This document provides an overview of photosynthesis and the dark reaction phase. It discusses key topics like the Calvin cycle, the structures of chloroplasts, and the C4 and CAM pathways. The Calvin cycle fixes carbon dioxide into organic molecules like glucose. It consists of carbon fixation, reduction reactions, and ribulose bisphosphate regeneration. The C4 pathway occurs in some plants and involves a four-carbon compound to concentrate carbon dioxide and prevent photorespiration. CAM plants open their stomata at night to fix carbon dioxide into malic acid, then release it for the Calvin cycle during the day.

1. Shri Shankaracharya Mahavidyalaya
Topic- Photosynthesis and Dark reaction
Submitted by
Miss Varsha Yadav
Assistant Professor
Department Of Botany

2. Photosynthesis and Dark reaction

3. content
Calvin Cycle
Hatch and Slack Cycle (C4
cycle)
Structure of
chloroplast
What is
Photosynthesis
Cam cycle
Reference

4. What is Photosynthesis
It is a process in which energy from sunlight is transformed into chemical
energy that is stored in carbohydrates and other organic molecules.
Two phase of photosynthesis.
1) Light reaction or light dependent phase.
- The reaction occurs in thylakoids.
- The products are ATP and NADPH.
2) Dark reaction or light independent phase
- The reaction occur in stroma.
- Product are Trioses, Tetroses, Pentoses, Hexoses & Heptose.

5. Structure of Chloroplast
 It is Double membrane contains thylakoids sacs.
 Chlorophyll, the green pigment within chloroplast gives Green
color of leaves.
 Chloroplast also contains stroma, a dense interior fluid.
Ultrastructure of chloroplast
Electron microscopic studies reveals that chloroplast is composed
of following two parts:-
1) Limiting membrane
2) Stroma

6. Overview of Calvin cycle

8. Overview
The Calvin cycle is part of photosynthesis, which occurs in two stages. In
the first stage, chemical reactions use energy from light to produce ATP
and NADPH. In the second stage (Calvin cycle or dark reactions), carbon
dioxide and water are converted into organic molecules, such as glucose.
Although the Calvin cycle may be called the "dark reactions," these
reactions don't actually occur in the dark or during nighttime. The
reactions require reduced NADP, which comes from a light-dependent
reaction.

9. What is Calvin
cycle
Calvin cycle is a set of light independent redox reactions
that occur during photosynthesis and carbon fixation to
convert carbon dioxide into the sugar glucose. These
reactions occur in the stroma of the chloroplast, which is
the fluid-filled region between the thylakoid membrane
and inner membrane of the organelle. Here is a look at
the redox reactions that occur during the Calvin cycle

10. The Calvin cycle consists of:
 Carbon fixation - Carbon dioxide (CO2) is reacted to produce
glyceraldehyde 3-phosphate (G3P). The enzyme RuBisCO catalyzes the
carboxylation of a 5-carbon compound to make a 6-carbon compound
that splits in half to form two 3-phosphoglycerate (3-PGA) molecules.
The enzyme phosphoglycerate kinase catalyzes phosphorylation of 3-
PGA to form 1,3-biphosphoglycerate (1,3BPGA).
 Reduction reactions - The enzyme glyceraldehyde 3-phosphate
dehydrogenase catalyzes reduction of 1,3BPGA by NADPH.
 Ribulose 1,5-bisphosphate (RuBP) regeneration - At the end of the
regeneration, the net gain of the set of reactions is one G3P molecule
per 3 carbon dioxide molecules

11. Calvin Cycle Chemical Equation
The overall chemical equation for the Calvin cycle is:
3 CO2 + 6 NADPH + 5 H2O + 9 ATP → glyceraldehyde-3-phosphate (G3P) +
2 H+ + 6 NADP+ + 9 ADP + 8 Pi (Pi = inorganic phosphate)
Six runs of the cycle are required to produce one glucose molecule.
Surplus G3P produced by the reactions can be used to form a variety of
carbohydrates, depending on the needs of the plant.

12. Three turns of the Calvin cycle are needed to make one G3P
molecule that can exit the cycle and go towards making glucose.
Let’s summarize the quantities of key molecules that enter and
exit the Calvin cycle as one net G3P is made. In three turns of the
Calvin cycle:
Summary of Calvin cycle reactants and products

13. Hatch and Slack Cycle (C4 cycle)
C4 pathway- It also involves carbon fixation to synthesize 4- carbon
compound oxaloacetic acid, C4 pathway occurs only in some plants to
prevent loss of energy by photorespiration.
 In the C4 cycle, the first stable compound is 4- Carbon compound,
namely Oxaloacetic acid (OAA). Hence it is called C4 cycle. The
primary CO2 acceptor is Phosphoenole pyruvic acid (PEP). It is a
process of CO2 fixation. CO2 fixation is fast and more efficient. This
pathway was worked out by Hatch and Slack in 1966. Hence the C4
cycle is also called as Hatch- Slack cycle.
 Fixation of one molecule of CO2 requires 5 ATP and 3 NADH.
 C4 cycle occurs in plants like maize, sorghum etc

14. Kranz anatomy
The special structure of leaves in C4
PLANTS (e.g. maize) where the tissue
equivalent to the spongy mesophyll
cells is clustered in a ring around the
leaf veins, outside the bundle-sheath
cells. (The term ‘Kranz’ means wreath
or ring in German). The bundle-sheath
cells contain large CHLOROPLASTS
whereas the spongy mesophyll cells
have few if any chloroplasts, unlike
their counterparts in C3 plants (see
MESOPHYLL).

