How does photosynthesis take place in lamplight


Definition: Use of sunlight to build organic molecules (carbohydrates, ATP) by green plants or bacteria.
Purpose: Conversion of light energy into chemical energy.

Plant photosynthesis

Oxygen development during photosynthesis by algae. © BKi 2015

In plant photosynthesis, a distinction is made between the Light reaction and the Dark reaction. The "dark reaction" does not have to take place in the dark, but it is independent of the supply of light. However, it is closely linked to the processes of the light reaction so that it comes to a standstill in the dark after a short time. In summary, the following process takes place:

hν 6 CO2 + 6 H.2O → C6H12O6 + 6 O2

C.6H12O6 stands for glucose, from which polysaccharides such as polysaccharides by polycondensation Strength or Cellulose being constructed:

n C6H12O6 → (C6H10O5)n + n H2O

The released oxygen comes from the water, it is a "waste product" of photosynthesis. Every year around 1011 t carbon dioxide (CO2) implemented by photosynthesis.

Light reaction

In the light reaction, water is split into oxygen, which escapes in gaseous form, and chemically bound hydrogen (as NADPH). Chemical energy is stored in the form of ATP. A potential difference of at least 1.23 V is required to split water. For this purpose, two photo systems coupled to one another are used: Photosystem I (PS I, P700) and Photosystem II (PS II, P680). The designations "P700" and "P680" stem from the fact that the absorption maxima are at 700 and 680 nm, respectively dark red Spectral range. The central component is a chlorophyll molecule or a "special pair" from two chlorophyll molecules. Other chlorophyll molecules serve as antenna pigments to collect the light. The green plants also need blue light, which is absorbed by carotene. The green light, which is useless for the plants, is, however, reflected; therein lies the reason for their green color.

To build up the potential difference mentioned, a membrane is required that Thylakoid membrane, in which the two photo systems PS I and PS II are embedded. Since four electrons have to be released to release an oxygen molecule, the cycle has to be run through four times, which can be shown with the help of flashes of light: Four flashes of light are required.

4 hν 2 H2O → O2 + 4 H.+ + 4 e-

The release of oxygen takes place on a manganese complex.

By absorbing light, P680 is put into an excited state, from which an electron is released that is taken over by a cascade of transport molecules. The chlorophyll was oxidized to a radical cation by the release of electrons, which is finally reduced again by the electrons released during the water splitting. The electron from the chlorophyll complex of the P680 is passed on to the P700, which is also excited, which in turn releases an electron. These electrons are absorbed by an iron-sulfur complex, passed on via ferredoxin and finally reduce NADP+ to NADPH:

NADP+ + H+ + 2 e- → NADPH

During this process, protons migrate from the outside of the thylakoid membrane (= light side) to the inside, while electrons migrate in the opposite direction. The resulting electrochemical potential The membrane is used to build up ATP, which serves as a chemical energy store:

ADP + Pi → ATP + H2O, ΔG> 0 (Pi stands for inorganic phosphate.)

Energetic conditions

A green leaf absorbs around 80% of the incident sunlight in the 400 - 700 nm spectral range. In total, plant photosynthesis uses almost 1 per thousand of the sunlight falling on the earth. To reduce one molecule of carbon dioxide (CO2) For a (-HCOH -) - carbohydrate unit, 2 NADPH molecules and 3 ATP molecules are required for their generation (reduction of NADP+ or reaction of ADP with orthophosphate) again 10 photons (Hν) are required. 60 photons are therefore required to build a hexose molecule. Photons with a wavelength of 700 nm have a molar energy of 171 kJ / mol, 60 mol photons of this wavelength consequently have a total energy of 10260 kJ.

Energy expenditure (ΔG ') the synthesis of one mole of hexose (glucose):

6 CO2 + 6 H.2O → C6H12O6 + 6 O2

ΔG ' = 2868 kJ / mol; according to this, the efficiency of photosynthesis (conversion of radiation energy into chemical energy) is about 28%.

NADPH is provided (coupled with ATP) by a non-cyclic electron transfer of the light reaction:

8 hν ⇒ 2 NADPH + 2 H+ + 2 ATP

Further ATP is synthesized from ADP with the help of a cyclic electron transfer:

2 hν ⇒ ATP

Dark reaction


  • Melvin Calvin, "The Path of Carbon in Photosynthesis. Nobel Lecture December 11, 1961", Angew. Chem., 74(3), 165-175 (1962)
  • Melvin Calvin, "The Path of Carbon in Photosynthesis. Nobel Prize Address, December 11, 1961", Angew. Chem. Int. Ed. Engl., 1(2), 65-75 (1962)
  • Nicholas Cox, Dimitrios A. Pantazis, Frank Neese, and Wolfgang Lubitz, "Biological Water Oxidation", Acc. Chem. Res.46(7), 1588-1596 (2013)
  • Gernot Renger, "Photosynthesis", in "Biophysik" (Ed. W. Hoppe et al.), Springer-Verlag, Berlin, 1982, section 13.1, p. 532ff.
  • U. L ttge et al., "Botanik", VCH, Weinheim, 2nd ed. 1994, chap. 8, p. 109ff.
  • R. Jones et al., "The Molecular Life of Plants", Wiley-Blackwell, Chichester, 2013, chap. 9, p. 284ff.


Author: BKi, © 2017