokinin balance root/shoot growth; ethylene drives positive-feedback fruit ripening.
Key terms
- System integration
- Coordination of an organism's separate parts so they collectively perform an overall function.
- Emergent property
- A property of a whole, integrated system that cannot be predicted from its parts studied alone.
- Baroreceptor
- A receptor that detects blood pressure via stretch of an artery wall.
- Chemoreceptor
- A receptor that monitors blood pH and the concentrations of oxygen and carbon dioxide.
- Enteric nervous system
- The network of neurons within the gut wall that controls peristalsis involuntarily.
- Circadian rhythm
- A roughly 24-hour biological cycle normally synchronized to the day/night cycle.
- Phototropism HL
- A directional growth response of a plant to light arriving from one particular direction.
- Phytohormone HL
- A chemical signalling molecule that controls plant growth, development and response to stimuli.
- Auxin efflux carrier HL
- A membrane protein positioned on one side of a plant cell that lets auxin exit only that side.
Sample exam questions
Three of the 70 multiple-choice questions for C3.1. Try each one before opening the answer.
Question 1. Which statement best describes 'system integration' in a multicellular organism?
- The random arrangement of different cell types within an organ
- The coordination of separate component parts so they collectively perform an overall function
- The process by which a single cell divides to form two daughter cells
- The replacement of damaged tissue with scar tissue
Show the answer
Answer: B. System integration is the coordination of cells, tissues, organs and body systems so that they work together to produce an overall function that no single part could achieve alone.
Question 2. Which of the following is the best example of why system integration is necessary in living systems?
- A single skin cell dividing by mitosis
- A red blood cell lacking a nucleus
- An enzyme having a specific active site
- A muscle contracting only when it receives a coordinated nerve impulse and adequate blood supply of oxygen and glucose
Show the answer
Answer: D. A muscle contraction depends on coordinated input from the nervous system (the signal to contract) and the circulatory system (delivering oxygen and glucose, removing waste) — illustrating that even a 'simple' function requires multiple systems to be integrated.
Question 3. A rise in blood pressure detected by baroreceptors leads, via the brainstem, to which change in heart rate?
- An increase in heart rate
- No change in heart rate
- A decrease in heart rate, as negative feedback to bring pressure back toward normal
- A change only in breathing rate, not heart rate
Show the answer
Answer: C. Baroreceptors detecting raised blood pressure send increased sensory input to the medulla, which responds by decreasing heart rate (and stroke volume) via increased parasympathetic output — negative feedback correcting the pressure back down.
Practise C3.1
Study notes, every question and full markschemes for C3.1 are in the app with Pro. Two lessons are completely free to try: A1.1 Water and B1.1 Carbohydrates and lipids.