Q.Match the following and mark the correct option. Column I: A. Fast muscle fibres; B. Slow muscle fibres; C. Actin filament; D. Sarcomere. Column II: i. Myoglobin; ii. Lactic acid; iii. Contractile unit; iv. I-band

Fast muscle fibres are the white fibres, which have little myoglobin and rely on anaerobic metabolism, so they build up lactic acid quickly and fatigue faster - matching them to ii (lactic acid). Slow muscle fibres are the red fibres, rich in the oxygen-storing pigment myoglobin, which lets them work steadily using aerobic metabolism - matching them to i (myoglobin). The actin filament is the thin filament that occupies the light, isotropic band of a myofibril, so it matches iv (I-band). The sarcomere is the stretch of myofibril between two successive Z-lines and is defined as the functional unit of contraction, matching it to iii (contractile unit).

So the correct matching is A-ii, B-i, C-iv, D-iii, which corresponds to option (C).

Muscle fibres are not all built for the same kind of work, and the two broad categories recognised are red and white fibres, distinguished mainly by how much of the oxygen-storing pigment myoglobin they contain.

• Fibres with a high content of myoglobin look reddish and are described as red or "slow" fibres. They are packed with mitochondria, which use the stored oxygen to make ATP aerobically, so they can sustain activity for a long time without tiring quickly. Column I's "slow muscle fibres" therefore link to Column II's myoglobin (i).
• Fibres with very little myoglobin look pale and are described as white or "fast" fibres. They have few mitochondria but plenty of sarcoplasmic reticulum, and they generate energy anaerobically. Repeated anaerobic activity leads to the breakdown of glycogen without oxygen, and this causes lactic acid to build up, which is exactly what produces fatigue. So "fast muscle fibres" link to lactic acid (ii).

The other two items concern the fine structure of the myofibril rather than fibre type:

• The actin filament is the thinner of the two contractile proteins and lies in the light band of a myofibril. Because this light band is also called the isotropic or I-band, actin filament links to I-band (iv).
• A sarcomere is the segment of a myofibril lying between two successive Z-lines, and it is explicitly treated as the smallest functional unit that contracts. So sarcomere links to contractile unit (iii).

Putting the four pairs together gives A-ii, B-i, C-iv, D-iii, which is listed as option (C).