1. Definition & Classification of Polysaccharides (2 Marks):
Polysaccharides (glycans) are complex carbohydrates composed of long chains of monosaccharide units bound together by glycosidic linkages. They act as structural or storage macromolecules. They are broadly classified into two categories based on their monomeric composition:
- Homopolysaccharides (Homoglycans): Contain only a single type of monosaccharide unit.
- Heteropolysaccharides (Heteroglycans): Contain two or more different types of monosaccharide units.

2. Homopolysaccharides (3 Marks):

• Starch: The major storage carbohydrate in plants. It is a mixture of two polymers of $\alpha$-D-glucose: Amylose (unbranched, $\alpha$-1,4 linkages) and Amylopectin (branched, $\alpha$-1,4 and $\alpha$-1,6 linkages every 24-30 residues).
• Glycogen: The major storage carbohydrate in animals, found mainly in the liver and muscles. It is structurally similar to amylopectin but much more extensively and compactly branched (branch points every 8-12 residues).
• Cellulose: A structural homopolysaccharide in plant cell walls. It consists of linear chains of $\beta$-D-glucose linked by $\beta$-1,4 glycosidic bonds. The $\beta$-linkage allows the chains to form strong, hydrogen-bonded microfibrils that provide immense tensile strength. Humans lack cellulase and thus cannot digest cellulose.
• Chitin: A structural polysaccharide forming the exoskeleton of arthropods and fungal cell walls. It is a linear polymer of N-acetylglucosamine (GlcNAc) linked by $\beta$-1,4 bonds.

3. Heteropolysaccharides (3 Marks):

• Glycosaminoglycans (GAGs): Linear polymers of repeating disaccharide units (usually a uronic acid and an amino sugar). They are highly negatively charged, allowing them to bind large amounts of water to form gel-like matrices in the extracellular matrix (ECM).
• Examples:
  • Hyaluronic Acid: Found in synovial fluid and vitreous humor, acting as a lubricant. (Only GAG not covalently attached to a core protein).
  • Heparin: An intracellular GAG found in mast cells; acts as a powerful natural anticoagulant by activating antithrombin III.
  • Chondroitin Sulfate: Provides tensile strength to cartilage, tendons, and ligaments.

4. Short Note on Lectins (2 Marks):
Lectins are a class of highly specific carbohydrate-binding proteins (excluding antibodies and enzymes). They possess specific binding sites that recognize and non-covalently bind to specific oligosaccharide sequences on the surface of cells.
- Functions: They play crucial roles in cell-cell recognition, adhesion, signaling, and immune responses (e.g., Selectins mediating white blood cell rolling during inflammation).
- Plant Lectins: Phytohemagglutinin (PHA) and Concanavalin A (ConA) are well-known plant lectins used in laboratories to agglutinate cells and stimulate lymphocyte division.

1. Primary Structure (2.5 Marks):
The primary structure is the linear, one-dimensional sequence of amino acids in a polypeptide chain, read from the N-terminus to the C-terminus. The exact sequence is dictated by the genetic code.
- Chemical Bonds: Peptide Bonds (covalent amide linkages) formed by a condensation reaction between the $\alpha$-carboxyl group of one amino acid and the $\alpha$-amino group of the next. Disulfide bonds (between cysteine residues) are also considered part of the primary structure in some classifications.

2. Secondary Structure (2.5 Marks):
The secondary structure refers to localized, regular folding patterns of the polypeptide backbone, independent of the side chains (R-groups).
- $\alpha$-Helix: A right-handed coiled structure where the carbonyl oxygen of residue $n$ forms a hydrogen bond with the amide hydrogen of residue $n+4$.
- $\beta$-Pleated Sheet: Consists of extended polypeptide chains (strands) lying side-by-side, held together by intermolecular hydrogen bonds. Can be parallel or anti-parallel.
- Chemical Bonds: Hydrogen Bonds between the backbone $-C=O$ and $-N-H$ groups.

