photosynthesis MCQs for UPSC Prelims

28 practice questions on photosynthesis from the Life Processes in Plants section of the UPSC Prelims syllabus. 28 come with a written explanation. Try the sample set below - the answer stays hidden until you ask for it.

9 Easy 14 Medium 5 Hard

Sample questions

Q1
hard

A plant is first kept in the dark until its leaves hold no starch. A strip of black paper is then clipped over part of one leaf and the plant is left in sunlight for a whole day. When that leaf is tested with iodine, the result will be that

  1. A the covered part turns blue-black while the uncovered part does not
  2. B no part of the leaf turns blue-black
  3. C the whole leaf turns blue-black
  4. D the uncovered part turns blue-black while the covered part does not
Show answer and explanation

Correct answer: D - the uncovered part turns blue-black while the covered part does not

Only the part reached by sunlight could make food, so only that part stores starch and gives the blue-black colour. The covered strip stayed in the dark, so it stays pale, and since one part did receive light, neither the whole leaf nor none of it can give the colour.

Q2
medium

A leaf that has been making food turns blue-black when iodine solution is dropped on it. This colour shows the presence of

  1. A chlorophyll
  2. B oxygen
  3. C starch
  4. D carbon dioxide
Show answer and explanation

Correct answer: C - starch

The blue-black colour appears only where starch is present, so the test shows that food has been made and stored in the leaf. Oxygen and carbon dioxide are gases that iodine does not detect, and chlorophyll is deliberately removed before the test is carried out.

Q3
medium

Read these statements about what a green leaf produces while it is making food in sunlight. (i) It releases oxygen into the air. (ii) It stores the food it makes in the form of starch. (iii) It releases carbon dioxide into the air. Which of them are correct?

  1. A (i), (ii) and (iii)
  2. B Only (i) and (iii)
  3. C Only (ii) and (iii)
  4. D Only (i) and (ii)
Show answer and explanation

Correct answer: D - Only (i) and (ii)

A leaf making food releases oxygen and stores the food as starch, so (i) and (ii) are correct. Statement (iii) is wrong, because carbon dioxide is taken in as a raw material while food is being made, not given out. Carbon dioxide is released by the plant during respiration, which is a different process.

Q4
medium

Before a leaf is tested for starch, its green colour is removed. This is done so that

  1. A the iodine does not damage the chlorophyll
  2. B the leaf becomes lighter and easier to hold
  3. C the leaf can go on making more starch during the test
  4. D the colour change produced by iodine can be seen clearly
Show answer and explanation

Correct answer: D - the colour change produced by iodine can be seen clearly

A green leaf would hide the blue-black colour, so the chlorophyll is taken out first and the result becomes easy to read. A plucked leaf makes no more starch, protecting the chlorophyll is not the aim, and the weight of the leaf is of no importance.

Q5
medium

The tiny pores present on the surface of a leaf allow

  1. A gases to pass into and out of the leaf
  2. B sunlight to travel into the leaf
  3. C water to be stored inside the leaf
  4. D minerals to be taken in from the air
Show answer and explanation

Correct answer: A - gases to pass into and out of the leaf

These pores are the doorways through which carbon dioxide enters and oxygen and water vapour leave the leaf. They are not water stores, sunlight falls on the whole leaf surface without needing an opening, and minerals are taken in by the roots from the soil.

Q6
easy

The green colouring matter present in the leaves of a plant is called

  1. A chlorophyll
  2. B nectar
  3. C starch
  4. D iodine
Show answer and explanation

Correct answer: A - chlorophyll

Chlorophyll is the green substance in leaves that captures the energy of sunlight for making food. Starch is the food that is stored afterwards, iodine is a liquid used to test for starch, and nectar is the sweet liquid found in flowers.

Q7
medium

The water that a leaf needs for making food reaches it

  1. A from the food already stored inside the seed
  2. B straight from the air whenever it rains
  3. C from the oxygen that the leaf gives out
  4. D from the soil, taken in by the roots and carried up the stem
Show answer and explanation

Correct answer: D - from the soil, taken in by the roots and carried up the stem

Roots draw water from the soil and the stem carries it up to the leaves, which is why a plant wilts and stops growing when the soil dries out. Rain water reaching a leaf surface is not what is used, a seed's store is used up early, and oxygen is a product and not a source of water.

Q8
medium

A potted plant is sealed inside a clear plastic bag along with a chemical that soaks up all the carbon dioxide from the air inside. The bag is then left in bright sunlight. The leaves will

  1. A make food only after the Sun has set
  2. B make no food, because one of the raw materials is missing
  3. C make food using the plastic of the bag instead
  4. D make more food than usual, because the air inside is now clean
Show answer and explanation

Correct answer: B - make no food, because one of the raw materials is missing

Carbon dioxide is one of the two raw materials, so removing it stops food from being made however bright the sunlight is. Clean air with no carbon dioxide is useless to a leaf, darkness would stop the process as well, and plastic cannot be used by a plant.

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