Biomolecules - Class 12 Chemistry

Class 12 Chemistry | Chapter 10

Biomolecules

Carbohydrates • Proteins • Vitamins • Nucleic Acids

1. Carbohydrates

Carbohydrates are primarily produced by plants and form a very large group of naturally occurring organic compounds. Some common examples are cane sugar, glucose, starch, etc.

Chemically, the carbohydrates may be defined as optically active polyhydroxy aldehydes or ketones, or the compounds which produce such units on hydrolysis.

1.1 Classification of Carbohydrates

Carbohydrates are classified on the basis of their behavior on hydrolysis:

Monosaccharides: Cannot be hydrolysed further to give a simpler unit. Example: Glucose, Fructose, Ribose.
Oligosaccharides: Yield two to ten monosaccharide units on hydrolysis. They are further classified as disaccharides, trisaccharides, etc. Disaccharides (e.g., Sucrose, Maltose, Lactose) are the most common.
Polysaccharides: Yield a large number of monosaccharide units on hydrolysis. Example: Starch, Cellulose, Glycogen, Gums. They are not sweet in taste, hence they are also called non-sugars.

💡 Reducing vs Non-Reducing Sugars:
Reducing sugars: Carbohydrates that reduce Fehling's solution and Tollens' reagent. All monosaccharides (aldoses & ketoses) and disaccharides where the aldehyde/ketonic group is free (e.g., Maltose, Lactose).
Non-reducing sugars: Do not reduce Tollens'/Fehling's. The aldehyde/ketonic group is bonded in glycosidic linkage (e.g., Sucrose).

1.2 Glucose (Aldohexose)

Preparation: Hydrolysis of cane sugar (sucrose) or starch. D-Glucose is the most abundant organic compound on earth.

Chemical Reactions proving the structure of Glucose:

With HI (Prolonged heating): Forms n-hexane, proving all 6 carbons are in a straight chain.
With Hydroxylamine (NH₂OH): Forms an oxime, confirming the presence of a carbonyl (>C=O) group.
With Bromine Water (Mild oxidizing agent): Oxidized to six-carbon carboxylic acid (Gluconic acid), proving the carbonyl group is an aldehyde.
With Acetic Anhydride: Forms glucose pentaacetate, confirming the presence of five -OH groups attached to different carbons.
With Nitric Acid (Strong oxidizing agent): Both glucose and gluconic acid oxidize to Saccharic acid, indicating the presence of a primary alcohol (-CH₂OH) group.

1.3 Cyclic Structure of Glucose

Open-chain structure couldn't explain: (1) Glucose doesn't give 2,4-DNP test or Schiff's test. (2) It doesn't form hydrogen sulphite addition product with NaHSO₃. (3) The pentaacetate of glucose doesn't react with hydroxylamine. (4) Glucose exists in two crystalline forms (α and β).

This led to the cyclic hemiacetal structure (Pyranose structure). The two cyclic hemiacetal forms of glucose (α-D-glucose and β-D-glucose) differ only in the configuration of the hydroxyl group at C1, called anomeric carbon. Such isomers are called anomers.

1.4 Disaccharides & Polysaccharides

Monosaccharides are joined by an oxide linkage formed by the loss of a water molecule. Such linkage between two monosaccharide units through an oxygen atom is called a Glycosidic Linkage.

Sucrose: α-D-Glucose (C1) + β-D-Fructose (C2). Non-reducing. Known as Invert Sugar because its hydrolysis changes the net optical rotation from dextro (+) to laevo (-).
Maltose: α-D-Glucose (C1) + α-D-Glucose (C4). Reducing sugar.
Lactose (Milk Sugar): β-D-Galactose (C1) + β-D-Glucose (C4). Reducing sugar.
Starch: Main storage polysaccharide of plants. Polymer of α-glucose. Consists of Amylose (water-soluble, unbranched chain, C1-C4 links, 15-20% of starch) and Amylopectin (water-insoluble, branched chain, C1-C4 links with C1-C6 branching, 80-85%).
Cellulose: Structural component of plant cell walls. Linear polymer of β-D-glucose joined by β-glycosidic linkage between C1 of one unit and C4 of the next.
Glycogen: Animal starch. Highly branched structure similar to amylopectin.

