Chloroplasts are organelles found in plant cells and eukaryotic photosynthetic organisms that conduct photosynthesis. They have a double membrane envelope and contain a stroma, thylakoids, and chloroplast DNA. Thylakoids contain light-absorbing pigments and perform the light reactions of photosynthesis, while the stroma is the site of the dark reactions where CO2 is fixed into sugars. Chloroplasts are essential for photosynthesis as they trap solar energy to produce ATP and NADPH via light reactions, and use these products to fix CO2 into carbohydrates via dark reactions, providing energy for plant growth.

1. CHLOROPLAST
By-
Asst. Prof. Thange D. S.
Department of Biochemistry,
N. A. C. & Sc. College,
Ahmednagar

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 conduct
photosynthesis.

3. STRUCTURE OF CHLOROPLAST
 They are planoconvex lens- like structures about 5µm diameter and 2 to 3 µm
thick.
 The chloroplast comprises the following
structures-
1. A double layered Envelope
2. A granular stroma or matrix
3. Thylakoids
4. Chloroplast DNA

4. ENVELOPE:-
 It is composed of a system of double membranes ,each of which is 50- 80 Å thick.
 The outer membrane is separated from the inner membrane by an intermembrane
space of about 10nm.
 Across this double membrane envelope, exchange of molecules between
chloroplast and cytosol occurs.

5. STROMA OR MATRIX:-
 The stroma fills most of the volume of the Chloroplast.
 It is a kind of gel-fluid phase that surrounds thylakoids (Grana).
 It contains about 50% of the protein of Chloroplast.
 It contains Ribosomes and DNA molecule.
 The stroma is the site of CO2 fixation and where the
synthesis of sugar, starch, fatty acids & some proteins
oocurs.

6. THYLAKOIDS:-
 The thylakoids consist of flattened and closed vesicles arranged
as membranous network.
 It may be stacked like a neat pile of coins forming GRANA.
 There may be 40-80 grana in the matrix of a Chloroplast.
 Thylakoids includes light-absorbing pigments, a complex chain
of electron carriers, & an ATP-synthesizing apparatus.
 Light reaction occurs in thylakoid membranes.

7. CHLOROPLAST DNA:-
 Chloroplast DNA (cpDNA) is also known
as plastid DNA (ptDNA).
 This is circular double stranded DNA
molecule.
 Chloroplast genome size is about
120-217kb .

8. FUNCTIONS OF CHLOROPLAST
 The main function of chloroplast is Photosynthesis.
 Process of photosynthesis consists of the following
two steps:
1) Light reaction: It is also known as Hill reaction,
photosynthetic electron transfer reaction or photochemical
reaction. In light reaction, solar energy is trapped in the form
of chemical energy of ATP & NADPH. During this, Oxygen
is evolved by photolysis.

9. 2) Dark reaction: It is also known as Calvin cycle, photosynthetic carbon reduction cycle
(PCR cycle), carbon fixation reaction or thermochemical reaction. Here, the reducing
capacity of NADPH & energy of ATP is utilized for the conversion of CO2 to
carbohydrate. It occurs in Stroma.
 Starch synthesis occurs within the stroma of chloroplast.
 From chloroplast, some of the intermediates are transported to cytosol for sucrose
synthesis.