Dark Reaction of Photosynthesis: 3 Pathways | Botany

There are three pathways of dark phase of CO₂ Fixation: 1. Calvin Cycle – C₃ Cycle 2. Hatch and Slack Cycle – C₄ Cycle 3. Crassulacean Acid Metabolism (CAM) Cycle.

Dark phase of photosynthesis i.e. CO₂ Fixation. Dark reactions are dependent on Enzymes 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.

Pathway # 1. 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).

i. 6 molecules of RuBP (Ribulose biphosphate, earlier called RuDP) combine with 6 molecules of CO₂ which produces 6 molecules of carbohydrate (hexose).

ii. One out of 6 molecules of Hexoses is consumed as food.

iii. Remaining 5 molecules of Hexoses are reconverted to RuBP (6 molecules) and thus completes the cycle.

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 its concentration is available in minimum quantity.

Inferences:

i) The law is applicable to only those reactions where the rate is governed by multiple factors like photosynthesis.

ii) The rate of such reaction is dependent on any one factor at specific time – Limiting factor or governing factor.

iii) The limiting factor will be that one which is available in minimum quantity i.e. Law of Minimum.

Pathway # 2. Hatch & Slack Cycle/C₄ Cycle:

i) It is found in Sugarcane, maize, sorghum, bajra etc. such plants are called C₄-plants.

ii) In 1965 Kortschak, Hartt & Burr working with C¹⁴O₂ 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.

iii) 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.

iv) C₄-Plants have ‘Kranz anatomy’ in leaves.

There are two types of chloroplasts:

(a) Normal or isomorphic chloroplast &

(b) Karnz type of chloroplast.

v) 1st stable product is 4-carbon compound oxalo-acetic acid hence the name C₄-cycle.

vi) 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 C₃-plants. Thereby resulting higher rate of photosynthesis.

vii) Most of the bad weeds of the world are C₄-plant.

Here CO₂ – acceptor is PEP i.e. phosphoenol pyruvate which is the substitute of RuBP of C₃ plant.

Plasmodesmata is the channel for the transfer of 4C compound from mesophyll to Bundle-sheath.

Depending on the process of decarboxylation of C₄-acids within the bundle-sheath, C₄ plant species are divided into three sub-groups viz.:

a. NADP-ME type;

b. PCK type, and

c. NAD-ME type.

a. NADP-ME Type:

Malate formed within the chloroplast of bundle sheath is decarboxylated via NADP-malic enzyme, hence called NADP-ME type. Here only chloroplast is involved.

b. PCK-Type:

Oxaloacetate is converted to aspartate by aspartate aminotransferase within cytoplasm. Then aspartate is transported to Bundle sheath cells. In Bundle sheath, aspartate is converted back to oxalo-acetate and oxaloacetate is decarboxylated to form PEP which yields pyruvate. Pyruvate is converted to Alanine and diffuses into mesophyll cells. It involves Chloroplast & cytoplasm.

c. NAD-ME Type:

Like PCK type, aspartate enters into the bundle sheath cells. It diffuses into mitochondria where it is converted to oxaloacetate and oxaloacetate is reduced to malate which is decarboxylated to pyruvate. Pyruvate diffuses out of mitochondria & enters into cytoplasm where it is converted to alanine. It involves Mitochondria, Cytoplasm & Chloroplast.

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

i) 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.

ii) PEP carboxylase is not sensitive to O₂ which has a competitive effect on RuBP carboxylase.

iii) C₄ plants lack photo respiration, hence photosynthetic rate is higher.

iv) In Mesophyll cells of C₃, Nitrogen (N) & Sulphur (S) reduction occur and so they compete for reducing power with photosynthesis.

v) In mesophyll cells of C₄, N & S-reduction occur but Calvin cycle is in Bundle sheath, therefore competition for reducing power is low.

vi) Photosynthates made in C₄ plants are readily transported to other parts.

vii) High temperature is optimum in C₄ plants for enzyme thereby increasing the photosynthetic turnover rate higher.

viii) High light saturation point (in C₄) combing higher electron transport and more generation of reducing power (NADPH₂) & ATP.

Pathway # 3. Crassulacean Acid Metabolism (CAM):

i) Occurrence:

Certain succulent plants of the family crassulaceae like cactus (cacti); pineapple, Onion, garlic, lili, sisal etc. All CAM-plants have succulent habit.

ii) Adaptability:

Extreme desiccation, such plants possess xerophytic characteristics like reduced leaves, thick cuticle, sunken stomata etc.

iii) 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.

iv) 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.