This document explains coordination and response in humans and plants for CAIE IGCSE Biology. It begins with the nervous system, dividing it into the central nervous system (brain and spinal cord) and peripheral nervous system (nerves and neurones). Different neurones—sensory, relay, and motor—are described, along with the reflex arc pathway and the role of synapses in transmitting impulses with neurotransmitters. The sense organs are outlined, focusing on the eye’s structure (cornea, iris, lens, retina, rods, cones, optic nerve) and processes like the pupil reflex and accommodation. Rods and cones are compared, showing differences in light sensitivity and colour vision. The document also covers hormones, including adrenaline, insulin, testosterone, and oestrogen, with emphasis on their roles and target organs. Homeostasis is explained through blood glucose control (insulin and glucagon) and temperature regulation (vasoconstriction, vasodilation, sweating, insulation). Disorders like diabetes and their treatments are highlighted. Finally, plant responses are introduced, with auxins controlling tropisms such as phototropism and gravitropism, showing how unequal auxin distribution leads to directional growth. With clear explanations of nervous, hormonal, and plant responses, this guide equips students with the knowledge to understand biological coordination and regulation.