C₄ cycle (Hatch & Slack Pathway)

Kortschak and Hartt first observed that 4C, OM (Oxaloacetic Acid) is formed during dark reaction in sugarcane leaves.

Hatch & Slack (1967) studied in detail and proposed pathway for dark reactions in sugarcane & maize leaves.

First stable product of this reaction is OAA., which is 4C, DCA (Dicarboxylic Acid), thus Hatch & Slack pathway is called as C₄ cycle or DCA 

C₄-cycle occurs in 1500 sps. of 19 families of angiosperm, but most of the plants are monocots, which belong to Graminae & Cyperaceae (Sugarcane, Maize, Sorghum, Oat, Chloris, Sedges, Bajra, Panicum, Alloteropsis etc.)

Atriplex hastata & A. patula are temperate sps, which are Crplants.

Dicots with C₄-cycIe are Euphorbia sps, Amaranthus, Chenopodium, Boerhavia, Atriplex rosea, Portulaca, Tribulus.        

Wheat and barley (monocot) are C₃ species rice sp. developed as C₄ plants by plant breeding scientists. 

Kranz (Wreath) anatomy - Present in leaves ofC₄ plants.

(i) Green bundle sheath cells (BS cells) present around the vascular bundles.

(ii) Dimorphic chloroplasts present in leaf cells. Chloroplast of B.S. cells or Kranz cells are larger and without grana, Mesophyll chloroplast are small and with grana. 

In the C₄-Plant, Crcycle occurs in bundle sheath cells, while C₄-cycle occurs in mesophylls.

Operation of Hatch and Slack pathway require cooperation of both photosynthetic cell i.e. mesophyll cells and BS cells.

Photosynthetically C₄ plants are more efficient as there is no Warburg effect or photorespiration in C₄-plants, because at the site of Rubis co (BS cells) less 02 is presents (mesophyll cells pumps more CO₂ for C₃ cycle).

C₄-plants are found in tropical habitats and have adapted themselves with high temperature, low water availability and intense light.

If concentration of O₂ increases artificially, then photorespiration may be started in C₄ plants.

ISt carboxylation in C₄-cycle occurs by PEP Case in mesophyll chloroplast, while second carboxylation or final CO₂ fixation by C₃ cycle occurs in bundle sheath cells.

CO₂ acceptor in C₄ mesophyll is PEP (Phosphoenol Pyruvate). (3C-compound), while RuBp in bundle sheath cells.

12 NADPH₂ (= 36 ATP) + 30 ATP = 66 ATP needed for production of 1 hexose (Glucose) in C₄-plants.

Pyruvate phosphate dikinase (PPDK) (ATP ® AMP) is a temperature sensitive enzyme of C, and CAM plants due to this C₄ plants better photosynthesizes at high tem

Special Features of C₄ Plants

1. C₄ plants are more efficient plants at present CO₂ concentration.

2. Present level of atmospheric CO₂ is generally not limiting factor for C₄ plants.

3. C₄ plants posses low CO₂ compensation points. (8-10 ppm)

4. The productivity (fertility) does not increase in C₄ plants, when CO₂ concentration increases because-

(a) Mesophyll cells provide more CO₂ for Calvin cycle.

(b) The concentration of CO₂ around the site of Rubisco is iltgber in C₄ plants, thus little or no chance of photorespiration. 

CAM-Plants

Cleary and Rouhani discovered CAM-process in members of   Crassulaceae family. Succulent xerophytie plants.

e.g. Kalanchoe, Crassula, Sedum, Klienia, Opuntia, Aloe, Agave, Euphorbia sps., Bryophyllums, Pineapple, Welwitschia (Gymnosperm) etc.

Primary acceptor of Ca₂ is PEP (Phosphoenol pyruvate) and oxaloacetic acid is the first product of Carboxylaiton reaction.

In CAM plants stomata are of scotoactive type, so initial CO₂ fixation is found in night but light reactions operates at day time. Final CO₂ fixation (C₃ cycle) occurs in day time.

PEPcase induces carboxylation reaction in night

PEP carboxylase & Rubisco are present in mesopbyU cells.

(No Kranz-anatomy)

In CAM plants 30 ATP and 12 NADPH₂ are required as assimilatory power for 1 glucose synthesis. 

• C₂Cycle

First of all Krotkov indicated that more CO₂ evolves during day time in 'C₃ plants.

Decker & Tio discovered photorespiration and clarified that C₂-cycle or glycolate pathway operates during day time in. Crplants & Rubisco acts as oxygenase at higher concentration of O₂ and low CO₂ concentration in the C₃ - green cells.