17. Steps of C4 pathway
 Carboxylation:It takes place in the chloroplast of mesophyll cells.
Phosphoenol pyruvate (PEP), a 3 carbon compound picks up CO2 and
changes into 4 C- Oxaloacetate in the presence of water. This reaction is
catalyzed by the enzyme, PEP Carboxylase. PEP
+ CO2+H2O -------- oxaloacetate (4C) + H3PO4 (PEP carboxylase)
 Breakdown:Oxaloacetate breaks down readily into 4 C- Malate and
Aspartate in the presence of the enzymes, transaminase and Malate
dehydrogenase. Oxaloacetate
(4C) ------- Aspartate (4C) +Malate (4C) (Transaminase, Malate
dehydrogenase). These
compounds diffuse from the mesophyll cells into sheath cells.
Next->

18.  Splitting: In the sheath cells Malate and Aspartate split enzymatically
to yield free CO2 and 3C- pyruvate. The CO2 is used in Calvin cycle in
the sheath cells.
Malate --------------- CO2 + Pyruvate (Decarboxylation)
 Phosphorylation:The pyruvate molecule is transferred to chloroplasts
of mesophyll cells, where it is phosphorylated to regenerate PEP in
the presence of ATP. This reaction is catalyzed by pyruvate
phosphokinase and the phosphoenol pyruvate is regenerated.
Pyruvate + ATP+ Pi --------------- PEP + AMP + Pyrophosphate (Pyruvate
phosphokinase)

19. Significance of C4
cycle
Fixation of CARBON dioxide is more efficient in C4 cycle than C3 cycle
because of following reasons
1) production in C4 plant 2-3 times grater than C3 plant.
2)C4 plant can be photosynthesize even in the presence of low
concentration of carbon dioxide.
3)

20. Cam cycle
Introduction and definition:-It occurs mostly in succulent plants
which grow under semi-arid conditions. The fixation of CO2 only at
night because the stomata is open only at these plants observe CO2
and convert into mallic acid.
At day time the decarboxylation of malic acid take place and CO2 is
released and used in C3 cycle.
It was first seen in crassulacea family e.g. Bryophyllum sedum and
kalanchoe and hence it is also called Crassulacean Acid Metabolism

21. The mechanism or cycle is reported on following families:
1) Dicotyledenous family:- Crassulacea, Azoaceae, Asclepiadaceae,
Caryophyllaeae,Chenopodium compositae, Canvolvulaceae,
Euphorpiaceae etc.
2) Monocotoledenous family:- Liliaceae, orchidaceae etc
3) Pteridophytes:- Polypediaceae.

22. 1) The stomata is remain closed at day and open at night.
2) CO2 fixation takes place in chlorophyll containing cell of leaves and stem
during night and malic acid synthesis take place.
3) malic acid is formed during night and stored in large vacuoles.
4) Decarboxylation of malic acid and converted into starch by C3 cycle.
5) the level of malate (malic) acid is increased and the level of gluecan
because decrease at night and vice versa.
6) Transpiration rate is low as well as parenchymatous cell are larger and
vacuolated.
7) function of vacuoles as a site of accumulation of organic acid.
8) They posses high level of gaseous exchanges and xerophytic characters.
Characteristics feature of CAM cycle:-

25. Mechanism of CAM cycle:-
CAM cycle take place in two parts:-
1) Acidification. 2) Deacidification
Acidification:- It takes please in following steps:-
1) Carbohydrates are converted into PEP (phosphenol pyruvic acid) through
glycosis.
2) The CO2 combines with PEP Erin the presence of phosphenol-Carboxylase
(PEP-C) enzyme to produce oxalicacetic acid (OAA)

26. 3) The OAA is now reduced to malate in the presence of mallate
dehydogenase enzyme and form NADPH+ ----> NADPH + H+
 Deacidification:- the carboxylation of malic acid into pyruvic acid and
CO2 in presence of light called Deacidification.
During day time malic acid is stored in vacuoles is diffused into cytoplast
and become Decarboxylated to produce pyrivic acid and carbon dioxide in
the presence of NADP malic acid.
One molecules of NADP is also reduced in this reaction.

27. 1) the CAM cycle is found in dry habitat plant due to sunken stomata.
2) Stomata in CAM plant remain closed in day time but photosynthesis
take place in the process of Deacidification.
3) As CAM plant are able to fix CO2 in dark, they survive longer with CO2
uptakes.
Significance of CAM cycle:-

28. References
Unified Botany
Biochemistry
V.Y. Tyagi - Botany 3rd year

29. Thank you