Question

In muscle, the functional unit of contraction, which is a portion of a myofibril, positioned in between two successive Z lines is?
A. I band
B. A band
C. H zone
D. I band with Z line

Answer 1

Hint: Myofibril is made up of two filaments- actin and myosin. Actin filaments are separated by an elastic fibre known as the Z line.

Complete answer: A myofibril or myofilament is a characteristic feature of muscle fibre and has a large number of parallelly arranged filaments.
Each myofilament has alternate light and dark bands. The light band is known as an I or isotropic band and contains actin whereas the dark band is known as an A or anisotropic band contains myosin.
Proteins are arranged as rod-like structure, along the longitudinal axis of the myofibrils. Actin filaments are also known as thin filaments while myosin filaments are known as thick filaments.
An elastic fibre known as Z line bisects the I band in the centre whereas the fibres of A band are also attached to a thin fibrous membrane known as M line. Along the length of myofibrils, A and I bands are arranged alternatively.
The portion of myofibril between two Z lines is known as a sarcomere.

Hence, the correct option is (A) I band.

Note: Striated muscle tissue contains a complex unit of contraction known as a sarcomere. A single sarcomere contains many myofibrils that are made of actin and myosin. The length of the muscle changes due to the modification of proteins which results in an overall change. These filaments help in conducting cellular movements.