AQA Bio muscle contraction Q

Describe the roles of calcium ions and ATP in the contraction of a
myofibril. MS: 1. Calcium ions diffuse into myofibrils from (sarcoplasmic) reticulum;
2. (Calcium ions) cause movement of tropomyosin (on actin);
3. (This movement causes) exposure of the binding sites on the
actin;
4. Myosin heads attach to binding sites on actin;
5. Hydrolysis of ATP (on myosin heads) causes myosin heads to
bend;
6. (Bending) pulling actin molecules;
7. Attachment of a new ATP molecule to each myosin head
causes myosin heads to detach (from actin sites)

In every textbook and even the PMT summary notes (this q is off pmt) we're taught that the myosin head has ADP which releases, causing the myosin to release from the actin binding site as it changes shape (this makes sense to me) but this mark scheme contradicts it completely, saying ATP is hydrolysed to make the myosin pull the actin(we are told myosin bends with no mention of ATP). So should I now use this information as knowledge to remember and literally ignore what all textbooks and notes online say?

Reply 1

GrilledChakin

Original post by Abaz3105

I was taught that the myosin head has ATP bound to it. To be honest, I think they refer to the same thing as ATP is hydrolysed to form ADP and Pi, and the ADP is then released from the myosin head, and the energy is used to bend the myosin head forward.

Just to be safe I would personally write both e.g. ATP is hydrolysed to form ADP which is released from the myosin head etc.