2. Proteins

Proteins are polymers of α-amino acids connected by peptide bonds.

2.1 Amino Acids

Contain amino (-NH₂) and carboxyl (-COOH) functional groups. 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 Zwitterion. In this form, amino acids show amphoteric behavior.

Essential amino acids: Cannot be synthesized in the body and must be obtained through diet (e.g., Valine, Leucine). Non-essential: Can be synthesized in the body (e.g., Glycine, Alanine).

Except for Glycine (which is optically inactive), all naturally occurring α-amino acids are optically active and have L-configuration.

2.2 Structure of Proteins

Proteins are linked by Peptide bonds (-CO-NH-) formed between the -COOH of one amino acid and -NH₂ of another with elimination of water.

Primary Structure: Sequence of amino acids in the polypeptide chain. Any change in this primary structure creates a different protein.
Secondary Structure: Refers to the shape in which a long polypeptide chain can exist. Arises due to regular folding of the backbone due to Hydrogen bonding between -C=O and -NH- groups.
- α-Helix: Chain coils into a right-handed screw (helix).
- β-Pleated sheet: All peptide chains are stretched out to nearly maximum extension and laid side by side, held by intermolecular H-bonds.
Tertiary Structure: Overall folding of the polypeptide chains (further folding of secondary structure) giving rise to Fibrous or Globular shapes. Stabilized by H-bonds, disulphide linkages, van der Waals and electrostatic forces.
Quaternary Structure: Spatial arrangement of two or more polypeptide chains (subunits) with respect to each other.

2.3 Denaturation of Proteins

When a protein in its native form is subjected to physical change (like change in temperature) or chemical change (like change in pH), the hydrogen bonds are disturbed. Due to this, globules uncoil and the helix gets uncoiled and the protein loses its biological activity. This is called denaturation.

During denaturation, 2° and 3° structures are destroyed, but 1° structure remains intact.
Examples: Coagulation of egg white on boiling; curdling of milk (caused due to formation of lactic acid by bacteria).

3. Vitamins

Organic compounds required in the diet in small amounts to perform specific biological functions for normal maintenance of optimum growth and health of the organism.

Classification of Vitamins

Fat Soluble Vitamins: Soluble in fat/oils but insoluble in water. Vitamins A, D, E, and K. They are stored in liver and adipose tissues.
Water Soluble Vitamins: B group vitamins (B1, B2, B6, B12 etc.) and Vitamin C. Must be supplied regularly in diet because they are readily excreted in urine and cannot be stored in the body (except B12).

Vitamin	Chemical Name / Sources	Deficiency Disease
Vitamin A	Retinol (Carrots, butter, milk)	Night blindness (Nyctalopia), Xerophthalmia
Vitamin B1	Thiamine (Yeast, milk, green veg)	Beri-Beri (loss of appetite, retarded growth)
Vitamin B2	Riboflavin (Milk, egg white, liver)	Cheilosis (fissuring at corners of mouth/lips)
Vitamin B12	Cyanocobalamin (Meat, fish, egg)	Pernicious anaemia (RBC deficient in haemoglobin)
Vitamin C	Ascorbic Acid (Citrus fruits, amla)	Scurvy (bleeding gums)
Vitamin D	Calciferol (Sunlight, fish liver oil)	Rickets (bone deformities in kids), Osteomalacia (adults)

4. Nucleic Acids (DNA & RNA)

Nucleic acids are long-chain polymers of nucleotides, hence called polynucleotides. They are responsible for heredity and protein synthesis.

4.1 Chemical Composition

Complete hydrolysis of DNA/RNA yields: (1) A pentose sugar, (2) Phosphoric acid, (3) Nitrogen-containing heterocyclic bases.

