انرژی روی زمین توسط خورشید تأمین می شود. گیاهان و ارگانیسم‌های خاص از طریق فرآیندی که فتوسنتز نامیده می‌شود، انرژی خورشید را توسط اندامک‌های سلولی ویژه‌ای به نام کلروپلاست به مولکول های آلی تبدیل می کنند. در این فصل ، من در مورد این روند به طور مفصل صحبت خواهم کرد. ---------------------------------------------------------------------- Life on earth is powered by sun. Plants and specific organisms convert the energy of the sun by specialized cellular organelles called chloroplasts into organic molecules through a process which is called photosynthesis. In this chapter, I will talk about this process in details.

1. Metabolism &
Transforming Energy
Photosynthesis
PART THREE
UNIVERSITY OF TEHRAN
Ahmad V.Kashani, PhD

2. Outlines
• Photosynthesis
• Modes of nutrition
• Site of photosynthesis in plants and
the structure
• Stages of photosynthesis
• Light reaction
• Calvin cycle
• Light reactions
• Light receptors
• Chlorophyll and other pigments
• Chlorophyll excitation
• Photosystems I & II
• Linear electron flow
• Circular electron flow
• Chemiosmosis in chloroplast
• Calvin cycle
• Phase I: Carbon fixation
• Phase II: Reduction
• Phase 3: Regeneration of the CO2
acceptor (RuBP)

3. Photosynthesis
• Chloroplast, the backbone of
photosynthesis
• The process of transforming and
converting sunlight energy and storing it
in form of sugar and other organic
molecules is called photosynthesis
• Modes of organism nutrition:
• Autotroph: or self-seeders
• Heterotroph: or other-feeding
• Photoautotrophs, organisms that use
light as a source of energy to synthesize
organic substances.

4. Photosynthesis
• Chloroplast is the site of photosynthesis
in plants.
• Chloroplasts are found mainly in the cells
of the mesophyll, the tissue in the
interior of the leaf.
• Carbon dioxide enters the leaf, and
oxygen exits, by way of microscopic
pores called stomata.
• Water absorbed by the roots is delivered
to the leaves in veins.
• Leaves also use veins to export sugar to
roots and other non-photosynthetic
parts of the plant.

5. Photosynthesis
• A typical mesophyll cell has about 30–40
chloroplasts, each measuring about 2–4
μm by 4–7 μm.
• Chloroplast
• Stroma
• Thylakoids
• Thylakoid space
• Granum (Grana)
• Chlorophyll, the green pigment that
gives leaves their color, resides in the
thylakoid membranes of the chloroplast.
(The internal photosynthetic membranes
of some prokaryotes are also called
thylakoid membranes)

6. Photosynthesis – Stages
• In the chloroplast, the thylakoid membranes
are the sites of the light reactions, whereas
the Calvin cycle occurs in the stroma:
1. The light reactions use solar energy to
make ATP and NADPH, which supply
chemical energy and reducing power,
respectively, to the Calvin cycle.
2. The Calvin cycle incorporates CO2 into
organic molecules, which are converted to
sugar.

7. Photosynthesis – Stages – Light Reaction
• Water molecule is split, providing a source
of electrons and protons (hydrogen ions, H+)
and giving off O2 as a by-product.
• Light absorbed by chlorophyll drives a
transfer of the electrons and hydrogen ions
from water to an acceptor called NADP1
(nicotinamide adenine dinucleotide
phosphate), where they are temporarily
stored.
• The light reactions use solar energy to
reduce NADP+ to NADPH by adding a pair of
electrons along with an H+.
• The light reactions also generate ATP, using
chemiosmosis to power the addition of a
phosphate group to ADP, a process called
photophosphorylation.
• Light energy is initially converted to
chemical energy in the form of two
compounds: NADPH and ATP.

8. Photosynthesis – Stages – Calvin Cycle
• Begins by incorporating CO2 from the
air into organic molecules already
present in the chloroplast, known as
carbon fixation.
• Then, reduction of fixed carbon
to carbohydrates by adding electron.
• The reducing power is provided by
NADPH, which acquired its cargo of
electrons in the light reactions
• To convert CO2 to carbohydrate, the
Calvin cycle also requires chemical
energy in the form of ATP, which is also
generated by the light reactions.
• In the Calvin cycle sugar is the final
product

9. Photosynthesis – Light reactions
• The Nature of Sunlight
• White light is a mixture of all
wavelengths of visible light.
• Visible light drives photosynthesis.

