Course review

C3.1 Integration of body systems

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Learning objective

C3.1.1—System integration

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• Integration lets interacting parts coordinate an overall biological function • Systems interact across molecular, cellular, organ, organism, and ecosystem levels

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C3.1.2—Hierarchy in multicellular organisms

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• Cells form tissues, organs, organ systems, and whole organisms • Emergent properties arise when subsystems interact, such as gut peristalsis and absorption

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C3.1.3—Integration of organs

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• Organs are integrated by nervous signals, hormones, and blood transport • Transport links nutrients, gases, wastes, hormones, and energy substrates between organs

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C3.1.4—Brain as information integration organ

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• The brain integrates sensory input and coordinates complex responses • Cerebral hemispheres, cerebellum, hypothalamus, and medulla have distinct roles

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C3.1.5—Spinal cord

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• The spinal cord links brain and peripheral nervous system • It integrates unconscious processes such as reflex arcs

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C3.1.6—Input through sensory neurons

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• Sensory neurons carry impulses from receptors to spinal cord and brain • Inputs may reach cerebral hemispheres for conscious perception

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C3.1.7—Output through motor neurons

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• Motor neurons carry impulses from CNS to skeletal muscle effectors • Voluntary muscle contraction is coordinated through cerebral hemispheres

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C3.1.8—Nerves as bundles

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• Nerves are bundles of sensory and motor nerve fibres in connective tissue • Mixed nerves carry impulses both to and from the CNS

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C3.1.9—Pain reflex arcs

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• Pain reflexes are rapid involuntary withdrawal responses • Reflex arcs use receptors, sensory neurons, relay neurons, motor neurons, and skeletal muscle

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C3.1.10—Cerebellum role

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• The cerebellum coordinates timing and force of skeletal muscle contractions • It maintains balance, posture, and smooth learned movements

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C3.1.11—Melatonin and sleep

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• The pineal gland secretes melatonin according to light-dark cycles • Melatonin modulates sleep timing as part of circadian rhythms

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C3.1.12—Epinephrine (adrenaline)

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• Adrenal glands secrete epinephrine during stress or danger • It increases heart rate, ventilation, and respiratory substrate availability

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C3.1.13—Hypothalamus and pituitary control

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• The hypothalamus links nervous inputs to endocrine control • Pituitary hormones regulate glands including thyroid, gonads, adrenals, and mammary glands

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C3.1.14—Heart rate feedback control

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• Baroreceptors monitor blood pressure; chemoreceptors monitor CO₂, pH, and O₂ • The medulla adjusts sinoatrial node activity by sympathetic and parasympathetic nerves

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C3.1.15—Ventilation rate feedback control

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• Chemoreceptors detect CO₂-driven pH changes in blood and cerebrospinal fluid • The medulla alters diaphragm and intercostal muscle activity to change ventilation rate

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C3.1.16—Peristalsis control

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• The CNS controls voluntary swallowing and egestion • The enteric nervous system coordinates involuntary peristalsis in gut smooth muscle

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C3.1.17 (HL)—Tropic responses in seedlings

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• Tropisms are directional growth responses in seedlings • Experiments compare shoot or root growth under lateral light or gravity stimuli

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C3.1.18 (HL)—Positive phototropism

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• Shoots show positive phototropism toward lateral light • Auxin redistribution causes greater elongation on the shaded side

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C3.1.19 (HL)—Phytohormones

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• Phytohormones are plant signalling chemicals controlling growth and development • Auxin, cytokinin, and ethylene coordinate responses to stimuli and internal state

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C3.1.20 (HL)—Auxin efflux carriers

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• PIN auxin efflux carriers actively move auxin between plant cells • Asymmetric carrier placement maintains auxin gradients during tropic responses

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C3.1.21 (HL)—Auxin promotes cell growth

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• Auxin activates proton pumping into the apoplast • Acidified cell walls loosen, allowing water uptake, elongation, and bending

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C3.1.22 (HL)—Auxin and cytokinin interactions

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• Auxin generally promotes root formation; cytokinin promotes shoot formation and division • Their ratio coordinates root-shoot growth and plant tissue culture outcomes

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C3.1.23 (HL)—Ethylene and fruit ripening

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• Ethylene is a gaseous phytohormone that promotes fruit ripening • Ripening fruit releases more ethylene, creating positive feedback in nearby fruit

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