Contractile protein - class-XII

• Skeletal muscle fiber is made up of two types of filaments. They are thick filament (myosin) and thin filament (actin). Contraction of a muscle fiber takes place by the sliding of the thin filaments over the thick filaments.
  
• Myosin (thick) filament is also a polymerized protein. Many monomeric proteins called meromyosins constitute one thick filament. 
  
• Each actin (thin) filament is made of two ‘F’ (filamentous) actins helically wound to each other. Each ‘F’ actin is a polymer of monomeric ‘G’ (Globular) actins. Two filaments of another protein, tropomyosin also run close to the ‘F’ actins throughout its length. A complex protein troponin is distributed at regular intervals on the tropomyosin.
  
• Hence, the important muscles proteins that help in the movement are actin and myosin, troponin and tropomyosin.
• So, the correct answer is 'All of the above'.

• Skeletal muscle fiber is made up of two types of filaments. They are thick filament (myosin) and thin filament (actin). Contraction of a muscle fiber takes place by the sliding of the thin filaments over the thick filaments.
  
• Myosin (thick) filament is also a polymerized protein. Many monomeric proteins called meromyosins constitute one thick filament. 
  
• Each actin (thin) filament is made of two ‘F’ (filamentous) actins helically wound to each other. Each ‘F’ actin is a polymer of monomeric ‘G’ (Globular) actins. Two filaments of another protein, tropomyosin also run close to the ‘F’ actins throughout its length
•  In the resting state a subunit of troponin masks the active binding sites for myosin on the actin filaments.
  
• So, the correct answer is 'Troponin'.

• Myofibrils are composed of two types of filaments i.e thin filament (actin) and thick filament (myosin).
  
• Actin filaments are thinner as compared to the myosin filaments, hence are commonly called thin and thick filaments respectively
  
• Each myosin (thick) filament is a polymerized protein. Many monomeric proteins called meromyosins constitute one thick filament (Myosin).
  
• Hence thick filaments in muscles are polymerized proteins of meromyosin.
• So, the correct answer is 'Meromyosin'.

Thin filaments of myofibril contain one filament of actin and two filaments of troponin, along with tropomyosin protein.

• The myofibril is composed of two types of filaments. They are thick filament(Myosin) and Thin filament(Actin).
  
• Increase in Ca++ level leads to the binding of calcium with a subunit of troponin i.e troponin-C on actin filaments and thereby remove the masking of active sites for myosin. Utilizing the energy from ATP hydrolysis, the myosin head now binds to the exposed active sites on actin to form a cross bridge.
  
• Each myosin (thick) filament is also a polymerized protein. Many monomeric proteins called Meromyosins constitute one thick filament. Each meromyosin has the globular head which is an active ATPase enzyme and has binding sites for ATP and active sites for actin. Hence ATPase activity in muscle fibre lies with Head of heavy meromyosin.
• So, the correct answer is 'Head of heavy meromyosin'.

Actin belongs to the family of globular multifunctional protein that forms microfilaments in muscle fibers. The associated proteins along with actin found in thin filaments are troponin and tropomyosin.however the binding sites are located in meromyosin.

• Myoglobin is a protein found in muscles of almost all mammals that binds to both iron and oxygen. 
• It is also made up of heme and globin protein. 
• The main function of myoglobin is binding to oxygen and carrying it to different muscle tissues.
• So, the correct answer is 'Muscles'.

I-bands (isotropic) contain only thin actin filaments, which is why they appear lighter under a microscope compared to the A-bands.

The layers of stomach wall include serosa, muscularis, submucosa, and mucosa these are involuntary in nature and show the presence of cell junctions also, the muscularis layer shows branched and multinucleated appearance.

Contractile fibres are those fibres which are capable in contraction of the muscles.

So, the correct  option is 'Myofibrils'

• Actin and myosin are the contractile proteins, and they interact with each other, causing contraction, while troponin and tropomyosin are regulatory proteins that regulate the interaction between actin and myosin. 
• Myofilaments(actin and myosin) are packed in functional units of striated muscle referred to as sarcomeres. 
• Hence, Troponin and tropomyosin are the regulatory protein in muscle contraction.
• So, the correct answer is 'Troponin and tropomyosin'.

In the sarcoplasmic reticulum of resting muscle, Ca2+ are held together by calmodulin. Calmodulin is also called as calcium-modulated protein, is a calcium-binding protein present in the cytoplasm. It is present in all eukaryotic cells.it's the function is to bind calcium ions in resting muscle. when the motor impulse is generated and reached to neuromuscular junctions calcium is released from calmodulin which facilitates muscle contraction.

Actin binding sites are located on the troponin complex, which regulates the interaction between actin and myosin during muscle contraction.

