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2.4.9 The sliding filament theory
Learning Objectives Success Criteria
To understand the sliding filament model
Outline the role of ATP in muscle contraction(Grade E - D)
Outline how the supply of ATP is maintained in the muscle(Grade C –B)
Explain using diagrams the sliding filament model of muscular contraction(Grade B – A)
Starter1. Blood vessels such as arterioles contain circular smooth
muscle. Contraction of this muscle constricts the vessel. Why do blood vessels not need longitudinal muscle to act against the circular muscle in order to cause dilation?
2. Suggest the advantage of the electrical activity of the heart being able to pass from atria walls to ventricle walls only at the AV node
Sarcomere – the smallest contractile unit of a muscle
Myofibrils are made up of actin and myosinMyofibrils appear striped due to alternating I-bands and A-bands
Microscopic Structure of Skeletal Muscle
Light bands are isotropic bands (I-bands) only actin is found in these bands
Dark bands are anisotropic bands (A-bands) actin and myosin overlap in these bands
Microscopic Structure of Skeletal Muscle
In the middle of each A-band is a lighter part called the H-zone
In the centre of each I-band is the Z-line, where the actin filaments join
The section of muscle between Z-lines is called a sarcomere
Microscopic Structure of Skeletal Muscle
10
(a) Name the protein present in the filaments labelled W and X.
(1 mark)
Figure 1 shows a diagram of part of a muscle myofibril.
The Sliding Filament MechanismActin and myosin slide
past one another when the muscle contracts
Evidence for this:Sarcomere gets shorter More overlapZ-lines get closer togetherI-band gets narrowerH-zone gets narrower
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Understanding the sarcomere’s bands
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The sliding filament theory
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How does the sarcomere change?
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The structure of myosin
The myosin filament is formed from a number of myosin proteins wound together. Each ends in a myosin head, which contains an ATPase.
myosin filament
myosin head
actin binding site
ATP binding site
ATPase head myosin neck
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The structure of actin
The actin filament is formed from a helix of actin sub-units. Each contains a binding site for the myosin heads.
Two other proteins are attached to the actin fibre:
troponin
tropomyosinmyosin head binding site
actin sub-unit
tropomyosin is wound around the actin troponin molecules are bound to tropomyosin and contain
calcium ion binding sites.
Muscle Contraction – Sliding Filament MechanismHeads of myosin form cross-bridges with the actin
filaments (attach to binding sites)Myosin heads flex together and pull the actin along the
myosinThey detachReturn to original angle and re-attach (uses ATP)Repeats 100 times a second
Muscle Contraction – Sliding Filament Mechanism
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What controls the sliding filaments?
TaskUsing the play doh demonstrate muscle contraction
Four sections1.Arrival of an action potential – Ca2+, troponin, tropomyosin,
actin-myosin crossbridge2.Movement of the actin filament – ATPase, ATP, power stroke3.Breaking of the cross bridge – ATP, myosin head4.Return to resting state – troponin, Ca2+, sarcoplasmic
reticulumUse keywords – write down a flowchart explaining each stage
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Maintaining ATP supply
1. Aerobic respiration in muscle cells mitochondria
Needs a supply of respiratory substrate and oxygen
2. Anaerobic respiration in muscle cell sarcoplasm
Produces lactate and can lead to fatigue/cramp.
3. Creatinine phosphate – another chemical present in muscle cells can donate its phosphate to recharge ADP back to ATP (supports a further 2-4 seconds
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Plenary
• Complete cloze sheet – summary sliding filament theory
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Sliding filament theory• When a nerve impulse arrives at a neuromuscular junction, calcium ions are
released from the sarcoplasmic reticulum.• The calcium ions diffuse through the sarcoplasm.• This initiates the movement of the protein filaments as follows:• Calcium ions attach to the troponin molecules causing them to move.• As a result, the tropomyosin on the actin filament shifts position, exposing
myosin binding sites on the actin filaments.• Myosin heads bind with myosin binding sites on the actin filament, forming
cross bridges.• When the myosin head binds to the actin, ADP and Pi on the myosin head are
released.• The myosin changes shape, causing the myosin head to nod forward. This
movement results in the relative movement of the filaments. The attached actin moves over the myosin.
• An ATP molecule binds to the myosin head. This causes the myosin head to detach.
• An ATPase on the myosin head hydrolyses the ATP forming ADP and Pi.• This hydrolysis causes a change in the shape of the myosin head. It returns to
its upright position. This enables the cycle to start again.
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Learning Objectives Success Criteria
• To understand the sliding filament model
• Outline the role of ATP in muscle contraction
(Grade E - D)
• Outline how the supply of ATP is maintained in the muscle
(Grade C –B)
• Explain using diagrams the sliding filament model of muscular contraction
(Grade B – A)