NDA (II) 2023 General Ability Test · Question 93 of 146

Correct — D, O₂. Oxygen is what photosynthesis makes, not what it needs. The reaction takes carbon dioxide and water and, driven by light energy trapped by chlorophyll, builds carbohydrate while releasing oxygen. The oxygen atoms that leave the leaf come from the water molecules that are split in the light reaction, which is why an experiment that labels the oxygen in water finds the label in the gas given off. Photosynthesis is, in that sense, the source of nearly all the free oxygen in the atmosphere, so listing oxygen among its requirements reverses the whole process.

• (a)Chlorophyll — Chlorophyll is the green pigment in the chloroplast that absorbs light and passes its energy into the reaction. Without it, light energy is never captured, which is why the pale parts of a variegated leaf test negative for starch.
• (b)CO₂ — Carbon dioxide supplies every carbon atom in the sugar the plant builds. It enters through the stomata, and a plant kept in air stripped of carbon dioxide makes no starch even in bright light.
• (c)Water — Water is drawn up by the roots and is split in the light reaction, providing the hydrogen used to reduce carbon dioxide and the oxygen that is released. It is a raw material of the reaction, not a product.

Photosynthesis is the process by which green plants convert light energy into chemical energy stored in carbohydrate. Its raw materials are carbon dioxide and water, its conditions are light and chlorophyll, and its outputs are carbohydrate and oxygen. It runs in the chloroplast in two stages — a light-dependent stage that splits water and captures energy, and a carbon-fixing stage that uses that energy to build sugar from carbon dioxide.

The trap is that plants really do need oxygen, so the option feels defensible. They need it for respiration, which happens in mitochondria in every living cell of the plant, day and night, and which is the reverse trade — oxygen and sugar in, carbon dioxide and water and energy out. Photosynthesis and respiration are complementary, and the exam repeatedly asks students to hold the two ledgers apart. A second reason students hesitate is the phrasing 'requirement', which invites thinking about what the plant needs rather than what the reaction consumes. Read the stem as being about the reaction itself and oxygen falls out immediately, because it stands on the product side of the equation.

• Photosynthesis needs carbon dioxide, water, light and chlorophyll, and it yields carbohydrate and oxygen.
• The oxygen released comes from the splitting of water, not from carbon dioxide.
• Photosynthesis takes place in the chloroplast, while respiration, which consumes oxygen, takes place in the mitochondrion.
• Green plants in a terrestrial ecosystem capture and convert only about one per cent of the sunlight falling on their leaves into food energy.

• Reasoning that plants need oxygen, so oxygen must be a requirement for photosynthesis; that need belongs to respiration.
• Assuming the released oxygen comes from carbon dioxide; it comes from water.
• Forgetting that light is as necessary as the three material inputs, even though it is not listed among these options.

Asked as a negative-form item — separate the products of a named process from its raw materials and conditions.

• practice — not a real PYQ

  The oxygen released during photosynthesis comes from

  • (a)carbon dioxide
  • (b)water
  • (c)chlorophyll
  • (d)glucose

  Answer(b) water — water molecules are split in the light reaction, and the oxygen atoms released form the gas that leaves the leaf.

• practice — not a real PYQ

  In a green plant, respiration differs from photosynthesis in that respiration

  • (a)occurs only in the leaves
  • (b)occurs only in daylight
  • (c)consumes oxygen and releases carbon dioxide
  • (d)takes place in the chloroplast

  Answer(c) consumes oxygen and releases carbon dioxide — it runs in mitochondria in every living cell, by day and by night.