🌚 Dark Reactions

• Dark phase of photosynthesis i.e. CO₂ Fixation.
• Dark reactions are dependent on Enzymes (Temperature dependent) but light reactions are dependent on pigments.
• In Dark reaction, CO₂ is reduced to carbohydrates with the help of the products of light reaction. i.e. ATP and NADPH₂.
• There are three pathways of dark phase of CO₂ Fixation.

1. Calvin Cycle: C₃ cycle
2. Hatch and Slack cycle: C₄ cycle
3. CAM cycle

Calvin Cycle/C3-cycle

• This cycle is predominantly found in Wheat, Rice, Barley, Pulses etc. and such plants are called C₃ plants because in the Calvin cycle, first stable product is C-3 compound (i.e. having three no. of carbons in a compound).
• 6 molecules of RuBP (Ribulose biphosphate, earlier called RuBP) combine with 6 molecules of CO₂ which produces 6 molecules of carbohydrate (hexose).
• One out of 6 molecules of Hexoses is consumed as food.
• Remaining 5 molecules of Hexoses are reconverted to RuBP (6 molecules) and thus completes the cycle.

Blackman’s Law: Law of Minimum or Law of limiting factor

• If light intensity is increased by twice but CO₂-Concentration is constant, there will be no increase in photosynthetic rate.
• It means CO₂ acts as a limiting factor because it’s concentration is available in minimum quantity.
• Inferences:
  • The law is applicable to only those reactions where the rate is governed by multiple factors like photosynthesis.
  • The rate of such reaction is dependent on any one factor at specific time — Limiting factor or governing factor.
  • The limiting factor will be that one which is available in minimum quantity i.e. Law of Minimum.

Hatch & Slack cycle/C4 cycle

• It is found in Sugarcane (Tropical Grasses), Maize, Sorghum, Bajra etc. such plants are called C₄ plants.
• In 1965 Kortschak, Hartt & Burr working with ¹⁴CO₂ on sugarcane leaves found C₄ dicarboxylic acid, malate & aspartate to be the major labelled products in very short periods of photosynthesis. This observation was confirmed by M.D. Hatch & C.R. Slack in 1967 in Queensland Australia.
• A sub-tropical species of Atriplex rosea exhibits C₄-cycle whereas the temperate species of the same genus Atriplex rosea has only the calvin cycle.
• C₄ Plants have Kranz anatomy in leaves.
• There are two types of chloroplasts
  • Normal or isomorphic chloroplast
  • Karnz type of chloroplast.
• 1st stable product is 4-carbon compound oxalo-acetic acid hence the name C₄ cycle.
• PEP carboxylase has high affinity for CO₂ and hence C₄ plants are able to absorb CO₂ strongly from a much lower CO₂ concentration than the plants. Thereby resulting higher rate of photosynthesis.
• Most of the bad weeds of the world are C₄ plant.

Reaction Centre: Division of labour

• Here CO₂ - acceptor is PEP i.e. Phosphoenol Pyruvate which is the substitute of RuBP of C₃ plant.

👉🏻 Physiological Differences in relation to dry matter production between C3 & C4 plants:

• The PEP carboxylase enzyme present in C₄ cycle has a very strong affinity for CO₂ as compared to RuBP carboxylase. Hence law of Minimum regarding the CO₂-concentration is not in operation for C₄-plants.
• PEP carboxylase is not sensitive to O₂ which has a competitive effect on RuBP carboxylase.
• C₄ plants lack photo respiration, hence photosynthetic rate is higher.
• In Mesophyll cells of C₃, Nitrogen (N) & Sulphure (S) reduction occur and so they compete for reducing power with photosynthesis.
• In mesophyll cells of C₄ N & S-reduction occur but Calvin cycle is in Bundle sheath, therefore competition for reducing power is low.
• Photosynthates made in C₄ plants are readily transported to other parts.
• High temperature is optimum in C₄ plants for enzyme thereby increasing the photosynthetic turnover rate higher.
• High light saturation point (in C₄) coming higher electron transport and more generation of reducing power (NADPH₂) & ATP.

Crassulacean Acid Metabolism (CAM)

• Occurence: Certain succulent plants of the family Crassulaceae like cactus (cacti); pineapple, Onion, garlic, lilli, sisal etc. All CAM-plants have succulent habit.
• Adaptability: Extreme desiccation, such plants possess xerophytic characteristics like reduced leaves, thick cuticle, sunken stomata etc.
• Special feature: Such plants have also the capability of fixing the CO₂ lost in respiration. Such plants behave like C₄-plants during the night and as C₃ plants during the day. These have slowest photosynthetic rates.
• Succulent Plants:
  • In botany, succulent plants, also known as succulents, are plants that have some parts that are more than normally thickened and fleshy (have low-surface to volume ratio), usually to retain water in arid climates or soil conditions.
  • Succulent plants may store water in various structures, such as leaves and stems.

    

• Biochemical reactions:

  

• In xerophytic plants, stomata open during the night & close during the day.
• When stomata are open, CO₂ is fixed by enzyme PEP-carboxylase to oxaloacetic acid and are stored as Malic Acid in the vacuole which breaks down in day time to release CO₂ for photosynthesis.

