Which one of the following statements is correct?
- A.In photosynthesis by green plants, carbon dioxide is used for making carbohydrate.
- B.During photosynthesis, plants use oxygen for making carbohydrate.
- C.The carbohydrate generated by plants comes from soil.
- D.The carbohydrate generated by plants comes from urea.
Correct answer
Explanation
The correct answer is A. In photosynthesis, green plants take carbon dioxide from the air and combine it with water, using sunlight, to make carbohydrates such as glucose.
The overall reaction is 6CO₂ + 6H₂O, in the presence of light and chlorophyll, giving C₆H₁₂O₆ + 6O₂. The carbon in every sugar, starch and cellulose molecule of a plant comes from atmospheric carbon dioxide, which enters the leaf through the stomata. The light reactions in the thylakoids split water and release oxygen, while the dark reactions (the Calvin cycle) in the stroma fix carbon dioxide into sugar. Jan van Helmont's willow-tree experiment first showed that a plant's mass does not come from the soil.
B is wrong: oxygen is a product of photosynthesis, not a raw material.
C is wrong: the soil supplies water and minerals, not the carbohydrate.
D is wrong: urea is a nitrogen fertiliser; it supplies nitrogen for proteins, not carbohydrates.
Exam tip: Photosynthesis uses CO₂ and H₂O, releases O₂; the oxygen comes from water.
Practice Questions
View allConsider the following animals :
1. Hedgehog
2. Marmot
3. Pangolin
To reduce the chance of being captured by predators, which of the above organisms rolls up/roll up and protects/protect its/their vulnerable parts?
- A.1 and 2
- B.2 only
- C.3 only
- D.1 and 3
Show answer
Hedgehogs and pangolins curl into a tight ball when threatened, covering the soft belly and face with spines or scales; marmots do not.
Read the full explanation →Which one of the following is used in preparing a natural mosquito repellent?
- A.Congress grass
- B.Elephant grass
- C.Lemongrass
- D.Nut grass
Show answer
Lemongrass and its close relative citronella grass yield aromatic oils that keep mosquitoes away, so they are the base of many natural repellents.
Read the full explanation →Which one of the following devices is non-ohmic ?
- A.Conducting copper coil
- B.Electric heating coil
- C.Semi conductor diode
- D.Rheostat
Show answer
A diode does not obey Ohm's law: the current through it is not proportional to the voltage across it. An ohmic conductor gives a straight-line graph of current against voltage, so its resistance stays constant at a fixed temperature.
Read the full explanation →What is the formula mass of anhydrous sodium carbonate? (Given that the atomic masses of sodium, carbon and oxygen are 23 u, 12 u and 16 u respectively)
- A.286 u
- B.106 u
- C.83 u
- D.53 u
Show answer
Anhydrous sodium carbonate is Na₂CO₃, and 2 × 23 + 12 + 3 × 16 = 106 u. Formula mass is the sum of the atomic masses of all the atoms in one formula unit of an ionic compound.
Read the full explanation →Which one of the following minerals is used as a fuel in nuclear power stations ?
- A.Bauxite
- B.Quartz
- C.Feldspar
- D.Pitchblende
Show answer
Pitchblende, or uraninite, is the chief ore of uranium, the fuel used in most nuclear power stations. Pitchblende is a heavy, black mineral made mostly of uranium oxide.
Read the full explanation →