A-LevelBiologyControl and coordination9700/42 · May/June 2017 · Q39 Marks

Fig. 3.1 shows a diagram of two sarcomeres of relaxed striated muscle. [Figure 3.1]

(a)(i)When the striated muscle contracts, state what happens to the length of: the I-band[1]
(a)(ii)the A-band.[1]
(b)Describe how the response of the sarcoplasmic reticulum to the arrival of an action potential leads to the contraction of striated muscle.[4]
(c)Glycogen storage disease type V (GSDV) is a metabolic disorder where stored glycogen cannot be broken down to release glucose, resulting in the production of only small quantities of ATP. People with GSDV are unable to exercise normally, as the lack of ATP affects the functioning of striated muscle. With reference to the sliding filament model of muscular contraction, suggest why a lack of ATP affects the functioning of striated muscle.[3]

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Mark scheme

Marking points from the official mark scheme (May/June 2017). The mark or mark type is shown in brackets.

[1] decreases / shortens / AW ;
[1] stays the same / nothing;
[max 4] 1. (when) sarcoplasmic reticulum / SR, depolarised; 2. calcium (ion) channels / voltage-gated channels, open ; 3. calcium ions, diffuse / move down a concentration gradient, (through open channels) ; 4. bind to troponin which changes shape ; 5. tropomyosin moves ; 6. binding sites exposed ; 7. allows myosin to bind (to actin) / cross bridge formation ; 8. ref. to power stroke / AW ;
[max 3] 1. no detachment of myosin heads ; 2. so no, energy transferred to myosin / ATPase activity / hydrolysis of ATP ; 3. so no, cross bridge formation ; 4. so no, power stroke / pulling of actin; 5. so no recovery stroke / myosin head does not return to original position ; 6. no pumping of calcium ions into SR ;

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