Important Points

• Structural material of Plants is Cellulose.
• Transportation of food in plants in the form of Sucrose.
• Storage form of carbohydrate in plants is Starch.

Difference between C3, C4 & CAM cycle/plant

Photorespiration/Glycolate Metaboism/C2-cycle

• The term ‘photorespiration’ was firstly used by Decker and re-Forted in tobacco plant.
• RuBP-carboxylase normally fix CO₂ and forms 2 molecules of PGA (Phosphoglyceric acid). But under High concentration of O₂ (i.e. high O₂/CO₂ ratio) induced by high light checks the production of two molecules of PGA and forms Glycolate (or phosphoglycolic acid) instead of one molecule of PGA.
• Phosphoglycolic acid or Glycolate is 2-carbon-compound which is the first stable compound hence the name Glycolate metabolism or C₂ cycle. In this process H₂O₂ (Hydrogen Peroxide) is formed which may be dissociated by the enzymes present in peroxisomes.
• Favourable Codiction:
  • High light level: It is a cause for increased photolysis of water (H₂O) which liberates O₂.
  • High O₂ level
  • Low CO₂ level
  • High Temperature
• Otto warburg in 1920 found that photosynthesis in algae was inhibited by O₂. This occurs in all C₃-plants. This effect is known as Warburg Effect. ^{FCI AGM 2021}
• Site of reactions in Respiration: Cytosol & Mitochondria but site in photorespiration are three. Here three organelles are involved viz.
  • Chloroplast
  • Peroxisome
  • Mitochondria
• Phosphoglycolic acid is dephosphorylated into glycolate which forms the substrate for photorespiration. Photorespiration of glycotate results in uptake of O₂ and release of CO₂.
• Photorespiration is light dependent because:
  • RuBP formation occurs much faster in light than in darkness because operation of Calvin cycle needs to form RuBP, requires ATP and NADPH₂ (both light dependent products).
  • Chloroplastic O₂, is more abundant in light due to photolysis of H₂O.
• Photorespiration is essentially absent in C4-plants because:
  • Rubisco (RuBP carboxylase) and other calvin cycle enzymes are present only in bundle sheath cells.
  • CO₂ concentration of Bundle sheath cells is maintained too high for O₂ to compete with CO₂ due to rapid dearboxylation of malate and aspartate transferred to Bundle sheath from mesophyll cells.
• Amino acids viz. serine and glycrine produced in the glycolate pathway are useful for protein synthesis.
• Reactions:

  

Factors Affecting Photosynthesis

Light

• It is the most important factor of photosynthesis. Light affects through its intensity, quality and duration.
• The amount of light received by the plant depends on its morphology. The actual requirement of the light intensity depends upon the type of plant and its habitat. Generally average sunlight is sufficient except on rainy or cloudy days. The rate of photosynthesis increases with the increase in light intensity until law of minimum operates.
• Extremely high light intensity has an inhibitory effect on the photo-synthesis and this phenomenon is called solarization.
• During solarization, photo-oxidation occurs in which certain cell constituents are oxidized by O₂ into CO₂.
• High light intensity also increases the transpiration rate, consequently reducing the water content of mesophyll cells which has also an inhibitory effect on photosynthesis.
• Low intensity causes stomatal closure which restricts the entry of CO₂.
• The light intensity at which the photosynthetic intake of CO₂ is equal to the respiratory output of CO₂, is called Compensation Point. Therefore net photosynthesis is zero at the compensation point.

• Visible part of the light spectrum i.e. wavelength between 400 nm to 750 nm is the only need for photosynthesis. Heavy absorption of light takes place in the red (Most effective light in photosynthesis) and next in the blue & violet. Continuous illumination of light also affects the photosynthesis.
  • Green: Least effective colour of light in driving photosynthesis
  • Red: Most effective colour of light in driving photosynthesis (Highest Rate of Photosynthesis)
  • Blue: Most effective colour of light for producing a phototraphic response.

CO₂

• The percentage of CO₂ in the air is 0.03% by volume. At optimum temperature and light intensity, photosynthesis is markedly increased with the increase in CO₂ concentration.
• But relatively high conc. of CO₂ reduces the photosynthetic rate.

Temperature

• There is a rapid increase in photosynthesis if temp. increases from 10-35 °C, provided other factors are not limiting. Photosynthetic rate declines with the higher temp. beyond the maximum limit.

Water

• It has no direct effect. The decrease in water content of leaves may cause partial or complete closure of stomatal openings which reduces the diffusion of CO₂.

O₂

• Oxygen accumulation may retard the photosynthesis.
• It is a competitive inhibitor of carboxylase thus inhibits photosynthesis in C₃ plants.

Mineral Nutrients

• Magnesium is a part of chlorophyll molecule and other nutrients are necessary for enzymic action and plant metabolism.
• All the above factors are external but some internal factors like chlorophyll content; protoplasmic factors & hydration, end product of photosynthesis may also affect the photosynthesis.

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