Photosynthesis is the process by which plants, algae, and some bacteria use sunlight, carbon dioxide, and water to produce oxygen and energy in the form of glucose. It occurs in two stages - the light-dependent reactions and the Calvin cycle. The light-dependent reactions use energy from sunlight to convert water to oxygen and produce ATP and NADPH. The Calvin cycle then uses the ATP and NADPH to fix carbon from carbon dioxide into organic molecules like glucose. Photosynthesis is essential for life as it produces the oxygen and food on which nearly all organisms depend.

1. Photosynthesis
Dr. Mohamed Badr

2. Photosynthesis
• Photosynthesis is the process by which autotrophic organisms use light
energy (photons) to make sugar and oxygen gas from carbon dioxide and
water .
• An anabolic (small molecules combined),
• Endergonic (stores energy) ,

3.  Makes organic molecules (glucose) out of inorganic materials
(carbon dioxide and water).
 It begins all food chains. Thus all life is supported by this
process.
 It also makes oxygen gas!!
 Responsible for removal of ~ 200 billion tons of C from the
atmosphere yearly.

4. Photosynthesis and respiration as complementary processes in the living world

5. In plants, photosynthesis takes place primarily in leaves, which consist of:
a. Upper and lower epidermis .
b. Stoma – pores.
c. Mesophyll cells (photosynthetic cells) which contains chloroplasts.

6. Chloroplast : Site of Photosynthesis

7. Stroma (aqueous fluid):
Has appropriate enzymes and
suitable PH for
Thylakoid membrane:
Has the ability to the absorption
of light by chlorophyll, synthesis
of , , and electron
transport.
Chloroplast Inner membrane: transports
for phosphate and sucrose
precursors.
Outer membrane:
permeable to small
molecules.
Granum:
Stack of thylakoids
Organelle where photosynthesis takes place.

8. Function of a Chloroplast
• The structure of the chloroplast allows the process of photosynthesis to
take place. Two stages of photosynthesis.
The light-dependent reactions Occurs in the Thylakoid membrane.
The Calvin cycle Occurs in stroma of chloroplast Stroma.

9. Chlorophyll Molecules
• Chlorophyll is the green pigment that captures light for
photosynthesis.
• Located in the thylakoid membranes which are found in
chloroplasts of plants.
• Chlorophyll have Mg+ in the center
• Chlorophyll pigments harvest energy (photons) by
absorbing certain wavelengths (blue-420 nm and red-660
nm are most important).
• There are two main types of chlorophyll (chlorophyll a &
chlorophyll b).

10.  Sunlight is actually white light made of all wavelength colors. sunlight is visible light.
 Different colors = different wavelengths of light
(The shorter the wavelength the higher the frequency, thus the higher the energy, The longer the
wavelength the lower the frequency , thus the lower the energy).
 violet wavelengths=380 nm=high frequency=high energy
 red wavelengths=750 nm=low frequency=low energy

11. Chloroplast
 Plants are green because the
green wavelength is reflected,
not absorbed.

12. Components of a Photosystem
 Photosystems are groups of photosynthetic pigments (including
chlorophyll) embedded within the thylakoid membrane.
1. Antennae Pigments (The light-harvesting complex )
– Chlorophyll a, b and carotene.
– 200 – 300 per photosystem.
2-Reaction Center.
–is a special pair of Chlorophyll a.
3- Primary Electron Acceptor.
 Photosystems are classed according to their
maximal absorption wavelengths (PS I = 700 nm, ;
PS II = 680 nm).

13. • Occurs in the Thylakoid membranes.
• converts light energy into chemical energy; produces ATP and NADPH
molecules to be used to fuel light independent reaction (The Calvin
cycle).
Stages of photosynthesis
The first stage: The Light Dependent Reactions(electron flow)

14.  The light dependent reactions begin in Photosystem II
 When a photosystem II absorbs light energy, delocalized electrons within the pigments become energized
or ‘excited‘ which become unstableand is released.
 These excited electrons are transferred to carrier molecules within the thylakoid membrane

15.  Excited electrons from Photosystem II (P680) are transferred to (ETC)within the thylakoid membrane
 As the electrons are passed through the chain they lose energy, which is used to translocate H+ ions into the
thylakoid.
 This build up of protons within the thylakoid creates an electrochemical gradient, or proton motive force.
 The H+ ions return to the stroma (along the proton gradient) via the transmembrane enzyme ATP synthase
(chemiosmosis).
 ATP synthase uses the passage of H+ ions to catalyze the synthesis of ATP (from ADP + Pi).
 This process is called photophosphorylation – as light provided the initial energy source for ATP
production.
 The newly de-energized electrons from Photosystem II are taken up by Photosystem I.