3. Tertiary Structure (2.5 Marks):
The tertiary structure is the complete, three-dimensional, biologically active conformation of a single polypeptide chain, encompassing the spatial relationship of all amino acid side chains.
- Chemical Bonds: Predominantly stabilized by side-chain interactions:

• Hydrophobic Interactions: Non-polar side chains cluster in the interior to avoid water (the primary driving force of protein folding).
• Hydrogen Bonds: Between polar side chains and the solvent.
• Ionic Bonds (Salt Bridges): Between oppositely charged side chains (e.g., Glutamate and Lysine).
• Disulfide Bonds: Covalent links between two cysteine residues.
• Van der Waals forces.

4. Quaternary Structure (2.5 Marks):
This structure exists only in proteins composed of two or more polypeptide chains (subunits). It refers to the spatial arrangement and non-covalent interaction of these subunits to form a functional multimeric protein complex (e.g., Hemoglobin is a tetramer $\alpha_2\beta_2$).
- Chemical Bonds: Same non-covalent interactions as tertiary structure: Hydrophobic interactions, Hydrogen bonds, and Ionic bonds at the subunit interfaces.

1. Biochemical Reactions of the TCA Cycle (6 Marks):
The Tricarboxylic Acid (TCA) cycle takes place in the mitochondrial matrix. It oxidizes Acetyl CoA into 2 molecules of $CO_2$, generating ATP/GTP and reducing equivalents (NADH, $FADH_2$).

• Step 1: Condensation. Acetyl CoA (2C) condenses with Oxaloacetate (4C) and water to form Citrate (6C), catalyzed by Citrate Synthase.
• Step 2: Isomerization. Citrate undergoes dehydration/rehydration via Aconitase to form Isocitrate.
• Step 3: 1st Oxidative Decarboxylation. Isocitrate is oxidized and decarboxylated by Isocitrate Dehydrogenase to form $\alpha$-Ketoglutarate (5C). $NAD^+$ is reduced to NADH + $H^+$, and one $CO_2$ is released.
• Step 4: 2nd Oxidative Decarboxylation. $\alpha$-Ketoglutarate undergoes oxidative decarboxylation via the $\alpha$-Ketoglutarate Dehydrogenase Complex to form Succinyl CoA (4C). Generates a second NADH + $H^+$ and $CO_2$.
• Step 5: Substrate-Level Phosphorylation. The high-energy thioester bond in Succinyl CoA is cleaved by Succinyl CoA Synthetase to form Succinate, coupled with the phosphorylation of GDP to GTP (or ADP to ATP).
• Step 6: Oxidation. Succinate is oxidized to Fumarate by Succinate Dehydrogenase (located in the inner mitochondrial membrane). FAD is reduced to $FADH_2$.
• Step 7: Hydration. Water is added across the double bond of Fumarate by Fumarase to form L-Malate.
• Step 8: Final Oxidation. L-Malate is oxidized to Oxaloacetate by Malate Dehydrogenase, generating the third NADH + $H^+$. The cycle is complete.

2. Regulation of the Citric Acid Cycle (4 Marks):
The TCA cycle is strictly regulated by energy status (ATP/ADP ratio and NADH/$NAD^+$ ratio) and substrate availability. The major regulatory enzymes are the irreversible steps:

• Citrate Synthase: Inhibited by ATP, NADH, Succinyl CoA, and its own product, Citrate. Stimulated by ADP.
• Isocitrate Dehydrogenase: The primary rate-limiting enzyme. Strongly allosterically stimulated by ADP and $Ca^{2+}$ (signaling muscle contraction). Strongly inhibited by ATP and NADH.
• $\alpha$-Ketoglutarate Dehydrogenase: Inhibited by its products, Succinyl CoA and NADH. Stimulated by $Ca^{2+}$.

