1) The leaves lack chlorophyll. In the absence of light, disintegration of chlorophyll pigment occurs. The carotene pigments are more stable. So leaves become pale.
2) The production of chlorophyll pigment is directly proportional to the amount of light available. When light is not available the colour of the leaf gradually changes to light green or yellow.
3) During this process Xanthophyll and carotenoid pigments become predominant and hence the colour.
4) Carotenoid pigment are accessory pigments and do not break down easily. They are said to be more stable than chlorophylls.

1) Many plants use a group of chemicals called sinapate esters to defend against the sun. These aromatic compounds are found in the upper cell layers of the plant tissues and sinapoyl Malate provides bulk of the UV protection.
2) Thus when the green leaves of the plants are absorbing visible part of the spectrum for photosynthesis, the chemicals help to absorb the incident, UV radiation thus preventing the UV from affecting other living organisms. Thus photosynthesis protects us from harmful radiation of the sun.

1) When photons are activated reaction centre of Pigment system II(P680), electrons are moved to the high energy level. Electrons from high energy state passes through series of electron carriers like Plastoquinone, Cytochrome complex, Plastocyanin and finally accepted by PSI (P700). During this movement of electrons from PS II to PS I ATP is generated.
2) PS I (P700) is activated by light, electrons are moved to high energy state and accepted by electron acceptor molecule Ferredoxin Reducing Substance (FRS). During the downhill movement through Ferredoxin, electrons are transferred to NADP⁺ and reduced into NADPH + H⁺ (H⁺ formed from splitting of water by light).
3) Electrons released from the Photosystem II are not cycled back. It is used for the reduction NADP⁺ in to NADPH ⁺ H⁺. During the electron transport it generates ATP and hence this type of photophosphorylation is called Non-Cyclic Photophosphorylation. The electron flow looks like the appearance of letter 'Z' and so known as Z scheme.
4) When there is availability of NADP⁺ for reduction and when there is splitting of water molecules both PS I and PS II are activated. Non-Cyclic electron transport PS I and PS II both are involved co-operatively to transport electrons from water to NADP⁺. In oxygenic species Non-cyclic electron transport takes place in three stages:
i) Electron transport from water to P 680: Splitting of water molecule produce electrons, protons and oxygen. Electrons lost by the PS II (P680) are replaced by electrons from splitting of water molecule.
ii) Electron transport from P 680 to P 700: Electron flow starts from P 680 through a series of electron carrier molecules like Plastoquinone (PQ), Cytochrome b6-f Complex, Plastocyanin (PC) and finally reaches P700 (PSI).
iii) Electron transport from P 700 to NADP+: PSI (P700) is excited now and the electrons pass to high energy level. When electron travels downhill through ferredoxin, NADP+ is reduced to NADPH+ H+.