Plastids

Plastids are found in all plant cells and in euglenoides. These are easily observed under the microscope as the sone specific pigments, thus imparting specific colours to the plants. Based on the presence or absence and type of pigments plastids can be classified into chloroplasts, chromoplasts and leucoplasts.

TYPES OF PLASTIDS 

Chromoplasts : In chromoplasts fat soluble carotenoid pigments like carotene, xanthophylls and others are present. This gives yellow, orange or red colour to the part of the plant. Chlorophyll either absent or occur in very less amount Chromoplasts occurs mainly in pericarp and petals. Red colour of tomatoes is due to the red pigment "Lycopene" of chromoplasts.

• Chromoplasts occur in petals but colour in petals is mainly due to water soluble pigments which are found in cell sap. eg. Anthocyanin

Chloroplasts :- The chloroplasts contain chlorophyll and carotenoid pigments which are responsible for trapping light energy essential for photosynthesis.

Leucoplasts:- The leucoplasts are the colourless plastids of varied shapes and sizes with stored nutrients.

Amyloplasts store carbohydrates (starch), e.g. potato: elaioplasts store oils and fats whereas the elaioplasts store proteins. Pigments and lamellar structure absents in Leucoplasts Generally occurs in non green and underground plant cells.

• Different types of plastids may transform from one form to another. Because genetic material is similar

Number, Shape & Size of chloroplasts :

• Majority of the chloroplasts of the green plants are found in the mesophyll cells of the leaves.

• Number varies from 1 per cell of the Chlamydomonas a green alga.to 20-40. T cell in the mesophyll.

• These are lens-shaped, oval spherical discoid. or even ribbon shaped,

• Length and width are also variable.

          Length= 5-10 m

           Width= 2-4 um

STRUCTURE OF CHLOROPLAST

Membrane : Like mitochondria the chloroplast are also double membrane bound Out of the two, the inner membrane is relative less permeable (Outer membrane contain porins

• The space limited by the inner membrane is caled the stroma.

Components of stroma:

(a) Thylakoids : In the stroma a number of organised flattened membranous sacs are present called thylakoids.

Thylakoids are arranged in stacks like the piles of coins called grana (singular granum or the intragranal thylakoids.

Each chloroplast! contains about 40-60 granum.

Stroma lamellae or Fret channel or Stroma thylakoids are flat membranous tubules connecting the thylakoids of the different granum.

The membrane of the thylakoids enclose a space called lumen.

Chlorophyll (photosynthetic pigments) are present in the thylakoids membrane.

A photosynthesis functional unit (Located in thylakoids membrane) contains of about 250 to 400 molecules of various pigments (Chl-a, Chl-b. Carotenes, Xanthophylls etc.) is called as Quantasome.

(b)Enzymes :  The stroma of the chloroplast contains enzymes required for the synthesis of carbohydrate. die enzymes of Calvin cycle or Dark reaction and protein synthesis.

(c) DNA :Stroma contain small double-stranded circular DNA molecules.

(d) Ribosome The Ribosome of the chloroplast are smaller (70s) than the cytoplasmic ribosOmes (80s) Chloroplasts have their own genetie system & complete protein synthesis machinery (ds - DNA, RNA, Ribosome, Enzymes Amino acids) but enzymes for photosynthesis are synthesised by nucleus and chloroplast thus chloroplasts are also called as semi autonomous organelle of the cell

FUNCTION :

Photosynthesis : The chloroplasts trap the light energy of sun and transform it into the chemical energy in the form glucose.

BIOGENESIS 

(1)From Proplastid

(2)  From binary fission of pre-existing plastids.

 ORIGIN : Endosymbiotic origin by a cyanobacterium.