Photosynthetic Unit

Photosynthetic Unit:

Photosynthetic Unit

Photosynthetic unit may be defined as the minimum number of pigment molecules capable of acting cooperatively in a photochemical act to evolve one molecule of O2 or to reduce one molecule of CO2 (Emerson and Arnold, 1932). This number was estimated to be approximately. 300 pigment molecules per reaction centre. However, findings that ratio of pigment system I (PSI) and pigment system II varies among different plants led Arntzen (1977) to propose separate package model.

According to this model, photosynthetic reaction centre and its associated light harvesting pigment molecules of one pigment system form one photosynthetic unit. Thus, separate photosynthetic units exist for PSI and PSII and their number also varies. Park and Beggins (1964) gave the term quantasome for the structural entities of photosynthetic units in thylakoids. However, recent findings disagree with the concept and discarded the view. Thus, photosynthetic unit is regarded only as a statistical unit.

Nature of Light

Solar radiation, according to electromagnetic wave theory (proposed by James Clark Maxwell, 1860), can be described as photons that travel in waves. The unit quantity of light energy in the quantum theory is called quantum, whereas the same of the electromagnetic field is called photon. Solar radiation can be divided on the basis of wave lengths. Radiation of shortest wavelength belongs to cosmic rays whereas that of longest wave length belong to radio waves. The different radiations on the basis of increasing wave length and decreasing energy per quantum are listed as cosmic rays, gamma rays, X-rays, ultra violet radiation, visible light, infra-red light, micro waves and radio waves

Solar radiation that hits the earth’s atmos- phere (about 42%) undergoes major modifications. A major part of it is filtered through various constituents of atmosphere, i.e., high energy radiations are screened out by the ozone layer in the atmosphere, and lower energy radiations are screened out by water vapour and carbon dioxide. Our atmos- phere, thus provides a window through which visible wavelengths pass on to the various life forms of the biosphere.

Visible Light:

It is that portion of solar radia- tion which our eyes can see. It consists of radiations having a wavelength between 380 to 760 nm. The biological systems make most effective use of the visible radiation. Visible light can further beresolved into light of different colours- violet (380 to 430 nm), indigo (430 to 470 nm), blue (470 to 500 nm), green (500 to 580 nm), yellow (580 to 600 nm), orange (600 to 650 nm) and red (650 to 760 nm).

How much photosynthetically active radiation is received by plants from the sun?

Ans. The total irradiance from the sun coming at the upper boundary of the atmosphere is 1,360 W m or watts per square meter (the solar constant). 1 W-2is equal to 1 joule per second (joule is the SI unit of energy). This irradiance includes ultraviolet and infrared wavelengths. Of this, only about 900 W m reach plants -2 because the rest is absorbed or scattered by water vapour, dust, CO2 and ozone present in the atmosphere. Of this, about half is in the infrared, roughly 5 percent is in the ultraviolet, andthe rest has wavelengths between 400 to 700 nm. Therefore, the plants receive about 400 to 500 W m-2 of photosynthetically active radiation from the sun.

Q. 2. Can photosynthesis occur under the light of ordinary fluorescent lamps?

Ans. Fluorescent lamps are commonly called tube rods. They produce light by fluorescence which is usually rich in blue and red wavelengths. Therefore, photosynthesis can occur under the light of ordinary fluorescent lamps provided it is about 400 to 500 Lux (10.76 Lux= 1 foot-candle).

Q. 3. Can photosynthesis occur under sodium and mercury vapour lamps?

Ans. These lamps emitt light of specific wave length when an electric current is passed through vapours. Sodium vapour lamps emitt orange wavelengths whereas mercury vapour lamps emitt blue and green wavelengths. Therefore, photosynthesis can occur under each of these lamps.

Absorption of Light by Pigments:

Light waves hit the object like rain falling on grounds. All the organic substances absorb light, but the coloured pigments absorb only visible light. The photosynthetic pigments are capable of absorb- ing various wavelengths of visible light. For ex-ample, chlorophyll a and chlorophyll b absorb blue and red light better than the light of other colours. Blue light carries more energy as compared to red light. Therefore, the red light, having less energy, is most effective in the process of photosynthesis. Because green light is only minimally absorbed, it is the least effective in photosynthesis and the leaves appear green in colour.

However, the other pigments, like carotenoids, are able to absorb light in violet- blue-green range. These pigments absorb their specific wavelengths of light and transfer their light energy to chlorophyll molecules.On receiving the required light energy, a chlorophyll molecule gets changed from ground state to excited state. This activated state is called excited singlet state. This state of chlorophyll molecule is unstable and lasts for about 10-12 seconds. If the energy of excited singlet state is not utilized during this short period, it is lost as heat and long wave radition. This release of radiation energy is known as fluorescence.

Chlorophyll molecules absorb both blue and red radiations but emit only red fluorescence. If the absorbed energy is not emitted as fluorescence, the excited singlet state of pigment molecule may come to the next higher energy level by losing some of its extra energy in the form of heat. This state is again untenable having half life of 10-9 seconds, called triplet excited state. This state may come back to ground state by releas ing radiation called phosphorescence

Demonstration of fluorescence by chlorophyll.


