Chloroplasts are organelles found in plant and algal cells that conduct photosynthesis. They contain their own DNA and can replicate independently. Chloroplasts have a double membrane structure and a thylakoid membrane system within a protein-rich stroma. They vary in shape and number per cell depending on the plant species. Chloroplasts capture sunlight using chlorophyll and convert it to chemical energy through photosynthesis, and also perform other functions like amino acid synthesis.

1. Chloroplast
PRAKASH DHAKAL
M. Sc. Public Health Microbiology
Tribhuvan University, Nepal

2. Chloroplasts (Chloro- green, plasts- living)
 Chloroplasts are found only in plant and photosynthetic algae. (Humans
and other animals do not have chloroplasts).
 The number of chloroplasts per cell varies depending upon the type of algae
and plant species.
 Although they exist within cells, chloroplasts are sometimes referred to as
“semi- autonomous organelles” because they contain their own DNA and can
move, change shape and divide by self.
Functions
 The main function of chloroplasts is to conduct photosynthesis, where the
photosynthetic pigment chlorophyll captures the energy from sunlight and
converts it and stores in the form of energy.
 Chloroplasts carry out a number of other function, including amino acid
synthesis and the immune response of plants.

4. Ultra Structure of chloroplast
 In different plants chloroplasts have different shapes, they vary from
spheroid, ovoid, discoid or club – shaped. It is vesicular, having a colorless
centre.
 It’s size varies from 2 to 3 µm in thickness and 5 to 10 µm long. In higher
plants there are usually 20 to 40 chloroplast per cell.
 However, in many algae there may be only one or a few chloroplasts, and
these may be star-shaped or a spiral ribbon or may have other forms.
 The chloroplast are double membrane bound organelles and are the site
of photosynthesis.
 The chloroplasts consists of the outer membrane, the inner membrane
and the Thylakoid system.

5.  The outer and the inner membrane of the chloroplast enclose a semi-gel
like fluid known as the stroma. This stroma makes up much of the volume
of the chloroplast, the thylakoids system floats in the stroma.
 Outer membrane - It is a semi-porous membrane and is permeable to
small molecules and ions, which diffuses easily. The outer membrane is
not permeable to larger proteins.
 Intermembrane space (Periplastidial space) - It is usually a thin
intermembrane space of about 10-20 nm and it is present between the
outer and the inner membrane of the chloroplast.
Inner membrane -
 The inner membrane of the chloroplast forms a border to the stroma. It
regulates passage of materials in and out of the chloroplast.

6.  In addition of regulation activity, the fatty acids, lipids and carotenoids are
synthesized in the inner chloroplast membrane.
Stroma (Matrix)
 Stroma is a alkaline, aqueous fluid which is protein rich and is present
within the inner membrane of the chloroplast.
 It has ribosomes and DNA. It also contains starch granules and many
proteins.
 In the stroma, disc like flattened structures called Thylakoid are found.
(Greek word "thylakos" which means 'sack‘). The collection of thylakoid is
called thylakoid systems.
 Chlorophyll is found in the thylakoids and is the site for the process of light
reactions of photosynthesis to happen.

7.  The thylakoids are arranged in stacks known as grana. Each granum
contains around 10-20 thylakoids.
 Thylakoids are of two types - granal thylakoids and stromal thylakoids.
 Granal thylakoids are arranged in the grana and are pancake shaped
circular discs, which are about 300-600 nm in diameter.
 The stromal thylakoids are in contact with the stroma and are in the form
of helicoid sheets.