What is Hormones? Definition, Types, Applications, Deficiency Diseases & Class 12 Notes

The definition of hormones, according to NCERT, 

Hormones are molecules that act as intercellular messengers in living organisms.

Endocrine glands produce hormones, which travel through the bloodstream to reach the target organs. They are made up of different molecules. The function of hormones is to maintain the balance of biological activities in the body.

There are different types of hormones based on their chemical composition and behaviour inside the body. Here we have mentioned three main types of hormones: steroid, peptide and amino acid-derived.

Steroid Hormones 

These hormones are lipid-based molecules produced from cholesterol. They are fat-soluble, meaning they can easily travel through the cell membrane. 

• Steroid Hormones Actions 
  • These hormones enter the cells and bind with receptor proteins inside the cytoplasm or nucleus. 

• • The hormone receptor complex attaches to the DNA and changes the expression of specific genes. 

• • The action of Steroid hormones is slow but long-lasting. 
• Examples of Steroid hormones 
  • Cortisol 

• • Estrogen & Progesterone 

• • Aldosterone 

• • Testosterone 

Peptide (Protein) Hormones

This hormone is made from chains of amino acids. It is water-soluble (hydrophilic) and cannot pass through the lipid cell membrane. 

• Action of Peptide Hormones

• • They bind to receptor proteins on the surface of target cells, activating the second messenger system, inside the cell (like cAMP or calcium ion).  

• • The second messenger triggers a chain of chemical reactions, switching enzymes on or off. 

• • The action of peptide hormones is fast but short-lived compared to steroid hormones. 
• Example of Peptide Hormones 
• • Insulin 

• • Oxytocin 

• • Growth Hormones 

• • Glucagon 

Amino Acid-Derived Hormones 

This hormone is modified from single amino acids. Some of these are water-soluble, others are fat-soluble. 

• Action of Amino Acid-Derived Hormones 
  • Based on solubility, they behave like peptide hormones or like steroid hormones. 

• • Usually, they act fast and control the energy levels, alternes, and daily rhythms. 
• Examples of Amino Acid-Derived Hormones 
  • Adrenaline (Epinephrine) 

• • Thyroxine (T4) and Triiodothyronine (T3) 

• • Melatonin

Some Important Links:

NCERT Class 11 notes	NCERT Class 11 Physics notes
NCERT Class 11 Chemistry notes	NCERT Class 11 Maths notes

There are different types of hormones in living organisms. The functions of hormones vary from each other. Here we have mentioned some important functions.

Regulating Metabolism 

• Hormones like thyroxine regulate metabolism - how fast the body uses glucose and proteins. It speeds up the chemical reaction to release energy in cells.

Controlling Growth and Development 

• Peptide hormones release the proteins for synthesis in bones and muscles. This helps in muscle growth, cell division, and bone elongation. 

Managing Stress and Energy 

• Hormones like adrenaline and cortisol provide energy and prepare the body for emergencies. Adrenaline activates a fight-or-flight reaction by increasing glucose breakdown, while cortisol (steroid hormones) changes protein and fat metabolism. 

Regulating Sleep and Mood 

• Melatonin is a type of hormone which is derived from tryptophan and controls the circadian rhythm by acting on brain receptors. On the other hand, serotonin and dopamine, derived from amino acids, are responsible for influencing mood. 
• These hormones control sleep cycles, memory and emotions. 

We know that hormones act like chemical messengers in the body. Below are some important characteristics of hormones.

Also Check: 

NCERT Class 12 notes	NCERT Class 12 Chemistry
NCERT Class 12 Maths notes	NCER Class 12 Physics notes

Hormones are Chemical Messengers

In living organisms, hormones are biochemical messengers, unlike nerves that carry electrical signals. They flow through the blood vessels and reach the distant parts of the body. Think of sending a letter to your friend via the post. It may take time to reach, but they reach the correct address. 

Produced by Endocrine Glands

• Hormones are produced in the blood by ductless glands (endocrine glands). For example. 

• The pituitary gland produces growth hormone, which is responsible for the growth of height, bone length, and muscles. 

• The thyroid gland makes thyroxine, which controls metabolism and body temperature. 

• Pancreases make insulin and glucagon that regulate blood sugar levels. 

Transported by Blood

• After being formed, hormones are dissolved in the blood and travel to the target organ or cell. 

Specific in Action

• Some hormones' actions are limited. They only act on the cells that have the right receptors. This is referred to as a lock-and-key mechanism. For example, insulin only acts on the liver and muscles because they have insulin receptors. 

