Introductory Crop Physiology (1+1)

Absorption spectra of chlorophyll

Absorption spectrum of Chrorophyll

• The absorption of different wavelengths of light by a particular pigment is called absorption spectrum.
• Chlorophylls absorb maximum light in the violet blue and red part of the spectrum.
• The absorption peaks of chlorophyll a are 410 and 660nm, for chlorophyll b 452 and 642nm.
• Carotenoids absorb light energy in blue and blue green part of the spectrum.
Transfer of light energy absorbed by accessory pigments to chlorophyll a
• All pigments except chlorophyll a are called as accessory pigments or antenna pigments.
• The light energy absorbed by accessory pigments is transferred to chlorophyll a molecule.
• The transfer of light energy from accessory pigments to chlorophyll a is called as resonance or forster transfer and takes part in primary photochemical reaction in photosynthesis. Chlorophyll a molecules also absorb light energy directly.
• As a result of absorbing the light energy, the chlorophyll molecule gets excited.

Excited states of atoms or molecules (fluorescence and phosphorescence)

• The normal state of the chlorophyll molecule or atom is called as ground state or singlet state.
• When an electron of a molecule or an atom absorbs a quantum of light, it is raised to a higher energy level which is called as excited second singlet state.
• This state is unstable and has a life time of 10-12 seconds.


• The electron comes to the next higher energy level by the loss of some of its extra energy in the form of heat.
• This higher energy level is called as excited first singlet state and is also unstable with a half life of 10-9 seconds.
• From the first singlet state, the excited electron may return to the ground state in two ways viz., either losing its remaining extra energy in the form of heat or in the form of radiant energy.
• The second process is called fluorescence.
• The chlorophyll molecules exit the extra energy in the form of fluorescent light when they are exposed to incident light.
• Fluorescent light is of longer wavelength than the incident light.

The excited molecule or the atom may also lose its excitation energy by internal conversion and comes to another excited state called as triplet state which is metastable with a half life of 10-3 seconds.
From the triplet state, the excited molecule or the atom may return to the ground state in three ways.

• By losing its remaining extra energy in the form of heat
• By losing extra energy in the form of radiant energy (phosphorescence) and the chlorophyll molecules emit phosphorescent light even after the incident radiant light is cut off. The phosphorescent light is of longer wavelength than incident light and also fluorescent light.
• Electrons carrying the extra energy may be expelled from the molecule and is consumed in some further photochemical reaction and the fresh normal electron returns to the molecule.
Quantum requirement and quantum yield

• Light rays consist of tiny particles called photons and the energy carried by a photon is called quantum.
• The number of photons (quantum) required to release one molecule of oxygen in photosynthesis is called quantum requirement.
• On the other hand, the number of oxygen molecules released per photon of light in photosynthesis is called as quantum yield.
• The quantum yield is always in fraction of one

TOP