10. Photosynthesis – Light receptors
• The chlorophyll molecules of
chloroplasts absorb violet-blue (380
nm) and red light (750 nm) and reflect
or transmit green light.
• This is why leaves appear green.
• Spectrophotometer is an instrument
that can measure the ability of a
pigment to absorb various
wavelengths of light

11. Photosynthesis – Light reactions
Chlorophyll & other pigments
• Based on the absorption spectra, there are three
types of pigments in chloroplasts:
• Chlorophyll a, the key light-capturing pigment
that participates directly in the light reactions;
• The accessory pigment chlorophyll b;
• a separate group of accessory pigments called
carotenoids.
• Functions of Carotenoids
• They may broaden the spectrum of colors that can
drive photosynthesis
• A more important function is photoprotection:
These compounds absorb and dissipate excessive
light energy that would otherwise damage
chlorophyll or interact with oxygen, forming
reactive oxidative molecules that are dangerous to
the cell.

12. Photosynthesis – Light reactions
Chlorophyll excitation
• Absorption of a photon causes a
transition of the chlorophyll molecule
from its ground state to its excited state.
• The photon boosts an electron to an
orbital where it has more potential
energy.
• If the illuminated molecule exists in
isolation, its excited electron immediately
drops back down to the ground-state
orbital, and its excess energy is given off
as heat and fluorescence (light).

13. Photosynthesis – Light reactions
photosystem
• When a photon strikes a pigment
molecule in a light-harvesting
complex, the energy is passed from
molecule to molecule until it reaches
the reaction-center complex.
• Here, an excited electron from the
special pair of chlorophyll a molecules
is transferred to the primary electron
acceptor.
• The primary electron acceptor, which is a
molecule capable of accepting electrons
and becoming reduced.

14. Photosynthesis – Light reactions
photosystem I & II
• Two types of photosystems:
• Photosystem I
• Photosystem II
• Named after their order of discovery
although Photosystem II functions
first in the light reaction chain
• Reaction-center complex contains:
• A special pair of chlorophyll
• Photosystem II -> P680
• Photosystem I -> P700
• Certain proteins
• Nearly Identical chlorophylls in both
photosystems

15. Photosynthesis – Light reactions
Linear Electron Flow
Pq
Pc

16. Photosynthesis – Light reactions
Circular Electron Flow
• An alternative pathway
• Only PS-I is involved not PS-II
• The electrons cycle back from
ferredoxin (Fd) to the cytochrome
complex,
• Then via a plastocyanin molecule (Pc)
to a P700 chlorophyll in the PS-I
reaction-center complex
• No production of NADPH and no
release of oxygen that results from
this process.
• But, cyclic flow does generate ATP.

17. Photosynthesis – Chemiosmosis
In Mitochondrion vs in Chloroplast
• In both kinds of organelles,
electron transport-chains pump
protons (H+) across a membrane
from a region of low H+
concentration to one of high
H+ concentration
• The protons then diffuse back
across the membrane through ATP
synthase, driving the synthesis of ATP.

18. Photosynthesis – Chemiosmosis

19. Photosynthesis – Calvin Cycle
• Unlike the citric acid cycle which is
catabolic, oxidizing acetyl CoA and
using the energy to synthesize ATP,
the Calvin cycle is anabolic, building
carbohydrates from smaller molecules
and consuming energy.
• Carbon enters the Calvin cycle in the
form of CO2 and leaves in the form of
sugar.
• The cycle spends ATP as an energy
source and consumes NADPH as
reducing power for adding high-
energy electrons to make the sugar.

20. Photosynthesis – Calvin Cycle
• the carbohydrate produced directly
from the Calvin cycle is not glucose.
• It is a three-carbon sugar called
glyceraldehyde 3-phosphate (G3P).
• For the net synthesis of one molecule
of G3P, the cycle must take place
three times, fixing three molecules of
CO2—one per turn of the cycle.
• Calvin cycle is divided into three
phases:
• Carbon fixation
• Reduction
• Regeneration of the CO2 acceptor.

21. Outlines
• Photosynthesis
• Modes of nutrition
• Site of photosynthesis in plants and
the structure
• Stages of photosynthesis
• Light reaction
• Calvin cycle
• Light reactions
• Light receptors
• Chlorophyll and other pigments
• Chlorophyll excitation
• Photosystems I & II
• Linear electron flow
• Circular electron flow
• Chemiosmosis in chloroplast
• Calvin cycle
• Phase I: Carbon fixation
• Phase II: Reduction
• Phase 3: Regeneration of the CO2
acceptor (RuBP)

22. Questions…?
THANK YOU!