photosynthesis Biochemistry. Absorption of radiant energy

you are: photosynthesis Biochemistry

Light can cause photochemical effect only after it will be absorbed. The substances absorbing visible light, are called as pigments. Absorption by a pigment of quantum of light (photon) is defined by character of distribution elektronov in a molecule of the given pigment; which lengths of waves will be absorbed by a pigment, depends on number and from an arrangement of double communications in its molecule, and also from presence at it of aromatic rings. As a result of absorption of a photon by a pigment distribution elektronov in its molecule changes also a pigment passes in other, ' activated ', the form a little. As between length of a wave of absorbed light and structure of absorbing substance there is a certain communication, we can establish characteristics of the pigment playing, a photoreceptor role in certain photochemical reaction, under the data showing as its activity depending on length of a wave changes.

Directing on green sheet monochromatic light of various length of the wave, received by means of an optical filter, a prism or difraktsionnoj lattices, and measuring the speed of photosynthesis corresponding to each length of a wave, it is possible to be convinced that light in dark blue (~ 420 nanometers) and red (~670 nanometers) spectrum areas provides the greatest efficiency of photosynthesis, and light in green (~ 500-600 nanometers) areas the least (fig. 4.7). (Dependence of relative efficiency of process on length of a wave) it is possible to explain such spectrum of action, proceeding from a spectrum of absorption of a chlorophyll, the main pigment hloroplastov.

Ekstragirovannyj from leaves intensively absorbs a chlorophyll just in those ranges of lengths of waves which are most effective in photosynthesis (fig. 4.8). This similarity between ' an absorption spectrum ' a chlorophyll and ' an action spectrum ' photosynthesis is one of the best proofs of that the role main retseptornogo a pigment in photosynthesis plays a chlorophyll. Separate features of a spectrum of action of photosynthesis specify that in light absorption at this process participate as well yellow pigments - karotinoidy which along with a chlorophyll in a considerable quantity contain in hloroplastah. For lack of a chlorophyll karotinoidy are incapable to carry out photosynthesis, therefore it is considered to be that activated by light karotinoidy transfer the energy absorbed by them to a chlorophyll which finally and performs actually photosynthetic work.

Calculations show that the small part of molecules of a chlorophyll is actually involved in carrying over elektronov at photosynthesis only. Its other molecules carry out only a role svetosobirajushchego a complex, or svetosobirajushchej aerials. The transmission of energy from karotinoidov to a chlorophyll and from one molecule of a chlorophyll to another results from process which name resonant carrying over. Molecules participating in this process should be densely packed, that oscillatory energy could be transferred directly from one molecule to another. In granah hloroplastov at the higher plants photosynthetic pigments and are packed, about what we already spoke in gl. 2; therefore the energy absorbed by one of pigments, can be easily transferred molecules of some other pigments.

Purple photosynthesizing bacteria have a pigment bakteriohlorofill - structural analogue of a chlorophyll. This pigment absorbs in green and infra-red sites of a spectrum, i.e. In those areas where intensive absorption is not peculiar to the higher green plants. Red, brown and blue-green seaweed contains along with a chlorophyll as well considerable quantities of pigments from group fikobilinov (fikoeritrin, fikotsianin, allofikotsianin and others, related to bilious pigments of animals);

Besides, at them are present and karotinoidy, such, as fukoksajatin and peridinin. The listed pigments form associations which at this seaweed play a role of the main light-absorbing system. Fikoeritrin absorbs in blue-green area of a spectrum and consequently it seems red whereas fikotsianin and allofikotsianin most intensively absorb in yellow and red ranges and are accordingly painted in dark blue or green colour (fig. 4.9). The spectrum of action of photosynthesis at this seaweed (fig. 4.10) considerably differs from a spectrum of green plants.

The Pigments of seaweed attached to fibers, are grouped in structural units, fikobilisomy, settling down

In hloroplastah on that party lamell which is turned to strome. All pigments fluoresce, i.e. Each of them absorbs photons of certain energy and certain length of a wave and lets out photons of smaller energy with some bolshej in the length of a wave. Thus, as a result of a number of certificates of absorption and light emission light energy finally is transferred to a chlorophyll.

Fikobiliny on which share it is necessary at some seaweed to 60% of all fiber, form effective enough svetosobirajushchuju system though the transmission of energy between these pigments occurs nevertheless not so effectively, as in hloroplastah the higher plants. There is seaweed at which the parity various fikobilinov varies depending on spectral structure of light. Such seaweed at illumination by light of different lengths of waves changes the colouring. This phenomenon has received the name of chromatic adaptation. If, for example, to grow up such seaweed on red to light, at them will prevail fikotsianin if to shine with their green light the main place will occupy fikoeritrin. Similar prisposobljaemost allows the seaweed growing on different depth, to absorb enough light necessary for photosynthesis though light with depth increase changes the spectral structure as the part of its energy at passage through a sheet of water is absorbed or dissipates molecules of water or the particles weighed in water. Not all seaweed possesses ability to chromatic adaptation. Some of them survive on different depth thanks to that with increase in depth they synthesise more pigment.