Q.Define a sarcomere.

[!TLDR]

A sarcomere is the repeating structural and functional unit of a myofibril, lying between two successive Z lines.

[!ANSWER]

The segment of a myofibril between two consecutive Z lines.

A sarcomere is the basic repeating unit of a myofibril, defined as the segment lying between two successive Z lines. It contains a central A band (with thick, myosin filaments), flanking I bands (with thin, actin filaments only), and an H zone at the centre of the A band. It is the shortening of many such sarcomeres, arranged end to end along a myofibril, that produces the overall shortening of a contracting muscle fibre.

[!ANSWER]

The segment of a myofibril between two consecutive Z lines.

Recall the sarcomere's defining boundary markers -- the two Z lines at either end.

Describing a sarcomere as 'the whole myofibril' rather than one repeating segment of it; a single myofibril contains many sarcomeres placed end to end.