Key Points

Living organisms respond to environmental stimuli using specialized systems. Animals use both the nervous system (electrical impulses) and endocrine system (hormones), while plants primarily use chemical coordination.

A neuron is the structural and functional unit of the nervous system. It consists of dendrites for receiving information, a cell body, and an axon for transmitting information as an electrical impulse.

A synapse is the microscopic gap between two adjacent neurons. At the synapse, the electrical impulse is converted into a chemical signal that diffuses across the gap to the next neuron.

A reflex action is a sudden, involuntary, and rapid response to a stimulus. It is a protective mechanism that does not involve conscious thought from the brain.

The neural pathway of a reflex action is called the reflex arc. The path is: Receptor $\rightarrow$ Sensory Neuron $\rightarrow$ Spinal Cord (Relay Neuron) $\rightarrow$ Motor Neuron $\rightarrow$ Effector (muscle or gland).

The human brain is divided into three main regions: the fore-brain, mid-brain, and hind-brain, each with specialized functions.

The fore-brain is the main thinking part of the brain. It controls voluntary actions and has separate areas for processing sensory information like hearing, sight, and smell.

The hind-brain consists of the cerebellum, medulla, and pons. The cerebellum controls posture, balance, and precision of voluntary movements, while the medulla controls involuntary actions like blood pressure and vomiting.

Plants exhibit two types of movements: growth-independent (nastic), which are rapid responses to stimuli like touch, and growth-dependent (tropic), which are slow, directional movements.

Tropic movements are directional growth responses in plants. Examples include phototropism (response to light), geotropism (response to gravity), and hydrotropism (response to water).

Phytohormones are chemical compounds that regulate plant growth and development. Major plant hormones include auxins, gibberellins, cytokinins, and abscisic acid.

Auxin is a plant hormone that promotes cell elongation. In phototropism, auxin diffuses to the shady side of the stem, causing cells there to grow longer and the stem to bend towards light.

Auxins, gibberellins, and cytokinins are growth-promoting hormones. Abscisic acid (ABA) is a growth-inhibiting hormone that causes effects like the wilting of leaves.

The endocrine system provides chemical coordination through hormones secreted by endocrine glands directly into the blood. These hormones act on specific target organs.

Secreted by the adrenal glands, adrenaline prepares the body for 'fight or flight' situations. It increases heart rate, breathing rate, and blood flow to muscles.

Insulin is a hormone produced by the pancreas that regulates blood sugar levels. Its deficiency leads to high blood sugar and causes diabetes.

The thyroid gland produces thyroxine, which regulates metabolism. Iodine is essential for the synthesis of thyroxine, and its deficiency can lead to a condition called goitre.

The secretion of hormones is regulated by a feedback mechanism. For example, a rise in blood sugar stimulates the pancreas to secrete more insulin, and as sugar levels fall, insulin secretion is reduced.