Water goes up through xylem vessels in tall trees. The process in plants and properties of water that help in upward movement are :
- A.Respiration and cohesion
- B.Transpiration pull, cohesion and adhesion
- C.Root pressure, cohesion and adhesion
- D.Transpiration and adhesion
Correct answer
Explanation
The correct answer is B, Transpiration pull, cohesion and adhesion. Evaporation from the leaves pulls a continuous column of water up the xylem, and cohesion and adhesion keep that column from breaking.
When water evaporates through the stomata, the cells of the leaf draw water from the xylem behind them, creating a strong suction called transpiration pull. Water molecules cling to one another through hydrogen bonds (cohesion), so the pull passes down the whole column to the roots. They also cling to the walls of the narrow xylem tubes (adhesion), which holds the column up against gravity. This cohesion-tension model explains how water reaches the top of trees taller than 100 metres.
Option A is wrong because respiration releases energy in cells and does not lift water. Option C is wrong because root pressure is weak, is seen mainly at night or in small plants, and cannot lift water up tall trees. Option D is wrong because it leaves out cohesion, without which the column would break.
Exam tip: Ascent of sap = transpiration pull + cohesion + adhesion.
Practice Questions
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- A.By heating wood at high temperature in absence of air
- B.By heating coal at high temperature in absence of air
- C.By burning hydrocarbons in a limited supply of air
- D.By heating coal at high temperature in presence of air
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Read the full explanation →The equivalent weight of oxalic acid in C₂H₂O₄·2H₂O is
- A.45
- B.63
- C.90
- D.126
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Read the full explanation →Which one of the following is not a monatomic element?
- A.Copper
- B.Helium
- C.Iodine
- D.Barium
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Read the full explanation →Which one of the following reactions does not result in the evolution of hydrogen gas ?
- A.Reaction of zinc metal with dilute sulphuric acid solution
- B.Mixing water to Plaster of Paris
- C.Heating zinc metal with sodium hydroxide solution
- D.Reaction of potassium metal with water
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Read the full explanation →Which one of the following statements is not correct?
- A.Most carbon compounds are good conductors of electricity.
- B.Bonding in organic compounds is covalent.
- C.Graphite is used as a lubricant.
- D.Diamond is an allotrope of carbon.
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Most carbon compounds are poor conductors of electricity, because their atoms are joined by covalent bonds and they do not form ions or free electrons that could carry a current.
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