IB Biology HL C3.1 Integration of body systems Notes

Integration lets interacting parts coordinate an overall biological function. A useful answer traces input, communication, integration, output, and response across molecular, cellular, organ, organism, and ecosystem levels.

Integration means separate parts acting as one coordinated system.

Cells form tissues, organs, organ systems, and whole organisms. Emergent properties arise when subsystems interact, such as gut peristalsis and absorption: no single smooth-muscle cell digests a meal, but coordinated tissues and organs move and absorb food.

Higher levels do things that isolated cells cannot do alone.

Organs are integrated by nervous signals, hormones, and blood transport. Nerves send fast targeted messages, hormones send slower blood-borne messages, and transport links nutrients, gases, wastes, hormones, and energy substrates between organs.

Fast nerves, slower hormones, and material transport integrate organs.

The brain integrates sensory input and coordinates complex responses. The cerebral hemispheres combine inputs for perception, learning, and memory, while the cerebellum, hypothalamus, and medulla have distinct roles in movement, endocrine control, and autonomic regulation.

The spinal cord links brain and peripheral nervous system. It integrates unconscious processes such as reflex arcs, so a withdrawal response can begin before the brain has produced conscious awareness of pain.

Sensory neurons carry impulses from receptors to spinal cord and brain. Receptor cells convert stimuli into nerve impulses; some inputs are integrated in spinal reflexes, while others reach the cerebral hemispheres for conscious perception.

Motor neurons carry impulses from CNS to skeletal muscle effectors. For voluntary movement, cerebral hemispheres coordinate output so muscles are stimulated to contract in a controlled pattern.

Motor output turns CNS processing into muscle contraction.

Nerves are bundles of sensory and motor nerve fibres in connective tissue. Mixed nerves carry impulses both to and from the CNS, so one nerve can contain afferent sensory fibres and efferent motor fibres.

Mixed nerves carry two traffic directions in one bundle.

Pain reflexes are rapid involuntary withdrawal responses. Reflex arcs use receptors, sensory neurons, relay neurons, motor neurons, and skeletal muscle, so the limb can withdraw before conscious decision-making.

A spinal reflex protects first; conscious perception follows.

The cerebellum coordinates timing and force of skeletal muscle contractions. It compares intended movement with sensory feedback so balance, posture, and smooth learned movements can be adjusted continuously.

Support Cerebellum Role with an inspectable mobile visual.

The pineal gland secretes melatonin according to light-dark cycles. Darkness raises melatonin, while light input from the retina via the hypothalamic suprachiasmatic nucleus suppresses secretion, helping set the sleep-wake rhythm.

Adrenal glands secrete epinephrine during stress or danger. Epinephrine circulates in blood and prepares the body for vigorous activity by increasing heart rate, ventilation, blood flow to skeletal muscles, and respiratory substrate availability.

The hypothalamus links nervous inputs to endocrine control. It regulates the pituitary gland, whose hormones influence other endocrine glands including thyroid, gonads, adrenals, and mammary glands.

The hypothalamus-pituitary link turns nervous information into endocrine control.

Baroreceptors in the aortic arch and carotid sinuses monitor blood pressure; chemoreceptors monitor CO₂, pH, and O₂. The medulla coordinates autonomic output to the heart, changing sinoatrial node activity, heart rate, and stroke volume.

Heart-rate control is sensory input plus medulla output to the heart.

Chemoreceptors detect CO₂-driven pH changes in blood and cerebrospinal fluid. The medulla sends signals to the diaphragm and intercostal muscles to change ventilation rate and ventilation volume, removing CO₂ faster when exercise raises respiration.

CO₂-driven pH change is the key monitored variable.

The CNS controls voluntary swallowing and egestion. Between those endpoints, the enteric nervous system coordinates involuntary peristalsis in gut smooth muscle so material moves continuously through the alimentary canal.

Tropisms are directional growth responses in seedlings. Experiments compare shoot or root growth under lateral light or gravity stimuli. A strong investigation records qualitative observations with labelled diagrams and quantitative observations by measuring angle of curvature, then considers reliability, accuracy, and precision.

Shoots show positive phototropism toward lateral light. Auxin redistribution causes greater elongation on the shaded side, so shaded cells elongate more than illuminated cells and the shoot bends toward the light source.

Phytohormones are plant signalling chemicals controlling growth and development. Auxin, cytokinin, and ethylene coordinate responses to stimuli and internal state, so plants can integrate growth direction, root-shoot balance, and fruit ripening without a nervous system.

