Mechanism of Photosynthesis in Alage and Leaves

In this article we will discuss about the mechanism of photosynthesis in algae and leaves.

Photosynthesis in Alage:

Photosynthesis in Algae and Liverworts:

Unicellular algae are aquatic and hence have abundant water and light available as raw materials. The third raw material CO₂ is soluble in water and forms bicarbonate and carbonate. The solubility of CO₂ in water is low and the diffusion rate of all three forms of inorganic carbon is also low. Thus, availability of CO₂ to the organism is restricted and this limits photosynthesis in aqueous environments.

Consequently most algae have evolved CO₂-concentrating mechanism a system for actively transporting inorganic carbon into the cell. Most liverworts inhabit moist conditions and have a flattened, lobbed thallus.

In such a state the photosynthetic cells are located along the upper region of the thallus where they have access to air. On the lower surface of the thallus the cells are devoid of chlorophyll and hence function as storage or absorptive cells. It is noteworthy that upper surface has polygonal sections having air chamber beneath.

Each polygonal section has a central pore which opens in the chamber below. Within each chamber large numbers of photosynthetic cells are protruding having a direct access to the air. The thallus has a body plan resembling leaf of higher plant.

Thallus is also thin and well spread to trap maximum light. The slow aqueous diffusion path of CO₂ is enhanced by direct exposure of the photosynthesis cells to air spaces inside the thallus.

Photosynthesis in Leaves:

Photosynthesis as an Oxidation-reduction Reaction:

The perusual of the above equation will show that photosynthesis is fundamentally an oxidation- reduction reaction. In the equation the reaction is shown between CO₂ and water to produce glucose which is hexose. It may be remembered that glucose is not the first product of photosynthesis. The above equation also reveals that equal molar quantities of CO₂ and O₂ and used are evolved, respectively.

Photosynthesis consists of two successive series of reactions. The first reaction requires light and is called light or Hill reaction while the second reaction is called or Blackman reaction or calvin cycle. Of the two reactions, the former is a photochemical reaction while the latter is a thermochemical reaction.

The unit of photosynthesis is supposed to consist of two types of centres, photosystem II (Fig. 13-10). The two systems are excited at different wavelengths of light and are linked through redox catalyst.

Further, it may be remembered that the light reaction involves two processes, photophosphorylation and photolysis of water. In the former reaction there is conversion of light energy into chemical energy. As far as photophosphorylation is concerned, it is of two types: cyclic photophosphorylation and noncyclic photophosphorylation.

It may be worth mentioning at this moment that thermochemical reaction is accomplished through a seriers of steps known as Calvin and Benson cycle. Alternate pathways are also available for the carbon fixation.

In the following a detailed account of different pathways and systems is given:

The summary equation of the photosynthetic process is given below:

The Emerson Enhancement Effect:

Emerson and his colleagues while using algae as the system found two pigment systems and also two light reactions which participate in photosynthesis. When exposed to a light wavelength more than 680 nm (700—720 nm) a specific rate of photosynthesis was observed. Likewise when the exposure was given at wavelength less than 680 nm (650—680 nm) some effect on photosynthesis was observed.

When the system was exposed to light of both wavelengths simultaneously, the effect on photosynthesis exceeded the sum of the two effects caused separately. This provided evidence that the two pigment systems worked in co-operation with each other and the increase in photosynthesis was due to synergism. This is called Emerson enhancement effect.

It is believed that the two photochemical systems are associated with two pigment systems i.e., pigment system I and pigment system II. Chlorophyll a exists in two forms, one with an absorption maxima at short red wavelength of 673 nm (chl a 673) and the other with an absorption maxima at long red wavelength of 683 nm (chl a 683). Another form of chlorophyll ‘a’ with an absorption maxima at 700 nm has also been reported and it is present in much smaller amounts than chl a 673 and chl a 683.

This chlorophyll is called p 700 (Fig. 13-11). Pigment system I contains the long red wavelength absorbing form chl a 683, with p 700 as the energy collecting pigment and the carotenoids, while pigment system II contains chlorophyll b and the short red wavelength form chl a 673 in higher plants and green algae or phycobilins and other chlorophyll a in the blue green and red algae. In summary, it may be mentioned that chlorophyll a acts as electron donor while the role of chlorophyll b is controversial.

