IGCSE Biology Paper-1: Specimen Questions with Answers 125 - 126 of 208

Question 125

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Question

MCQ▾

When solution X is tested with iodine solution, a blue-black colour is observed. A different solution, Y, is added to a new sample of solution X and the mixture is shaken and left for 30 minutes at 40 °C. When tested with iodine solution, an orange brown colour is observed.

What are solutions X and Y?

Choices

Choice (4)
a.X-starch, Y-lipase
b.X-maltose, Y-amylase
c.X-starch, Y-amylase
d.X-maltose, Y-lipase

Answer

c.

Explanation

  • Amylase, which is commonly found in saliva and germinating seeds. It catalyzes the breakdown of starch. When amylase reacts with starch, it cuts off the disaccharide maltose (two glucose molecules linked together). As the reaction progresses, less starch will be present and more sugar (maltose) will be present. The activity of amylase can be observed by using iodine. Because iodine reacts with starch to form a dark brown/purple colour. As amylase breaks down starch, less and less starch will be present and the colour of the solution (if iodine is added) will become lighter and lighter.
Positive and Negative Iodine Test

Positive and Negative Iodine Test

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Question 126

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Question

MCQ▾

The diagram shows the stem of a plant. A strip of the outer tissue including the phloem has been removed.

Transport in the Plant Affected

Transport in the Plant Affected

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How is transport in the plant affected?

Choices

Choice (4)
a.Water cannot pass to the leaves.
b.Water cannot pass to the roots.
c.Dissolved salts cannot pass to the leaves.
d.Amino acids and sucrose cannot pass to the roots.

Answer

d.

Explanation

  • In ringing experiment, a complete ring of tissue is removed outside the vascular cylinder at a height from ground in a dicot plant. As the xylem is intact water can move freely to the aerial parts from roots. But the sugars cannot move down to roots from aerial parts, because of the removal of phloem tissue, which occurs in the outer cylinder and is removed in ringing experiment.
  • A prominent swelling is observed just above the point of removal of ring of tissue due to accumulation of sugars. This experiment proves the respective roles of xylem and phloem in transportation of water and sugars.
Water and Sugars

Water and Sugars

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