Control and Coordination in Living Organisms
This domain asks how an organism detects a change and produces an ordered response. The Madhyamik paper tests neuron structure, the reflex arc, brain regions, plant and animal hormones, and tropic versus nastic movements. Expect one-mark definitions, sequence questions ('arrange the path of a reflex') and three-mark explanations.
Nervous coordination
- Neuron: dendrites receive, the axon conducts away, the myelin sheath insulates and speeds conduction at the nodes of Ranvier.
- A synapse transmits chemically through a neurotransmitter, so transmission is one-way.
Reflex action
- Path: stimulus, receptor, sensory neuron, spinal cord (association neuron), motor neuron, effector (muscle or gland).
- A reflex is involuntary and protects the body before the brain registers the pain.
Brain regions
- Cerebrum: intelligence, memory, voluntary movement. Cerebellum: balance and posture. Medulla oblongata: heartbeat, breathing, blood pressure. Hypothalamus: temperature, hunger, thirst and control of the pituitary.
Hormones
- Pituitary growth hormone (excess gigantism, deficit dwarfism); thyroxine needs iodine (deficiency goitre); insulin from the pancreas lowers blood sugar (deficiency diabetes mellitus); adrenaline prepares the fight-or-flight response.
- Plants: auxin for phototropism and apical dominance, gibberellin for stem elongation and germination, cytokinin for cell division, abscisic acid for stomatal closure and dormancy.
Worked example
A shoot on a windowsill bends towards the light. Auxin migrates away from the illuminated side, so it accumulates on the shaded side. Cells there elongate more than on the lit side, and the shoot curves towards the window.
Traps that cost marks
- Auxin collects on the shaded side, not the lit side.
- The medulla, not the cerebellum, controls heartbeat.
- Tropism is directional and growth-based; nastic movement such as the Mimosa leaf folding is non-directional and reversible.