An animal is a network of organ systems. The body works because they talk to each other constantly — chemistry and current.
Every sub-topic below feeds at least one of these 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?
The required syllabus content for C3.1, in order. Each card is one lesson-sized checkpoint.
This is a necessary process in living systems. Coordination is needed for component parts of a system to collectively perform an overall function.
For example, a cheetah becomes an effective predator by integration of its body systems.
Integration of organs in animal bodies by hormonal and nervous signalling and by transport of materials and energy
The brain as a central information integration organ
The spinal cord as an integrating centre for unconscious processes
Input to the spinal cord and cerebral hemispheres through sensory neurons
Output from the cerebral hemispheres to muscles through motor neurons
Nerves as bundles of nerve fibres of both sensory and motor neurons
Pain reflex arcs as an example of involuntary responses with skeletal muscle as the effector
Role of the cerebellum in coordinating skeletal muscle contraction and balance
Modulation of sleep patterns by melatonin secretion as a part of circadian rhythms
Epinephrine (adrenaline) secretion by the adrenal glands to prepare the body for vigorous activity
Control of the endocrine system by the hypothalamus and pituitary gland
Feedback control of heart rate following sensory input from baroreceptors and chemoreceptors
Feedback control of ventilation rate following sensory input from chemoreceptors
Control of peristalsis in the digestive system by the central nervous system and enteric nervous system
Multicellular life depends on a hierarchy of subsystems working together. Integration at every level produces emergent properties — the whole becomes greater than the sum.
A cheetah is an effective predator because its sensory, muscular, skeletal, cardiovascular and nervous systems are integrated. None of them alone is sufficient. The integration produces an emergent property — fast pursuit hunting — that no component possesses on its own.
Animals integrate organs using two parallel communication systems plus a transport system to move materials between them.
| Feature | Nervous system | Endocrine system |
|---|---|---|
| Signal type | Electrical (action potentials) | Chemical (hormones) |
| Carrier | Neurons | Bloodstream |
| Speed | Milliseconds | Seconds to days |
| Duration | Brief | Sustained |
| Targets | Specific cell/tissue | All cells with matching receptor |
The bloodstream also transports the substances themselves — O₂, CO₂, glucose, hormones, nutrients, antibodies, urea, heat — making integration of physically separated organs possible.
Four named brain regions, plus the spinal cord with its reflex arcs.
Learning, memory, language, conscious thought, sensory perception, motor control. The outer wrinkled layer of the cerebrum.
Coordinates skeletal muscle contractions and balance. Processes positional and motion information to refine movement.
Controls heart rate, ventilation rate, blood pressure. The brainstem's "autopilot" — keeps you alive without thinking.
The hypothalamus monitors body conditions and signals the pituitary, which releases hormones to control other endocrine glands. Thermoregulation, water balance, growth, reproduction all loop through here.
Touch a hot stove. Pain receptors (nociceptors) in the skin → action potential in a sensory neuron → interneuron in the spinal cord's grey matter → motor neuron → muscle contracts → hand pulls away. Three neurons, no brain involvement, milliseconds. Speed prioritised over conscious processing.
Melatonin: daily rhythm, hours-long. Epinephrine: stress response, minutes.
Secreted by the pineal gland. Blue light suppresses it. Levels rise in the evening (sleepiness) and fall in the morning (wakefulness). Establishes circadian rhythms — sleep, body temperature cycles, hormone cycles.
Released by adrenal glands during stress or excitement. Effects: bronchioles dilate (more O₂); liver converts glycogen → glucose; heart beats faster and harder; blood is redirected to muscles; pupils dilate. The body is prepared for intense physical effort within seconds.
The hypothalamus monitors body state and sends releasing factors to the pituitary, which then releases hormones controlling other endocrine glands.
A two-stage system: hypothalamus → pituitary → target endocrine glands. Examples of chains: hypothalamus → pituitary releases TSH → thyroid releases thyroxine → controls metabolic rate. Or hypothalamus → pituitary releases ACTH → adrenal cortex releases cortisol → stress response.
