Essay Answer

Question 1

Acetylcholine receptor:

Answer 1

• Acetylcholine is released from the somatic motor neuron at neuromuscular junction through exocytosis
• The acetylcholine binds to acetylcholine receptor channels of the muscle fibers
• When the acetylcholine gated channels open, they allow both sodium and potassium to cross the membrane.
• The action potential travels down the T-tubules and causes sequential opening of voltage gated channels
• The transduction of the electrical signal alters conformation of receptor’s protein shape
• The protein shape change triggers calcium release from the sarcoplasmic reticulum via calcium gated, which is a modified endoplasmic reticulum that wraps around each myofibril.

Question 2

Sarcomere

Answer 2

• Most of the time, the parallel thick and thin filaments within the sarcomeres are connected by myosin crossbridges that span the space between the filaments.
• During contraction, the overlapping myosin and actin filaments of fixed length slide past one another in an energy-requiring process, resulting in contraction. This is visualized through the shortening of the sarcomere.
• The mitochondria are essential for ATP production.
• Muscle fiber require ATP constantly: during contraction for crossbridge movement and release, during relaxation to pump calcium back into the sarcoplasmic reticulum.

Question 3

3. Myosin

Answer 3

• Myosin uses ATP in the sliding filament mechanism
• Sliding filament - The overlapping actin & myosin filaments of fixed lengths passing one another in an energy requiring process
• Contracts muscles with Actin
• Binds on to Actin to create a power stroke
• Power stroke = myosin crossbridges and swivels and pushes the actin filaments toward the sarcomere
• @ the end of a power stroke each myosin head releases actin then swivels back to a new actin molecule
• It has an ATP ase which is an enzyme that hydrolyzes ATP
• ATP comes from mitochondria

Question 4

Sarcoplasmic reticulum

Answer 4

• Modified version of endoplasmic reticulum that wraps around myofibrils
• Concentrates and sequesters calcium
• Calcium can be released from a gated-channel protein
• Sarcoplasmic reticulum pumps calcium back into the lumen using a calcium ATPase
• When calcium binds onto troponin, the myosin is able to bind with actin
• Organelles such as sarcoplasmic reticulum & mitochondria are in the muscle cells
• Calcium is diffused into the cytoplasm

Question 5

Contractile Ring/Cytokinesis

Answer 5

• Cytokinesis is the physical process of cell division, which divides the cytoplasm of a parental cell into two daughter cells.
• It occurs concurrently with two types of nuclear division called mitosis and meiosis
• Mitosis and each of the two meiotic divisions result in two separate nuclei contained within a single cell.
• The cell cycle is made up of G1, S, G2, mitosis, and cytokinesis.
• Cytokinesis starts during the nuclear division phase called anaphase and continues through telophase.
• A ring of overlapping arrays of actin and myosin filaments called the contractile ring forms around the equator of the cell just beneath the plasma membrane. Through the same sliding filament mechanism responsible for muscle contraction.

Question 6

Troponin

Answer 6

• Binds to Calcium
• As the free cytosolic calcium concentration decreases, the equilibrium between bound and unbound calcium is disturbed and calcium is released from troponin
• Controls the position of tropomyosin
• For the initiation of sliding filaments, troponin needs to be bound onto calcium