(a) Classify carbohydrates by giving suitable examples. Also, mention characteristics of each category. (20 marks) (b) Explain…

(a) Classification of carbohydrates and characteristics of each class

Carbohydrates are polyhydroxy aldehydes or ketones, or compounds that yield them on hydrolysis. They are divided into four groups. Monosaccharides: simple sugars that cannot be hydrolysed further. They are sweet, crystalline and soluble in water. Trioses, tetroses, pentoses (ribose) and hexoses (glucose, fructose and galactose) are classed by carbon number, and as aldoses (glucose) or ketoses (fructose) by functional group. All are reducing sugars. Disaccharides: two monosaccharides joined by a glycosidic bond, soluble and sweet. Maltose (glucose and glucose) and lactose (glucose and galactose) are reducing, and sucrose (glucose and fructose) is non-reducing because both anomeric carbons are in the bond. Oligosaccharides: three to ten monosaccharide units, such as raffinose and stachyose, which are found in pulses. They often occur on cell surfaces in glycoproteins. Polysaccharides: long chains of hundreds or thousands of units, not sweet, not crystalline, and mostly insoluble, and non-reducing. Homopolysaccharides: starch (amylose and amylopectin, plant storage), glycogen (animal storage), cellulose (β-1,4-glucose, plant wall) and chitin (N-acetylglucosamine, in arthropod cuticle and fungal walls). Heteropolysaccharides: hyaluronic acid and chondroitin sulphate (connective tissue) and heparin (anticoagulant). Tests: reducing sugars reduce Benedict's or Fehling's solution, and starch gives a blue colour with iodine.

(b) Structure of haemoglobin and its role in carbon dioxide transport

Structure: haemoglobin is a conjugated protein of the red blood cell, a tetramer of two α and two β globin chains (α₂β₂) of 141 and 146 amino acids. Each chain holds a haem group, a porphyrin ring with a central ferrous iron atom (Fe²⁺), which binds one oxygen molecule. The chains are held by non-covalent bonds and the molecule shows a quaternary structure that changes from tense to relaxed state on oxygen binding. The molecular weight is about 64,500. Role in carbon dioxide transport: Bicarbonate (about 70 per cent): CO₂ entering a red cell combines with water through carbonic anhydrase to form carbonic acid, which splits into H⁺ and bicarbonate. Haemoglobin buffers the H⁺ (Bohr effect, which helps oxygen release), and bicarbonate leaves into the plasma in exchange for chloride (the chloride shift). Carbaminohaemoglobin (about 20 to 25 per cent): CO₂ binds to the amino groups of globin. Deoxyhaemoglobin binds more CO₂ (Haldane effect). Dissolved in plasma: about 7 per cent. In the lungs the reactions are reversed, and CO₂ is released and exhaled.

(c) Techniques of in vitro fertilisation

In vitro fertilisation is the union of egg and sperm outside the body, followed by transfer of the embryo. IVF with embryo transfer: eggs are collected after ovarian stimulation, fertilised with sperm in a dish, the embryos are grown for a few days and embryos are transferred to the uterus (intra-uterine transfer, usually for embryos of more than eight cells). A test-tube baby is born. ZIFT (zygote intra-fallopian transfer): the zygote or an early embryo of up to eight blastomeres is placed in the fallopian tube. GIFT (gamete intra-fallopian transfer): eggs and sperm are transferred to the fallopian tube, where fertilisation takes place in the body. It needs healthy tubes. ICSI (intra-cytoplasmic sperm injection): a single sperm is injected into the egg, for male infertility. Others: intra-uterine insemination (IUI), artificial insemination of semen into the uterus, and frozen embryo transfer. In India, ART is regulated by the Assisted Reproductive Technology (Regulation) Act, 2021.