Essay for Botany Students on the Process of Photosynthesis

Photosynthesis has two major steps: (i) trapping of light energy and conversion of trapped energy to chemical energy and (ii) fixation of C0₂ to produce simple sugars utilising the energy fixed in the earlier step.

Trapping and conversion of light energy, mediated through reactions known as light reactions of photosynthesis, occurs only in the presence of light. Fixation of CO occurs both in the presence and absence of light. The reactions allowing the assimilation (fixation) of C0₂ do not require the presence of light. So this phase or step of photosyntheisis is known as dark phase or dark reaction of photosynathesis.

Conversion of light energy to chemical energy:

Light Absorption (Absorption Spectra): Light is the visible part of the electromagnetic radiation that comes from the sun visible light consists of a spectrum of wavelengths ranging from 390 nm to 700 nm. Different colours in visible light are due to different bands of wavelengths as slown in.

The photosynthetic pigments can absorb light. Different pigments absorb different colour or wavelength of light differently.

The representation of the amount of absorption by any specified pigment (s) under different wavelengths gives the absorption spectrum of the pigment (s) Generally, a specified pigment shows highest absorption at a particular wavelength. Such a maximum absorption is considered as absorption peak or absorption maxima. The absorption maxima for China and Chip beyond green wavelength of light are 670 nm and 644 nm, respectively.

The effectiveness of different wavelengths of light on the photosynthetic activity of a system (chloroplast/ green cell/leaf) is known as Action spectrum.

The action spectrum speaks about how the absorption of light is related to the conversion of absorbed light to chemical / photosynthetic product. The absorption spectrum is closely related to action spectrum of a photosynthetic system.

If one compares the absorption spectrum of chlorophyll with that of a system, there is an indication that, where there is no absorption by chlorophyll, there is also action (photosynthesis). It is due to the presence of accessory pigments, which absorb light energy at that wavelength and transfer the absorbed energy to chlorophylls.

Oxygen- a product of light reaction:

Isolated chlorplasts under illumination produce oxygen. In 1937, Hill demonstrated the production of oxygen from isolated chloroplasts in the presence of suitable oxidant (electron acceptor) such as ferricyanide. Such oxidant is commonly known as Hill oxidant. Using non­radioactive heavy isotopes of oxygen (¹⁸0) in water, Ruber, Hassid and Kamen proved oxygen was derived form H₂0. So in the presence of light, chlorophyll and an electron acceptor, water splits to produce oxygen the phenomenon is known as photolysis of water. In the green cell system the electron acceptor is the oxidised nicotinamide adenine dinuceltide phosphate (NADP).

Systems involved in conversion of light energy:

The reactions involved in the process of conversion of light energy are known as photochemical reactions. These reactions are carried out through two distinct systems. The preliminary evidence in support of the involvement of two distinct systems in light reaction comes from the experiment of Emerson popularly known as Emerson effect.

Emersion effect:

Emerson has measured the number of oxygen molecules evolved per quantum of light energy absorbed by the photosynthesising system. It is known as quantum yield. The quantum yields under different wavelengths of light give an interesting result.

There is a pronounced decrease in the quantum yield at wavelengths greater than 680 nm in the red zone of the light spectrum this decline in photosynthesis in known as red drop. Simultaneous or alternate use of 680 nm and a shorter wavelength produces a higher quantum yield than when two wavelengths were used separately. This enhancement in photosynthetic yield is known as Emerson effect.

Wavelength (nm)

The quantum yield of photosynthesis drops abruptly when exitation wavelength is greater than 680 nm

Photo systems:

The result of Emerson’s experiment shows that there must be two reactions: one exclusively works at more than 680 nm, whereas the other works at a lower wavelength. Such reactions are mediated through two distinct groups of pigments known as pigment system I and pigment system II. The pigment systems carrying definite reactions are called photosystem I (PS I) and photosystem II (PS II). These systems are demonstrated to be present separately in the thylakoid membrane.

(a) Photosystem I (PS I)

PS I is a lighter and smaller particle. These particles are present both in stroma and grana thylakoid lamellae. PS I particle has more amount of chlorophyll a in comparison to chlorophyll and other accessory pigments. It contains a special chlorophyll a molecule having absorption maxima at 700 nm known as P₇₀₀. It contains ferredoxin. PS I has the capability to reduce the coenzyme NADP⁺ to NADPH+

(b) Photosystem II (PS II)

PS II is large and heavier than PS I. It is rich with chlorophyll. These particles are predominantly present in grana lamellae. It contains a special type of chlorophyll a molecule having the asborption maxima at 680 nm. Such special moelcule is designated as P₆₈₀. PS II contains plastoquinone, cytochromes and plastocyanin. The system is responsible for photolysis of water. ATP is generated in this system by a process of phosphorylation. Such a process of phosphorylation using light energy is called photophosphorylation.