 Some Other Characteristics of Hormones 

1. Controlled by feedback mechanisms 
2. Chemically diverse 
3. Help maintain homeostasis 
4. Regulate vital physiological processes 
5. Short-Lived Action 
6. Effective in very small quantities 

 So far, you have understood what hormones are and their types; now, we will discuss how hormones exactly send the signal to the target cells. The mechanism of hormones depends on their chemical nature. Below are the step-by-step mechanism actions. 

General Mechanism of Hormone Actions

• First of all, hormones are produced by endocrine glands. Then they dissolved in the blood and reached the target cells. Hormones know which cell to target because each cell has receptors (like a specific address).  
• After binding, hormones activate a series of chemical reactions inside the cell. This changes the behaviour of cells, like increasing metabolism, producing proteins, or regulating growth.

Two Main Pathways of Hormone Mechanism 

• Lipid-Soluble Hormones (Steroid Hormones & Thyroid Hormones) 
  • These hormones are fat-soluble and can easily pass through the cell membrane.  
  • They bind to the intracellular receptors inside the cell. 
  • The hormone receptor attaches to DNA and influences gene expression. 
  • This forms a new protein that changes cell activity. 
  • Effect: The effect of the hormone is slow but long-lasting. 
  • Examples of lipid-soluble hormones are cortisol, estrogen, testosterone, and Thyroxine.

• Water Soluble Hormones (Peptide, Protein, and Amino Acid Derived Hormones) 
  • These hormones cannot pass through the lipid membrane. 
  • They bind to the receptors on the cell surface, activating the second messenger system (like cyclic AMP or calcium ions). 
  • The second messenger activates the enzymes inside the cell and produces quick changes in cell function. 
  • Effect of Water-Soluble Hormones: It affects fast, but it lasts for a short period.

Fig: Binding of Water-Soluble Hormones

Feedback Control in Hormone Mechanism

The actions of hormones are regulated by feedback loops: 

1. Negative feedback: Balance the hormone level. (e.g. insulin & glucagon maintain blood sugar).
2. Positive feedback: It is rare but amplifies the effect.

The endocrine system is like a chemical messaging network. It uses hormones rather than electrical signals to control and coordinate activities like growth, stress response, metabolism and reproduction. Some of the major endocrine glands and the hormones they produce.

Also Read: NCERT Notes for Class 11 & 12

Hypothalamus

It is located at the base of the brain. The hypothalamus acts as the master controller; it links the nervous system to the endocrine system. Some of the major hormones are: 

• TRH (Thyrotropin-Releasing Hormone) 

• CRH (Corticotropin-Releasing Hormone) 

• GnRH (Gonadotropin-Releasing Hormone) 

• GHRH (Growth Hormone-Releasing Hormone) 

Pituitary Gland (Master Gland) 

The pituitary gland is located below the hypothalamus. It controls endocrine glands. The major hormones of the pituitary gland are 

• Anterior Pituitary: 

• Growth Hormones 

• Prolactin 

• Thyroid-Stimulating Hormone 

• Luteinizing Hormone 

• Follicle-Stimulating Hormone 

• Adrenocorticotropic Hormone 

• Posterior Pituitary 

• Oxytocin 

• Antidiuretic Hormone (ADH / Vasopressin) 

Thyroid Gland

This gland is located in the neck region (in front of the trachea). The major thyroid gland hormones are. 

• Thyroxine (T4 and Triiodothyronine (T3) 

• Calcitonin 

Some other major endocrine glands and their hormones 

1. Adrenal Gland 
2. Pancreas 
3. Parathyroid Gland 
4. Pineal Gland 
5. Thymus Gland 

The functioning of hormones depends on various factors that influence their role. Here, we have explained the factors affecting hormone function. 

Hormone Concentration 

The reaction of the hormone is proportional to its concentration in the blood. Low levels may lead to insufficient stimulation of target organs, and the excess level leads to negative feedback inhibition. For example, low insulin may lead to high blood sugar, while excess insulin may cause hypoglycemia. 

Blood Flow of Target Tissue 

We know that hormones travel to the target cell through the bloodstream, so proper blood circulation is essential. For example, poor blood circulation may affect the functions of tissues. 

Age, Sex, and Genetic Factors 

Hormone levels and response may vary with age and sex. Genetic factors may affect hormone production, metabolism or receptor. For example, growth hormones are high in children and decrease with age. 

Environment and Lifestyle Factors 

Physical activities, sleep, diet, and stress can affect hormone levels and action. For example, metabolism and immunity are affected when chronic stress is increased.