Describe the structure & function of chloroplasts found in higher plants with help of labelled diagram.

Question

Describe the structure & function of chloroplasts found in higher plants with the help of a labelled diagram.

Answer 1

Plastids were discovered and named by E.Haeckel (1865 – 1865). Schimper (1883-85) named the plastids as chioroplast which participate in photosynthesis. Plastids are bounded by two unit membranes and are found in the cytoplasm of eukaryotic plant cells. The chioroplasts are green due to chlorophyll and are slightly flat and round. They synthesize and store many bio-chemical substances. They are absent in animals, fungi & prokaryotic cells. The blue-green algae (cyanobacteria) have similar primitive structures called chromospheres.

Structure of Chloroplast:

They are bounded by two membranes made up of proteins and fats. Their thickness is 25 to 70A° and they have a broad inter-membranous space called periplastidial space. The dimensions of the chloroplasts is 5 to l0p x 2 to 4p.m. The inner jelly like fluid part of the chloroplast is called as stroma which consists of protein, starch, plastoglobuli, 70S ribosome, DNA and water etc.

The stroma consists of bundles of membranous lamellae called thylacoids. These thylakoids are arranged in stacks known as grana (Singular-granum). Every granum includes 2 to 100 thylacoids. The chlorophyll is found in the thylacoids and they are sites for the process of light reactions of photosynthesis. The thylacoids are interconnected by stroma lamellae. The inner surface of membranes of thylacoids are attached with minute granules called quantasome.

The name quantasome was given by Rodric B. Park and John Biggins (1994). Every quantasome consists of about 230 pigment molecules including photosynthetic pigments and redox carriers. The diameter of quantasome is 150 to 180A. The chemical composition of chloroplast is proteins (40 to 50%),’Lipids (23 to 25%), RNA (4%), DNA (0.03%), Chlorophyll (1 to 2%), carotinoids and trace elements like iron, zinc, manganese, magnesium, copper etc.

When the plants are kept in dark for longer period, the chloroplast form colourless etioplasts which contain lipids and volatile oils. These etioplasts reform chloroplasts when exposed to light.

Functions of Plastids:

1. The chloroplasts synthesize carbohydrates food through photosynthesis. The light reaction takes place in grana and the dark reaction in the stroma.
2.  Leucoplasts help in storage of proteins, starch and oil. Such as amvloplasts, elaiplasts & aleuroplasts.
3. Chloroplasts are also the site for photorespiration. The photorespiration needs chloroplasts, peroxysomes and mitochondria.
4. They release oxygen by degrading water.
5. They consume atmospheric CO₂ and clean the