Q68 — Pressure-Flow Model of Phloem Sucrose Transport

Theoretical 2 Real exam question — full text reproduced under IBO's CC BY-NC-SA 4.0 license

The picture shows the tissues involved in the transport of sucrose in plants: the sieve tube cells and the companion cells.

Diagram of phloem loading and unloading. At the source end, a mesophyll source cell (labeled A, containing sucrose) transfers sucrose via a companion cell into the sieve tube (labeled B). Sucrose then moves down the sieve tube through further companion-cell/sieve-tube pairs (labeled C at the sink-end sieve tube) into a root cell (labeled D, containing sucrose) at the sink. Sucrose loading at the source (cells A and B) and unloading at the sink (cells C and D) along the sieve tube–companion cell system.

Use the information and figures to determine whether the following statements are true or false.

A. The water potential in cell B is the highest

B. The concentration of sucrose in cell A is the highest

C. The difference in pressure potential is the driving force of the transport of sucrose along sieve cells.

D. Sucrose is transported from the mesophyll cell to B cell without energy expenditure

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Question reproduced from IBO 2022, Theoretical Paper 2, licensed under CC BY-NC-SA 4.0 — attributed to the International Biology Olympiad. Open the full exam PDF · Community solutions (unofficial)