Sugar: In DNA → β-D-2-deoxyribose. In RNA → β-D-ribose.
Bases: Purines = Adenine (A), Guanine (G). Pyrimidines = Cytosine (C), Thymine (T, only in DNA), Uracil (U, only in RNA).

4.2 Structure

Nucleoside: Base + Sugar (attached at C-1' of sugar).
Nucleotide: Base + Sugar + Phosphoric acid (attached at C-5' of sugar).
Polynucleotide (Primary structure): Nucleotides are joined together by phosphodiester linkages between the 5' and 3' carbon atoms of the pentose sugar.
Secondary Structure of DNA: James Watson and Francis Crick gave a double-strand helix structure for DNA. Two nucleic acid chains are wound about each other and held together by hydrogen bonds between specific pairs of bases. Adenine pairs with Thymine (A=T), and Guanine pairs with Cytosine (G≡C).

4.3 RNA Structure & Types

In secondary structure of RNA, helices are present which are single stranded. Sometimes they fold back on themselves. Types of RNA: Messenger RNA (m-RNA), Ribosomal RNA (r-RNA), Transfer RNA (t-RNA).

🎓 NEET Previous Year Questions

Q1. [NEET 2022] Which of the following is an example of an aldohexose? (a) Fructose (b) Ribose (c) Glucose (d) Erythrose

Answer Glucose is a six-carbon sugar (hexose) containing an aldehyde group (aldo). Hence, an aldohexose. Fructose is a ketohexose. Ribose is an aldopentose.

Q2. [NEET 2021] During denaturation of proteins, which of the following structures is/are destroyed? (a) Primary structure only (b) Secondary and tertiary structures (c) Primary, secondary and tertiary structures (d) Secondary structure only

Answer During denaturation, the hydrogen bonds are disturbed causing globules to uncoil. Thus, the secondary and tertiary structures are destroyed, but the primary structure (peptide bonds) remains intact. Correct is (b).

Q3. [NEET 2020] Which one of the following vitamins is water-soluble? (a) Vitamin C (b) Vitamin D (c) Vitamin E (d) Vitamin K

Answer Fat soluble are A, D, E, K. Water soluble are B-complex and C. Correct is (a) Vitamin C.

Q4. [NEET 2019] Which of the following statements is not true about glucose? (a) It is an aldohexose (b) On heating with HI, it forms n-hexane (c) It is present in furanose form (d) It does not give 2,4-DNP test.

Answer The cyclic structure of glucose is a six-membered ring containing oxygen, known as Pyranose, not Furanose (which is a 5-membered ring found in Fructose). Hence (c) is not true.

Q5. [NEET 2018] In DNA, the linkages between different nitrogenous bases are:

Answer The two complementary DNA strands are held together by Hydrogen bonds between specific nitrogenous base pairs (A=T via two bonds, G≡C via three bonds).

💡 Rapid Revision

Anomers vs Epimers: Anomers (like α and β glucose) differ specifically at the C-1 hemiacetal carbon.
Sucrose Inversion: Sucrose is dextrorotatory (+), but on hydrolysis it gives dextro-glucose and laevo-fructose. Since laevo-fructose > dextro-glucose, the mixture is laevorotatory (-). The sign of rotation inverts.
Zwitterion: Most amino acids exist primarily as dipolar zwitterions in neutral aqueous solution. Net charge is zero.
Base Pairing Rule: In DNA, A pairs with T (2 H-bonds) and C pairs with G (3 H-bonds). In RNA, A pairs with U.

Numericals & Solutions - Biomolecules - Class 12 Chemistry

CLASS 12 CHEMISTRY | NCERT SOLUTIONS

Chapter 10 — Biomolecules

22 Solved Questions — Carbohydrates, Proteins, DNA & Vitamins

Note: Biomolecules is entirely theoretical and structural. "Numericals" here refer to standard reasoning-based, structure-elucidation, and definitional questions often asked for 2 or 3 marks in board exams and competitive reasoning.

