DEFINE SARCOMERE AND LABEL ITS BANDS(CBSE 12TH BIOLOGY IMPORTANT QUESTION)

 

A sarcomere is the structural and functional unit of a myofibril in a striated muscle. It is the segment between two Z-lines and is responsible for muscle contraction through the sliding filament mechanism.

Bands of a Sarcomere (with Labels):

|←-- Sarcomere --→|

Z-line        I-band        A-band    H-zone   M-line H-zone  A-band   I-band   Z-line

  |              |            |         |          |        |          |            |            |

--Z--------I---------A-----H-----M----H------A--------I--------Z-- (repeats)

Description of Bands and Lines:

Z-line (Z-disc)-Ends of a sarcomere. Actin filaments are anchored here.

I-band (Isotropic band)-Light band with only actin (thin) filaments. Spans two sarcomeres.

A-band (Anisotropic band)-Dark band with overlapping actin and myosin. Remains constant during contraction

H-zone-Central part of A-band containing only myosin (thick filaments), disappears during contraction.

M-line-Middle of H-zone, holds myosin filaments together.

Important Points:

• The sarcomere shortens during muscle contraction.

• The A-band remains unchanged, while I-band and H-zone shorten.

• Z-lines move closer during contraction.