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Essay on Plant Metabolism (2622 Words)

Estimates reveal that 90% world’s photosynthesis is done by marine water and fresh algae.

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1. Light Reaction (Grana)

H2O ATP + NADPH2 + O2 ^

l.a > Non cyclic Phosphorylation

l. b > Cyclic Phosphorylation

1. ñ > Pseudocyclic Phosphorylation

2. Dark Reaction (Stroma)

ATP + NADH2 + CO2 > C6H12 O6

2. a > C3 Cycle

2. b > C4 Cycle

2. ñ > CAM

Photosynthetic Apparatus:

The chloroplasts in green plants constitute the photosynthetic apparatus. The chloroplast is bounded by two membranes consisting of lipid bilayer and proteins. Internally the chloroplast is filled with a hydrophilic matrix called as stroma in which are embedded grana. Each granum consists of disk shaped grana lamella. In cross section these lamella are paired to form sac like structures and have been called as thylakoids.

Chlorophylls and other photosynthetic pigments are found in the form of protein pigment complexes mainly in thylakoid membranes of grana. The grana are sites of primary photochemical reaction. Dark reaction of photosynthesis occurs in stroma.

Photosynthetic Pigments:

Photosynthetic Pigments are of three types:

1. Chlorophylls 2. Carotenoids 3. Phycobillins.

Different pigments absorb light of different wavelengths.

(Refer Fig. 1 above) chlorophyll mostly occurus the grana lambella. Seven types chlorophyll are known.

Red Drop and Emersons Effect:

Proposed by Robert Emerson, (1943) while working on quantum yield of Photosynthesis in monochromatic light of different wavelengths in chlorella noticed a sharp decrease in quantum yield at wavelengths greater than 680 nm. Because this decrease in the quantum yield took place in red part of the spectrum and this phenomenon is called red drop.

Emerson and Lewis later found that, inefficient far-red light in chlorella, beyond 680 nm could be made fully efficient if supplemented with light of shorter wavelengths. The quantum yield from the two combine beams of light was found to be greater than the sum effect of both beams used separately. This enhancement of photo synthesis called as emersons enhancement effect.

Two Pigment Systems:

Photosynthesis is driven by two pigment systems called pigment system I (PSI) and PS1I. Wavelength of light shorter than 650 nm affect both the pigment systems and greater than 680 nm affect only PSI (700 nm).

(I) PSI Relatively Weak Fluorescent:

Consists of chlorophylls, carotenoids, cytochrome 7, molecule of P700, Plastocyanin, cyt b, FRS (Ferredoxin reducing substance) and rich in chlorophyll-a, iron and copper.

i. Controls, the process of producing a strong reductant to reduce NADP to NADPH + H+

ii. Mainly found in stroma Lamellae

(II) PSII – Strongly Fluorescent:

i. In addition to chlorophylls, carotenoids, moecule of P 680, Plastoquinone, four mn molecules bound to one or more proteins. Cyt b, cyt b6, and chloride.

ii. Concerned with generation of strong oxidant and weak reductant coupled with the release of oxygen.

Production of Assimilatony Powers in Photo Synthesis:

ATP and NADPH are the assimilatory Powers. The process of NADP in to NADPH + H+ may be denoted as ETS in Photosynthesis or Reduction of NADP, while the process of formation ATP from ADP and inorganic phosphate using light energy called photo phosphorylation.

Light Reaction:

The light reaction and its concerned activities are found in thylakoids and grana.

6 CO2 – C6HI2O6 + 6O2

The light reaction phase is a complicated process with several important events.

Events in Light Reaction —

(i) Absorbtion of light energy by chloroplast Pigments:

Different chloroplast pigments absorb light in different region on the visible part of the spectrum.

(ii) Transfer of light energy from Accessory Pigments to Chlrophyll-a:

All the Photosynthetic Pigments except Chlorophyll-a, are called accessory pigments. Light energy absorbed by them is transferred by resonance to Chl-a which alone take part in the primary photo chemical reaction. Chl-a alone can absorb the light energy directly. In PS1I, reaction centre is P680 and in PSI its P700.