Note: The Pyruvate Dehydrogenase complex, while strictly not part of the TCA cycle, dictates Acetyl CoA entry and is heavily regulated (inhibited by Acetyl CoA, ATP, NADH).

1. Classification of Diabetes Mellitus (3 Marks):
Diabetes Mellitus (DM) is a group of metabolic disorders characterized by chronic hyperglycemia. It is primarily classified into:

• Type 1 Diabetes Mellitus (T1DM): An autoimmune destruction of the pancreatic $\beta$-cells, leading to an absolute deficiency of insulin. Patients are prone to Diabetic Ketoacidosis (DKA) and require lifelong exogenous insulin therapy. (Formerly known as Insulin-Dependent DM).
• Type 2 Diabetes Mellitus (T2DM): Characterized by peripheral insulin resistance followed by a progressive decline in insulin secretion. Highly associated with obesity, genetics, and sedentary lifestyle. (Formerly known as Non-Insulin Dependent DM).
• Gestational Diabetes Mellitus (GDM): Glucose intolerance with onset or first recognition during pregnancy due to placental hormones inducing insulin resistance.
• Specific Types (e.g., MODY): Maturity-Onset Diabetes of the Young (monogenic defects in $\beta$-cell function) or secondary diabetes due to exocrine pancreatic diseases.

2. Complications of Diabetes Mellitus (4 Marks):
Complications arise due to chronic hyperglycemia inducing endothelial dysfunction, oxidative stress, and the accumulation of Advanced Glycation End Products (AGEs).
- Microvascular Complications:

• Diabetic Retinopathy: Damage to retinal blood vessels, leading to blindness.
• Diabetic Nephropathy: Glomerular damage, proteinuria, leading to end-stage renal disease (ESRD).
• Diabetic Neuropathy: Nerve damage causing sensory loss (stocking-glove distribution), pain, and autonomic dysfunction.

- Macrovascular Complications:

• Cardiovascular Disease: Accelerated atherosclerosis leading to myocardial infarction (heart attack).
• Cerebrovascular Disease: Increased risk of stroke.
• Peripheral Vascular Disease: Poor circulation leading to diabetic foot ulcers and amputations.

3. Note on Insulin Release in the Pancreas (3 Marks):
Insulin is synthesized and secreted by the $\beta$-cells of the Islets of Langerhans. The stimulus-secretion coupling mechanism is as follows:
1. Glucose Entry: Blood glucose enters the $\beta$-cell via the GLUT-2 transporter (which is not insulin-dependent).
2. Metabolism: Glucose is phosphorylated by Glucokinase and undergoes glycolysis and the TCA cycle, raising the intracellular ATP/ADP ratio.
3. Channel Closure: The rise in ATP closes ATP-sensitive Potassium ($K_{ATP}$) channels on the cell membrane, preventing $K^+$ efflux.
4. Depolarization: The trapped $K^+$ causes the membrane to depolarize.
5. Calcium Influx: Depolarization opens Voltage-Gated Calcium ($Ca^{2+}$) Channels. $Ca^{2+}$ rushes into the cell.
6. Exocytosis: The sudden spike in intracellular $Ca^{2+}$ triggers the fusion of pre-formed insulin secretory vesicles with the plasma membrane, releasing insulin into the bloodstream.

1. Introduction (1 Mark):
Beyond their primary role as the building blocks of proteins, amino acids serve as crucial precursors for a vast array of specialized, biologically active nitrogenous compounds, including neurotransmitters, hormones, pigments, and coenzymes.

2. Derivatives of Tyrosine (3 Marks):
Tyrosine is a highly versatile precursor. Important derivatives include:
- Catecholamines (Neurotransmitters/Hormones): Tyrosine is hydroxylated and decarboxylated to form Dopamine (motor control, reward), which is further hydroxylated to Norepinephrine and methylated to Epinephrine (adrenaline - fight or flight response).
- Thyroid Hormones: Iodination of tyrosine residues on thyroglobulin produces $T_3$ (Triiodothyronine) and $T_4$ (Thyroxine), essential for regulating basal metabolic rate.
- Melanin: Tyrosine is oxidized by the enzyme tyrosinase to form melanin, the primary pigment in skin, hair, and eyes, protecting against UV radiation.

