Question

Differentiate between A-band and I-band.

Answer 1

Hint: Skeletal muscle tissue forms skeletal muscles that bind and regulate locomotion of bones or skin and any movement that can be controlled consciously. Since it can be regulated by thought, voluntary muscle is also called skeletal muscle. Skeletal muscles have a long and cylindrical appearance; skeletal muscle tissue has a striped or striated appearance when viewed under a microscope.

Complete answer: The striated appearance in muscle tissue is a product of repeated bands of actin and myosin proteins which are present all along the width of myofibrils. Dark A bands and light I repeat bands along myofibrils, and the cell alignment of myofibrils creates the striated or banded appearance of the entire cell. Each band has a dense line called a Z disc or a Z line that runs vertically through the centre. The boundary of units called sarcomeres, the functional units of skeletal muscle, is marked by the Z discs. The space between two consecutive Z discs is one sarcomere, comprising one full A band and two halves of an I band, one on each side of the A band. A myofibril is made up of several sarcomeres that run along its length and the myofibrils and muscle cells shorten as the sarcomeres individually contract. Myofibrils are made of structures called myofilaments that are smaller. Two main types of filaments are available: thick filaments and thin filaments; each has distinct compositions and locations. Only the A band of myofibril develops thick filaments. Thin filaments bind to a protein called alpha-actinin in the Z disc and occur along the whole length of the I band and partway into the A band.
Difference between A band and I band:

A band	I band
These are the Sarcomere's anisotropic bands.	The isotropic bands of the sarcomere are these.
They appear under the microscope as a dark band.	Under the microscope, they appear as light bands.
In the centre, a band has a broad light zone called the H zone.	They have a cross with a thick dark membrane called a Z line in the middle section.
A ban includes mainly myofilaments and parts of secondary myofilaments.	Filaments of secondary myofilaments comprise them.
The length of a band remains constant during muscle contractions.	During muscle contractions, they shorten.

Note: The other two types of muscles are cardiac muscles and smooth muscles. Cardiac muscles form the heart, which causes the heart to beat rhythmically, whereas smooth muscles are present in the intestines and oesophagus, which helps in digestion.