(a)[3]
State the functions of $P$, $Q$ and $R$.
(b)[1]
Circle the membrane width marked as W in Fig. 5.1.
(c)[2]
Membranes, including the cell surface membrane, are described as having a fluid mosaic structure. Explain what is meant by fluid mosaic.
(d)[3]
Aquaporins are membrane channel proteins in plant and animal cells. They allow water to move across membranes. Explain why they are needed.