• Each myosin (thick) filament is also a polymerized protein. Many monomeric proteins called Meromyosins constitute one thick filament.
• Each meromyosin has two important parts, a globular head with a short arm and a tail. The globular head is an active ATPase enzyme and has binding sites for ATP and active sites for actin.
• The globular head of a myosin molecule that projects from a myosin filament in muscle and is held to attach temporarily to an adjacent actin filament and draw it into the A band of a sarcomere between the myosin filaments to form a cross bridge.
  
• Cross bridges between actin and myosin are broken up by Binding of ATP to the myosin head and The myosin, releasing the ADP and P1 and muscle goes back to its relaxed state.
• So, the correct answer is 'Binding of ATP to the myosin head'.

Muscle is a soft tissue found in most animals. Muscle cells contain protein filaments of actin and myosin that slide past one another, producing a contraction that changes both the length and the shape of the cell. Therefore, (d) is the correct answer. 

• Skeletal muscle fiber is made up of two types of filaments. They are thick filament and thin filament.
  
• Thick filament is made up of a protein called Myosin.
  
• Each myosin (thick filament) is made up of Many monomeric proteins called Meromyosins.
  
• Hence Major protein in the thick filaments of skeletal muscle fiber is Myosin.
• So, the correct answer is 'Myosin'.

• Each myofibril (muscle filament)has alternate dark and light bands on it. 
• The light bands contain actin and are called I-band or Isotropic band, whereas the dark band called ‘A’ or Anisotropic band contains Myosin.
• Myosin filaments are also called as Thick filaments.
• Hence Myosin filaments are localized in A-Band.
  
• So, the correct answer is 'A-Band'.

Troponin is a regulatory protein, not a fibrous protein. Fibrous proteins are typically structural, like collagen or keratin.

• Skeletal muscle fiber is made up of two types of filaments. They are thick filament(myosin) and thin filament(actin). 
  
• Each actin (thin) filament is made of two ‘F’ (filamentous) actins helically wound to each other. Two filaments of another protein, tropomyosin also run close to the ‘F’ actins throughout its length. A complex protein Troponin is distributed at regular intervals on the tropomyosin.
  
• Hence Component of actin filament of a sarcomere is Actin, troponin, and tropomyosin.
• So, the correct answer is 'Actin, troponin, and tropomyosin'.

• The myofibril is composed of two types of filaments. They are thick filament(Myosin) and Thin filament(Actin).
  
• Each actin (thin) filament is made of two ‘F’ (filamentous) actins helically wound to each other. Each ‘F’ actin is a polymer of monomeric ‘G’ (Globular) actins. Two filaments of another protein, tropomyosin also run close to the ‘F’ actins throughout its length. A complex protein Troponin is distributed at regular intervals on the tropomyosin. In the resting state tropomyosin partially covers myosin binding site on actin. Tropomyosin is a fibrous protein and Troponin is a globular protein.
• So, the correct answer is 'Troponin-fibrous protein'

• Myofibril is made up of two types of filaments.They are thin filament(actin) and thick filament(myosin).
  
• Each actin (thin) filament is made of two ‘F’ (filamentous) actins helically wound to each other. Each ‘F’ actin is a polymer of monomeric ‘G’ (Globular) actins. Two filaments of another protein, tropomyosin also run close to the ‘F’ actins throughout its length. A complex protein Troponin is distributed at regular intervals on the tropomyosin.
  
• Each myosin (thick) filament is also a polymerized protein. Many monomeric proteins called Meromyosins, constitute one thick filament. Globular head of meromyosin consists of Heavy meromyosin (HMM).
  
• So, the correct answer is 'i, ii and iii are correct'

• The myofibril is composed of two types of filaments. They are thick filament (Myosin) and Thin filament (Actin).
  
• Each actin (thin) filament is made of two ‘F’ (filamentous) actins helically wound to each other. Each ‘F’ actin is a polymer of monomeric ‘G’ (Globular) actins. Two filaments of another protein, tropomyosin also run close to the ‘F’ actins throughout its length. A complex protein Troponin is distributed at regular intervals on the tropomyosin. In the resting state tropomyosin partially covers myosin binding site on actin.
  
• Increase in Ca++ level leads to the binding of calcium with a subunit of troponin i.e troponin-C on actin filaments and thereby remove the masking of active sites for myosin. Utilizing the energy from ATP hydrolysis, the myosin head now binds to the exposed active sites on actin to form a cross bridge.
• Hence In a resting muscle fibre, tropomyosin partially covers myosin binding sites on actin.
• So, the correct answer is 'Myosin binding sites on actin'.

• Myofibril is made up of two types of filaments. They are thick filament (myosin) and thin filament (actin). Actin filaments are thinner as compared to the myosin filaments, hence are commonly called thin and thick filaments respectively
  
• Each myosin (thick) filament is a polymerized protein. Many monomeric proteins called meromyosins constitute one thick filament.
  