Mechanism of conversion of light energy:

PS I and PS II participate in series to convert light energy to chemical energy. Both the systems trap light energy and carry out their respective photochemical reactions. The smallest group of pigment molecules with their accessories, which carries out a photochemical reaction or a photochemical act, is known as photosynthetic unit.

The pigments in PS II absorb light and funnel it to the P_6g0, which in turn is oxidised. It comes back to its original state by receiving the electron producted by photolysis of water.

Chlorophyll

The quinone like substances present in PS II receives the electron that comes out from P₆₈₀. The electron then moves down through plastoquinone, cytochromes and plastocyanin

Simultaneously the pigments of PS I also receive the light energy and funnel it to P_?(X). The P₇₀₀ becomes excited and oxidised lossing an electron. The oxidised P₇₀₀ comes back to its ground state by receiving the electron from plastocyanin of PS II. Ferredoxin an iron containing protein receives the electron that comes out from P₇₀₀. The reduced ferredoxin reduces NADP to NADPH and H⁺ in the presence of H⁺ produced by photolysis of water.

The working of both the PS I and PS II along with the site of production of ATP and reduced coenzyme can be represented in a scheme known as ‘Z’-scheme, proposed by Hill and Bendal (1960).

Photophosphorylation:

The major event in the conversion of light energy is the production of ATP by photophosphorylation. Depending on the involvement of either two or one photosystem and the sites of electron flow to induce photophosphorylation, there are two types of photophosphonylation: viz; noncyclic and cyclic.

(a) Noncyclic photophosphorylation:

Both the photosystems (PS II and PS I) are involved as mentioned in the Z-scheme. The electron that comes out from P_6g0-chlorophyll a molecules of PS II never goes back to P_6g0. It moves through electron carrier molecules of PS II and ultimately passes to oxidised P₇₀₀ of PS I. The electron from PS I is accepted by ferredoxin which then reduces NADP⁺ to produce reduced coenzyme, NADPH and H⁺. ATP is produced at the site of plastoquinone to cytochrome.

(b) Cyclic Photophosphorylation

In cyclic photophosphorylation, only the PS I is involved. The pigments of PS I trapping the light energy funnel it to P₇₀₀, which gets excited. The excited P₇₀₀ loses electron and gets oxidized.

The electron being received by ferredoxin moves to plastoquinone instead of reducing NADP. From plastoquinone the electron moves down through different electron carriers and finally reaches the oxidised P_7(X). Thus some electrons coming out from P₇₀₀ go back to oxidised P_70Q.

Thus the movement of electron is cyclic and during the movement, there is production of ATP at two sites, one at ferredoxin to plastoquinone and other at plastoquinone to cytochrome. So this process of photophosphorylation is cyclic.

In light reaction i.e. during the conversion of light energy, two important energy rich molecules are produced: ATP and NADPH.

Carboxylative Phase:

It this phase, C0₂ is accepted by a five-carbon acceptor molecule known as ribulose diphosphate (RuDP) which is also known as ribulose bisphosphate (RuBP). Ribulose bisphosphate carboxylase oxygenase (Rubisco) is the key enzyme for the carboxylation of RuBP. The 6-carbon sugar, produced by caroboxylation, is immediately hydrolysed into two stable three-carbon sugars known as phosphoglyceric acid (PGA).

Regenerative Phase:

A series of reactions are carried out in this phase to generate RuBP, the CO, acceptor molecule. During this phase one molecule of PGAL is converted to one molecule of dihydroxyacetone phosphate (DHAP) by the enzyme triose phosphate isomerase. DHAP condenses with PGAL to produce fructose 1,6 diplosphate, the sugar product of photosynthesis. The sequence of reactions of Calvin cycle is summarised in.

C₄Pathway:

C0₂ assimilation, as elucidated by Calvin shows that the 3-carbon product, phosphoglyceric acid (PGA) is the first stable product in the process of carboxylation. But certain tropical plants carry out the CO, assimilation by producing the first stable product as a four- carbon dicarboxylic acid, identified as oxaloacetate (OAA). The pathway of C0₂ assimilation in these tropical plants is known as C₄-pathway. M.D. Hatch and C.R. Slack (1967) elucidated the detailed reactions carried out in C₄ pathway C₄ pathway is also known as Hatch-Slack pathway.