📝 Carbohydrates & Glucose Structure (Q1 – Q8)

2 MarksQ1. What are reducing sugars? Give two examples.

✓ Solution
Carbohydrates that reduce Fehling's solution and Tollens' reagent are called reducing sugars. They have a free aldehydic or ketonic group.
All monosaccharides (aldoses and ketoses) are reducing sugars.
Examples: Glucose, Fructose, Maltose, Lactose.

3 MarksQ2. Write chemical reactions to show that the open structure of D-glucose contains: (i) Straight chain (ii) Five alcohol groups (iii) Aldehyde group.

✓ Solution
(i) Straight chain: Heating glucose with HI for a long time gives n-hexane, showing all 6 carbons are linked in a straight chain.
(ii) Five alcohol groups: Acetylation of glucose with acetic anhydride gives glucose pentaacetate, confirming the presence of five -OH groups attached to different carbons.
(iii) Aldehyde group: Glucose gets oxidized to six-carbon acid (gluconic acid) by a mild oxidizing agent like bromine water, confirming the carbonyl group is an aldehyde.

3 MarksQ3. What is the basic structural difference between starch and cellulose?

✓ Solution
Both are polymers of glucose but differ in the anomer used and linkage type.
Starch: Composed of α-D-glucose units. It consists of two components: Amylose (linear, C1-C4 α-glycosidic linkages) and Amylopectin (branched, C1-C4 with C1-C6 branching).
Cellulose: Composed of β-D-glucose units. It is a strictly linear polymer joined by β-glycosidic linkages between C1 of one unit and C4 of the next. This gives it a tough, rigid fibrous structure that humans cannot digest.

2 MarksQ4. Enumerate the reactions of D-glucose which cannot be explained by its open chain structure.

✓ Solution
1. Despite having an aldehyde group, glucose does not give Schiff's test or 2,4-DNP test.
2. It does not form the hydrogen sulphite addition product with NaHSO₃.
3. The pentaacetate of glucose does not react with hydroxylamine (NH₂OH), showing the aldehyde group is blocked/locked in a cyclic form.

2 MarksQ5. What are anomers? Give an example.

✓ Solution
Anomers are a special type of epimers (diastereomers) that differ in configuration only at the specific hemiacetal/acetal carbon (also called the anomeric carbon, which is C1 in aldoses and C2 in ketoses).
Example: α-D-Glucose and β-D-Glucose are anomers. They differ solely in the orientation of the -OH group at carbon-1.

2 MarksQ6. Why is sucrose called a non-reducing sugar?

✓ Solution
Sucrose is a disaccharide made of α-D-glucose and β-D-fructose.
In sucrose, the glycosidic linkage involves the reducing groups of both monosaccharides (C1 of glucose and C2 of fructose). Because both reducing groups are mutually bonded and not free, sucrose does not reduce Tollens' or Fehling's reagents.

2 MarksQ7. Define Invert Sugar.

✓ Solution
Sucrose is initially dextrorotatory (+66.5°). On hydrolysis with acid or invertase enzyme, it yields an equimolar mixture of D-(+)-glucose (+52.5°) and D-(-)-fructose (-92.4°).
Because the laevorotation of fructose is greater than the dextrorotation of glucose, the net optical rotation of the mixture becomes laevorotatory (-). This inversion in the sign of rotation gives the mixture the name "invert sugar".

2 MarksQ8. What happens when D-glucose is treated with Nitric acid?

✓ Solution
Nitric acid is a strong oxidizing agent. It oxidizes both the aldehyde group (at C1) and the primary alcohol group (at C6) of glucose into carboxylic acids.
The product formed is a dicarboxylic acid called Saccharic acid (or glucaric acid).

