How animals and plants sense and respond to change
In animals, the nervous system detects changes and coordinates responses. Receptors in sense organs pick up stimuli, and neurons carry electrical impulses to the brain or spinal cord. A reflex action, such as pulling your hand away from a hot object, is a quick automatic response controlled by the spinal cord through a reflex arc. The brain has three main parts: the forebrain for thinking and senses, the midbrain, and the hindbrain, whose cerebellum controls balance and medulla controls breathing and heartbeat.
Plants have no nervous system but respond through hormones and growth movements. A shoot bends towards light, which is phototropism, and roots grow downwards with gravity, which is geotropism. Touch-sensitive plants like Mimosa fold their leaves quickly. Plant hormones include auxins for growth towards light, gibberellins for stem elongation, cytokinins for cell division and abscisic acid, which slows growth. In humans, endocrine glands release hormones: insulin controls blood sugar, thyroxine needs iodine, and adrenaline prepares the body for emergencies.
Why it matters
This topic explains why we blink when something comes near our eyes, why iodised salt prevents goitre, and why people with diabetes may need insulin. Farmers use plant hormones to ripen fruit and produce seedless grapes. Wearing helmets protects the brain, which controls every part of the body. It connects to human physiology in Class 11.
Key terms
- Reflex action
- A quick, automatic response to a stimulus without conscious thought.
- Phototropism
- The growth movement of a plant part in response to light.
- Auxin
- A plant hormone that promotes cell elongation and growth towards light.
- Endocrine gland
- A gland that releases hormones directly into the blood.