The plants, which exhibit the C₄ pathway, are maize, sugarcane, sorghum, Amarcmthus, Portulaca etc. These plants are known as C₄ plants. These plants show peculiarities in leaf anatony. A concentric ring of mesophyll tissue is present around the vascular bundle.

The bundle sheath cells are large with chloroplasts, which may or may not have grana. But the chloroplasts of mesophyll cells have well developed grana.

The arrangement of tissue around the vascular bundle gives a particular pattern of anatomy of leaf, known as Kranz anatomy. Such an anatomy has a physiological significance.

In C₄ plants both the Calvin cycle and Hatch-Slack cycles are functional for CO, fixatio Mesophyll cells are with all requisites to carryout C₄ cycle, whereas bundle sheath cells carryo C₃ cycle.

Mechanism:

In mesophyll cells, the CO, acceptor molecule is phosphoenol pyruvate (PEP). In the presence of phosphoenol pyruvate carboxylase, the PEP reacts with CO, and produc oxaloacetate. Oxaloacetate is converted to malate in the mesophyll cells. Malate enters the bundlesheath cells, where it gets decarboxylated to produce pyruvate and CO,. The reaction mediated by the enzyme NADP malic enzyme. The released CO, is trapped by the Calvi cycle working in the bundle sheath cells.

The efficiency of CO, assimilation in C₄ plants is more because the C₄ cycle is capable fixing CO, efficiently even at a lower concentration. Moreover, at low concentration of C0₂ a high concentration of 0″.Rubisco facilitates photorespiration for which there is less productioi of photosynthate.

C₃ and C₄ Pathways:

C0₂ assimilation occurs through C, and C₄ pathways in which the first stable product is 3-carbon sugar or 4-carbon dicarboxylic acid, respectively. C₄ cycle is mostly present in temperate and tropical plants. C₄ cycle is operated in certain tropical plants. C₃ pathway was elucidated by Calvin whereas C₄ pathway was elucidated by Hatch and Slack. Table 6.1 presents the major differences between C₃ and C₄ plants showing C₃ and C₄ pathways, respectively.

1. All the green cells exhibit the 1. Mesophyll cells exhibit C₄ pathway, where of C₃ pathway. As bundle §heath cells, C₃ pathway.

2. There is only one CO, acceptor 2. There are two CO, acceptors, the RuBP molecule, the RuBP. And PEP.

3. First stable product is phosphoglyceric 3. First stable product is oxaloacetate (OAA) acid (PGA).

4. Leaves show normal anatomy. 4. Leaves show a special type anatomy known as Kranz anatomy.

5. There is only one carboxylating enzyme, 5. There are two carboxylating enzymes, PEP Rubisco, which carries out carboxylation. Carboxylase and Rubisco

6. Oxygen has an inhibitory effect on 6. Oxygen does not have an inhibitory CO, assimilation. Effect on CO, assimilation

7. Photorespiration is present 7. Photorespiration is absent.

Magnitude and efficiency of photosynthesis:

Photosynthesis provides food to all organisms besides, other requirements such as fire wood, timber, fibres, medicines etc. are also met by green plants, which grow autotrophic ally carrying out the photosynthetic process. Moreover, photosynthesis is a natural process, which produces oxygen for other organisms. About 170 million tonnes of dry matter is produced annually by this process. The process runs basically through light energy that comes from the sum and C0₂ and H₂0 from the environment.

The atmosphere contains only about 0.3% C0₂ by volume. But this amount is enoromou: even to carryout the process of photosynthesis for many hundred years, even if there is nil addition of CO, to the atmosphere. However, there is always an addition of C0₂ to out environment from different sources, which makes the photosynthetic C0₂ reservoir never ending.

Enoromous amount of light energy from the sun is available on the earth surface. Only

Of the available light energy is used by green plants to carry out photosynthesis and this is sufficient to meet the requirement of heterotrophs.

Studies on photosynthesis is carried out to enhance its efficiency to use light energy and| to fix CO,. Photosynthesis is regulated by the informations stored both in nuclear and chloroplast DNA. If manipulation can be done at the DNA level, there is every possibility to enhance the efficiency of photosynthesis.