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આ MCQ મોડ્યુલ આના પર આધારિત છે: Proteins Amino Acids
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Proteins Amino Acids
🎓 Class 12
Chemistry
CBSE
Theory
Ch 10 – Biomolecules
⏱ ~14 min
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આ મૂલ્યાંકન આના પર આધારિત હશે: Proteins Amino Acids
મૂલ્યાંકન બનાવવામાં તેમની સામગ્રી સામેલ કરવા ચિત્રો, PDF અથવા Word દસ્તાવેજ અપલોડ કરો.
Proteins — Amino Acids and the Peptide Linkage
Proteins are the most abundant biomolecules of the living system. Chief sources are milk, cheese, pulses, peanuts, fish and meat. They occur in every part of the body and form the fundamental basis of the structure and functions of life, and are also required for growth and maintenance of the body. The word protein is derived from the Greek proteios, meaning primary or of prime importance.
The defining statement. All proteins are polymers of α-amino acids. Everything in this part and the next follows from that one sentence.
10.2.1 Amino Acids
Amino acids contain amino (–NH₂) and carboxyl (–COOH) functional groups. Depending on the relative position of the amino group with respect to the carboxyl group, amino acids can be classified as α, β, γ, δ and so on.
Only α-amino acids are obtained on hydrolysis of proteins. In an α-amino acid both functional groups are attached to the same carbon — the α-carbon — which also carries a hydrogen and a variable side chain R. They may contain other functional groups as well, located in that side chain.
Names and symbols
All α-amino acids have trivial names, which usually reflect a property of the compound or its source. Glycine is so named since it has a sweet taste (Greek glykos, sweet), and tyrosine was first obtained from cheese (Greek tyros, cheese). Amino acids are generally represented by a three-letter symbol, and sometimes a one-letter symbol is used.
Table 10.2 — Natural amino acids (* = essential)
| # | Name | Side chain R | 3-letter | 1-letter |
|---|---|---|---|---|
| 1 | Glycine | –H | Gly | G |
| 2 | Alanine | –CH₃ | Ala | A |
| 3 | Valine* | (H₃C)₂CH– | Val | V |
| 4 | Leucine* | (H₃C)₂CH–CH₂– | Leu | L |
| 5 | Isoleucine* | H₃C–CH₂–CH(CH₃)– | Ile | I |
| 6 | Arginine* | HN=C(NH₂)–NH–(CH₂)₃– | Arg | R |
| 7 | Lysine* | H₂N–(CH₂)₄– | Lys | K |
| 8 | Glutamic acid | HOOC–CH₂–CH₂– | Glu | E |
| 9 | Aspartic acid | HOOC–CH₂– | Asp | D |
| 10 | Glutamine | H₂N–CO–CH₂–CH₂– | Gln | Q |
| 11 | Asparagine | H₂N–CO–CH₂– | Asn | N |
| 12 | Threonine* | H₃C–CHOH– | Thr | T |
| 13 | Serine | HO–CH₂– | Ser | S |
| 14 | Cysteine | HS–CH₂– | Cys | C |
| 15 | Methionine* | H₃C–S–CH₂–CH₂– | Met | M |
| 16 | Phenylalanine* | C₆H₅–CH₂– | Phe | F |
| 17 | Tyrosine | (p)HO–C₆H₄–CH₂– | Tyr | Y |
| 18 | Tryptophan* | indolyl–CH₂– | Trp | W |
| 19 | Histidine* | imidazolyl–CH₂– | His | H |
| 20 | Proline | entire structure (cyclic) | Pro | P |
10.2.2 Classification of Amino Acids
Acidic, basic and neutral
Amino acids are classified as acidic, basic or neutral depending upon the relative number of amino and carboxyl groups in the molecule.
| Class | Relative numbers | Example from Table 10.2 |
|---|---|---|
| Neutral | equal number of amino and carboxyl groups | glycine, alanine, valine |
| Basic | more amino groups than carboxyl groups | lysine, arginine |
| Acidic | more carboxyl groups than amino groups | glutamic acid, aspartic acid |
Essential and non-essential
Amino acids which can be synthesised in the body are known as non-essential amino acids. Those which cannot be synthesised in the body and must be obtained through diet are known as essential amino acids — marked with an asterisk in Table 10.2.
From the table the essential amino acids are valine, leucine, isoleucine, arginine, lysine, threonine, methionine, phenylalanine, tryptophan and histidine. The remaining ten — glycine, alanine, glutamic acid, aspartic acid, glutamine, asparagine, serine, cysteine, tyrosine and proline — are non-essential.
