Paper 1

Question 1

How does an action potential cause muscle contraction?

Answer 1

Action potential arrives at sarcolemma causing Na+ channels to open so depolarisation occurs so Ca2+ channels open and calcium is released from the sarcoplasmic reticulum. Ca2+ binds to troponin which causes it to change shape and tropomyosin is moved out of the way of myosin binding sites. Myosin heads can now bind to actin filaments forming a cross bridge and myosin will pull actin filaments past the to contract.

Question 2

What are the products of the light dependent stage of photosyntheses and how are they used in the Calvin cycle?

Answer 2

Products of light independent stage (non cyclic) = O2, NADPH, ATP
In the Calvin cycle, 2 x NADPH –> 2 x NADP and 2 x ATP –> 2 x ADP to convert GP to TP. ATP –> ADP to regenerate RuBP from 5/6 TP.