3. Derivatives of Tryptophan (2 Marks):
- Serotonin (5-HT): Tryptophan is hydroxylated and decarboxylated to produce Serotonin, a key neurotransmitter regulating mood, sleep, and appetite.
- Melatonin: Serotonin is further acetylated and methylated in the pineal gland to form Melatonin, which regulates circadian rhythms.
- Niacin (Vitamin $B_3$): A minor but crucial pathway converts tryptophan to the coenzymes $NAD^+$ and $NADP^+$. (Note: 60mg of tryptophan yields ~1mg of niacin).

4. Derivatives of Other Key Amino Acids (4 Marks):
- Glycine: Condenses with Succinyl CoA to initiate the synthesis of Porphyrins (e.g., Heme in hemoglobin). Also contributes to the synthesis of Creatine (energy storage in muscle) and Purines.
- Histidine: Decarboxylated by histidine decarboxylase to form Histamine, a potent vasodilator involved in allergic responses and gastric acid secretion.
- Arginine: Cleaved to produce Nitric Oxide (NO), a critical signaling molecule that induces smooth muscle relaxation (vasodilation), and Urea.
- Glutamate: Decarboxylated to form $\gamma$-Aminobutyric Acid (GABA), the primary inhibitory neurotransmitter in the central nervous system.

1. The Sources of the Two Nitrogens (2 Marks):
Ammonia is highly toxic. The body safely excretes it by synthesizing Urea ($NH_2-CO-NH_2$) in the liver. The two nitrogen atoms in a single urea molecule come from two distinct sources:
- First Nitrogen: Originates from free ammonia ($NH_4^+$), primarily generated by the oxidative deamination of glutamate.
- Second Nitrogen: Donated directly by the amino acid Aspartate.

2. Pre-Urea Cycle Steps (Transamination & Deamination) (2 Marks):
- Amino acids transfer their $\alpha$-amino group to $\alpha$-ketoglutarate, forming Glutamate (Transamination).
- In the liver mitochondria, Glutamate Dehydrogenase catalyzes the oxidative deamination of Glutamate, releasing free ammonia ($NH_4^+$) and regenerating $\alpha$-ketoglutarate.

3. The Urea Cycle Steps (Mitochondrial Phase) (3 Marks):
- Step 1 (Rate-Limiting): Free ammonia ($NH_4^+$) and bicarbonate ($HCO_3^-$) condense using 2 ATP to form Carbamoyl Phosphate, catalyzed by Carbamoyl Phosphate Synthetase I (CPS-I). This incorporates the first nitrogen.
- Step 2: Carbamoyl phosphate donates its carbamoyl group to Ornithine to form Citrulline, catalyzed by Ornithine Transcarbamylase (OTC). Citrulline is then transported into the cytosol.

4. The Urea Cycle Steps (Cytosolic Phase) (3 Marks):
- Step 3: In the cytosol, Citrulline condenses with Aspartate (the donor of the second nitrogen) to form Argininosuccinate, catalyzed by Argininosuccinate Synthetase. This consumes 1 ATP (cleaved to AMP, counting as 2 ATP equivalents).
- Step 4: Argininosuccinate Lyase cleaves Argininosuccinate into Arginine (which now contains both nitrogens destined for urea) and Fumarate (which links to the TCA cycle).
- Step 5: Finally, Arginase hydrolyzes Arginine to yield Urea and regenerate Ornithine. Ornithine re-enters the mitochondria to begin the cycle anew, while Urea is transported to the kidneys for excretion.

