CHLOROPLAST:
-> They are double membrane bound organelles present in plant cells and algae .
-> They are the site of photosynthesis where light energy is converted into chemical energy .
-> It contains green pigment chlorophyll which captures the sunlight .
-> They are semi autonomous organelles as they have their own DNA and ribosomes .
-> They were first clearly described by HUGO VON MOHL in 1837 and later in 1864 JULIUS VON SACHS discovered chloroplast involvement in photosynthesis.
-> The endosymbiotic theory suggests that chloroplast originated from Cyanobacteria .
( ENDOSYMBIOTIC THEORY- It explains that some organelles in eukaryotic cells, like mitochondria and chloroplasts , originated as free living prokaryotic organisms that were engulfed by larger host cell. )
-> They are found only in green parts of the plants mainly in Mesophyll cells of the leaves .
*SHAPE AND SIZE :
-> Chloroplast shape varies discoid , lens shaped , cup shaped or ribbon like depending on organism .
Based on organism in which it is present the shape varies.
-> The size of chloroplast is 4-10 micrometers in diameter and 1-3 micrometres thick.
*STRUCTURE :
1) ENVELOPE- Outer double membrane which is smooth , permeable to small molecules and ions.
2) INNER MEMBRANE - It is selectively permeable as it contains the transport proteins.
3) INTER MEMBRANE SPACE - It is a narrow space of approximately 10 to 20nm between outer membrane and inner membrane .
4) STROMA- It is protein rich fluid inside the chloroplast . It contains circular DNA , 70S ribosomes , enzyme for dark reaction (KELVIN CYCLE) , starch grains and lipid droplets .
-> It is site for co2 fixation and synthesis of carbohydrates .
5) THYLAKOID SYSTEM - It is the network of flattened membrane sacs called as Thylakoid .
-> The stacks of thylakoid form grana . (singular: granum)
-> Grana are connected by stroma lamellae .
-> The thylakoid membrane contains chlorophyll A and B, carotenoids, photosystem I & II and ATP synthase enzyme .
-> It is the site for light reaction for photosynthesis .
6) THYLAKOID LUMEN - It is an internal space of thylakoid where hydrogen ions accumulate during light reaction for ATP synthesis .
* CHEMICAL COMPOSITION:
-> Chloroplast contains about 50-60% protein , 20-30% lipids , 5-10% pigments (such as chlorophyll, carotenoids) , and small amounts of DNA , RNA , and minerals.
* FUNCTIONS :
A) PHOTOSYNTHESIS - Chloroplast performs both stages of photosynthesis .
-> LIGHT REACTION: It occurs in thylakoid membrane . It requires light and water .
-> DARK REACTION (KELVIN CYCLE) : It occurs in stroma and use ATP and NADPH to fix Co2 into the glucose .
B) STORAGE AND TRANSPORT: Glucose synthesised in photosynthesis is stored as starch grains in stroma .
c) SYNTHESIS OF BIOMOLECULES : Chloroplast synthesised fatty acids , amino acids and pigments .
D) SELF REPLICATION: Due to their own DNA and ribosomes they can synthesise some proteins and can replicate independently .
E) RELEASE OF OXYGEN : Chloroplast release oxygen during photolysis of water essential for life on earth .
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