Biology: The Dynamic Science (Mind...

4th Edition
Peter J. Russell + 2 others
ISBN: 9781305389892



Biology: The Dynamic Science (Mind...

4th Edition
Peter J. Russell + 2 others
ISBN: 9781305389892
Textbook Problem

In translocation of sucrose-rich phloem sap:

a. the sap flows toward a source as pressure builds up at a sink.

b. crassulacean acid metabolism reduces the rate ofphotosynthesis.

c. companion cells use energy to load solutes at a source and thesolutes then follow their concentration gradients to sinks.

d. sucrose diffuses into companion cells whereas H+ simultaneously leaves the cells by a different route.

e. companion cells pump sucrose into sieve-tube elements.

Summary Introduction


The long-distance transport mechanism of organic compounds is called translocation. Translocation occurs from the source of the production of the organic substance via loading it into sieve tubes to the sink, where it is unloaded. Sucrose is the most commonly transported organic substance via the long-distance transport mechanism.


Explanation/justification for the correct answer:

Option (c) states that companion cells use energy to load solutes at a source, and the solutes follow their concentration gradients to sinks. Through active transport, sucrose is loaded into the companion cells and then to the sieve tubes using the energy of companion cells against the concentration gradient. As the sieve tubes become more concentrated, the inward movement of water occurs via osmosis, thus leading to an increase in the turgor pressure. Under high pressure, the phloem sap translocates by bulk flow to the sink, where the sink takes up the sucrose, which results in a decreased pressure and concentration of sucrose. Thus, solutes finally reach the sinks where they are used or stored. Hence, option (c) is correct.

Explanation for the incorrect answers:

Option (a) states that the sap flows toward the source as the pressure builds up at the sink. The phloem sap loads into the sieve tubes at the source against the concentration gradient, thus leading to translocation via bulk flow into the sink...

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