Structures and Functions in Living Organisms · 7 question types
Past paper frequency (2018 to 2024)
This topic accounts for approximately 14% of your exam marks.
Nervous system structure, reflex arcs, and hormones are all commonly examined.
Every living organism has to detect what is happening around it and respond in a way that keeps it alive. A change in the environment that an organism can detect is called a stimulus; the action the organism then takes is called the response.
Every coordinated response uses the same five-step pathway:
stimulus → receptor → coordinator → → response
| Stage | What it is | Examples |
|---|---|---|
| Stimulus | A change the organism can detect | Light, sound, temperature, touch, pressure, chemical signals |
| Receptor | A cell or organ that detects the stimulus | Light-sensitive cells in the eye; touch receptors in the skin; taste buds; the inner ear |
| Coordinator | A control centre that processes information and decides what to do | Brain, spinal cord, pancreas |
| Effector | A muscle or gland that carries out the response | Skeletal muscle (contracts); sweat gland (secretes sweat); endocrine gland (releases a hormone) |
| Response | The action taken | Pulling a hand away, sweating to cool down, releasing insulin |
In humans, two systems carry out this coordination: the nervous system (fast, electrical signals) and the endocrine system (slower, chemical signals). They work side by side, each one good for different jobs.