Photochemical, Light or Hill Reaction:

This is the first reaction in photosynthesis which takes place in the presence of light and involves two reactions, photophosphorylation and photolysis of water. Chorophyll absorbs light energy; which is transferred and used in an electron transfer chain. Photophosphorylation is a process by which light energy is transformed into chemical energy. It is a light dependent reaction but does not require CO₂.

Over the years, it has come to be appreciated that NADP⁺ reduction and ADP phosphorylation during the initial light reactions produce NADPH and ATP and these are utilized during the calvin reaction when the reduction of CO₂ occurs. Soon after it also became abundantly evident that O₂ evolution and phosphorylation reactions were localized within the grana of the chloroplast.

On the contrary, the calvin reactions took place in the stroma region. One of the most important enzymes and also the abundant enzyme on planet, is ribulose 1, 5-bisphosphate carboxylase. This enzyme is involved in the linking of the light and calvin reactions of photosythesis. It is generally present in stroma along with the thylakoid membrane surfaces.

In the late 1950s it became evident that two separate photosystems existed in higher plants and algae and these interacted in plant photosynthesis. Hobert Emerson while working on Chlorellci observed that wavelengths beyond 680 nm decreased photosynthesis even though chlorophyll a could absorb them.

This decrease in photosynthetic efficiency was said to be red drop effect. The red drop effect was suggested to be due to only one photosystem being in operation. Subsequently Duysens and R. Hill formulated the series scheme for photosynthesis (Fig. 13-12A).

Photosystems I and II:

The important component of photosystem I (PS I) in higher plants and algae is absorption of light by chlorophyll a at an absorption maximum at 683 nm. During PSI, there is no oxygen evolution. In fact all the organisms where oxygen is evolved during photosynthesis definitely posses PSII which includes chlorophyll a with an absorption maximum at 673 nm.

In addition, it also contains either chlorophyll b or some other chlorophyll or phycobilin as an accessory pigment. In any case, the important light absorbing pigment is chlorophyll a. The accessory pigments of PSII absorb light and channelize it to chlorophyll a.

It needs special mentioning that chlorophyll a is single kind of pigment that directly takes part in the conversion of light energy into chemical energy in plants. On the contrary, photosynthetic bacteria possess PSI only. Several bacteriochlorophylls (e.g., bacteriochlorophyll a, b, c, and d) present in different species of bacteria are concerned with light absorption function and trap light energy into chemical form.

Pigment System I:

The photosynthetic unit or quantasome of each photosystem comprises several chlorophyll molecules. It is assumed that several chlorophyll molecules are so grouped to constitute light harvesting molecules [Fig. 13-14, 15].

However, it is one chlorophyll molecule which constitutes the reaction centre. In other words, energy conversion takes place in this molecule. The antenna chlorophyll molecules act only to absorb photons of light.

The photon is either trapped by the reaction centre molecule or is lost as heat or fluorescence. In the absence of Mg²⁺ atoms from the photosynthetic bacterial antenna chlorophyll molecules cause no effect and the latter continue to harvest photons. However, if Mg is removed from the reaction centre molecule, photosynthesis stops.

We may further add that reaction centre acting as a trapping centre is advantageous since it is at a low energy level than the other chlorophyll molecules. One reaction centre is also useful since it requires only one acceptor molecule (Phe) for the several chlorophyll molecules constituting the photosynthetic unit.

Pigment system I comprises 200 to 400 molecules of 4 major spectral form of 663, 669, 677, 684, chlorophyll a and 695 and a molecule of pigment P 700 and nearly 50 molecules of carotenoids are as follows:

Photons migrate from chlorophyll a to P 700. It is the P 700 which traps light photons and releases electrons. It is the heart of PSI. It may be cleared at this point that P 700 is a special form of chlorophyll a and has maximum absorption band at 700 nm. From the above discussion, it may be concluded that P 700 acts as an electron lead from PSI pigment to electron acceptors.

Pigment System II:

PhotosystemI is located on the inner surface of the thylakoid (Figs. 13-15, 17) membranes and photosystem II on the outer surface. Photosystem II contains about 200 molecules of chlorophyll a (673 nm). The pigment at the reaction centre is P 680.

In addition 200 molecules of chlorophyll b, c, d, or depending upon the species in question are present. It is during photosystem II that oxygen is evolved. The redox potentials of the photosynthesis are also different. One may add that photosystems act as electron pumps.

The sequence of events discovered led to the naming of PSI and PSII even though the sequences is from PSII→PSI. In evolution, it is assumed that PSII arose when PSI was in existence.