The oxygen evolved during photosynthesis comes from splitting of
- A.water
- B.carbon dioxide
- C.oxygen
- D.light
Correct answer
Explanation
The correct answer is A, water. In photosynthesis, the oxygen released into the air comes from water molecules that are split using light energy.
Photosynthesis takes place in the chloroplasts of green plants. In the light reaction, chlorophyll absorbs sunlight, and this energy splits water molecules into hydrogen ions, electrons and oxygen. This splitting is called photolysis of water. The oxygen escapes into the air, while the hydrogen is used, along with carbon dioxide, to make glucose in the dark reaction or Calvin cycle. Experiments with water containing the heavy oxygen isotope O-18 proved that the released oxygen comes from water, not from carbon dioxide.
B is wrong because the oxygen atoms of carbon dioxide end up in glucose and water, not in the oxygen gas released.
C is wrong because oxygen is the product of the process, not the substance that is split.
D is wrong because light is a form of energy; it drives the splitting of water but is not itself split.
Exam tip: 6CO2 + 12H2O → C6H12O6 + 6O2 + 6H2O; the released O2 comes from water (photolysis).
Practice Questions
View allWhich one of the following regions in plants is responsible for the growth of its roots and shoots?
- A.Collenchyma
- B.Endodermis
- C.Cortex
- D.Apical meristem
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Apical meristems sit at the tips of roots and shoots, and their dividing cells make the plant grow in length.
Read the full explanation →Out of different types of tissues found in plants, which one of the following is a complex tissue?
- A.Parenchyma
- B.Sclerenchyma
- C.Xylem
- D.Collenchyma
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Xylem is made of more than one type of cell working together, which is what makes a tissue complex. Plant permanent tissues are of two kinds.
Read the full explanation →Transport of Na+ and Ca2+ across the plasma membrane is an example of
- A.antiport
- B.symport
- C.uniport
- D.aquaporins
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In the sodium-calcium exchanger, Na+ ions enter the cell while Ca2+ ions are pushed out, so the two ions cross the membrane in opposite directions.
Read the full explanation →Which one of the following could be the melting point of iron?
- A.25°C
- B.37°C
- C.500°C
- D.1500°C
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Pure iron melts at about 1,538°C, so 1,500°C is the only option close to its melting point. Iron is a metal with strong metallic bonds, so a great deal of heat is needed to break up its solid structure.
Read the full explanation →Blood is a type of
- A.Epithelial tissue
- B.Muscular tissue
- C.Nervous tissue
- D.Connective tissue
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Blood is a fluid connective tissue, with cells floating in a liquid matrix called plasma. Connective tissues link and support other parts of the body, and they have cells spread out in a matrix.
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