Phytohormones coordinate plant responses across different tissues.

PIN auxin efflux carriers actively move auxin between plant cells. Auxin can diffuse into cells but becomes trapped after ionization unless efflux carriers export it; asymmetric carrier placement maintains auxin gradients during tropic responses.

Polar PIN placement turns local auxin export into a tissue-level gradient.

Auxin activates proton pumping into the apoplast. The lower pH loosens cross-links between cellulose fibres and promotes expansin activity; potassium ion and water entry then increase turgor so acidified cell walls loosen, allowing water uptake, elongation, and bending.

Auxin promotes elongation by acidifying the cell wall before water-driven expansion.

Auxin generally promotes root formation; cytokinin promotes shoot formation and division. Shoot tips produce auxin transported to roots, root tips produce cytokinin transported to shoots, and their ratio coordinates root-shoot growth and plant tissue culture outcomes.

Ethylene is a gaseous phytohormone that promotes fruit ripening. Ripening fruit releases more ethylene, creating positive feedback in nearby fruit so ripening is rapid and synchronized.

Support Ethylene and Fruit Ripening with an inspectable mobile visual.

Body-system integration lets interacting parts coordinate an overall biological function; systems interact across molecular, cellular, organ, organism, and ecosystem levels. Cells form tissues, organs, organ systems, and whole organisms; emergent properties arise when subsystems interact, such as gut peristalsis and absorption. Organs are integrated by nervous signals, hormones, and blood transport; transport links nutrients, gases, wastes, hormones, and energy substrates between organs. The brain integrates sensory input and coordinates complex responses; cerebral hemispheres, cerebellum, hypothalamus, and medulla have distinct roles. The spinal cord links brain and peripheral nervous system; it integrates unconscious processes such as reflex arcs. Sensory neurons carry impulses from receptors to spinal cord and brain; inputs may reach cerebral hemispheres for conscious perception. Motor neurons carry impulses from CNS to skeletal muscle effectors; voluntary muscle contraction is coordinated through cerebral hemispheres. Nerves are bundles of sensory and motor nerve fibres in connective tissue; mixed nerves carry impulses both to and from the CNS. Pain reflexes are rapid involuntary withdrawal responses; reflex arcs use receptors, sensory neurons, relay neurons, motor neurons, and skeletal muscle. The cerebellum coordinates timing and force of skeletal muscle contractions; it maintains balance, posture, and smooth learned movements. The pineal gland secretes melatonin according to light-dark cycles; melatonin modulates sleep timing as part of circadian rhythms. Adrenal glands secrete epinephrine during stress or danger; it increases heart rate, ventilation, and respiratory substrate availability. The hypothalamus links nervous inputs to endocrine control; pituitary hormones regulate glands including thyroid, gonads, adrenals, and mammary glands. Baroreceptors monitor blood pressure; chemoreceptors monitor CO2, pH, and O2; the medulla adjusts sinoatrial node activity by sympathetic and parasympathetic nerves. Chemoreceptors detect CO2-driven pH changes in blood and cerebrospinal fluid; the medulla alters diaphragm and intercostal muscle activity to change ventilation rate. The CNS controls voluntary swallowing and egestion; the enteric nervous system coordinates involuntary peristalsis in gut smooth muscle.

Use this for questions that combine nervous, endocrine, transport, and feedback control in body-system integration.

Plant signalling integrates directional growth, phytohormones, and responses to stimuli. Tropisms are directional growth responses in seedlings; experiments compare shoot or root growth under lateral light or gravity stimuli. Shoots show positive phototropism toward lateral light; auxin redistribution causes greater elongation on the shaded side. Phytohormones are plant signalling chemicals controlling growth and development; auxin, cytokinin, and ethylene coordinate responses to stimuli and internal state. PIN auxin efflux carriers actively move auxin between plant cells; asymmetric carrier placement maintains auxin gradients during tropic responses. Auxin activates proton pumping into the apoplast; acidified cell walls loosen, allowing water uptake, elongation, and bending. Auxin generally promotes root formation; cytokinin promotes shoot formation and division; their ratio coordinates root-shoot growth and plant tissue culture outcomes. Ethylene is a gaseous phytohormone that promotes fruit ripening; ripening fruit releases more ethylene, creating positive feedback in nearby fruit.

Use this for HL questions on tropisms, auxin transport/action, cytokinin balance, and ethylene ripening.