Heart rate is constantly adjusted in response to blood pressure and blood chemistry. The medulla is the integration centre.
Chemoreceptors in the carotid arteries and the medulla monitor blood pH. The respiratory control centre adjusts ventilation accordingly.
During exercise, muscle respiration releases more CO₂ → CO₂ reacts with H₂O in blood → carbonic acid → pH falls. Chemoreceptors detect this and signal the respiratory control centre in the medulla. Output goes to the diaphragm and intercostal muscles → rate and depth of ventilation increase → more CO₂ exhaled → pH returns to ~7.35–7.45.
Voluntary control at the ends (swallowing, egestion); involuntary control of everything in between by the enteric nervous system.
Swallowing food and egestion of faeces are under voluntary control by the central nervous system. But moving food along the gut by peristalsis — coordinated waves of smooth muscle contraction — is involuntary. It's controlled by the enteric nervous system, a branch of the autonomic nervous system embedded in the gut wall. The enteric system has ~500 million neurons; large enough to be called the "gut's brain".
An extra 7 sub-topics for HL — same syllabus, deeper mechanism.
Observations of tropic responses in seedlings
Positive phototropism as a directional growth response to lateral light in plant shoots
Phytohormones as signalling chemicals controlling growth, development and response to stimuli in plants
Auxin efflux carriers as an example of maintaining concentration gradients of phytohormones
Promotion of cell growth by auxin
Interactions between auxin and cytokinin as a means of regulating root and shoot growth
Positive feedback in fruit ripening and ethylene production
A tropism is a directional growth response to a stimulus. Phototropism — directional growth in response to light — is the classic example.
Plant shoots show positive phototropism: they grow toward the light source. Roots show negative phototropism (growing away from light) and positive geotropism (growing toward gravity).
Germinate radish seedlings. Grow in cardboard boxes with light holes in different positions. Record qualitative observations (drawings) of growth direction; measure quantitative angle of curvature with a protractor. Reliability through replicate trials and standard deviation analysis.
Phytohormones — plant hormones — control growth, development and responses to stimuli. Concentration gradients within tissues do the actual signalling.
Auxin can diffuse freely into plant cells but, once inside, is modified so that it cannot diffuse back out. The only way for auxin to leave a cell is via auxin efflux carriers — membrane proteins that actively pump auxin out.
If all cells in a tissue place their efflux carriers on the same side, auxin is actively transported from cell to cell across the tissue. The result: a controllable concentration gradient of auxin within the plant — different concentrations in different regions, exactly where they're needed.
Auxin makes cells grow by indirectly loosening the cellulose cell wall, allowing the cell to elongate.
Phototropism mechanism: light from one side causes auxin to be redistributed via efflux carriers toward the shaded side. Cells on the shaded side accumulate more auxin → grow longer than cells on the lit side → the shoot curves toward the light.
Plant growth is coordinated by the balance between two hormones produced at opposite ends of the plant.
This balance is exploited in tissue culture: by manipulating the ratio of these two hormones, plant biologists can drive callus cells to produce either shoots or roots — essential for plant cloning and crop development.
Ethylene gas stimulates fruit ripening — and ripening fruit produces more ethylene. A self-amplifying loop that synchronises ripening across a whole crop.
Ethylene is a gaseous phytohormone (the IUPAC name is ethene — C₂H₄). Once ripening starts in one fruit, the ethylene released triggers ripening in neighbouring fruits, which release more ethylene, accelerating the process across the whole tree (or crate of bananas).
Commercial growers exploit this: harvest fruit unripe, ship it green, then expose to ethylene gas to ripen it on arrival. "One bad apple spoils the bunch" is literal — over-ripe fruit produces enough ethylene to over-ripen neighbours too.
If you can't define one of these in a sentence, that's where to revise next.
“What are examples of branching (dendritic) and net-like (reticulate) patterns of organization?”
“What are the consequences of positive feedback in biological systems?”
“In what ways are biological systems regulated?”