Chloroplasts

 


Ultrastructure of Chloroplasts:

A  chloroplasts has three main parts _ envelope, matrix and thylakoids with pyrenoid and stigma  being two  additional  structure present in the chloroplasts of some algae.


Chloroplast Envelope: 

A  chloroplasts is covered by an envelope  made up  of two smooth membranes. The membranes are trilaminar lipoprotein structure  separated by an intermembrane space of 100 _ 200A° width. The outer membrane may be attached to endoplasmic reticulum. At places the inner membrane is connected  to thylakoids. The outer membrane is more permeable than the inner membrane. The inner membrane has more of  proteins including carrier proteins. 

Matrix:

The ground substance of a chloroplast  known as  matrix or stroma is a semifluid colloidal complex containing   DNA, RNA, ribosomes, plastoglobuli and enzymes .DNA of chloroplast or cp  DNA   is naked, circular  or occasionally  linear and chloroplast may have several copies of it. DNA makes the chloroplast genetically autonomous because it can both replicate  and transcribe to form  RNA.  Chloroplast ribosomes  are 70 S and they resemble the ribosomes  of prokaryotes.  Ribosomes help the chloroplasts to  synthesize most of the   enzymes required by it.



  The important  enzymes present in  chloroplast are those that take part in synthesis of photosynthetic pigments, photolysis of water, photophosphorylation, dark assimilation of CO2, synthesis and degradation of starch, synthesis of lipids, etc. Plastoglobuli are lipid droplets  which may contain some  enzymes, vitamins K and quinones. The chloroplasts matrix of higher plants may store starch temporarily,  as starch grains known  as assimilation starch. In  green algae ( e.g., Spirogyra, Ulothrix), the chloroplasts possess special  starch storing structure  called pyrenoids. In higher plants, pyrenoids are replaced by amyloplasts. 

Thylakoid:

Thylakoids are membrane lined flattened sacs which run throughout  the stroma or matrix of the chloroplast. They take part in photosynthesis. They are the structural elements of the chloroplast which generally run parallel but may show interconnections. They may also be attached to the inner membrane of chloroplasts envelope. 

  In the chloroplasts  of higher plants, thylakoids are stacked at places to form grana, each granum having 2_ 100 thylakoids, 40_ 60 grana may occur in a   chloroplasts. Grana are absent in algal chloroplasts which, therefore are called agranal. 

   Thylakoids are differentiated  into two types_ granal thylakoids and stroma or intergranal thyakoids. A granum is attached to only a few stroma or nongranal Thylakoids,  though it is made up of upto 100 Thylakoids. It is, thus, believed that the Thylakoids get folded and bifurcated in the region of grana. Space present in a granal Thylakoid is called loculus while that of stromal Thylakoids is  termed Fret channel. 

 Thylakoid membranes  possess photosynthetic pigments and coupling factors, latter being  involved in  ATP  synthesis. Photosynthetic pigments, which include chlorophyll a, chlorophyll b, carotenes and xanthophylls occur in specific groups called phptosystems ( previously quantasomes). There are two photosystems, one found in stromal Thylakoids and nonappressed parts of granal Thylakoids and second found in appressed  parts of granal Thylakoids. 


Functions:

1): They act as centres of photosynthesis. 

2): They are able to trap sun energy and change it into chemical energy which is used by all living organisms.

3): They pick up  carbon dioxide  and release oxygen  during photosynthesis thus keeping the percentage of the two balanced in the atmosphere. 

4): They store starch either temporarily ( in higher plants) or permanently  ( in several algae).

5): Chloroplasts of some algae provide photosensitivity because of the presence of stigma or eye spot.

6): The reducing power produced during light reaction ( NADPH2) is used in the reduction of nitrate and synthesis of amino acids.

7): Spinach chloroplasts help in   fatty acid  synthesis. 

8): They store fat in the form of plastoglobuli. 

9): They can be changed into chromoplasts.

Autonomy:

Chloroplasts possesses a great degree of functional autonomy  as it has own  DNA  which can show both replication and   transcription ( or produce RNA).  The plastid manufactures some of  proteins,  its own  enzymes  and other  biochemicals because  of the 70 S  ribosomes  which can help  translate  the coded information  contained in mRNA transcribed over chloroplasts   DNA . New chloroplasts aris either from division of pre_ existing  chloroplasts  or by the division of the proplastids, the precursors of chloroplasts. 

































































Stomach               Pancreas

RNA                    Nucleic Acids







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