💡 Proteins & Amino Acids (Q9 – Q16)

2 MarksQ9. What are essential and non-essential amino acids? Give one example of each.

✓ Solution
Essential Amino Acids: Amino acids which cannot be synthesized in the human body and must be supplied through diet. Example: Valine, Leucine, Lysine.
Non-essential Amino Acids: Amino acids which can be synthesized in the human body. Example: Glycine, Alanine.

2 MarksQ10. Define Zwitterion.

✓ Solution
In aqueous solution, the carboxyl (-COOH) group of an amino acid can lose a proton, and the amino (-NH₂) group can accept that proton. This internal acid-base transfer creates a dipolar ion containing both positive (-NH₃⁺) and negative (-COO⁻) charges, called a Zwitterion.
In this form, the net charge is zero, and it shows amphoteric behavior.

3 MarksQ11. Differentiate between Globular and Fibrous proteins.

✓ Solution
Fibrous Proteins: Consist of thread-like linear polypeptide chains running parallel, held together by hydrogen and disulphide bonds. They are generally insoluble in water and act as structural materials.
Examples: Keratin (hair, nails), Myosin (muscles).
Globular Proteins: The polypeptide chains fold upon themselves to form a spherical/globular shape. The polar groups point outwards, making them generally soluble in water. They perform metabolic functions.
Examples: Insulin, Albumin, Haemoglobin.

2 MarksQ12. What is a peptide linkage?

✓ Solution
A peptide linkage (or peptide bond) is an amide bond (-CO-NH-) formed between the carboxyl group (-COOH) of one α-amino acid and the amino group (-NH₂) of another α-amino acid with the elimination of a water molecule. It links amino acids together to form proteins.

3 MarksQ13. Explain the term "denaturation of proteins". What happens to the different levels of protein structure during this process?

✓ Solution
When a protein strictly in its native form is subjected to physical change (temperature, radiation) or chemical change (pH shift, heavy metals), the precise arrangement of hydrogen bonds is disrupted.
This causes the protein globules to uncoil and the helices to open up, leading to the loss of its biological activity. This is denaturation.
Structural Impact: The secondary and tertiary structures are completely destroyed. However, the primary structure (the sequence of amino acids linked by strong covalent peptide bonds) remains intact.

2 MarksQ14. Why are amino acids usually high-melting crystalline solids rather than volatile liquids?

✓ Solution
Amino acids exist primarily as dipolar Zwitterions (H₃N⁺-CH(R)-COO⁻) in their crystalline lattice, displaying strong electrostatic (ionic) interactions between molecules, rather than weak van der Waals forces acting on neutral molecules. Thus, they behave like ionic salts, leading to high melting points.

2 MarksQ15. Name the two types of secondary structures of proteins. What type of bond stabilizes them?

✓ Solution
The two main types are the α-helix and the β-pleated sheet.
Both structures are stabilized primarily by Hydrogen bonding between the carbonyl oxygen (-C=O) of one amino acid residue and the amide hydrogen (-NH-) of another.

2 MarksQ16. What is the difference between α-amino acid and β-amino acid?

✓ Solution
In an α-amino acid, both the amino group (-NH₂) and the carboxyl group (-COOH) are attached to the same carbon atom (the α-carbon).
In a β-amino acid, the amino group is attached to the carbon adjacent to the α-carbon (the β-carbon). Proteins strictly consist of polymers of α-amino acids.

📈 Nucleic Acids & Vitamins (Q17 – Q22)

3 MarksQ17. Differentiate between DNA and RNA biologically and chemically.

✓ Solution
Chemically:
1. Sugar: DNA contains β-D-2-deoxyribose. RNA contains β-D-ribose.
2. Bases: DNA contains Thymine (T) alongside A,C,G. RNA contains Uracil (U) instead of Thymine.
3. Structure: DNA is predominantly double-stranded helix. RNA is single-stranded (though it can fold back on itself).
Biologically:
1. DNA handles replication and storage of genetic information.
2. RNA reads DNA to control protein synthesis (mRNA, tRNA, rRNA).