The light dependent uptake of 02 & release of CO₂ in C₃ photosynthetic cell is called photo-respiration.

Photorespiration is not linked with ATP generation (in place ATP are consumed) as ordinary dark respiration, thus it is harmful or wasteful process linked with C₃ cycle.

It occurs in chloroplast, peroxisomes & mitochondria (three' cell organelle reaction)

During photo respiration, 75 percent of the carbon lost by the oxygenation of RuBP is recovered, because two molecules of glycine (2C + 2C = 4C) form one molecule of serine (3C). During this one carbon releases in form of CO₂ in mitochondria thus 25 percent carbon is lost.

This serine molecule changes into PGA via different reactions of C₂ cycle.

H₂O₂ (Peroxisome) and NH3 (Mitochondria) are produced in photorespiration.

Glycine (Peroxisome) and serine (mitochondria) are also formed in photorespiration

It is assumed that in C₃ plants, if photorespiration does not occur, it will increases O₂ cone which oxidises (Photooxidation or Solarization) the different protoplasmic parts of photosynthetic cell at high light intensity. 

Factors Affecting Photosynthasis

1. Light:

(a) Light Quality or wavelength: Maximum photosynthesis takes place in red light than in blue light. But rate of photosynthesis is highest in white light. Minimum in green light

(b) Light Intensity: Rate of photosynthesis is greater in" intense light than diffused light- But at higher light intensity photooxidation (solarization) occurs and photosynthetic apparatus may get destroyed.

P/R (Photosynthesis: Respiration) Ratio at mid "day is 10: 1; but can reach upto 20: 1 ratio. At the time of evening & morning rate of photosynthesis equals to respiration, this situation is called as light compensation point.Intensity of light, at which rate of photosynthesis, becomes equal (or compensate) to the rate of respiration in plants (Dark respiration + photorespiration) is known as light compensation point (Net photosynthesis or net primary productivity at this point is zero.)

Plants which are adapted to grow in high intensity of light is called heliophytes & plants which are adapted to grow in shade is sciophytes.

(c) Duration of Light: On the basis of effect of light on plants may be LOP & SOP.

Rate of photosynthesis is greater in intermittent light than continuous light - Warburg.

2. Temperature: Optimum temp. for photosynthesis is 20-35°C. At high temp rate of photosynthesis decreases due to denaturation of enzymes. Conifers & lichens' can perform photosynthesis at -20°C to - 35°C, while thermal algae Oscillatoria at 70-80°C. Generally different habitat plants show different response to photosynthesis on a given temperature.

3. CO₂: An increase in CO₂ cone. upto 1 % rate of photosynthesis is increased. Higher CO₂ concentration is toxic to plant & also closes stomata.

C₄ -Plants can photosynthesize at low CO₂ cone. (upto 10 ppm). "CO₂ cone." at which CO₂ fixation in photosynthesis is equal to volume" of CO₂ released in respiration is "CO₂ compensation point", when plant saturated with full light.

CO₂ compensation point for C₄ plants is 8-10 ppm, while for C₃ plants it is 40- 100 ppm.

4.O₂: High O₂ conc. reduces photosynthesis due to photorespiration.

5. Water: Less availability of water reduces the rate of photosynthesis (stomata get closed)

6. Chlorophyll: The amount of CO₂ in grams absorbed by1 gm. of chlorophyll in 1 hour is called as photosynthetic number or assimilatory number (Willstatter & Stoll).

7. Product: Rate of photosynthesis decreases, when sugar accumulates in mesophyll cells.

8. Leaf: Various leaf factors like leaf age and leaf orientation effect the rate of photosynthesis.

In young & nature leaves photosynthesis is more than old (senescent) leaves.

9. Inhibitors: DCMU (Diuron/Dichlorophenyl Dimethyl Urea) CMU (Monuron), PAN, Atrazine, Simazime, Bromocil. Isocil-­inhibit the photosynthesis by blocking PS-II. They stop e^- flow between P-680 & PQ.In cyclic ETS diquat, paraquat (Viologen dyes) inhibit e^- flow between P-700 & Fd.All these chemicals are used as herbicides, which mostly block ETS.

10. Minerals: Mg and Nitrogen are essential for structure of chlorophyll and enzymes. Thus reduction in N₂ and Mg supply to plants effects adversely the rate of photosynthesis.Rubisco alone accounts for more than half of total leaf nitrogen. Generally all essential element affect the rate of photosynthesis.

Concept of three cardinal points (Von Sachs): The effect of the various external factors on the rate of biological processes were centred around the attempts to establish minimum, optimum and maximum values known as cardinal points.

