Biology is Too Hard
And the Mark Schemes are never clear
Muscle Contraction - Exam Practice
The question on the right relates to figure 1 as well. It shows the distribution of actin and myosin filaments when the muscle is relxed and contracting. This is a basic test of which protein filament is moving during contraction. It is to make sure that you know it is the myosin that 'grabs' the actin and pulls it towards the M line.
The question on the right is showing you a sarcomere - it says 'part of a muscle myofibril' in the question.
The protein labelled W is myosin and the protein labelled X is actin - job done - you either know this or you dont' - pure AO1 stuff!
Part C to this question is a 4 marker. The examiner is looking for simple straight forward stuff on Ca2+ ions in muscle contraction - so be sick all over the paper if you know it. Leave nothing out. Whilst most of you will get MP1 you may not get MP2&3 because you haven't followed it all through. A lot of candidates nationally will miss MP4 because Ca2+ ions activating ATPase is often overlooked.
The key with the next question is too read the question fully. At first you might think it is about nerve impulses. It is a 3 mark question and the trick is to spot the phrase 'shown in the diagram'. This tells you that the nerve impulse has happened and you are picking up from that - i.e. release of Ca2+ ions from the sarcoplasmic reticulum. But if you are unsure write everything including depolrisation in the T-tubules right through to muscle contraction.
Rigor mortis is a condition where a dead body becomes rigid and remains in the position it died in. It is really difficult to change the position after a certain time. The examiner is testing your knowledge that ATP is needed to bind to the myosin head and when it is hydrolysed to ADP and Pi it re-cocks the head. If you are dead you cant respire so you dont produce ATP
The question says a relaxed muscle fibril so no change there. The question is about why you can see light and dark regions in a sarcomere. So all you need to do for 2 marks is describe where actin and myosin proteins cross over and where they don't. Luckily this question is only 2 marks as the first mark point is a bit rubbishy and probably something that no one would think to write. Everything else on their is gettable I reckon.
Again a popular question - asking you how the banding patterns change when the sarcomere contracts. The examiner is trying to test you on 'what moves when muscle contracts?' It is the actin that moves and the myosin stays where it is. So the H zone will narrow as more actin overalps with myosin. The lighter bands will also narrow as more actin moves into the A band, and this means because there is more overlap the dark band will widen.
Not the worlds most friendliest question this. For one reason only the dreaded D word - DATA! Panic stations. Thankfully the first question is all about what ATPase does during muscle contraction. Good, saved. ATPase hydrolyses ATP to ADP and Pi. In actual fact the binding of ATP to the cross-bridge causes the myosin head to detach, whilst the hydrolysis of ATP and the energy released causes the head to recock. This allows the myosin to once again bind to the actin, which is where the examiner is gettting the phrase ' used to form actomyosin bridges'. Actually ATPase has nothing much to do with myosin detachment but it seems to be such a massive misconception they have decided to award it. Remember everytime ATP is hydrolysed energy is released to do some 'active' process - so its alwways worth saying in your answer ' ATP is hydrolysed to ADP & Pi and energy is released.
Unfortunately you were never going to get away with it. The data needs explaining. ATPase activity is low in B because tropomyosin is present. The ATPase will only work on cross-bridges that have already detached and have ATP bound to them. After that - nothing - as tropomyosin is blocking the binding sites on the actin. But that is why we are seeing a bit of ATPase activity. The presence of calcium ions does 2 things to increase ATPase activity - 1) it binds to Troponin and moves the tropomyosin off the binding sites on the actin protein and 2) it activates the enzyme itself - so we see a lot of activity in C. This why A has less activity than C as well incidentally.