Plant and Animal Hormones – Chemical Coordination

Introduction: Chemical Messages in Living Bodies

In Part 2 we met the nervous way of sending information — fast, electrical, and targeted to a specific muscle. But nerves cannot help with slow, steady changes such as growing taller, ripening a fruit or raising blood sugar for hours. These jobs are run by a second, slower postal service — hormones. A hormone is a chemical message, released in tiny amounts into the body fluid (sap in plants, blood in animals), that travels to a distant target and changes its behaviour.

6.2.2 Plant Hormones — Phytohormones

Plants do not have glands. They make hormones in tissues that are actively dividing — tips of shoots, roots and new leaves. These chemicals diffuse to other parts and control growth, flowering, fruit ripening and even leaf-drop.

6.3 Hormones in Animals — The Endocrine System

In animals, hormones are manufactured by endocrine glands scattered throughout the body. These are ductless — the hormone is poured directly into the blood.

Pituitary — The Master Gland

The pea-sized pituitary hangs just below the brain. It releases the growth hormone (GH), which controls the growth of bones and muscles during childhood. Too little GH in childhood causes dwarfism; too much causes gigantism. The pituitary also controls several other endocrine glands — that is why it is called the "master gland".

Thyroid — Needs Iodine

The butterfly-shaped thyroid in the neck secretes thyroxine, which regulates the body's metabolism (the rate at which food is burned for energy). Thyroxine contains iodine as an essential element. If the diet is low in iodine, the thyroid enlarges to try to capture more, producing a visible swelling at the neck called goitre. This is why table salt is iodised in India.

Adrenal — Fight-or-Flight

Two adrenal glands sit atop the kidneys. In moments of sudden fear, danger or excitement, they release adrenaline. Adrenaline speeds up the heart-beat, increases breathing rate, widens airways and diverts blood from the skin and digestive system to the skeletal muscles — preparing the body to fight or flee.

Pancreas — The Sugar Controller

The pancreas is interesting because it is both a digestive and an endocrine gland. Special cells in it (islets of Langerhans) secrete insulin. Insulin lowers blood-sugar level by helping body cells take up glucose. When the pancreas fails to make enough insulin, blood sugar stays high — a condition called diabetes mellitus. Such patients are often given insulin injections.

Testes and Ovaries — The Gonads

• Testes (in males) produce testosterone, responsible for male secondary sexual characters — facial hair, deeper voice, broader shoulders at puberty.
• Ovaries (in females) produce estrogen (female secondary characters — breast development, wider pelvis) and progesterone (regulates menstrual cycle and supports pregnancy).

Feedback Mechanism — How Hormones Stop at the Right Level

If hormones kept pouring out non-stop, the body would be thrown out of balance. Endocrine glands therefore work under a feedback mechanism, usually negative feedback: once the job is done, the hormone level itself signals the gland to stop.

The classic example is the insulin–glucose loop: