Plastids:

Double walled, DNA containing largest organelle in plant cells discovered by Haeckel

Type of plastids:

1. Chromoplasts:  

• Plastids containing different types of pigments (e.g. carotenes, Xanthophylls etc.) Chlorophylls either absent or occur in very less amount.
• Red colour of red chillies and red tomatoes is due to a red pigment Lycopene a type of carotene.
• Yellowish-orange colour of fruits is due to α–carotene, β-carotene and γ–carotene.
• β-carotene is precursor of vitamin A, and its rich source are carrot roots, Chromoplasts also occur in petals but colours of petals are mainly due to water soluble pigments which occur in cell sap e.g. Anthocyanin-Blue, violet or red pigment; Anthoclor-Yellow pigment.
• Chromoplasts containing various pigments (phycoerythrin and phycocyanin) are found in algae.

2. Leucoplasts:

Devoid of pigments and internal lamellar structures. Store food in different forms like starch (Amyloplasts), Fat and oil (Elaioplasts) and protein (Aleuroplasts).

3. Chloroplasts:

 

Green plastids discovered by Sachs

• Double unit membrane bound cell organelles next to nucleus in size.
• Shape of chloroplasts vary in different plants. Discoidal or oval – Higher plants; Girdle shaped – Ulothrix; Cup shaped – Chlamydomonas; Reticulate – Oedogonium; Spiral – Spirogyra; Stellate – Zygnema.
• The average diameter in higher plants is 4 to 6 μm.
• The number of chloroplasts is relatively constant in different plant. Algae, such as Chlamydomonas, often possess a single huge chloroplast.

Structure:

Conversion of Plastid:

Three main components; Envelope, stroma and thylakoids.

1. Envelope made of a double limiting membrane across which the molecular interchange with the cytosol occurs.
2. Stroma fills most of the volume of the chloroplasts and is a kind of gel-fluid phase that surrounds the thylakoids.
3. Thylakoids consist of flattened vesicle arranged as a membranous network. The outer surface of the thylakoid is in contact with the stroma, and its inner surface encloses an intrathylakoid space.

• In higher plants thylakoids are arranged in stack (10-15 thylakoid) called grana (singular-granum) Each chloroplast contain about 20-100 grana. Lamellae which connect grana are called stromal thylakoids or fret channels.
• In thylakoids, chlorophyll, carotenoid molecules and a reaction centre are assembled, forming two photosystems (I and II). Each photosystem is associated with an electron transport system and with structural proteins.
• Chlorophyll, the main pigment, is an asymmetrical molecules with a porphyrin head composed of four pyrrole rings and forming a complex with a Mg atom.

1. Photosystem-I: The pigment system (light harvesting system) is more abundant in membrane of stoma thylakoids and nonappressed parts of granal thylakoids. Trap centre is a special chlorophyll a molecules called P₇₀₀.
2. Photosystem-II: The pigment system is more abundant in the appressed parts of granal thylakoids. Trap centre is a special chlorophyll a molecule called P₆₈₀.
3. Cytochrome b-f complex: The complex is made of one cyt f, two cyt b, one FeS centre and a polypeptide. It takes part in electron transport. Mobile electron carriers include ferredoxin, plastocyanin and plastoquinone.
4. ATP-synthetase: It is more commonly present in nonappressed membranes of grandal thylakoids and stroma thylakoids.