Chloroplast: Structure & functions

Structure of chloroplast and its functions

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.