PHOTOSYNTHESIS(Part-3) - Botany,Class - 11 - Class 11 PDF Download

C4 plants pathway and Hatch & Slack Pathway - photosynthesis in higher plants, Biology, Class 11

DIVERSITY IN DARK REACTIONS

CO₂ concentrating mechanism/Co-operative photosynthesis/Dicarboxylic acid cycle (DCA cycle) /C₄ cycle/Hatch & Slack Pathway

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

• Hatch & Slack Australia (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 cycle.

• 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.) Rice sps.

• Atriplex hastata & A. patula are temperate sps,. which are C₃–plants.

• Dicots with C₄–cycle are Euphorbia sps., Amaranthus, Chenopodium, Boerhavia, Atriplex rosea, Portulaca, Tribulus.

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

• Kranz (Wreath) anatomy – Present in leaves of C₄ 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.

• Rubisco present in BS cells, while PEPCase in mesophyll cells.

•  In the C₄–Plant, C₃–cycle occurs in bundle sheath cells, while C₄–cycle occurs in mesophylls.

•  Thus 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, Because at the site of Rubisco (BS cells) no O2 is release & (mesophyll cells pumps more CO₂ for C₃ cycle).

•  C₄–plants found in tropical habitats and 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.

•  First carboxylation in C₄–cycle occurs by PEPCase in mesophyll cytoplsam, while second carboxylation or final CO₂ fixation by C₃ cycle occurs in bundle sheath cells.

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

•  12 NADPH₂ & 30 ATP needed for production of 1 Hexose (Glucose) in C₄–plants.

•  Pyruvate phosphate dikinase (PPDK) (ATP → AMP) is a temperature sensitive enzyme ofC₄ and CAM plants due to this C₄plants better photosynthesizes at high temperature.

•  C₄plant evolve by Anatomical, physiological & genetical modified.

•  Oleary and Rouhani discovered CAM–process in members of Crassulaceae family. Succulent xerophytie plants. Eg. are .– Kalanchoe, Bryophyllum, Sedum, Kleinia, Opuntia, Crassula, Agave, Aloe, Euphorbiasps, Pineapple, Welwitschia (Gymnosperm) etc.

•  Primary acceptor of CO₂ is PEP (Phosphoenol pyruvate) and oxaloacetic acid is the first product of carboxylation 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 present in mesophyll cells. (No Kranz–anatomy)

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

•  CAM plants exhibits ecophysiological adaptation with xeric habits.

Photosynthetic carbon oxidation cycle/C₂ Cycle/Photorespiration/Glycolate–Metabolism

• Term was given by ‘Krotkov’

•  First of all Krotkov et. al 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 C₃–plants & Rubisco acts as oxygenase at higher concentration of O₂ and low CO₂ concentration in the C₃ – green cells.

•  The light dependent uptake of O₂ & 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 organalle reaction).

• During photorespiration, 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) produced in photorespiration. 

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

•  Scientists are trying to change C₃ spe. into C₄ sps.

•  It is assumed that in C₃ plants, if photorespiration does not occur, then increases O₂ conc. which may oxidise (Photooxidation or Solarization) the different protoplasmic parts of photosynthetic cell at high light intensity.

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Factors affecting Photosynthesis - photosynthesis in higher plants, Biology, Class 11 

FACTORS AFFECTING PHOTOSYNTHESIS
(1) Light

(a) Light Quality or wavelength → Maxm 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 called as light compensation point.

•  Intensity of light, at which rate of photosynthesis, becomes equal (or compensate) with the rate of respiration in plants is known aslight 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 LDP & SDP.

•  Product 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 –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 (0.03%/314 ppm)

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

•  C₄–Plants can photosynthesize at low CO₂ concn (upto 10 ppm). “CO₂ concn at which CO₂ fixation in photosynthesis is equal to volume of CO^₂ released in respiration is"CO2compensation 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 concn. 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 by 1 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 likeleaf ageandleaf orientationeffect the rate of photosynthesis.

•  In young & mature 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 of factors, 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), Rhodospirillum, 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 denoted by – B–890 or 870

•  Evolution of O₂ is not related to bacterial photosynthesis, because water is not e– donor and PS II is absent.

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

•  Olson 1970 gave a non cyclic scheme in bacterial photosynthesis.

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• Bacteria has only one pigment system, PS I.