(iii) Activation of Chl-a molecule by Photons of Light:

When P680 or P700 forms chlorophyll – a molecule in two pigment systems receive a photon of light, it becomes an excited molecule having more energy than ground state energy and finally expels it energy along with an e+ and a+ charge comes on the Chl-a which now becomes oxidised.

Chl-a > Light Excited triplet state of chl-a > (Chl-a)+ + e–

(iv) Photolysis of water and O2 Evolution (Oxidation of water):

Associated with PSII and catalysed by presence of Mn++ and Cl– ions. PSII receives light and water molecules split into OH– and H+ ions (Photolysis of water). OH” ions unite and form some more water molecules again and release O2 and electrons.

2H2O > 4H++O2+4e–

(v) Transfer e– from PS II to PS I, (Pc):

Plasto cyanin is blue, cu containing protein art as Mobile carrier.

(vi) Electron transport and the production of Assimilatory power (NADPH + H+ + ATP):

When the expelled e– travelling through number of electron carriers either cycled back or is consumed in reducing NADP+ to NADPH + H+. The extra light energy liberated along with the e– is utilised in the formation of ATP molecules at certain places during its transport, the process is called photo­phosphorylation.

There are 3 types of photophosphorylation, viz.,

In non cyclic photo phosphorylation, the flow of electron is unidirectional, that is, electron denoted by PS11 after passing through plasto quinones, Cyt b6, Cyt F, plastocyanin and PSI eventually reaches ferredoxin which in turn denotes to reduce NADP. The reduced NADP is utilised for the reduction of CO2. The electron does not complete the cycle. It starts from PSII reduction. So the ATP synthesis resulting from this type of non cyclic electron transport chain is known as non cyclic photophosphorylation. Water molecule is utilised as a source of electron in this system. In this process, two molecules of ATP are formed per two molecules of NADP reduced.

During light reaction, the protons accumulate inside the thylakoid membrane resulting in a proton Gradient. The energy released by the protons when they diffuse across the thylakoid membrane into the stroma is used to produce ATP.

2. Cyclic PhotoPhosphorylation:

Here ATP formed from PSI and wavelength of light greater than 680 nm is used. Here initially energy is provided from absorbed light Quantum instead of glucose. Electrons used here do not come from water. In the cyclic photophosphorylation light lifts the electron from P700 or feredoxin. The excited electron returns to P700 through two to three transfer steps to decreasing redox potentials. It is during such a downhill migration of the electron that enough energy is released for ATP Synthesis. In this process ATP can be formed between cytochrome b6 and cytochrome and between ferredoxin and cytochrome b6.

3. Pseudocyclic Photophosphorylation:

Arnon and ñî workers demostrated, even in the absence of CO2 and NADP, if chlorophyll molecules are illuminated it can produce ATP from ADP and Pi in presence of FMN and oxygen. The process otherwise is called oxygen dependent FMN catalylsed photophosphroylation, which involves reduction of FMN with the production of oxygen.

FMN is reoxidisable Hydrogen acceptor with the effect that the reduced FMN is be oxidised by oxygen. Thus the process can continue repeatedly to produce ATP.

Here the source of both hydrogen for the reduction of FMN and of oxygen for its oxidation in the water.

Dark Reaction:

It occurs in Stroma —

The dark reaction process of photosynthesis has been named variously such as calvin cycle, Blackman reaction, carbon assimilation, Reductive pentose phosphate cycle etc.

It consists of —

(a) Synthesis of carbohydrate

(b) Regeneration of ribulose diphosphate

It can otherwise be called as Reductive Pentose Phosphate Pathway. (RPP) the cycle includes 3 phases.

1. Carboxylation of the ribulose 1.5- bisposphate forming 3-phospho glycerate.

2. Redution of 3-phosphoglycerate forming Glyceraldehyde-3-P04.

3. Regeneration of ribulose 15-bisphosphate from Glyceraldehye 3-ÐÎ4.

For the fixation of 6CO2 molecules into one hexose sugar molecule through calvin cycle, 12

(NADPH + H+) and 18 ATPs are required,

6 CO2 + 12 (NAPH + H+) + 18 ATP > F-6-P + 12 NADPH+B + 18 ADP+ 17Pi + 11 H20

C4 – Dicarboxylic Acid Pathway Hatch and Slack Pathway:

In this pathway, C4-dicarboxylic acids are the earliest products and so its called C4–path way, which is another carbon reduction pathway demonstrated by Hatch and Slack.

