Biology 9700 · AS & A Level · Transport mechanisms
Transport mechanisms — practice question
Phloem transfer cells are altered companion cells that transport sucrose and other assimilates from mesophyll tissue into phloem sieve tube elements. Fig. 5.1 shows an electron micrograph of a phloem transfer cell (magnification $\times 10\,000$).
(a)[2]
Calculate the actual distance across the transfer cell from X to Y. Show your working and state your answer to the nearest micrometre.
(b)[5]
With reference to Fig. 5.1, explain how phloem transfer cells assist the transfer of sucrose from the mesophyll of a leaf into a phloem sieve tube element.
(c)[4]
Explain how sucrose in the phloem sap is transported from sieve tube elements in leaves to cells in a sink, such as a root.
Worked solution & mark scheme
This 11-mark question has a full step-by-step worked solution and mark scheme. One marking point: “A correct measurement is $9\,\mu m$” …