Plastids

               PLASTIDS

👉 Plastics are found in all plant cells and in euglenoids. These are easily observed under the microscope as they are large. They bear some specific pigments,thus imparting specific colours to the plants. Based on the presence or absence and type of pigments plastids can be classified into Chloroplast, Chromoplast and Leucoplasts.

TYPES OF PLASTIDS

(1) Chromoplasts:- In chromoplasts fat soluble carotenoid pigments like carotene, xanthophyllas and others are present. This gives yellow, orange or red colour to the part of the plant. Chlorophylls 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.

👉 Chromoplast occurs in petals but colour in petals is mainly due to water soluble pigments which are found in cell SAP eg.:- Anthocyanin

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

(3) Leucoplasts:- The Leucoplasts are the colourless plastids of varied shapes and sizes with stored nutrients: Amyloplasts store carbohydrates (starch),eg.,potato;elaioplasts store oils and fats whereas the aleuroplasts store proteins. Pigments and lamellar structure absents in Leucoplasts.Generally occurs in on 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 20-40 per cell in the mesophyll.

👉 These are land-shaped, oval, spherical, discoid or even ribbon shaped.

👉 Length and width are also variable.

        Length = 5-10 micron metre

        Width  =  2-4 micron metre 

   STRUCTURE OF CHLOROPLASTS 

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

👉 The space limited by the inner membrane is called the stroma.

       Components of stroma:

(a) Thylakoids:-

👉 In the stroma a number of organised flatted membraneous sacs are present called Thylakoids.

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

👉 Each chloroplasts contains about 40-60 granum.

👉 Stroma lamellae or fret channel or stroma thylakoids are flat membraneous 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 thailikoids 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,(i.e. 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)

👉 Chloroplast have their own genetic system & complete Protein synthesis machinery (ds-DNA, RNA, ribosome, enzymes, amino acids) but enzymes for photosynthesis are synthesised by both genes of nucleus and chloroplast thus chloroplast 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.

Ribosomes

Cytoskeleton