Hormones, Biological Importance & Tests

Hormones — Chemical Messengers

Hormones are molecules that act as intercellular (chemical) messengers. They are produced by the endocrine glands and carried by the blood to target organs, where they regulate metabolism, growth and other body processes. They are needed in very small amounts, and the body keeps their levels carefully balanced.

Hormones are of several chemical types:

• Steroid hormones — e.g. the sex hormones estrogen and testosterone, and cortisol (from cholesterol).
• Polypeptide/protein hormones — e.g. insulin and glucagon (control blood glucose).
• Amino-acid-derived hormones — e.g. adrenaline (epinephrine) and thyroxine (from the amino acid tyrosine).

[NEET Important] Know the famous examples: insulin lowers blood glucose (its deficiency causes diabetes mellitus); thyroxine (needs iodine) controls metabolic rate (deficiency causes goitre); adrenaline is the "fight-or-flight" hormone.

Key Point: hormones are chemical messengers from endocrine glands that regulate body processes; examples — insulin (blood glucose), thyroxine (metabolism), adrenaline (fight-or-flight), sex hormones (steroids).

Biological Importance of the Biomolecules

A quick consolidation of what each class of biomolecule does:

• Carbohydrates — the body's main energy source (glucose) and energy store (starch in plants, glycogen in animals); cellulose is a structural material and dietary fibre.
• Proteins — structural (keratin, collagen), functional (enzymes, antibodies, some hormones, transport proteins like haemoglobin); built from amino acids.
• Enzymes — biological catalysts that make life's reactions possible.
• Vitamins — needed in small amounts for health; often act as co-factors for enzymes.
• Nucleic acids — store and transmit genetic information (DNA) and direct protein synthesis (RNA).
• Hormones — chemical messengers that coordinate body functions.

Key Point: carbohydrates = energy/structure; proteins = structure/function; enzymes = catalysts; vitamins = small-amount health factors; nucleic acids = genetic information; hormones = messengers.

The Chapter's Key Tests & Distinctions

A consolidated toolbox of the distinctions this chapter relies on:

• Reducing vs non-reducing sugar: a reducing sugar gives a positive Fehling's (red Cu$_2$O) and Tollens' (silver mirror) test; sucrose is non-reducing (no test), while glucose, fructose, maltose and lactose are reducing.
• Starch vs cellulose: both are glucose polymers, but starch (alpha-glucose) gives a blue-black colour with iodine and is digestible, while cellulose (beta-glucose) is not digestible by humans.
• Amino acid (zwitterion): an amino acid is amphoteric (reacts with both acid and base) and has a high melting point — signs of the zwitterion.
• Nucleoside vs nucleotide: a nucleotide has a phosphate group; a nucleoside does not.
• DNA vs RNA: DNA has deoxyribose + thymine (double-stranded); RNA has ribose + uracil (single-stranded).

Key Point: Fehling/Tollens detect reducing sugars (sucrose negative); iodine gives blue-black with starch; amino acids are amphoteric zwitterions; nucleotide = nucleoside + phosphate; DNA (deoxyribose/T) vs RNA (ribose/U).

Solved Examples

Example 1: What are hormones?

Define hormones and state where they are produced.

Solution: Hormones are chemical messengers produced by the endocrine glands and carried by the blood to target organs, where they regulate body processes. They are needed in very small amounts.

Example 2: Insulin

What does insulin do, and what disease results from its deficiency?

Solution: Insulin (a polypeptide hormone) loweres blood glucose; its deficiency causes diabetes mellitus.

Example 3: Thyroxine

Which hormone controls metabolic rate, and which element does it need?

Solution: Thyroxine (from the thyroid gland) controls the metabolic rate; it requires iodine (a deficiency of iodine causes goitre).

Example 4: Reducing sugar test in practice

How would you show that glucose is a reducing sugar but sucrose is not?

Solution: Add Fehling's solution (or Tollens' reagent) and warm: glucose gives a red Cu$_2$O precipitate (or silver mirror), while sucrose gives no reaction (it is non-reducing).

Example 5: Starch with iodine

What is observed when iodine solution is added to starch?

Solution: A characteristic blue-black colour appears — the standard test for starch.

Example 6: Why amino acids are amphoteric

How would you demonstrate the amphoteric nature of an amino acid?

Solution: Show that it reacts with both an acid and a base — the -COO⁻ of the zwitterion accepts a proton from acid, and the -NH$_3^+$ donates a proton to base.

Example 7: Adrenaline

What is the common role of adrenaline (epinephrine)?

Solution: It is the "fight-or-flight" hormone — released in stress/emergency, it raises heart rate, blood pressure and blood glucose to prepare the body for action.

Example 8: Energy store

Name the storage carbohydrate that provides energy in animals and where it is stored.

Solution: Glycogen, stored mainly in the liver and muscles, is broken down to glucose to provide energy.

Example 9: Distinguish a nucleoside from a nucleotide

How does a nucleotide differ from a nucleoside?

Solution: A nucleotide contains a phosphate group (sugar + base + phosphate), whereas a nucleoside has only the sugar + base (no phosphate).

Example 10: A test scheme

You have glucose, sucrose and starch solutions. How would you identify each?

Solution: Add iodine — only starch gives a blue-black colour. Of the remaining two, add Fehling's/Tollens' — glucose (reducing) gives a positive test (red Cu$_2$O / silver mirror), while sucrose (non-reducing) does not.