It is now ready to start a new cycle but further along the thin filament push

It is now ready to start a new cycle but further

This preview shows page 259 - 262 out of 522 pages.

filaments back again). It is now ready to start a new cycle, but further along the thin filament. [push oars into starting position.] One ATP molecule is split by each cross bridge in each cycle, which takes a few milliseconds. During a contraction, thousands of cross bridges in each sarcomere go through this cycle thousands of times, like a millipede running along the ground. Fortunately the cross bridges are all out of synch, so there are always many cross bridges attached at any time to maintain the force. Control Of Muscle Contraction How is the cross bridge cycle switched off in a relaxed muscle? This is where the regulatory proteins on the thin filament, troponin and tropomyosin, are involved. Tropomyosin is a long thin molecule, and it can change its position on the thin filament. In a relaxed muscle is it on the outside of the filam ent, covering the actin molecules so that myosin cross bridges can’t attach. This is why relaxed muscle is compliant: there are no connections between the thick and thin filaments. In a
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A level Biology BIOCHEMISTRY contracting muscle the tropomyosin has moved into the groove of the double helix, revealing the actin molecules and allowing the cross bridges to attach. Contraction of skeletal muscle is initiated by a nerve impulse, and we can now look at the sequence of events from impulse to contraction (sometimes called excitation contraction coupling ). 1. An action potential arrives at the end of a motor neurone, at the neuromuscular junction. 2. This causes the release of the neurotransmitter acetylcholine. 3 This initiates an action potential in the muscle cell membrane. 4. This action potential is carried quickly throughout the large muscle cell by invaginations in the cell membrane called T-tubules . 5. The action potential causes the sarcoplasmic reticulum (large membrane vesicles) to release its store of calcium into the myofibrils. 6. The calcium binds to troponin on the thin filament, which changes shape, moving tropomyosin into the groove in the process. 7. Myosin cross bridges can now attach and the cross bridge cycle can take place. Relaxation is the reverse of these steps. This process may seem complicated, but it allows for very fast responses so that we can escape from predators and play the piano. QUESTIONS (a) The diagram below shows the arrangement of fibres within a muscle sarcomere
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A level Biology BIOCHEMISTRY (i) Name the parts of the muscle sarcomere labelled A, B and C A= MYOSIN; B= ACTIN; C= Z LINE/DISC (all 3 = 2 marks, 2 = 1 mark) (ii) Describe what would happen to the regions labelled D and E when the muscle contracted. D would get smaller; E would get larger (iii) Describe the role of Calcium ions in muscle contraction (iv) Describe the role of ATP in muscle contraction provides the energy; to change the configuration of myosin heads (swivel ok) ; which move the actin over the myosin filament (any 2) 1. Fur colour in the Canadian Bigfoot is controlled by three alleles Cb, Cr, and c. (c is recessive)
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