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1. Photosystems: 

a. Two kinds of photosystems are present in thylakoid membranes of chloroplasts. 

b. Each has its own set of light harvesting chlorophyll and carotenoid molecules. 

c. Chlorophyll and accessory pigments help to capture light energy over large area and pass it on to the photocenters. 

d. Thus, a photon absorbed anywhere in the harvesting zone of P-680 center can pass its energy to the P680 molecule. 

e. The cluster of pigments which transfer their energy to P680 absorb at or below 680nm. 

f. Together with P-680 they form Photosystem – II. 

g. Likewise, P-700 forms Photosystem – I along with pigment molecule which absorbs light at or below 700 nm.

2. Photosystem II: 

a. Photolysis of water and release of oxygen takes place in this system. 

b. In this process, when PS-II absorbs light, electrons are released and chlorophyll molecule is oxidized. 

c. Electrons emitted by P680 (PS-II) are ultimately trapped by P700 (PS -1). 

d. Oxygen is the byproduct by the photosynthesizing plants. 

e. Protons accumulate inside the thylakoid resulting in a Proton gradient. 

f. When the protons diffuse across the thylakoid membrane into stroma against the H+ gradient, energy, is released. 

g. This is used to produce ATP. 

3. Photosystem 1: 

a. Upon absorption of light quanta by PS-I (P700) reaction center emits energy rich electrons. 

b. These flow down a chain of electron carriers to NADP along with the proton generated by splitting of water. 

c. This result in the formation of NADPH. 

d. Hydrogen attached to NADPH is used for reduction of CO₂ in dark reaction also called as reducing power of the cell.

(a) Mg, Fe, Cu, Zn, Mn, K and nitrogen

(d) A – (iii); B – (iv); C – (ii); D – (i)

(c) four carbons and one nitrogen atom

The oxidized P-680 regains its electrons by the photolysis of water as follows:

1. 4H₂O → 4H⁺ + 4OH^- 

2. 4OH^- → 4(OH) + 4e^- 

3. 4OH^- → 2H₂O + O₂ 

4H₂O → 2H₂O + O₂ ↑ + 4H⁺ 4e^- Overall reaction

(b) electrons, protons and oxygen

(c) OAA and 3PGA

(c) proton, electron and oxygen

(c) porphyrin and phytol respectively

Oxygen not be a limiting factor for photosynthesis.

(i) 1. Cuticle

2. Upper Epidermis

3. Palisade tissue/chloroplast

4. Xylem

5. Stomata

(ii) One vein has been shown.

(iii) Xylem – helps in the conduction of water.

Stomata – through which exchange of gases takes place.

1. Root

2. Photon

3. Carbon-dioxide

4. Chlorophyll

The specific function of light energy in the process of photosynthesis is to Activate chlorophyll. 

(a) In the grana of chloroplast 

Cyclic photophosphorylation: 

a. Illumination of photosystem-I causes electrons to move continuously out of the reaction center of photosystem-I and back to it. 

b. The cyclic electron-flow is accompanied by the photophosphorylation of ADP to yield ATP. This is termed as Cyclic photophosphorylation. 

c. Since this process involves only pigment system I, photolysis of water and consequent evolution of oxygen does not take place.

Robert Hill proved that the source of oxygen evolved during photosynthesis is water and not carbon dioxide. Hence, it is called Hill’s Reaction. 

1. In this experiment, Hill cultured isolated chloroplasts in a medium containing CO₂ free water, haemoglobin and ferric compound. 

2. Ferric salts and haemoglobin were added in the medium as hydrogen and oxygen acceptors respectively. 

3. When the suspension was illuminated, he observed that haemoglobin turned into oxyhaemoglobin (red colour). 

4. This confirmed that water must have oxidized releasing O₂, that reacted with haemoglobin. Reduction of ferric compound was also indicated by change in colour. 

5. The H₂O molecule oxidized to evolve O₂ as a byproduct. Thus, Hill proved that the source of evolving O₂ is H₂O  and not CO₂ . 

6. This process of splitting up of water molecules under the influence of light in the presence of chlorophyll is called Photolysis of water or Hill Reaction.

1. Chlorophyll-a is an essential photosynthetic pigment as it converts light energy into chemical energy and acts as a reaction centre. 

2. Initially, it lies at ground state or singlet state but when it absorbs or receives photons (solar energy), it gets activated and goes in excited state or excited second singlet state. 

3. In the excited state, chlorophyll-a emits an electron. The emitted electron is energy rich, i.e. has extra amount of energy. 

4. Due to the loss of electron (e_- ), chlorophyll-a becomes positively charged. This is the ionized state. 

5. Chlorophyll-a molecule cannot remain in the ionized state for more than 10‘9 seconds. Hence the photochemical reaction or electron transfer occurs very fast. 

6. The energy rich electron is then transferred through various electron acceptors and donors (carriers). 

7. During the transfer, the electron emits energy which is utilized for the synthesis of ATP. This shows that light energy is converted into chemical energy in the form of ATP.