C3.1 Integration of body systemsIB Biology HL: Revision notes

Organisms are a hierarchy of subsystems (cells → tissues → organs → systems) that must be coordinated. Integration gives emergent properties — for example, a cheetah hunts effectively only because its body systems work together.

Animal organs are integrated by nervous signalling (fast, targeted electrical impulses), hormonal signalling (hormones carried in the blood to all tissues; only cells with receptors respond; slower and longer-lasting) and transport of materials and heat in the blood.

• The brain integrates information from several inputs and is the site of learning and memory; conscious processes involve the cerebral hemispheres.
• The spinal cord integrates unconscious processes.
• Sensory neurons carry impulses from receptors to the CNS; motor neurons carry impulses to muscles, which contract.
• A nerve is a bundle of sensory and motor fibres (myelinated and unmyelinated) in a protective sheath.
• Pain reflex arc: free nerve ending in the hand → sensory neuron → one interneuron in spinal cord grey matter → motor neuron → skeletal muscle.
• The cerebellum coordinates skeletal muscle contraction and balance.

• Melatonin from the pineal gland rises in darkness and falls in morning light, setting the circadian sleep–wake cycle.
• Epinephrine from the adrenal glands raises heart rate, ventilation and blood glucose and diverts blood to skeletal muscle, preparing for vigorous activity.
• The hypothalamus controls the pituitary gland, which regulates other endocrine glands.
• Heart rate: baroreceptors (aorta, carotid arteries) monitor blood pressure; chemoreceptors monitor pH, O₂ and CO₂. The medulla sends impulses to the heart to change heart rate and stroke volume.
• Ventilation: CO₂ forms carbonic acid and lowers blood pH; brainstem chemoreceptors trigger faster, deeper contraction of the diaphragm and intercostal muscles.
• Peristalsis: swallowing and egestion are voluntary (CNS); peristalsis between is involuntary (enteric nervous system).

Positive phototropism is a directional growth response of plant shoots towards lateral (one-sided) light, which helps the leaves absorb more light for photosynthesis.

When investigating tropisms in seedlings:

• Qualitative observations are descriptive (e.g. "shoots bent towards the light"), often recorded as annotated drawings.
• Quantitative observations are numerical, such as the angle of curvature in degrees.

Precision is how fine and repeatable a measurement is; accuracy is how close it is to the true value. Improve them by measuring from photographs against a fixed reference line, using finer-scaled instruments, using larger samples of seedlings and repeating measurements; reliability improves with repeats and controlled conditions (same age of seedlings, same light intensity and duration).

Phytohormones are signalling chemicals that control growth, development and responses to stimuli; plants use a variety of them.

Auxin can diffuse freely into plant cells but cannot diffuse out. It leaves only through auxin efflux carriers in the plasma membrane. These can be placed on one side of a cell; if all cells put their carriers on the same side, auxin is actively transported from cell to cell and becomes concentrated in one part of the plant.

Acid growth: auxin promotes secretion of H⁺ ions into the apoplast. The acidified cell wall has its cross-links between cellulose molecules loosened, so the wall stretches under turgor and the cell elongates.

In phototropism, auxin is moved to the shaded side of the shoot; cells there elongate more, so the shoot bends towards the light.

Root tips produce cytokinin, which is transported to the shoots; shoot tips produce auxin, which is transported to the roots. The interaction between these two phytohormones ensures that root and shoot growth are integrated — each part responds to signals about the other.

Ethylene (IUPAC name ethene) stimulates the changes in fruit during ripening, and ripening stimulates more ethylene production. This positive feedback makes ripening rapid and synchronized: all the fruits on a plant ripen together, which attracts seed-dispersing animals.