IGCSE Biology Paper-4: Specimen Questions with Answers 29 - 32 of 279

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Passage

The transformations that nitrogen undergoes as it moves between the atmosphere, the land and living things make up the nitrogen cycle.

Question 29 (2 of 3 Based on Passage)

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Diagram shows a part of nitrogen cycle. Name the step of nitrogen cycle and label the part A and B.

Diagram Shows a Part of Nitrogen Cycle

Explanation

This is the first step of nitrogen cycle called as nitrogen fixation . In which atmospheric nitrogen is converted into ammonia.

Part A – Nitrogen fixation by lightning

Part B-Nitrogen fixation by bacteria in root nodules of leguminous plants

Question 30 (3 of 3 Based on Passage)

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What is the role of denitrifying bacteria in nitrogen cycle?

Explanation

Denitrifying bacteria present in the soil break down nitrates and return nitrogen gas into the air.

Question 31

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The pH of rainwater is little below than 7 . Why?

Explanation

Rainwater is usually acidic in nature because carbon di oxide dissolved in it to form carbonic acid. Sometimes SO2 and NO2 are also added in atmosphere to make rainwater more acidic.

Question 32

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Draw the double helix structure of DNA.

Explanation

Figure Shows Double Helix Structure of DNA

The DNA molecule is right-handed double helix called B-form. It has a diameter of 2 nm and a pitch of 3.4 nm with 10 base pairs per turn. The sugar – phosphate are antiparallel with respect to their 5 ⇾ 3 orientations and the purine – pyrimidine base pairs form across the axis of the helix.

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