The plant dye henna (mehndi) imparts an orange-red colour to skin and hair because its colouring molecule reacts mainly with which chemical components of keratinised tissues?

Introduction:
Henna, also called mehndi, is a natural dye obtained from the plant Lawsonia inermis. It is widely used in many cultures to colour hands, feet and hair with an orange-red to brown shade. The colour is not simply painted on the surface but results from a chemical reaction between henna pigment and components of the skin and hair. This question asks which specific chemical components are mainly involved in that reaction.

Given Data / Assumptions:
- Henna contains the dye molecule lawsone. - The colour appears on keratin rich tissues such as the outer layer of skin, nails and hair. - Options include proteins and amino acids, lipids, carbohydrates and amino acids only. - We assume normal human skin and hair composition, where keratin is the key structural protein.

Concept / Approach:
The colouring action of henna is based on the binding of lawsone to keratin, a fibrous structural protein present in skin and hair. Lawsone forms stable bonds with amino groups present in the amino acids that make up keratin proteins. Thus, the reaction is essentially between the henna pigment and the proteins and amino acids in keratinised tissues. Lipids and carbohydrates in the skin are not the primary binding targets for this dye. Therefore, the best description is that henna reacts with proteins and amino acids.

Step-by-Step Solution:
Step 1: Recall that skin and hair contain keratin, which is made of long chains of amino acids forming proteins. Step 2: Understand that lawsone, the active dye in henna, forms stable bonds with amino groups in these proteins. Step 3: Recognise that amino acids are the building blocks of proteins, so effectively both proteins and amino acids participate. Step 4: Eliminate lipids and carbohydrates because they are not primarily responsible for binding and fixing the henna pigment. Step 5: Conclude that the best answer summarising this chemistry is proteins and amino acids.

Verification / Alternative check:
Cosmetic science and dermatology references explain that henna staining is a result of a Michael addition reaction between lawsone and amino groups in keratin. This leads to the formation of a stable complex that grows deeper in colour over several hours and then gradually fades as the keratin layers are shed. These explanations repeatedly mention keratin proteins and amino acid residues, not lipids or carbohydrates, which confirms our choice.

Why Other Options Are Wrong:
Lipids: Present in skin as sebum and membrane components, but they are not the main chemical partners for lawsone in henna dyeing. Carbohydrates: Occur in small amounts in skin glycoproteins and glycocalyx, but they are not the primary binding sites for henna pigment. Amino acids only: This option ignores the fact that amino acids are mostly present as part of keratin proteins in the living tissue. Describing proteins and amino acids together gives a more accurate biological picture.

Common Pitfalls:
A common confusion is to think that any pigment binds to lipids or to the surface oils of the skin, leading to the wrong choice of lipids. Another pitfall is not remembering that keratin itself is a protein rich in amino acid side chains that can form chemical bonds. A helpful memory tip is to associate hair dyeing and protein treatment together, since many cosmetic products refer to protein rich treatments that strengthen hair, reinforcing the link between dyes and proteins.

Final Answer:
Henna gives an orange-red colour by reacting mainly with Proteins and amino acids present in keratinised tissues of skin and hair.