1. Preparatory Steps (Activation and Transport) (3 Marks):
Before degradation, fatty acids must be activated in the cytosol and transported into the mitochondrial matrix.
- Activation: Fatty acid reacts with ATP and Coenzyme A, catalyzed by Fatty Acyl-CoA Synthetase (Thiokinase), to form a high-energy Fatty Acyl-CoA. This consumes 2 ATP equivalents.
- Transport: The inner mitochondrial membrane is impermeable to long-chain acyl-CoA. The Carnitine Shuttle transfers the acyl group to Carnitine (via CPT-I), transports it across the membrane, and reattaches it to mitochondrial CoA (via CPT-II).

2. The Four Repeated Steps of $\beta$-Oxidation (5 Marks):
Once inside the matrix, the fatty acyl-CoA undergoes a cyclic sequence of four reactions, sequentially cleaving two-carbon units as Acetyl CoA from the carboxyl end.
- Step 1 - First Oxidation: Acyl-CoA Dehydrogenase catalyzes the oxidation of the $\alpha$ and $\beta$ carbons, creating a trans-double bond and forming trans-$\Delta^2$-Enoyl CoA. The electrons reduce FAD to $FADH_2$.
- Step 2 - Hydration: Enoyl CoA Hydratase adds a molecule of water across the double bond, introducing a hydroxyl group at the $\beta$-carbon to form L-$\beta$-Hydroxyacyl CoA.
- Step 3 - Second Oxidation: $\beta$-Hydroxyacyl CoA Dehydrogenase oxidizes the hydroxyl group to a keto group, forming $\beta$-Ketoacyl CoA. $NAD^+$ is reduced to NADH + $H^+$.
- Step 4 - Cleavage (Thiolysis): $\beta$-Ketothiolase (Thiolase) cleaves the bond between the $\alpha$ and $\beta$ carbons using a free Coenzyme A. This releases one molecule of Acetyl CoA and a new Fatty Acyl-CoA shortened by two carbons.

3. The Cycle Repetition (2 Marks):
The shortened Fatty Acyl-CoA immediately re-enters the cycle at Step 1. These four steps (Oxidation $\rightarrow$ Hydration $\rightarrow$ Oxidation $\rightarrow$ Cleavage) are repeated until the entire even-chain fatty acid is entirely degraded into Acetyl CoA molecules. For a 16-carbon Palmitic acid, the cycle repeats 7 times to yield 8 Acetyl CoA, 7 $FADH_2$, and 7 NADH.

1. Foundational Concept (2 Marks):
Unlike pyrimidines, where the ring is formed before attaching to a sugar, the purine double-ring is built atom-by-atom directly onto a ribose sugar scaffold. The primary end product of this 10-step pathway is Inosine Monophosphate (IMP).

2. Formation of the Scaffold (2 Marks):
Ribose-5-phosphate (derived from the Pentose Phosphate Pathway) is activated by ATP to form 5-Phosphoribosyl-1-Pyrophosphate (PRPP). This step is catalyzed by PRPP Synthetase and acts as the foundation for the purine ring.

3. The Committing Step (2 Marks):
PRPP reacts with Glutamine, replacing the pyrophosphate with an amino group to form 5-Phosphoribosyl-1-amine, catalyzed by Glutamine:PRPP Amidotransferase. This is the rate-limiting and committed step of purine synthesis.

4. Building the Purine Ring (Contributions) (4 Marks):
The ring is assembled on this amine group. A strict examiner will look for the precise sources of the atoms in the purine ring:
- N1: Aspartate
- C2 and C8: Formyl-Tetrahydrofolate (THF) - $N^{10}$-formyl-THF
- N3 and N9: Amide group of Glutamine
- C4, C5, and N7: Glycine (contributes an entire small molecule)
- C6: $CO_2$ (Respiratory bicarbonate)

After 9 highly regulated enzymatic steps (requiring 6 ATPs), the ring closes to form Inosine Monophosphate (IMP). IMP is then divergently converted into either AMP (requires GTP and Aspartate) or GMP (requires ATP and Glutamine) maintaining a balanced pool of purine nucleotides.

