Chloroplasts, essential organelles for photosynthesis in plants and eukaryotic organisms, are complex structures composed of three membranes that create distinct internal compartments. They contain their own circular DNA, which encodes proteins necessary for photosynthesis, including the enzyme ribulose bisphosphate carboxylase (rubisco). Photosynthesis in chloroplasts occurs in two phases: light reactions in the thylakoid membrane and dark reactions in the stroma, with ATP synthesis driven by an electrochemical gradient created during these processes.

1. CHLOROPLAST
by Bibrita Bhar
M. Sc. 1st year student of
School of Biotechnology
Madurai Kamaraj
University
HOUSE OF
PLANTPHOTOSYNTHETIC
MACHINERY

2. INTRODUCTION
The term Chloroplast was first described by Nehemiah Grew and
Antonie Van Leeuwenhoek.
“Chloro” means green while“ Plast” means living.
Chlorophyll pigments present in the chloroplast imparts the
green colour to plants.
Chloroplasts are present in plants and other eukaryotic organisms
that conducts photosynthesis.

3. INTRODUCTION
Responsible for photosynthesis, are in many respects similar to mitochondria.
Chloroplasts are larger and more complex than mitochondria, and they
perform several critical tasks in addition to the generation of ATP.
Chloroplasts synthesize amino acids, fatty acids, and the lipid components of
their own membranes.
The reduction of nitrite (NO2
-) to ammonia (NH3), an essential step in the
incorporation of nitrogen into organic compounds, also occurs in chloroplasts.

4. STRUCTURE
Plant chloroplasts are large organelles (5 to 10 μm long).
Chloroplast envelope: 1) the inner membrane 2)outer membranes and 3)thylakoid membrane.
The thylakoid membrane forms a network of flattened discs called thylakoids, which are frequently
arranged in stacks called grana. Because of this three-membrane structure, the internal organization of
chloroplasts is more complex than that of mitochondria.
The three membranes divide chloroplasts into three distinct internal compartments: (1) the
intermembrane space between the two membranes of the chloroplast envelope; (2) the stroma, which
lies inside the envelope but outside the thylakoid membrane; and (3) the thylakoid lumen.
Chloroplasts are highly structured, membrane-rich organelles.
Outer membrane
Inner membrane
Thylakoids
Granum
Stroma
Outer membrane
Inner membrane
Thylakoids
Granum
Stroma

5. STRUCTURE
Distinctive composition of
lipid of thylakoid
membrane: only 10%
phospholipids; the majority,
80%, are uncharged mono- and
digalactosyl diacylglycerols,
and the remaining 10% are the
sulfolipids sulfoquinovosyl
diacylglycerols (quinovose is 6-
deoxyglucose).
The acyl chains of these lipids
have a high degree of
unsaturation, which gives the
thylakoid membrane a highly
fluid character.

6. CHLOROPLAST GENOME
Like mitochondria, chloroplasts
contain their own genetic system,
reflecting their evolutionary origins
from photosynthetic bacteria.
The genomes of chloroplasts consist
of circular DNA molecules present in
multiple copies per organelle.
Chloroplast genomes are larger and
more complex than those of
mitochondria, ranging from 120 to
160 kb and containing approximately
120 genes..
Function Number of
genes
Genes for the genetic
apparatus
rRNAs (23S, 16S, 5S, 4.5S) 4
tRNAs 30
Ribosomal proteins 21
RNA polymerase subunits 4
Genes for photosynthesis
Photosystem I 5
Photosystem II 12
Cytochrome bf complex 4
ATP synthase 6
Ribulose bisphosphate
carboxylase
1

7. CHLOROPLAST GENOME
The chloroplast genome encodes approximately 30 proteins
that are involved in photosynthesis, including components of
photosystems I and II, of the cytochrome bf complex, and of
ATP synthase.
In addition, one of the subunits of ribulose bisphosphate
carboxylase (rubisco) is encoded by chloroplast DNA. Rubisco
is the critical enzyme that catalyzes the addition of CO2 to
ribulose-1,5-bisphosphate during the Calvin cycle.
Not only is it the major protein component of the chloroplast
stroma, but it is also thought to be the single most abundant
protein on Earth, so it is noteworthy that one of its subunits is
encoded by the chloroplast genome.

8. CHLOROPLAST & PHOTOSYNTHESIS
Photosynthesis occurs in two distinct phases:
1. The light reactions, which use light energy to generate NADPH and ATP.
The light reactions occur in the thylakoid membrane and involve processes that resemble
mitochondrial electron transport and oxidative phosphorylation.
In photosynthetic prokaryotes, which lack chloroplasts, the light reactions take place in the
cell’s inner membrane or in highly invaginated structures derived from it called
chromatophores.
2. The dark reactions, actually light-independent reactions, which use NADPH and ATP
to drive the synthesis of carbohydrate from CO2 and H2O.
In eukaryotes, the dark reactions occur in the stroma through a cyclic series of enzyme-
catalyzed reactions.

9. Light reactions occur in
the thylakoids (PSII) and
stroma lamella (PSI).
Dark reactions occur
in the stroma

10. Chlorophylls a and b
Ring structure in “head”
(absorbs light)
Tail
When a photon strikes its energy
can be transferred to an electron
in the “head” region. The
electron is excited, raised to a
higher electron shell, with greater
potential energy
PRINCIPLE PHOTORECEPTOR

11. LHC light harvesting complex
LHCI, PSI, and ATP
synthase are all in the
stroma lamella or on the
edge of a grana
ORGANISATION OF FOUR MAJOR PROTIENS

12. Protons diffuse to the site of ATP synthase
Transfer of electrons and protons in the thylakoid membrane

13. CHEMIOSMOTIC GENERATION OF ATP IN CHLOROPLASTS AND
MITOCHONDRIA
In terms of its role in generation
of metabolic energy, the
thylakoid membrane of
chloroplasts is thus equivalent
to the inner membrane of
mitochondria. The inner
membrane of the chloroplast
envelope does not function in
photosynthesis.
Instead, the chloroplast ETC is
located in the thylakoid
membrane, and protons are
pumped across this membrane
from the stroma to the thylakoid
lumen. The resulting
electrochemical gradient then
drives ATP synthesis as protons
cross back into the stroma.

14. GENERATION OF ATP IN CHLOROPLASTS AND ATP SYNTHASE
Ion concentration
differences and electric
potential differences across
membranes are a source of
energy that can be utilized
As a result of the light
reactions the stroma has
become more alkaline
(fewer H+ ions) and the
lumen more acid (more H+
ions)
Hydrophilic
Hydrophobic
The internal stalk and
much of the enzyme
complex located in the
membrane rotates during
catalysis.
The enzyme is actually a
tiny molecular motor

15. PROTEIN IMPORT INTO THE CHLOROPLAST STROMA
Proteins are targeted for import into
chloroplasts by a transit peptide at their
amino terminus. The transit peptide
directs polypeptide translocation
through the Toc complex in the
chloroplast outer membrane and the
Tic complex in the chloroplast inner
membrane. This peptide is then
removed by proteolytic cleavage within
the stroma. Both cytosolic and
chloroplast chaperones (Hsp60 and
Hsp70) are required for protein import.

16. PROTEIN IMPORT INTO THE CHLOROPLAST LUMEN
Proteins are imported into the
thylakoid lumen in two steps.
The first step is import into the
chloroplast stroma. Cleavage of
the transit peptide then exposes
a second hydrophobic signal
sequence, which directs protein
translocation across the
thylakoid membrane.

17. THANK YOU