ICSE Class 10 Answered

(i) Structure of the chloroplast:

(ii) The photochemical phase of photosynthesis is light dependent. A series of chemical reactions occur in quick succession initiated by light; therefore, the phase is called the photochemical phase. It takes place in the chlorophyll-containing thylakoids of the chloroplast.

It occurs in the following steps:

1) Activation of chlorophyll: On exposure to light, chlorophyll becomes excited and absorbs photons (a photon is the smallest unit of light energy) to get activated.

2) Photolysis of water: The energy absorbed is used in the splitting of water molecules into hydrogen ion (H⁺) and hydroxide ion (OH⁻). During this process, electrons are released.

2H₂O → 4H⁺ + O₂ + 4e^-

3) Production of the reducing agent: NADP, i.e. nicotinamide dinucleotide phosphate, picks up the hydrogen ion and is reduced to NADPH.

NADP + e⁻ + H⁺ → NADPH

4) Formation of molecular oxygen: Hydroxide ion is oxidised to hydrogen peroxide which decomposes to form molecular oxygen (O₂). Molecular oxygen is released during photosynthesis.

2O → O₂

5) Photophosphorylation: Electrons are used to convert ADP (adenosine diphosphate) into an energy-rich compound ATP.

ADP + Phosphate → ATP

(inorganic)

(iii) 

1. I would select white light because photosynthesis is maximum in white light, while it is minimum in green light as green light is reflected by green plants.
2. I would select a destarched plant because during the experiment, any starch formed in the leaf will indicate that the starch is synthesised by the plant during photosynthesis by using light.