sliding filament theory

describe the process of muscle contraction according to the sliding filament theory:

• action potential travels through T tubules which are in contact w/ sarcoplasmic reticulum which stores Ca²⁺

• action potential causes Ca²⁺ channels to open on the sarcoplasmic reticulum allowing Ca²⁺ to diffuse into myofibrils from sarcoplasmic reticulum

• Ca²⁺ causes movement of tropomyosin so myosin binding site no longer blocked on actin filament

• myosin heads attach to binding sites on actin, forming a cross bridge

• hydrolysis of ATP on myosin heads causes myosin heads to bend

• myosin heads change angle and perform power stroke, pulling actin along and releasing ADP + Pi

• this causes the sarcomere to shorten, leading to muscle contraction

describe the process of muscle relaxation according to the sliding filament theory:

• ATP attaches to myosin head and causes myosin heads to detach, breaking the cross bridge

• myosin hydrolyses ATP - catalysed by ATP hydrolase - to return to its original position (relaxation)

• myosin head now has a molecule of ADP attached and will reattach itself further along the actin filament

• cycle repeats as long as Ca²⁺ conc remains high (i.e. as long as there is an action potential)

• ATP used to actively transport Ca²⁺ into sarcoplasmic reticulum, stopping the contraction process

what is tropomyosin? what is its significance?

• tropomyosin - fibrous protein twisted around 2 actin chains

• covers binding site on actin in relaxed myofibril

describe how the sarcomere changes during contraction:

• sarcomeres shorten, causing a change in the light and dark banding

• thin actin filaments slide over thick myosin filaments

• Z line pulled closer together

• I band shortens

• A band remains constant

• H zone narrows

during vigorous exercise, the pH of skeletal muscle tissue falls, this fall in pH leads to a reduction in the ability of Ca²⁺ to stimulate muscle contraction - suggest how (3)

any 3 from:

• low pH changes shape of Ca²⁺ receptor

• fewer Ca²⁺ bind to tropomyosin

• fewer tropomyosin molecules move away

• fewer actin binding sites revealed

• fewer cross bridges can form/fewer myosin heads can bind

the diagram shows the arrangement of actin and myosin in a sarcomere. one form of muscle disease is caused by a mutated allele of a gene. this leads to production of myosin molecules that are unable to bind to other myosin molecules.

if myosin molecules are unable to bind to other myosin molecules, this prevents muscle contraction - use the diagram and your knowledge of how muscles contract to suggest why (3)

• can’t form myosin/thick filaments

• can’t pull/move actin

• myosin moves

• can’t move actin towards each other