Spinach leaves, Pestle and morter, 80% acctone, calcium carbonate, Buchner funnel, test tube, source of light.


fluorescence by chlorophyll.

Take 25 gm of fresh spinach leaves in a Pestle and morter. Crush them with 10 ml of 80% acetone. Add a pinch of calcium carbonate and again crush. Filter the extract on a Buchner funnel. The deep green-coloured filtrate containing chlorophylls is obtained. Pour the filtrate in a test tube. Place the test tube before the source of light and observe.


The solution appears green when placed between the source of light and eyes of observer, i.e., in transmitted light. The solution appears red when source of light is placed behind the observer and solution is placed in front of observer i.e., in reflected light. The phenomenon is called fluorescence .

Absorption Spectrum and Action Spectrum :

Absorption spectrum of a particular pigmentis the curve plotted on a graph paper representing the amount of light absorbed at each wave length by that pigment. To determine the absorption spectrum of a particular pigment, a purified sample of that pigment is placed in the path of visible light inside an instrument called a spectrophotometer. This instrument measures the amount of light that passes through the pigment solution and from this it can be calculated how much light of a particular wave length was absorbed.

Then the result is plotted on a graph which represents the absorption spectrum of that pigment. The absorption spectra of chlorophyll a and chlorophyll b are shown in . The figure shows that these pigments absorb maximum in blue and red regions of visible light. The chlorophyll a molecules absorbing maximum at different wave lengths are often known after the wave lengths they absorb maximum. For example, Chl 4670, Chl a680, Chl a700, etc.

Action spectrum is the curve plotted on a graph paper representing the amount of O, evolved or the amount of CO₂ fixed or any other action of photosynthesis at different wave lengths of light. It has been observed that the photosynthesis occurs maximum in blue and red regions of visible light.It is possible to determine the relative contribution of a particular pigment in the process of photosynthesis by comparing the absorption spectra with the action spectra. In higher plants, the absorption spectra of chlorophyll a and chlorophyll b run almost parallel to the action spectra of photosynthesis depicting their role in the process.

Two Steps of Photosynthesis (Light Reaction and Dark Reaction)

:The present state of our knowledge regarding photosynthesis proves that it is not a one step process, but it is a two step process. The first step is dependent on light and hence called light reaction or photochemical reaction. In this step, the as- similatory power is generated in thylakoids. This step is followed by the second step in which the light is not directly required but the assimilatory power generated in the light reaction is used up for the fixation of CO2 into carbohydrate. This second step is called dark reaction or thermochemical reaction which takes place in the stroma of the chloroplasts.

Hill Reaction (Evidence in support of light reaction)

hill reaction

Robin Hill (1937) demonstrated that dry leaves powder, when exposed to light in presence of water, liberates a small amount of oxygen. This was later demonstrated by Hill and Scarisbrick (1940) using isolated chloroplastst. Hill and Scarisbrick ob- served that isolated chloroplasts could release O2 in the light if certain oxidants such as ferric oxalate or other Fe3+ salts were present in the medium.

The Fe3+ salts get reduced to Fe²+ salts by the electrons taken from light- driven split of water. Other compounds such as quinones, dichlorophenol in- dophenol dye, etc., can also be reduced by illuminated chloroplasts. Ferricyanide is reduced to ferrocyanide. This light-driven split of water in the absence of CO2 fixation and reduction of ferric salts to ferrous salts is known as Hill reaction. Therefore, Hill reac- tion is not the actual light reaction of photosynthesis but it is an evidence in support of 300light reaction.

Activity . Demonstration of Hill reaction

.Requirements :

Fresh spinach leaves, homogeniser with cooling device, centrifuge, muslin cloth, 0.25 M Sucrose solution, funnel, test tubes, dropper, dichlorophenol indophenol dye.


 Demonstration of Hill reaction

: Wash the Spinach leaves several times with water and place them in a plastic bag inside the ice chamber. Homogenize about 25 gm of leaves with 25 ml of 0.25 M sucrose solution in a homogeniser with cooling device. Filter the green suspension through several layers of muslin cloth and centrifuge the supernatant at 600 g for 2 min. Take the supernatant and centrifuge in cold for 10 min at 100 g. Discard the supernatant and suspend the chloroplast pellet in sucrose solution in a test tube. Pour a small amount of blue coloured 2, 6- Dichlorophenol indophenol dye and expose the suspension to light.


Blue colour of 2, 6- dichlorophenol indophenol dye disappears showing that the dye gets reduced. Simultaneously, some O2 bubbles also come out.


The blue coloured dye is oxidized and it becomes colourless only when it gets reduced. Reduction of dye in the suspension indicates that water is photoxoidized by suspended chloroplasts 2 H2O- →4H+4e+0₂ ↑

Quantum Requirement and Quantum Yield:

The number of light quanta needed for the production of one molecule of oxygen is called quantum requirement. It has been estimated that a minimum 8 quanta are required to evolve one mol. of O2 or to reduce one mol. of CO2. Thus, quantum requirement is 8. Quantum yield is defined as the number of O2 molecules produced per quantum oflight absorbed. It is 1/8 or 12%.