Law of minimum (Liebig): According to it, when a process is governed by a number of separate factors, then the rate of process is controlled by that factor present in minimum amount.

Law of limiting factors: (Blackman) - It is the modification of Law of minimum by Liebig. "When a process is conditioned to its rapidity by a number off actors, then rate of process is limited by the pace of the slowest factor" (CO₂ light, chlorophyll, water, temp.)CO₂ becoming limiting in clear sky, but light limiting in cloudy days.Atmospheric CO₂ is not limiting factor for C₄ plants & submerged hydrophytes.

Bacterial Photosynthesis

Certain bacteria are capable for photosynthesis.

Eg: Chlorobium (Green Sulphur), Chromatium (Purple Sulphur), Rhodospirilium, Rhodopseudomonas (Purple non sulphur).

Cyclic-photophosphorylation is an important method in bacterial photosynthesis.

Absorption of infra red spectrum takes place during bacterial photosynthesis thus no red drop.

Pigment system of bacteria is denoted by - B-890 or 870

Evolution of O₂ is not related to bacterial photosynthesis, because water is not e^- donor (H₂S)

Only one ATP is produced in each turn of cyclic photophosphorylation, in bacteria.

Olson 1970 gave a non cyclic scheme in bacterial photosynthesis.      

6CO₂ +12H₂S  C₆H₁₂O₆ + 6H₂O + 12S

Bacteria has only one pigment system, PS I (PS II is absent).

Chemosynthesis

Some forms of bacteria obtain energy by chemosynthesis. This process of carbohydrate formation in which organisms use chemical reactions to obtain energy from inorganic compounds is called chemosynthesis. Such chemoautotrophic bacteria do not require -light and synthesizes all organic cell requirements from CO₂ and H₂O and salts at the expense of oxidation of inorganic substances like (H₂, NO₃^-, SO₄^2- or carbonate). Some examples of chemosynthesis are:

Nitrifying bacteria:

e.g., Nitrosomonas, Nitrosococcus, Nitrobacter etc.

Sulphur bacteria:

e.g., Beggiatoa, Thiothrix and Thiobacillus.

Iron bacteria:

e.g., Ferrobacillus, Leptothrix and Cladothrix.

Hydrogen bacteria:

e.g., Bacillus pentotrophus

Carbon bacteria:

e.g., Carboxydomonas, Bacillus oligocarbophilus.

NOTE:

Function of accessory pigment carotene is.:

- Converts elementary or nascent oxygen to molecular/ gaseous O₂.

O + O (elementary oxygen) + Carotene ® Epoxide complex

-       Protects photooxidation (photodamaging) of pigment system.

-       Precursor of vit.-A.

-       Oxidation to form ABA hormone in guard cells.

Chlorophyll pigment soluble in organic solvents like acetone, ether etc. (anthocyanin is non photosynthetic water soluble pigment, which present in vacuole).

Chloroplast in bundle sheath of Bermuda grass is also granal type.

Photolysis of water occurs at + 0.8 E°

In cyanobacteria (BGA), photosynthesis occurs on chlorosomes or lamellisome or carboxysome.

PS-I is strong reductant as PS-I has good ability to reduce NADP⁺ while PS-II is a strong oxidant because it has extreme power of oxidation & photolysis of water molecule.

Wilmott's bubbler apparatus proves that oxygen is evolved during photosynthesis.

Cytochromes are Iron - porphyrin protein discovered by MacMunn (termed by Keilin).

Pigments except chlorophyll, presents in Quantasomes are called as accessory or antenna pigment of light harvesting complex (LHC).

Electroosmotic theory - By Spanner and Jones for translocation of sugars.

Chollet and Ogren (1975) - Recognised 3 categories of C, plants.

(i) Maize and Sugarcane type: In this category malate is transported to bundle sheath cells and its decarboxylation gives CO₂ for C₃ cycle.

(ii) Panicum and Chloris type: In this category malate is transported into bundle sheath cells, but this changes into oxaloacetate, which gives CO₂ for C₃ cycle.

(iii) Atriplex type: In this category the aspartate is transported into bundle sheath cells, where it changes into malate, which provides CO₂ for C₃ cycle.

Significance of Photosynthesis

Photosynthesis is vital process for life on planet earth as it is the only process, that links the physical and biological world by conversation of solar energy into organic matter, which make bulk of the dry matter of any organism.

Presence of O₂ in the atmosphere is also an outcome of photosynthesis. This oxygen is helpful to living organisms. in two ways:

(i) Oxidative break down of organic food matter (respiration,

(ii) Making ozone (O₃), in outer layer of atmosphere which helps in stopping the highly destructive U.V. rays.

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