Characteristics of C4 Cycle:

i. The carbon fixation reactions are spatially seperated like mesophyll and bundle sheath cells.

ii. The cycle is less efficient in itself when compared to C3 mode. Its because the fixation of 1 CO2 mol in C3 mode requires 2 NADPH + 3 ATP, while in C4 mode, 2 NADPH + 5 ATP are required.

iii. But, C4 plants are photosynthetically efficient than C3 plants because of the absence of Photorespiration in C4 plants.

Significance of Hatch and Slack Pathway:

(i) It’s a modification of calvin cycle and is advantageous to plants growing in dense, stands of tropical vegetation where CO2 concentration much reduced.

(ii) There has been a reduction of atmospheric CO2 concentration since the evolution of photosynthesis which prompted C4 plants to select this pathway.

(iii) Discovery of C4 cycle, indicated the existence of yet undiscovered reaction other than the conventional calvin cycle.

Table. 4. Difference between C3 and C4 plants

C3 Plants:

1. Wheat, oats, Barely, rice, cotton, Beans, spinach, sunflower etc.

2. ‘C’ Pathway in photosynthesis is only C3 pathway (i.e.) calvin cycle only.

3. Phosphoglyceric acid [PGA] is the first stable product.

4. RUBP-bisphosphate Primary CO2 acceptor.

5. Leaves have diffused mesophyll and only one type of chloroplasts.

6. In each chloroplast two pigment systems are present.

7. Calvin cycle occurs in mesophyll chloroplast.

8. CO2 compensation point 50-150 PPm CO2.

9. Photorespiration present and easily detectable.

10. The efficiency of CO2 absorption at low concentration is far less. So they are less efficient.

11. Only C3 cycle is found.

12. Optimum temperature 10-25°C

13. 18 ATP for one glucose molecule

C4 plants:

1. Sugarcane, maize, sorghum and amarnathus.

2. C4 dicarboxylic acid pathway (hatch and slack pathway).

3. OAA (oxalo acetic acid) – I stable product.

4. PEP is the Primary CO2 acceptor.

5. Leaves have kranz anatomy with compact mesophyll around vascular bundles (Bundle sheath) and dimorphic chloroplasts present.

6. In bundle sheath cells chloroplasts, PSII absent. So, dependent on mesophyll chloroplasts for the supply of NADPH + H+.

7. Calvin cycle occurs in bundle sheath cells chloroplasts.

8. 0-10 PPm CO2.

9. Photorespiration is present only to a slight degree or absent.

10. Efficiency is higher and are more efficient.

11. Both C3 and C4 cycles found.

12. Optimum temperature is 30-45°C.

13. 30 ATP for one Glucose molecule.

Crassulacean Acid Metabolism (CAM Cycle):

Here the CO2 is concentrated at the site of Rubisco, which is found in crassulaceae, Euphorbiaceae, Orchidaceae and agavaceae. This mechanism enables plants to improve water use efficiency and so, these plants have a competitive advantage in dry environment.

Unlike C4 plants, the formation of C4 acids, is both temporally and spatially seperated.

In CAM plants, stomata open at night and closed during most of the day The distinctive diurnal fluctuation in acidity in plants showing CAM due to changes in amounts of vascular malic acid in mesophyll cells.

During night, malic acid synthesised using CO2 which accumulates in the vacuole. During day this malic acid consumed resulting in decrease of acidity. Citric and isocitric acid also contribute to acidity, but their amount is negligible and these do not show consistent diurnal patterns of fluctuation as shown by malic acid.

Synthesis of Malate during night or Dark CO2 Fixation:

Large amounts of starch are consumed during acidification which indicates that carbohydrates are the source of malate synthesis

C6H12O6 + 2 CO2 > 2 C4H6O5 (malate)

Malate is synthesized during night in reaction in which some malate product derived from carbohydrate reserves e.g. pyruvate or PEP is carboxylated to produce malate either directly or first forming oxalo acetic acid which is then reduced to malate.