Physical properties and the zwitter ion
Amino acids are usually colourless, crystalline solids. They are water-soluble, high-melting solids and behave like salts rather than simple amines or carboxylic acids.
This behaviour is due to the presence of both an acidic (carboxyl) and a basic (amino) group in the same molecule. In aqueous solution the carboxyl group can lose a proton and the amino group can accept a proton, giving rise to a dipolar ion known as a zwitter ion. This is neutral overall but contains both positive and negative charges.
Why the zwitter ion explains the physical properties. A doubly charged species is held in the crystal by strong electrostatic (ionic) forces rather than weak van der Waals forces. That is precisely why amino acids melt at high temperatures and dissolve readily in water, and why they behave like salts — which is the examinable link between structure and property in this section.
In the zwitter ionic form, amino acids show amphoteric behaviour, as they react both with acids and with bases.
With acid: ⁻OOC–CHR–NH₃⁺ + H⁺ → HOOC–CHR–NH₃⁺ (cation)
With base: ⁻OOC–CHR–NH₃⁺ + OH⁻ → ⁻OOC–CHR–NH₂ + H₂O (anion)
With base: ⁻OOC–CHR–NH₃⁺ + OH⁻ → ⁻OOC–CHR–NH₂ + H₂O (anion)
Optical activity
Except glycine, all other naturally occurring α-amino acids are optically active, since the α-carbon atom is asymmetric. These exist in both D and L forms. Most naturally occurring amino acids have the L-configuration. L-amino acids are represented by writing the –NH₂ group on the left-hand side.
Why glycine is the exception. In glycine the side chain R is simply –H. The α-carbon therefore carries two identical hydrogen atoms, so it is not asymmetric and glycine has no optical activity. Every other amino acid has four different groups on the α-carbon.
10.2.3 Structure of Proteins — the Peptide Linkage
Proteins are polymers of α-amino acids connected to each other by a peptide bond or peptide linkage. Chemically, the peptide linkage is an amide formed between the –COOH group of one molecule and the –NH₂ group of another.
The reaction between two molecules of similar or different amino acids proceeds through combination of the amino group of one molecule with the carboxyl group of the other. This results in the elimination of a water molecule and formation of a peptide bond –CO–NH–. The product is called a dipeptide, because it is made up of two amino acids. For example, when the carboxyl group of glycine combines with the amino group of alanine we get the dipeptide glycylalanine.
If a third amino acid combines with a dipeptide, the product is called a tripeptide, containing three amino acids linked by two peptide linkages. Similarly, four, five or six amino acids linked give a tetrapeptide, pentapeptide or hexapeptide. When the number of amino acids is more than ten, the products are called polypeptides.
When does a polypeptide become a protein? A polypeptide with more than a hundred amino acid residues, having a molecular mass higher than 10,000 u, is called a protein. However, the distinction is not very sharp — polypeptides with fewer amino acids are likely to be called proteins if they have a well-defined conformation, such as insulin, which contains 51 amino acids.
Fibrous and globular proteins
Proteins can be classified into two types on the basis of their molecular shape.
| Feature | Fibrous proteins | Globular proteins |
|---|---|---|
| Chain arrangement | polypeptide chains run parallel, held by hydrogen and disulphide bonds, forming a fibre-like structure | chains coil around to give a spherical shape |
| Solubility in water | generally insoluble | usually soluble |
| Examples | keratin (hair, wool, silk), myosin (muscles) | insulin, albumins |
| Typical role | structural | dynamic — enzymes, hormones, transport |
🧪 Activity 10.3 — Why amino acids behave like saltsL4 Analyse
Glycine and chloroacetic acid have almost the same molecular mass, yet behave completely differently. This activity uses that comparison to establish the zwitter ion.
Predict: Glycine (H₂N–CH₂–COOH, M = 75) melts at about 535 K and dissolves freely in water. Chloroacetic acid (Cl–CH₂–COOH, M = 94.5) melts at about 336 K. Why should the lighter compound melt nearly 200 K higher? Write your idea first.
- Draw glycine as written above and identify one acidic group and one basic group in the same molecule.
- Transfer the proton from the –COOH to the –NH₂ and redraw the species. Mark both charges.
- State what kind of forces will hold these species together in the solid.
