5.5.16 The Sliding Filament Model Flashcards
What is the structure of thick and thin filaments in a myofibril?
The thick filaments within a myofibril are made up of myosin molecules
These are fibrous protein molecules with a globular head
The fibrous part of the myosin molecule anchors the molecule into the thick filament
In the thick filament, many myosin molecules lie next to each other with their globular heads all pointing away from the M line
The thin filaments within a myofibril are made up of actin molecules
These are globular protein molecules
Many actin molecules link together to form a chain
Two actin chains twist together to form one thin filament
A fibrous protein known as tropomyosin is twisted around the two actin chains
Another protein known as troponin is attached to the actin chains at regular intervals
How do muscles contract according to the sliding filament model?
Muscles cause movement by contracting
During muscle contraction sarcomeres within myofibrils shorten as the actin and myosin filaments move past each other
This is known as the sliding filament model of muscle contraction and occurs via the following process
An action potential arrives at the neuromuscular junction
Calcium ions are released from the sarcoplasmic reticulum into the sarcoplasm by diffusion
Calcium ions bind to troponin molecules, stimulating them to change shape
This causes troponin and tropomyosin proteins to change position on the actin filaments
Myosin binding sites are exposed on the actin molecules
The globular heads of the myosin molecules bind with these sites, forming cross-bridges between the two types of filament
The myosin heads bend and pull the actin filaments towards the centre of the sarcomere, causing the muscle to contract a very small distance
The movement of the myosin heads is known as the power-stroke
When the myosin heads bend, it releases a molecule of ADP
ATP binds to the myosin head, allowing it to detach from actin
The myosin head acts as an ATPase enzme, hydrolysing ATP into ADP and Pi; the energy released during this reaction allows the myosin head to return to its original position
The myosin head can now bind to a new binding site on the actin filaments
The myosin heads move again, pulling the actin filaments even closer to the centre of the sarcomere and causing the sarcomere to shorten further
As long as troponin and tropomyosin are not blocking the myosin-binding sites and the muscle has a supply of ATP, this process repeats until the muscle is fully contracted
What is the role of ATP and phosphocreatine?
A supply of ATP is required for muscle contraction
ATP binding allows myosin to detach from actin and ATP hydrolysis allows the myosin heads to return to their original shape; both of these processes are essential to allow the process described above to repeat
The return of calcium ions to the sarcoplasmic reticulum occurs via active transport
Resting muscles have a small amount of ATP stored that will only last for 3-4 seconds of intense exercise
The mitochondria present in the muscles fibres are able to respire aerobically and produce ATP but this is slow and can take a considerable amount of time
Anaerobic respiration, which is faster than aerobic, still takes 10 seconds before it even begins to produce any ATP
Phosphocreatine is a molecule stored by muscles that can be used for the rapid production of ATP
A phosphate ion from phosphocreatine is transferred to ADP
ADP + phosphocreatine → ATP + creatine
Different muscle fibre types contain different limited amounts of phosphocreatine
It allows for muscles to continue contracting for a short period of time until the mitochondria are able to supply ATP
For example, it would be utilised by the muscles of a 100m sprinter as sprinting involves an intense level of muscle contraction
For prolonged activity, once the supply of phosphocreatine has been used up then the rate of muscle contraction must equal the rate of ATP production from both aerobic and anaerobic respiration