2 MarksQ18. What are the different types of RNA found in the cell?

✓ Solution
There are three main types of RNA, each serving a specific function in protein synthesis:
1. Messenger RNA (m-RNA): Carries genetic info from DNA to ribosomes.
2. Ribosomal RNA (r-RNA): Forms the structure of ribosomes.
3. Transfer RNA (t-RNA): Transports specific amino acids to the ribosome during protein assembly.

2 MarksQ19. Name the vitamin deficiency responsible for: (i) Scurvy (ii) Night Blindness (iii) Beri-Beri.

✓ Solution
(i) Scurvy (bleeding gums) is caused by deficiency of Vitamin C (Ascorbic acid).
(ii) Night Blindness (Nyctalopia) is caused by deficiency of Vitamin A (Retinol).
(iii) Beri-Beri (nerve damage, weakness) is caused by deficiency of Vitamin B₁ (Thiamine).

2 MarksQ20. Why must Vitamin C be supplied regularly in our diet?

✓ Solution
Vitamin C (Ascorbic acid) is a water-soluble vitamin.
Because it is soluble in water, it cannot be stored in the body's fat tissues and is readily excreted in urine. Therefore, it must be replenished constantly through the diet (citrus fruits, amla) to prevent scurvy.

2 MarksQ21. Define nucleotide and nucleoside.

✓ Solution
Nucleoside: A unit formed by the attachment of a nitrogenous base exclusively to the 1' position of a pentose sugar (Sugar + Base).
Nucleotide: When a phosphoric acid group is attached to the 5' position of the sugar moiety of a nucleoside, the entire unit (Sugar + Base + Phosphate) is called a nucleotide. It is the building block of nucleic acids.

2 MarksQ22. What are the common types of secondary structure of proteins determined by?

✓ Solution
The secondary structure (α-helix and β-pleated sheet) is determined by the regular folding of the backbone of the polypeptide chain.
This specific folding is dictated and stabilized by intramolecular and intermolecular hydrogen bonds between the peptide bonds (-C=O...H-N-).

✍ Score Guide — 22 Questions
Biomolecules questions heavily target the exact distinction between RNA/DNA bases, the 1°/2°/3°/4° structure of proteins after denaturation, Vitamin solubility charts, and Glucose confirmation reactions with HI, HNO₃, and Bromine water.

High-Yield Facts & Formulas: Biomolecules

Carbohydrates definition
Optically active polyhydroxy aldehydes or ketones or compounds which yield these on hydrolysis.

Reducing Sugars
Saccharides containing free aldehydic or ketonic groups; reduce Tollen's and Fehling's reagents. All monosaccharides are reducing.

Non-reducing Sugar
Sucrose is a non-reducing sugar because both reducing groups are occupied in glycosidic linkage.

Glucose (Aldohexose)
Contains one aldehydic group, five hydroxyl groups (1 primary, 4 secondary) and 6 carbon atoms.

Anomers
Diastereoisomers of cyclic saccharides that differ only in configuration at C1 (aldehyde) or C2 (ketone) e.g., α- and β-glucose.

Mutarotation
Spontaneous change in specific rotation of an optically active compound towards an equilibrium value. (Shown by glucose).

Fructose (Ketohexose)
Naturally occurring fructose is L-fructose but D-series is used in biologically active forms.

Glycosidic linkage
C-O-C bond linking two monosaccharide units together.

Invert Sugar
Equimolar mixture of glucose and fructose obtained by hydrolysis of sucrose (specific rotation changes from +ve to -ve).

Polysaccharides
Starch, Cellulose, Glycogen. Not sweet, insoluble in water and non-reducing.

Amylose vs Amylopectin
Amylose is water soluble (15-20%), linear. Amylopectin is water insoluble (80-85%), branched.

Glycogen (Animal Starch)
Stored in liver, muscles and brain. Similar structure to amylopectin but more highly branched.

Amino Acid Configuration
All naturally occurring amino acids (except glycine) are chiral and have L-configuration.