- Repeat steps 1–3 for chloroacetic acid, which has no basic group.
- Now write the two equations showing glycine reacting with HCl and with NaOH.
Glycine exists as a zwitter ion, so its crystal is held together by ionic forces.
Internal proton transfer converts H₂N–CH₂–COOH into ⁻OOC–CH₂–NH₃⁺, which carries a full negative and a full positive charge while remaining overall neutral. A lattice of such dipolar ions is held by strong electrostatic attractions, essentially like an ionic salt — hence the high melting point, the crystalline solid state, the ready solubility in water and the general observation that amino acids behave like salts rather than simple amines or carboxylic acids.
Chloroacetic acid has no basic group, so no internal proton transfer is possible. Its molecules are held only by hydrogen bonding and weaker dipole forces, and it melts far lower despite being the heavier molecule.
The amphoteric equations from step 5:
with acid — ⁻OOC–CH₂–NH₃⁺ + H⁺ → HOOC–CH₂–NH₃⁺ (the carboxylate accepts the proton, giving a cation);
with base — ⁻OOC–CH₂–NH₃⁺ + OH⁻ → ⁻OOC–CH₂–NH₂ + H₂O (the ammonium group gives up its proton, leaving an anion).
Because the zwitter ion can respond to either reagent, amino acids are amphoteric. This is also why they act as natural buffers in the body, resisting change in pH in both directions.
Intext question
Intext 10.4 — Why amino acids have higher melting points and solubility than the corresponding halo acids
An amino acid contains both an acidic –COOH group and a basic –NH₂ group in the same molecule, so an internal proton transfer takes place to give the dipolar zwitter ion, ⁻OOC–CHR–NH₃⁺. The solid is therefore composed of doubly charged species held together by strong electrostatic forces, much as in an ionic salt, and a large amount of energy is needed to break that lattice — hence the high melting point. The charged ends are also strongly hydrated by water, so solubility in water is high. A halo acid such as chloroacetic acid has no basic group, cannot form a zwitter ion, and its molecules are held only by weaker hydrogen bonding and dipole interactions, giving a much lower melting point.
Competency-Based Questions
A nutritionist is designing a protein supplement for schoolchildren. She has data on three amino acids: P has one –NH₂ and two –COOH groups; Q has two –NH₂ and one –COOH group; R has one of each and its side chain is simply a hydrogen atom. She must also decide which amino acids must be supplied by the diet itself.
1. Classify P, Q and R as acidic, basic or neutral, and name one real amino acid fitting each. L2 Understand
P is acidic — more carboxyl than amino groups; examples are glutamic acid and aspartic acid. Q is basic — more amino than carboxyl groups; examples are lysine and arginine. R is neutral — equal numbers of each; since its side chain is –H it is glycine.
2. R is the only naturally occurring amino acid that is optically inactive. Explain. L4 Analyse
R is glycine, whose side chain is a hydrogen atom. Its α-carbon therefore carries –NH₂, –COOH and two identical hydrogen atoms. A carbon must have four different groups to be asymmetric, so glycine's α-carbon is not a chiral centre and glycine shows no optical activity. In every other amino acid the side chain R differs from H, making the α-carbon asymmetric — which is why all the others are optically active, most of them occurring naturally in the L-configuration.
3. Which amino acids must the supplement definitely contain, and why? L3 Apply
It must contain the essential amino acids — those which cannot be synthesised in the body and must be obtained through diet. From Table 10.2 these are valine, leucine, isoleucine, arginine, lysine, threonine, methionine, phenylalanine, tryptophan and histidine. The non-essential ones such as glycine, alanine, serine and glutamic acid can be made by the body itself, so their absence from a supplement is not critical.
4. Two amino acids, glycine and alanine, are allowed to react. How many different dipeptides are possible if each may supply either functional group, and what links them? L4 Analyse
Four dipeptides are possible: Gly–Gly, Ala–Ala, Gly–Ala (glycylalanine) and Ala–Gly (alanylglycine). The last two are different compounds, because it matters which acid contributes the –COOH and which contributes the –NH₂. Each is joined by a peptide linkage –CO–NH–, chemically an amide, formed with the elimination of a water molecule between the carboxyl group of one and the amino group of the other.
