State two differences in structure between a phloem sieve tube element and a xylem vessel element.
Sucrose is carried in phloem sieve tubes. A student tested a sample of sap taken from inside a phloem sieve tube to find out whether sucrose was present. One step in the procedure instructed the student to heat the phloem sap with hydrochloric acid. Explain why this step is needed in the procedure.
Explain how a low helper T-lymphocyte count could reduce the body's capacity to make antibodies against $V.\ cholerae$.
Fig. 4.1 illustrates a stage in sucrose loading into the phloem. Companion cells have many mitochondria and ribosomes. Explain the functions of mitochondria and ribosomes in the stage shown in Fig. 4.1.
Describe and explain how sucrose moves from the cell wall of the companion cell into the cytoplasm.