Consumption of malate in Light Deacidification:

During the day, when acidified organs exposed to light, rapid consumption of malate occurs resulting in deacidification (due to releases Co2 from malate). The malate may be decarboxylated in two ways.

(i) Malate + NADP + – NADP- malic enzyme- Pyruvate – + CO2 + NADPH + H+

(ii) (a) Malate + NDA+ – Malate dehydrogenese – Oxaloacetate + NADH + H+

(b) Oxalo acetate + ATP – PEP-carboxy / Kinase – PEP

Pyruvate and PEP utilised for carbohydrates Synthesis during the day. Pyruvate first converted to PEP in the presence of Pyruvate Po4 dikinase.

Pyruvate + ATP + Pi >PEP + Amp + PPi

PEP in both types of CAM plants is converted into 3 PGA by release reactions of glycolysis.Then, 3-PGA is utilised in calvin cycle.

Photo Respiration:

Otherwise known as photo respiratory carbon oxidation cycle (PCO). Rubisco, the most abundant protein in the world is having a distinct property, that it can catalyse both the carboxylation and oxygenation of RuBp. oxygenation is the primary reaction in a process known as photorespiration.

It’s the process, in which, Co, is lost from cells that are simultaneously, fixing Co2 by the calvin cycle. The cycle acts as a scavenger operation to recover the fixed Co2 lost by the oxygenase reaction of rubisco. Three organelles are involved in the process viz., chloroplast peroxisomes and mitochondria.

i. Photorespiration only in chlorophyllous tissues of plants. Glycolate is the chief metabolite and also its substrate. Other important metabolites are the amino acids glycine and serine.

ii. Like usual mitochondrial respiration, its also an oxidative process where oxidation of glycolate occurs with subsequent

iii. Glycolate cycle/C2-cycle.

Factors Affecting:

1. C02 compensation point

2. C02 concentration when high, photo respiration is also high and vice versa.

3. Inhibitors of Glycolic acid oxidase such as a-hydroxy-sulphonates inhibit the process of photorespiration.

Significance:

1. It’s a double wasteful process

2. Resulting in considerable decrease in photosynthetic productivity.

3. Unlike usual mitochondrial respiration, neither reduced co-enzymes are generated nor the oxdiation of glycolate coupled with the formation of ATP molecules.

4. Moreover, there is consumption of reduced coenzyme and ATP.

5. However, the knowledge on photo respiration is important. Therefore by manipulating different atmospheric conditions, use of inhibitiors of glycolic acid oxidase such as a-hydroxy sulphonates and through genetic control, the process can be regulated and photosynthetic productivity can be increased.

Factors Affecting Photosynthesis:

External Factors —

1. Light:

It affects the rate of phososynthesis in three ways:

(a) Light Quality:

Due to heavy absorption of light rays in the red part of the spectrum of chlorophylls, maximum photosynthesis takes place in red light.

(b) Light intensity:

The rate of photosynthesis is greater in intense light than in the diffused light.

(c) Duration of light:

The rate of photosynthesis is greater in intermittent light than in continuous light.

2. Carbon dioxide:

An increase in CO2 concentration upto about 1% increases the rate of photo synthesis.

3. Temperature:

Photosynthesis will stop in some plants at about freezing point and in some plants beyond 40-50°C.

4. Water:

The rate of photosynthesis may go down if the plants are inadequately supplied with water.

Internal Factors:

1. Chlorophyll content:

The rate of photosynthesis should increase with an increase in the chlorophyll content.

2. Protoplasmic factors:

Proper hydration of the protoplasm is essential for protosynthesis.

3. Accumulation of the end products of photosynthesis:

Accumulation of carbohydrates in the photosynthesizing cells retards the rate of photosynthesis.

4. Anatomy of leaf:

The thickness of the cuticle and epidermis, structure and distribution of stomata, distribution and relative proportion of chlorophyllous and non chlorophyllous mesophyll tissue all influence the rate of photosynthesis.

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