16. •Excited electrons from Photosystem I may be transferred to a carrier molecule and used to reduce NADP+
•This forms NADPH – which is needed (in conjunction with ATP) for the light independent reactions
•The electrons lost from Photosystem I are replaced by de-energized electrons from Photosystem II
•The electrons lost from Photosystem II are replaced by electrons released from water via photolysis
•Water is split by light energy into H+ ions (used in chemiosmosis) and oxygen (released as a by-product).

17. Summary of the Light reactions
Light reactions:
 Chemical energy compounds (ATP& NADPH) are made
from light energy.
 Water is split into the following:
 Electrons- donated to PSII to replace lost electrons.
 Hydrogen ions – carried to ATP synthase to provide
energy for the production of ATP.
 O2 – released in to atmosphere as a by–product.

18. The second stage: The Calvin Cycle
Stages of photosynthesis

19. • The Calvin cycle is somewhat like a sugar factory within a chloroplast. It is
called a cycle because, the starting material is regenerated each time the
process occurs. In this case, the starting material that gets regenerated is a
compound called Ribulose Bisphosphate (RuBP), a sugar with five
carbons.
• These reactions occur in the Stroma of chloroplast.
• With each turn of the Calvin cycle, there are chemical inputs and outputs.

20. • The Calvin Cycle consists of a series of reactions that reduce
carbon dioxide to produce glyceraldehyde-3-phospahte.
1. Carbon Fixation
2. Reduction
3. Regeneration
Steps of Calvin Cycle

21. Step 1: Carbon Fixation
 CO2 (1c) + Ribulose 1,5- bisphosphate (5c) = short lived 6C intermediate
 6C molecule split into two 3C molecule known as 3-Phosphoglycerat (PG).
 Reaction type: synthesis
 Enzyme: Ribulose bisphosphate carboxylase /oxygenase (Rubisco).

22. The first step in the Calvin cycle

23. Step 2: Reduction of Glycerate -3-phosphate
 ATP phosphorylates each 3-carbon molecule.
 Reaction type: phosphorylation.
 Enzyme: Phosphoglycerate Kinase.
 NADPH used to synthesize G3P
 Reaction type: redox
Enzyme: Glyceraldehyde-3-phosphate Dehydrogenase

24. • Glyceraldehyde-3-P Dehydrogenase catalyzes reduction of the carboxyl
of 1,3-bisphosphoglycerate to an aldehyde, with release of Pi, yielding
glyceraldehyde-3-P.
• the chloroplast Glyceraldehyde-3-P Dehydrogenase uses NADPH as e-
donor.
The second step in the Calvin Cycle

25.  The 6 G3P produced 1 of them exits the cycle to eventually become glucose .
 The other 5 G3P continue in the cycle to help regenerate the starting substances (Ribulose
1,5- bisphosphate). For every three turns of the cycle, five molecules of glyceraldehyde-3-
phosphate are used to re-form 3 molecules of ribulose 1,5- biphosphate.
 Enzymes: Triose Phosphate Isomerase, Aldolase, Fructose Bisphosphatase, Transketolase,
Epimerase, Ribose Phosphate Isomerase &Phosphoribulokinase.
Step 3: Regeneration of Ribulose Bisphosphate

27. • Light-dependent and light-independent (Calvin cycle) reactions are
interdependent:
The light dependent reactions convert light energy to chemical energy.
● Produce ATP&NADPH.
The calvin cycle makes sugar from carbon dioxide (CO2).
●ATP generated by light reactions provides the energy for sugar synthesis.
●The NADPH produced by light reactions provides the electrons for
reductions of CO2.
The light-dependent reactions cannot continue producing NADPH
without the NADP+ from the Calvin Cycle, or ATP without ADP, etc. etc.
•Thus, each set of reactions relies on the products of
the other.

29. Difference between Light and Dark Reaction
Dark ReactionLight Reaction
It can take place in the presence or absence of
sunlight.
It takes place only in the presence of light.
It is a biochemical phase.It is a photochemical phase
It takes place in the stroma of the chloroplast.It takes place in the grana of the chloroplast.
Glucose is produced. Co2 is utilized in the dark
reaction.
The water molecules split into hydrogen and
oxygen.
The hydrogen of NADPH combines with CO2
NADP utilizes H+ ions to form NADPH.
Photolysis does not occur.Photolysis occurs in PS-II.
Glucose is the end product. ATP and NADPH help in
the formation of glucose.
The end products are ATP and NADPH.

30. Factors Affecting Photosynthesis
• Light
The rate of photosynthesis increases as light intensity increases until all the
pigments are being used.
The reactions cannot increase beyond this point.
• Temperature
Temperatures too high or too low can affect the enzymes assisting the
chemical reactions.
• Water
A shortage of water can slow or even stop photosynthesis.
Plants that live in dry conditions have adaptations to live in these conditions
waxy coatings on their leaves.