Essential Amino Acids
Cannot be synthesized in body; must be obtained from diet (e.g., Valine, Lysine, Leucine).

Zwitter ion
Amino acid in neutral pH exists as a dipolar ion (charge balanced). Isoelectric point is pH at which no migration in electric field.

Peptide Bond
-CO-NH- group joining two amino acid units. Formed by condensation.

Protein Structure (Primary)
Linear sequence of amino acids in a polypeptide chain.

Secondary Structure
α-helix or β-pleated sheet. Formed by hydrogen bonding between peptide groups.

Tertiary Structure
Overall folding of the polypeptide chain into a 3D shape.

Denaturation of Protein
Unfolding of protein structure due to heat, radiation or chemical change. Lose 2° and 3° structure; 1° remains.

Enzymes
High-molecular weight proteins which are specific, efficient biological catalysts.

Vitamin A (Retinol)
Deficiency cause Xerophthalmia and Night Blindness.

Vitamin B1 (Thiamine)
Deficiency cause Beri-beri.

Vitamin B12 (Cyanocobalamin)
Deficiency cause Pernicious Anemia. Contains Cobalt.

Vitamin C (Ascorbic acid)
Deficiency cause Scurvy. Found in citrus fruits. Destroyed by cooking.

Vitamin D (Calciferol)
Deficiency cause Rickets in children and Osteomalacia in adults. "Sunshine vitamin".

Vitamin K (Phylloquinone)
Deficiency cause increased blood clotting time (hemorrhage).

Nucleic Acid Backbone
Chain of alternating sugar and phosphate groups. Bases are attached to sugar units.

DNA Nitrogenous Bases
Purines: Adenine(A), Guanine(G). Pyrimidines: Cytosine(C), Thymine(T).

RNA Nitrogenous Bases
Purines: A, G. Pyrimidines: Cytosine(C), Uracil(U).

Base Pairing (Chargaff's Rule)
In DNA: A pairs with T (2 H-bonds); G pairs with C (3 H-bonds). [A+G] = [T+C].

Double Helix
Proposed by Watson and Crick. Two polynucleotide chains wound around each other.

DNA Replication
Semiconservative process resulting in two identical DNA double helices.

ATP (Adenosine Triphosphate)
The universal energy currency of cells.

Insulin
A globular protein/hormone consisting of 51 amino acids in two chains.

Hemoglobin
Transport protein containing Iron (Fe²⁺) in a heme group.

Molisch's Test
General test for all carbohydrates. Violet ring formed with α-naphthol and H₂SO₄.

Barfoed's Test
Distinguishes monosaccharides (reaction occurs fast) from disaccharides (slow).

Seliwanoff's Test
Distinguishes aldoses from ketoses. Ketoses give cherry red color with resorcinol.

Iodine Test
Starch gives blue-black color; Amylopectin/Glycogen give reddish-violet. Cellulose gives no color.

Ninhydrin Test
General test for α-amino acids; gives purple color (Ruhemann's Purple).

Xanthoproteic Test
Test for aromatic amino acids (Tyrosine, Tryptophan); gives yellow color with HNO₃.

Biuret Test
Tests for peptides bonds; violet color with alkaline CuSO₄.

Glucose structure proof
Cyanohydrin formation proves >C=O; Oxime formation proves carbonyl; Pentaacetate proves 5 -OH groups.

Pyranose vs Furanose
6-membered ring (pyran) vs 5-membered ring (furan).

Cellulose
Major constituent of cell wall of plant cells. β-D-glucose units with (1,4) linkage. Humans cannot digest it.

Hormones
Intercellular messengers. Some are steroids (Estrogens), some amino acid derivatives (Epinephrine), some peptides (Oxytocin).

Genetic Code
Sequence of triplets (codons) in DNA/mRNA which determines protein sequence.

Phosphodiester linkage
Linkage between 5' of one sugar and 3' of the next in nucleic acids.

Optical inactivity in Glycine
Glycine is the only α-amino acid without a chiral center.

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