F215:04:09 The sliding filament model Flashcards
Where are sarcomeres?
they are the span between one Z-line to the next
How long is the sarcomere in a relaxed state?
2.5um
Why are Z lines close together when a muscle is contracted?
because the lengths of the I-band and H-zone are reduced
But the A band does not change in length during contraction
Describe how the different zones change in the sarcomere when muscles contract
the lengths of the I-band and H-zone are reduced
But the A band does not change in length during contraction
What are the 2 types of filament involved in contraction?
actin and myosin
Describe actin
Two strands coiled around each other like a twisted double string of beads
Each strand is composed of of G-actin subunits
Then Tropoyosin molecules coil round the F actin to reinforce it
What is the purpose of Tropomyosin and where is it found?
Found coiled round actin
Acts as a reinforcement
What is tropomyosin?
a rod shaped protein found coiled round actin
It has a troponin complex attached to each tropomyosin molecule
What is troponin made up of?
3 polypeptides, one binds to actin, one to tropomyosin and the other to calcium ions
Why is it important that troponin binds to tropomyosin and actin?
Ti keep the tropomyosin in place around the actin
What are thick filaments bundles of?
the protein myosin, whose heads stick out from opposite ends of the filament
What does each myosin molecule consist of?
a tail and 2 protruding heads
what is the power stroke?
When the head groups bend causing the thin filament to be pulled along and so overlap with more thick filament.
Its part of muscle contractions and ADP and Pi are released
What are the 4 stages of muscle contraction?
- Myosin head groups attach to the surrounding actin filaments forming a cross-bridge
- The head group then bends, causing the thin filament to be pulled along and so overlap more with the thick filament (power stroke) ADp and Pi are released
- The cross bridge is then broken as new ATP attaches to the myosin head
- The head group moves backwards as the ATP is hydrolysed to ADP and Pi. It can then for a cross bridge with the thin filament further along and start again
What is a cross bridge?
When the myosin head groups attach to the surrounding actin filaments forming the cross bridge
How is the whole muscle shortened during the contraction of a muscle?
several million cross bridges are being continuously made and broken causing the thin filaments to slide past the thick filaments to shorten the sarcomere, which then shortens the whole muscle
How is calcium involved in muscle contraction
As binding sites for the myosin head group on the actin fibre are covered by tropomyosin subunits, they cannot attach to myosin head groups.
When an action potential arrives, calcium ions are released from the sarcoplasmic reticulum and bind to the troponin molecules, which changes it shape and uncovers the binding sites for actin-myosin
when happens when the nervous stimulation stops in terms of calcium?
The calcium ions are actively transported back into the sarcoplasmic reticulum by carrier proteins on the membrane which leads to muscle relaxation
Why is ATP needed in the power stroke/.
As when myosin head groups have attached to the actin binding site and bends, the molecules are in their most stable form, so ATP is needed to break this and reset the myosin head forwards
Is a muscle fibre a cell or an organelle?
A cell
Are myofibrils cells or organelles?
organelles
How much ATP is available in a muscle fibre at any one time?
Enough to support 1-2 seconds of contraction
How is continued contraction maintained?
ATP must be generated as quickly as it is used up, this is carried out by 3 mechanisms:
- aerobic respiration
- Anaerobic respiration
- Transfer from creatine phosphase in the muscle cell sarcoplasm
How does aerobic respiration produce ATP to allow contraction to be maintained?
Aerobic respiration takes place in muscle cell mitochondria
The level at which this process can regenerate ATP is dependent on the supply of oxygen to the muscles and the availability of respiratory substrates
How does anaerobic respiration produce ATP to allow contraction to be maintained?
It takes place in the muscle cell of the sarcoplasm
Its quite a quick process but leads to a build up on lactic acid which is toxic
The lactic acid enters the blood and stimulated an increase in blood supply to the blood
What are the pros and cons of using anaerobic respiration to maintain muscle contraction
Pros: quick The lactic acid enters the blood and stimulated an increase in blood supply to the blood Cons: Lactic acid is toxic
How does the transfer from creatine phosphase in the muscle cell sarcoplasm produce ATP to allow contraction to be maintained?
As the phosphate group from creatine phosphate can be transferred to ADP to form ATP very quickly by the action of the enzyme creatine phosphotransferase
The supply of creatine phosphate is sufficient to support muscular contraction for a further 2-4 seconds
How long will the supply of creatine phosphate is sufficient to support muscular contraction for?
a further 2-4 seconds
What enzyme ensures the phosphate group from creatine phosphate is transferred to ADP to form ATP very quickly?
creatine phosphotransferase
List 3 uses of ATP in muscle cells
Contraction of sarcomeres in the power stroke
Protein synthesis
Suggest why anaerobic exercise such as weight lifting does not help weight loss
As it leads to the production of lactate during explosive exercise
On completion of exercise, most of the lactate is converted back into pyruvate.
Anaerobic respiration is carried out during anaerobic exercise which can only use glucose as a respiratory substrate, to respire fat/fatty acids, aerobic respiration and exercise must be carried out
Suggest which process for ATP regeneration is most important for a marathon runner
Aerobic respiration
Suggest which process for ATP regeneration is most important for a 100m sprinter
creatine phosphate regeneration of ATP