1. Overview and Arrangement (3 Marks):
The Electron Transport Chain (ETC) is located in the Inner Mitochondrial Membrane (IMM). The components are arranged sequentially based on their Standard Reduction Potential ($E_0^\prime$). Electrons flow spontaneously downhill from carriers with a more negative reduction potential (e.g., NADH) to carriers with a more positive reduction potential, ultimately reaching Oxygen, the final electron acceptor with the highest positive potential.

2. The Key Protein Complexes (5 Marks):
The ETC consists of four large multi-protein transmembrane complexes:

• Complex I (NADH:Ubiquinone Oxidoreductase): Accepts electrons from NADH, passes them through FMN and Fe-S centers, and reduces Coenzyme Q. It pumps 4 protons ($H^+$) from the matrix to the intermembrane space.
• Complex II (Succinate Dehydrogenase): The only complex shared with the TCA cycle. It accepts electrons from FADH$_2$ (via Succinate oxidation) and passes them to Coenzyme Q. It does not pump protons.
• Complex III (Cytochrome bc1 Complex): Transfers electrons from reduced Coenzyme Q (Ubiquinol) to Cytochrome c via the Q-cycle. It pumps 4 protons ($H^+$).
• Complex IV (Cytochrome c Oxidase): Accepts electrons from Cytochrome c and transfers them to molecular oxygen ($O_2$), reducing it to water ($H_2O$). Contains Cytochromes a and a3 and Copper centers. It pumps 2 protons ($H^+$).

3. The Mobile Electron Carriers (2 Marks):
Connecting the massive complexes are two highly mobile carriers:
- Coenzyme Q (Ubiquinone): A small, lipid-soluble molecule freely diffusing within the hydrophobic core of the IMM membrane. It shuttles electrons from Complex I and II to Complex III.
- Cytochrome c: A small, water-soluble peripheral membrane protein located on the intermembrane space surface. It shuttles electrons from Complex III to Complex IV.

1. Functional Interdependence (3 Marks):
Photosynthesis is classically divided into two parts, but they are absolutely coupled.
- Light-Dependent Reactions (Thylakoids): Harvest solar energy to split water, release $O_2$, and synthesize the energy-carrying molecules ATP and NADPH.
- Light-Independent Reactions / Calvin Cycle (Stroma): Strictly dependent on the ATP and NADPH generated above to reduce $CO_2$ into carbohydrates (e.g., G3P). Without the light reactions replenishing these carriers, the Calvin cycle halts instantly.

2. Biochemical Coordination via Stromal Environment (4 Marks):
The Calvin cycle enzymes are highly active in the light and inactive in the dark. Light actively alters the stromal environment to turn the Calvin Cycle "ON":
- pH Changes: Light causes protons ($H^+$) to be pumped from the stroma into the thylakoid lumen. This raises the stromal pH from ~7.0 (dark) to ~8.0 (light). Rubisco and other Calvin cycle enzymes have an optimal pH of 8.0.
- Magnesium Ion ($Mg^{2+}$) Efflux: To balance the electrical charge of the pumped protons, $Mg^{2+}$ ions are pushed out of the thylakoid into the stroma. Rubisco activity is highly dependent on high $Mg^{2+}$ concentrations.

3. Coordination via the Thioredoxin System (3 Marks):
Light causes electrons to flow through Photosystem I to Ferredoxin.
- The enzyme Ferredoxin-Thioredoxin Reductase uses these electrons to reduce Thioredoxin.
- Reduced Thioredoxin then reduces critical disulfide bonds (-S-S- to -SH) on key Calvin cycle enzymes (e.g., Sedoheptulose-1,7-bisphosphatase, Rubisco Activase), changing their conformation from an inactive state to an active state.
- In the dark, these enzymes revert to their oxidized, inactive forms, ensuring the plant doesn't wastefully consume ATP/NADPH when light energy is unavailable.