• Hence myosin is the thick filament of muscle.
• So, the correct answer is 'Myosin'.

• Myofilament is the filaments of myofibrils, constructed from proteins, principally myosin or actin.
  
• They are made up of thick and thin myofilament, which helps to give it a striped appearance.

Skeletal muscle is composed of muscle fibres which have smaller units called myofibrils. There are three types of proteins make up each myofibril; they are contractile, regulatory and structural proteins. By contractile proteins, we mean actin (thin filament), a globular protein assembling with a long beady structure consisting chains and myosin (thick filament).

The sarcomere is the functional unit of a myofibril, defined as the segment between two Z-lines, containing a complete A-band and two half I-bands.

Skeletal muscle is composed of muscle fibres which have smaller units called myofibrils. There are three types of proteins make up each myofibril; they are contractile, regulatory and structural proteins. By contractile proteins, we mean actin (thin filament) and myosin (thick filament).

Thick filaments in muscle fibers are composed of polymerized myosin molecules, specifically the meromyosin subunits.

Tropomyosin is a protein found in the actin filaments in muscles. The molecule contains two elongated strands that run along the length of the filament. When the muscle is at rest, the tropomyosin molecule covers the binding site of the actin molecule, where interaction with myosin occurs, On contraction of the muscle, the tropomyosin is displaced by another protein, troponin, allowing the interaction of actin with myosin. So, the correct answer is (c).

Following are the incorrect statements about the molecular arrangement of actin and myosin in myofibrils:

(i) Actin is present in thin filament - True

Actin is the protein which helps in the muscle contraction. It present in two forms G actin (globular actin) and F actin (filament actin). In muscles, actin filament is formed by the two long strand of bead-like twisted actin molecule. 

The entire cross section of the muscle fibre is taken up by long, cylindrical strands of contractile proteins called myofibrils. One myofibril is divided into segments called sarcomeres. These are the contractile units of a muscle. A dark stripe called a Z disc marks the ends of one sarcomere and the beginning of the next.

The main components of protein in muscle is actin and myosine that form thin and thick fibers respectively.

Actin and myosin are the structural filaments that play the main role in muscle contraction by sliding filament theory.

Actin is the thin filaments of the muscles. They are a polymer of G-actin monomers arranged in chains and two such chains are wound in a helix. So, the given statement is true.

Actin is the thin filament of the muscles. It is a polymer made of many monomers. The monomers of actin are globular in shape and hence called Globular-actin or G-actin. Monomers together form straight chains of polymer action. The polymer action is called F-actin.

Skeletal muscle is composed of muscle fibres which have smaller units called myofibrils. There are three types of proteins make up each myofibril; they are contractile, regulatory and structural proteins. By contractile proteins, we mean actin (thin filament) and myosin (thick filament).

Myosin is the primary motor protein in muscle fibers, forming the thick filaments that interact with actin to cause contraction. While actin is also a major constituent, myosin is the most abundant protein in the muscle fiber.

• The myofibril is composed of two types of filaments. They are thick filament(Myosin) and Thin filament(Actin).
  
• Increase in Ca++ level leads to the binding of calcium with a subunit of troponin i.e troponin-C on actin filaments and thereby remove the masking of active sites for myosin. Utilizing the energy from ATP hydrolysis, the myosin head now binds to the exposed active sites on actin to form a cross bridge.
  
• Each myosin (thick) filament is also a polymerized protein. Many monomeric proteins called Meromyosins constitute one thick filament. Each meromyosin has the globular head is an active ATPase enzyme and has binding sites for ATP and active sites for actin.
  
• Hence actin binding sites are located over meromyosin'.
• So, the correct answer is 'Meromyosin'.

• Skeletal muscle fiber is made up of two types of filaments. They are thick filament and thin filament.
  
• Thick filament is made up of a protein called Myosin. Actin filaments are thinner as compared to the myosin filaments, hence are commonly called thin and thick filaments respectively
  
• Each myosin (thick filament) is made up of Many monomeric proteins called Meromyosins.
• Hence the thick myofilament from the given contractile proteins is myosin.
• So.the correct answer is 'Myosin'.

• Each myofibril consists of a series of contractile units (sarcomeres) connected at a protein border (the Z line).
•  Thin filaments(actin) of adjacent sarcomeres are present in the Z line between the thin filaments are thick filaments(myosin). 
• The protein tropomyosin winds around the thin filaments and covers the myosin binding sites. Hence, In skeleton muscle, Z-line is connected to actin.

• The main contractile components of a muscle fibre include:
•  Thick filaments - The major protein of a thick filament is myosin. 
• Thin filaments - The major proteins of a thin filament include actin, as well as troponin and tropomyosin. Hence, Major protein in thick filaments of skeletal muscle fibre is myosin.