(a(i))[1]
The thyroxine concentration in the blood normally varies around a set point. Name the mechanism that keeps the concentration of thyroxine in the blood close to its set point.
(a(ii))[1]
State the part of the striated muscle fibre that releases $\text{Ca}^{2+}$.
(a(iii))[4]
Describe how $Ca^{2+}$, troponin and tropomyosin work in the contraction of striated muscle.
(a(iv))[2]
Cyclic AMP (cAMP) also has a role in the response of liver cells to glucagon. Describe how cAMP functions in the response of liver cells to glucagon.
(b)[3]
A rise in the blood concentration of thyroxine can cause a condition known as insulin resistance (IR). IR lowers the sensitivity of target cells, such as muscle and liver cells, to insulin. Suggest how reduced sensitivity to insulin affects target cells.