1. Photosynthesis Dr. Mohd Ashaq Malik Assistant Prof. of Botany
Govt PG College Rajouri

2. BASICS OF PHOTOSYNTHESIS
Almost all plants are photosynthetic autotrophs, as are some bacteria and protists
Autotrophs generate their own organic matter through photosynthesis
Sunlight energy is transformed to energy stored in the form of chemical bonds
(c) Euglena
(d) Cyanobacteria
(b) Kelp
(a) Mosses, ferns, and
flowering plants

3. Light Energy Harvested by Plants & Other Photosynthetic Autotrophs
6 CO2 + 6 H2O + light energy → C6H12O6 + 6 O2

4. WHY ARE PLANTS GREEN? Plant Cells have Green Chloroplasts
The thylakoid membrane of the chloroplast is impregnated with photosynthetic pigments (i.e., chlorophylls, carotenoids).

5. THE COLOR OF LIGHT SEEN IS THE COLOR NOT ABSORBED
Chloroplasts absorb light energy and convert it to chemical energy
Reflected
light
Light
Absorbed
light
Transmitted
light
Chloroplast

6. AN OVERVIEW OF PHOTOSYNTHESIS
Photosynthesis is the process by which autotrophic organisms use light energy to make sugar and oxygen gas from carbon dioxide and water
Carbon dioxide
Water
Glucose
Oxygen gas
PHOTOSYNTHESIS

7. AN OVERVIEW OF PHOTOSYNTHESIS
The light reactions convert solar energy to chemical energy
Produce ATP & NADPH
Light
Chloroplast
NADP
ADP
+ P
The Calvin cycle makes sugar from carbon dioxide
ATP generated by the light reactions provides the energy for sugar synthesis
The NADPH produced by the light reactions provides the electrons for the reduction of carbon dioxide to glucose
Calvin
cycle
Light
reactions

8. Photophosphorylation

10. PHOTOSYNTHESIS 6CO2 + 6H2O C6H12O6 + 6O2 Sunlight provides ENERGY
CO2 + H2O produces
Glucose + Oxygen
6CO2 + 6H2O
C6H12O6 + 6O2

12. Steps of Photosynthesis
Light hits reaction centers of chlorophyll, found in chloroplasts
Chlorophyll vibrates and causes water to break apart.
Oxygen is released into air
Hydrogen remains in chloroplast attached to NADPH
“THE LIGHT REACTION”

13. Steps of Photosynthesis
The DARK Reactions= Calvin Cycle
CO2 from atmosphere is joined to H from water molecules (NADPH) to form glucose
Glucose can be converted into other molecules with yummy flavors!

14. Calvin cycle

15. Calvin cycle-Carbon fixation
Carbon from CO2 is added to organic acceptor and can then be used for synthesis
Ribulose biphosphate (RuBP) is the acceptor
5 carbon sugar
Rubisco (Ribulose biphosphate carboxylase) is the enzyme that adds the new carbon to RuBP
Most common enzyme on earth
Results in a 6 carbon molecule that immediately degrades into two three carbon molecules-3 phosphoglycerate s
has carboxyl group

16. Calvin Cycle-Reduction
ATP is used to phosphorylate 3-Phosphoglycerate
New molecule is 1,3-Bisphosphoglycerate
Activated intermediate
Electrons from NADPH are used to reduce 1,3-Bisphosphoglycerate
Results in a carbonyl group
Sugars have carbonyl group
G3P is produced and it is a 3 carbon sugar
For every 6 G3P molecules produced-5 are recycled and one can be used for anabolic rxns

17. Calvin Cycle-Regeneration
Uses 5 molecules of G3P to produce 3 molecules of RuBP
Requires ATP to perform this task
This is why more ATP is needed than NADPH
Without regeneration of G3P, this would not be a cyclical reaction
RuBP is ready to act as Carbon acceptor for fixation stage

18. Where does CO2 come from Plants get CO2 from air via stomata
Have guard cells that open and close stomata
Also guard against loss of water vapor