5. Insulin has only 51 amino acid residues and a molecular mass below 10,000 u, yet it is called a protein. Evaluate the criterion being used. L5 Evaluate
The usual criterion is that a polypeptide with more than a hundred amino acid residues and molecular mass above 10,000 u is called a protein. Insulin meets neither condition, so on a purely numerical reading it would be classed as a polypeptide. However, NCERT states explicitly that the distinction between a polypeptide and a protein is not very sharp. Polypeptides with fewer amino acids are still likely to be called proteins if they possess a well-defined conformation of the kind a protein has — and insulin does, being a globular protein with a precise three-dimensional shape on which its hormonal activity depends. The lesson is that the size cut-off is a convenient guideline rather than a strict definition; the functionally meaningful criterion is whether the molecule folds into a specific, biologically active structure.
Assertion–Reason Questions
For each pair choose: (A) Both A and R are true and R is the correct explanation of A. (B) Both A and R are true but R is not the correct explanation of A. (C) A is true but R is false. (D) A is false but R is true.
Assertion (A): Amino acids are high-melting crystalline solids that behave like salts.
Reason (R): In the solid and in aqueous solution they exist as zwitter ions carrying both a positive and a negative charge.
Answer: A. Internal proton transfer produces a dipolar ion, and a lattice of such ions is held by strong electrostatic forces, giving salt-like behaviour, high melting points and good water solubility.
Assertion (A): Glycine is optically inactive.
Reason (R): Glycine does not contain an asymmetric carbon atom.
Answer: A. With R = –H the α-carbon bears two identical hydrogens, so it has only three different groups and is not a chiral centre. Every other natural α-amino acid is optically active.
Assertion (A): A peptide linkage is formed by addition of a water molecule between two amino acids.
Reason (R): The peptide linkage –CO–NH– is chemically an amide.
Answer: D. The assertion is false — a peptide bond is formed by the elimination of a water molecule between the –COOH of one amino acid and the –NH₂ of another, not by addition. The reason is a true statement: the –CO–NH– linkage is indeed an amide.
Coming next. Part 4 builds upward from the peptide chain to the four levels of protein structure — primary, secondary (α-helix and β-pleated sheet), tertiary and quaternary — then denaturation and Section 10.3 on enzymes.
Frequently Asked Questions
What is an alpha amino acid and why are only these found in proteins?
An α-amino acid has both the amino (–NH₂) and carboxyl (–COOH) groups attached to the same carbon, the α-carbon, which also carries a hydrogen and a variable side chain R. Amino acids can in principle be α, β, γ or δ depending on where the amino group sits relative to the carboxyl group, but only α-amino acids are obtained on hydrolysis of proteins.
What is a zwitter ion and why does it explain the properties of amino acids?
In aqueous solution the carboxyl group of an amino acid loses a proton and the amino group accepts one, giving a dipolar ion called a zwitter ion that is neutral overall but carries both a positive and a negative charge. Because the solid is built from such doubly charged species held by strong electrostatic forces, amino acids are high-melting crystalline solids that dissolve readily in water and behave like salts rather than like simple amines or carboxylic acids.
What is the difference between essential and non-essential amino acids?
Non-essential amino acids can be synthesised in the body. Essential amino acids cannot be synthesised in the body and must be obtained through diet. From Table 10.2 the essential ones are valine, leucine, isoleucine, arginine, lysine, threonine, methionine, phenylalanine, tryptophan and histidine.
Why is glycine the only optically inactive natural amino acid?
Optical activity requires an asymmetric carbon carrying four different groups. In glycine the side chain R is simply a hydrogen atom, so the α-carbon bears –NH₂, –COOH and two identical hydrogen atoms. It is therefore not a chiral centre. In every other amino acid the side chain differs from hydrogen, so the α-carbon is asymmetric and the acid is optically active, most naturally occurring ones having the L-configuration.
What is a peptide linkage and how is it formed?
A peptide linkage is the –CO–NH– bond joining two amino acids, and chemically it is an amide. It forms when the carboxyl group of one amino acid combines with the amino group of another with the elimination of a water molecule. Two amino acids give a dipeptide, three give a tripeptide with two peptide linkages, and more than ten give a polypeptide.
What is the difference between fibrous and globular proteins?
In fibrous proteins the polypeptide chains run parallel and are held together by hydrogen and disulphide bonds to give a fibre-like structure; they are generally insoluble in water and examples are keratin in hair, wool and silk, and myosin in muscles. In globular proteins the chains coil around to give a spherical shape; these are usually soluble in water and examples are insulin and albumins.
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JEE Advanced 2026 Paper 2 — Full Mock Test (54 Questions, 180 Marks)
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