(i) C₄ cycle/Di - carboxylic acid pathway/ Di - carboxylation pathway is an adaptive mechanism.
(ii) It takes place in two phases. First phase takes place in stroma of mesophyll cells, where the CO₂ acceptor molecule 3-Carbon compound, phosphoenol pyruvate (PEP) to form 4-carbon oxalo acetic acid (OAA). The first product is a 4-carbon and so it is named as C₄ cycle. Oxalo acetic acid is a dicarboxylic acid and hence this cycle is also known as dicarboxylic acid pathway.
(iii) Carbondioxide fixation takes place in two places one in mesophyll and another in bundle sheath cell.
(iv) C₄ plants requires 5 ATP and 2NADPH + H⁺ to fix one molecule of CO₂.

Stage: I Mesophyll Cells

(i) Oxaloacetic acid (OAA) is converted into malic acid or aspartic acid and is transported to the bundle sheath cells through plasmodesmata.
Stage: II Bundle Sheath Cells
(i) Malic acid undergoes decarboxylation and produces a 3 carbon compound Pyruvic acid and CO₂.
(ii) The released CO₂ combines with RUBP and follows the calvin cycle and finally sugar is released to the phloem.
(iii) Pyruvic acid is transported to the mesophyll cells.

1) It is advantageous for succulent plants to obtain CO₂ from malic acid when stomata are closed.
2) During day time stomata are closed and CO₂ is not taken but continue their photosynthesis.
3) Stomata are closed during the day time and help the plants to avoid transpiration and water loss.

1) Plants having C₄ cycle are mainly of tropical and sub-tropical regions and are able to survive in environment with low CO₂ concentration.
2) C₄ plants are partially adapted to drought conditions.
3) Oxygen has no inhibitory effect on C₄ cycle since PEP carboxylase is insensitive to O₂.

The characteristic feature of C₄ plants is the presence of dimorphic chloroplast:
a) Bundle sheath chloroplast: Larger chloroplast, thylakoids not arranged in granum and rich in starch.
b) Mesophyll Chloroplast: Smaller chloroplast, thylakoids arranged in granum and less starch.This is called Kranz Anatomy.

1) Emerson conducted experiment with chlorella using monochromatic light to measure quantum yield. (photosynthetic yield).
2) He found that the monochromatic light oflonger wavelength (far red light) when supplemented with shorter wavelength of light (red light) enhanced photosynthetic yield.
3) This enhancement of photosynthetic yield is referred to as Emerson's Enhancement Effect

1) Emerson conducted experiment in Chlorella using only one wavelength of light (monochromatic light) at a time and he measured quantum yield.
2) The fall in the photosynthetic yield beyond red region of the spectrum is referred as Red drop or Emerson's first effect.

Yellow (C₄₀H₅₆O₂) pigments are like carotenes but contain oxygen. Lutein is responsible for yellow colour change of leaves during autumn season. Ex: Lutein, Violaxanthin and Fucoxanthin.

Chlorophyll is synthesized from intermediates of respiration and photosynthesis. Succinc acid an intermediate of Kreb's cycle is activated by the addition of Coenzyme A and it reacts with a simple amino acid glycine and the reaction goes on to produce chlorophyll a. Biosynthesis of Chlorophyll a requires Mg, Fe, Cu, Zn, Mn, K and Nitrogen.

1) Photosynthetic organisms provide food for all living organisms on earth either directly or indirectly.
2) It is the only natural process that liberates oxygen in the atmosphere and balances the oxygen level.
3) Fuels such as coal, petroleum and other fossil fuels are from preserved photosynthetic plants.