Emerson Effects:

(a) Emerson’s first effect or red drop:

Emer- son (1957) measured quantum yield in Chlorella plants by exposing them to different wave lengths of monochromatic lights (lights of only one wave length) and plotted the results on a graph paper . The yield was almost constant in all the wavelengths but dropped suddenly in the region above 680 mu (red region). The fall in photosynthetic yield beyond red region is called “Red drop”or Emerson’s first effect.

(b) Emerson’s second effect of enhancement effect:

Emerson repeated the first experiment by supplying additional shorter wave length of light along with exposures of different wavelengths of lights. Emerson et al (1957) observed that quantum yield was enhanced by combined effect of short and long wave lengths of light even beyond the 680 mu. It is called the enhancement effect or Emerson’s second effect .Emerson’s experiments gave conclusive idea that process of photosynthesis involves two light reactions- one carried by short wavelength absorb- ing form of chlorophyll a (i.e., chla680) and the other by pigments including a long wavelength absorbing form of chlorophyll a (i.e., Chl a700).

Two Pigment Systems (Photosystem I and Photosystem II)

:Light reaction of photosynthesis involves the participation of two separate pigment systems of photosystems, i.e., PS I and PS II (named for the order in which they were discovered). Each photosystem has a pigment-protein complex posed of chlorophyll a, chlorophyll b, carotenoids and other components required in electron transport. Each pigment-protein complex consists of a core complex (having a reaction centre plus about 40-60 chlorophyll molecules) and an antenna complex.

Various pigment molecules in antenna complex absorb different wave lengths of light and transfer their absorbed energy to core complex which is finally concentrated in the reaction centre- Chlorophyll. The later takes part inphotochemical reaction. The role of antenna com- plex and core complex is significant in harvesting the maximum solar radiation over a large area and passing their absorbed energy to reaction centres. It is because the energy trapped by a single chlorophyll molecules is not enough to initiate the first chemical reaction that would occur in light.

(i) Photosystem I (PSI):

The PS I complex consists of single molecule of reaction centre – P700 (or chlorophyll a700), 200-400 chlorophylls, 50 carotenoids and two iron containing proteins similar to ferredoxin, called Fe ~ S proteins. PS Ihas more chlorophyll a molecules as compared to chlorophyll b and carotenoids. It is light green in colour and located both on the non appressed part of grana thylakoids as well as on Fret channels (stroma thylakoids) mostly towards the outer surface of membrane. PS I is associated both with cyclic and noncyclic electron transports and drives electrons from PS II to NADP+. However, it can carry on cyclic electron transport independently.

(ii) Photosystem II (PS II):

The PS II complex consists of single molecule of reaction centre P680 (or chlorophyll 4680), 200 chlorophylls, 50 carotenoids, quinone, Mn++, CI and an unknown water oxidizing enzyme. PS II has almost equal number of chlorophyll a and chlorophyll b molecules. It is dark green in colour and located mostly in the appressed parts of grana thylakoids towards the inner surface of membranes. PS II is associated with noncyclic electron transport, spliting of water and evolution of molecular oxygen.

Differences between PS I and PS IIPS I

4. This system is directly invovled with the photo-oxidation of water and evolution of molecular oxygen.PS I is located on the outer surface of non-appressed parts of grana thylakoids and Fret channels
2. PS I comprises of about 200 to 400 chlorophylls, 50 carotenoids, onc molecule of P700-2. PS II comprises of about 200 chlorophylls, 50 carotenoids and one molecule of P680-
3. It is light green in colour3. It is dark green’ in colour.
4. This system is not directly involved with the photo oxidation of water and evolu- on of molecular oxygen.4. This system is not directly involved with the photo-oxidation of water and evolu- on of molecular oxygen.
5. This system produces a strong reductant which reduces NADP to NADPH.5. PS II donates electrons to PS I when NADP+ is reduced.
6. PS I is involved both in cyclic and noncyclic electron transport..6. PS II is involved only in noncyclic electron transport.
7. Pigment molecules of PS I absorb at or below 700 nm wave length of light7. Pigment molecules of PS II absorb at or below 680 nm wave length of light.

Cyanobacteria and other photosynthetic bacterial members have no chloroplasts. They have photosynthetic pigments located in specialized membranes.

Certain carotenoids (especially violoxanthin, a xanthophyll) also occurin the chloroplast envelope, giving it a yellowish colour.

Carotenoids protect the chlorophyll pigments against oxidative destruction caused by O2 when irradiance levels are high.

Photosystem I (PSI) is located in stroma thylakoids and non-appressed regions of grana that face the stroma whereas most of the photosystem II (PS II) is present only in the appressed regions of grana thylakoids

.Manganese undergoes various oxidation states (i.e., Mn2+, Mn3+ and Mn4+) during photooxidation ofwater and evolution of molecular oxygen.

Both calcium (Ca2+) and chloride (CI) ions are essential for the photooxidation of water.PS II complex is more closely involved with oxidation of water which initiates electron transport in photosynthesis. Release of one O2 molecule requires oxidation of two H2O molecules and removal of four electrons . read more

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