Question

What happens to the actin and myosin filaments when a muscle contracts?

Answer 1

Hint: Myosin are ATP dependent proteins. Actins are present in concentrations of over 100μM.
All eukaryotic cells contain myosin isoforms.

Complete answer:
Actins are globular proteins which form microfilaments. They are found in all eukaryotic cells. Actin is an important part of many cellular processes like muscle contraction, organelle movement, cell movement and cytokinesis. There are three main groups of actin in vertebrates, namely:
- Alpha
- Beta
- Gamma
Tropomyosin covers the actin’s active site during the resting phase. This disables the actin-myosin reaction and leads to the contraction of muscles. Actin and myosin make up to $90\%$ of all muscle protein.
Myosins are a family of proteins responsible for muscle contraction in eukaryotic organisms. They utilize ATP as an energy source and are important factors for actin-based motility. Myosin structures are generally similar in almost all myosin-containing species.
The contraction-relaxation cycle of muscles has the following steps:
- The sarcolemma is depolarized and an action potential is transmitted through T-tubules.
- The calcium channels of the sarcoplasmic reticulum open.
- Increase in calcium concentrations cause a change in the structure.
- Myosin heads move over the thin filaments with or without the use of ATP.
- The sarcoplasmic reticulum captures the calcium and inhibits the actin-myosin interaction.

Note:
Myosins are structurally composed of a head, tail and neck.
Actins are made of two types:
- F-actin
- G-actin
Actin myofibrils consist of most of the cytoplasm of muscle cells. Myofibrils are made up of thin filaments of actin.