(a) Enlist various photosynthetic pigments. Give a detailed account of chlorophyll synthesis along with the enzymes involved in…

(a) Photosynthetic pigments, chlorophyll synthesis and role of carotenoids

Pigments: chlorophylls (a, b, c, d and bacterial chlorophylls), carotenoids (carotenes such as β-carotene and xanthophylls such as lutein and violaxanthin) and phycobilins (phycocyanin, phycoerythrin) in algae and cyanobacteria.

Chlorophyll synthesis (in the plastid):

1. Glutamate is charged to tRNA (glutamyl-tRNA synthetase), reduced by glutamyl-tRNA reductase to glutamate-1-semialdehyde and converted to δ-aminolevulinic acid (ALA) by GSA aminotransferase, the rate-limiting, light-regulated stage.
2. Two ALA condense to porphobilinogen (ALA dehydratase); four porphobilinogen join to hydroxymethylbilane (PBG deaminase), which cyclises to uroporphyrinogen III (uroporphyrinogen III synthase).
3. Uroporphyrinogen decarboxylase gives coproporphyrinogen III; coproporphyrinogen oxidase gives protoporphyrinogen IX; protoporphyrinogen oxidase gives protoporphyrin IX.
4. Mg-chelatase inserts Mg²⁺ to give Mg-protoporphyrin IX, which Mg-protoporphyrin methyltransferase methylates; the cyclase forms the fifth ring to give protochlorophyllide.
5. Light-dependent protochlorophyllide oxidoreductase (POR) reduces it to chlorophyllide a; chlorophyll synthase adds the phytol tail to give chlorophyll a, and chlorophyllide a oxygenase converts a to chlorophyll b.

Carotenoids act as accessory pigments absorbing 400–550 nm light and passing energy to chlorophyll, protect against photo-oxidation by quenching triplet chlorophyll and singlet oxygen, dissipate excess energy through the xanthophyll cycle (violaxanthin to zeaxanthin), and stabilise light-harvesting complexes.

(b) Reactions and enzymes of the Calvin and Hatch-Slack cycles

Calvin cycle (C3, stroma):

• Carboxylation: RuBP + CO₂ → 2 × 3-phosphoglycerate, by RuBisCO.
• Reduction: 3-PGA → 1,3-bisphosphoglycerate (phosphoglycerate kinase, ATP) → glyceraldehyde-3-phosphate (GAPDH, NADPH).
• Regeneration: triose phosphate isomerase, aldolase, fructose-1,6-bisphosphatase, transketolase, sedoheptulose-1,7-bisphosphatase, ribose-5-phosphate isomerase, ribulose-5-phosphate epimerase and phosphoribulokinase (ATP) regenerate RuBP.

Fixing 3 CO₂ to one triose phosphate needs 9 ATP and 6 NADPH.

Hatch-Slack cycle (C4): in mesophyll cytosol, PEP carboxylase fixes HCO₃⁻ on phosphoenolpyruvate to form oxaloacetate, which is reduced to malate (NADP-malate dehydrogenase) or transaminated to aspartate (aspartate aminotransferase). The C4 acid moves to bundle-sheath cells, where it is decarboxylated by NADP-malic enzyme (chloroplast), NAD-malic enzyme (mitochondria) or PEP carboxykinase (cytosol) to release CO₂ for RuBisCO and the Calvin cycle. The C3 residue (pyruvate or alanine) returns, and pyruvate phosphate dikinase regenerates PEP with ATP. It concentrates CO₂ and suppresses photorespiration, as in maize, sorghum and sugarcane.

(c) Pathways of carbohydrate metabolism in plants

Synthesis: photosynthetic triose phosphates give sucrose in the cytosol (sucrose phosphate synthase) for transport, and starch in the chloroplast or amyloplast (ADP-glucose pyrophosphorylase, starch synthase, branching enzyme).

Breakdown: starch is degraded by amylases and phosphorylase, and sucrose by invertase and sucrose synthase. Respiratory pathways: glycolysis in the cytosol (glucose to pyruvate), the TCA cycle and oxidative phosphorylation in mitochondria, the oxidative pentose phosphate pathway giving NADPH and ribose, and fermentation under anaerobic conditions. In germinating oilseeds, the glyoxylate cycle converts fat to sugar.