IGCSE Biology Paper-4: Specimen Questions with Answers 127 - 129 of 279

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Passage

Plants produce glucose in leaves and convert some of it to sucrose.

Question 127 (2 of 7 Based on Passage)

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In plants the movement of sucrose is usually continuous. However, after 2 hours the movement of sucrose in tube X in Fig. 30.2 stopped.

Suggest why the movement of sucrose in tube X stopped.

Explanation

This movement of water and sugar molecules will continue until the concentration of entire system becomes uniform and will stop once a state of equilibrium is achieved. But if sugar molecules are either removed or converted into starch, the movement will continue endlessly.

Question 128 (3 of 7 Based on Passage)

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Roots can be an example of a sink.

Explain why sometimes roots act as a source rather than a sink.

Explanation

Belowground organs of plants (e. g. roots and rhizomes) are sinks during plant growth since they cannot perform photosynthesis. Some organs are both a source and sink. Leaves are sinks when growing and sources when photosynthesizing. Rhizomes are sinks when growing but become sources in the spring when they provide energy for new growth.

Question 129 (4 of 7 Based on Passage)

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The movement of sucrose in plants can be modelled using laboratory apparatus.

Fig. shows the apparatus used to model the movement of sucrose in a plant:

  • Partially permeable bags were attached tightly to the ends of tube X.
  • The bag representing a source was filled with a colored sucrose solution.
  • The bag representing a sink was filled with water.
  • The containers and tube X and tube Z were filled with water.
Movement of Sucrose Solution

Fig. shows the position of the colored sucrose solution 45 minutes after the apparatus was set up.

The arrows show the direction of the movement of the liquids.

Sucrose Solution

(i) State the name of the tissue represented by tube X and the name of the tissue represented by tube Z in Fig.

X …

Z …

Explanation

X – the movement of sugars in the phloem begins at the source, where sugars are loaded (actively transported) into a sieve tube. Loading of the phloem sets up a water potential gradient that facilitates the mass movement in the phloem.

Z – Xylem, water in the adjacent xylem moves into the phloem by osmosis.

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