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 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 chloroplasts 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 a larger host cell).
-> They are found only in green parts of the plants, mainly in mesophyll cells of plants.
-> This is present in algae but absent in fungi and bacteria.
*SHAPE AND SIZE :
-> Chloroplast shape varies: discoid, lens-shaped, cup-shaped, or ribbon-like depending on the organism.
Based on the organism in which it is present, the shape varies.
-> The size of chloroplasts is 4-10 micrometres 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 transport proteins.
3) INTERMEMBRANE SPACE - It is a narrow space of approximately 10 to 20nm between the outer membrane and inner membrane.
4) STROMA- It is a protein-rich fluid inside the chloroplast. It contains circular DNA, 70S ribosomes, enzymes for the dark reaction (Calvin cycle), starch grains and lipid droplets.
-> It is the 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 thylakoids 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 of the light reaction for photosynthesis.
6) THYLAKOID LUMEN - It is an internal space of the thylakoid where hydrogen ions accumulate during the 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 the thylakoid membrane. It requires light and water.
It produces ATP, NADPH, and oxygen.
-> DARK REACTION (Calvin cycle): It occurs in the stroma and uses ATP and NADPH to fix CO2 into glucose.
B) STORAGE AND TRANSPORT: Glucose synthesised in photosynthesis is stored as starch grains in the stroma.
c) SYNTHESIS OF BIOMOLECULES: Chloroplasts synthesise 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: Chloroplasts release oxygen during the photolysis of water essential for life on Earth.
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