19. C4 Plants Way to avoid photorespiration
Takes place in grasses like corn
Two types of tissue for photosynthesis
First acts like antechamber-Mesophyll cell
PEP carboxylase has higher affinity for CO2 than Rubisco
Adds Carbon from CO2 onto PEP to form Oxaloacetate
Eventually CO2 is released into adjacent Bundle Sheath cell where Calvin takes place
Maintains high CO2 to O2 ratio in BSC
Rubisco can still use CO2

20. C4 Plants

21. CAM Plants Pineapples, cacti and other succulents Similar to C4
Way to get around photorespiration
Stomata open during night to take in CO2
Carbon is stored in an organic acid molecule
During daylight (when light cylce produces ATP and NADPH) organic acids degrade releasing CO2 into the Calvin Cycle
Crassulacean acid metabolism, also known as CAM photosynthesis, is a carbon fixation pathway that evolved in some plants as an adaptation to arid conditions.
In a plant using full CAM, the stomata in the leaves remain shut during the day to reduce evapotranspiration, but open at night to collect carbon dioxide (CO2).
The CO2 is stored as the four-carbon acid malate in vacuoles at night, and then in the daytime, the malate is transported to chloroplasts where it is converted back to CO2, which is then used during photosynthesis.
The pre-collected CO2 is concentrated around the enzyme RuBisCO, increasing photosynthetic efficiency.
The mechanism was first discovered in plants of the family Crassulaceae.

25. Photorespiration
Happens in many plants during hot daytime conditions-soybeanss
Guard cells close stomata to prevent water loss
Unable to take up new CO2 molecules
Oxygen:CO2 ration shifts
Rubisco uses O2 instead of CO2
Eventually yields CO2 for Calvin cycle but so much effort was expended it is a net loss for plant
Photorespiration
Photorespiration is a wasteful pathway that competes with the Calvin cycle. It begins when Rubisco acts on oxygen instead of carbon dioxide.

26. Photorespiration
Photorespiration (also known as the oxidative photosynthetic carbon cycle, or C2 photosynthesis) refers to a process in plant metabolism where the enzyme RuBisCO oxygenates RuBP, wasting some of the energy produced by photosynthesis.
The desired reaction is the addition of carbon dioxide to RuBP (carboxylation), a key step in the Calvin–Benson cycle, however approximately 25% of reactions by RuBisCO instead add oxygen to RuBP (oxygenation), creating a product that cannot be used within the Calvin–Benson cycle.
This process reduces the efficiency of photosynthesis, potentially reducing photosynthetic output by 25% in C3 plants.
Photorespiration involves a complex network of enzyme reactions that exchange metabolites between chloroplasts, leaf peroxisomes and mitochondria.

27. Significance of Photorespiration
Photorespiration is definitely not a win from a carbon fixation standpoint. However, it may have other benefits for plants.
There's some evidence that photorespiration can have photoprotective effects (preventing light-induced damage to the molecules involved in photosynthesis), help maintain redox balance in cells, and support plant immune defenses

28. Overview

29. Photosynthesis occurs in chloroplasts
In most plants, photosynthesis occurs primarily in the leaves, in the chloroplasts
A chloroplast contains:
stroma, a fluid
grana, stacks of thylakoids
The thylakoids contain chlorophyll
Chlorophyll is the green pigment that captures light for photosynthesis

30. The location and structure of chloroplasts
LEAF CROSS SECTION
MESOPHYLL CELL
LEAF
Mesophyll
CHLOROPLAST
Intermembrane space
Outer
membrane
Granum
Inner membrane
Grana
Stroma
Thylakoid compartment
Stroma
Thylakoid

31. Chloroplast Pigments Chloroplasts contain several pigments
Chlorophyll a
Chlorophyll b
Carotenoids
Xanthophylls
Figure 7.7

32. Summary—Light Dependent Reactions
a. Overall input
light energy, H2O.
b. Overall output
ATP, NADPH, O2.

33. Summary—Light Independent Reactions
a. Overall input
CO2, ATP, NADPH.
b. Overall output
glucose.

34. Review: Photosynthesis uses light energy to make food molecules
A summary of the chemical processes of photosynthesis
Chloroplast
Light
Photosystem II Electron transport chains Photosystem I
CALVIN CYCLE
Stroma
Electrons
Cellular respiration
Cellulose
Starch
Other organic compounds
LIGHT REACTIONS
CALVIN CYCLE

35. Thank you