The word Chloroplast is derived from the Greek word chloros meaning green. Chloroplast means green cell. Chloroplasts are the largest green colored organelles in the cytoplasm that produce sugary food in the process of photosynthesis. It is green in color because it contains more green pigment called chlorophyll. It is called the cell’s kitchen, carbohydrate food factory and energy conversion organ. A.F.W Schimper first observed it in plant cells in 1883 and named it chloroplast. Julius von Sachs (1832-1897), the father of plant physiology, discovered chloroplasts.
Location of chloroplasts
Green leaves, young green stems, unripe fruits, green shoots etc. contain chloroplasts in palisade cells. C3 plants have granulated chloroplasts. C4 plants have granulated and non-granulated chloroplasts.
Number of chloroplasts
Each cell contains one or more chloroplasts. Its number is 10-40 in higher plant cells. Spirogyra has 1-16 chloroplasts and Chara has hundreds of chloroplasts. Each cell of Mnium has 106 and Ricinus communis plant has 400,000 chloroplasts per square millimeter.
Size and shape Chloroplast
The lens-shaped chloroplasts of higher plant cells are 3-5 microns in diameter. Spiral chloroplasts of Spirogyra are longer than the length of the cell.
Chloroplasts of algae are more diverse. In higher plant cells, chloroplasts are lens-like. Chloroplasts vary in shape in lower class plant cells. Spiral (Spirogyra), star-shaped (Zygnema), cup-shaped (Chlamydomonas), round (Pithophora), reticulate (Oedogonium), ribbon or ring-like (Ulothrix) etc.
Origin of Chloroplast
In lower plants, old chloroplasts divide to form new chloroplasts. They originate from primitive plastids in higher plants. In the presence of sunlight, the original plastid transforms into a mature chloroplast. In the absence of sunlight, chloroplasts transform into leucoplasts.
Structure of Chloroplast
- Membrane: Each chloroplast is covered by a bilayer membrane. Outer cover and inner cover. The outer layer is called the outer layer and the inner layer is called the inner layer. The space between the two coats is called the periplastidal space. Distance of periplastidal space is 6-8 nm. The outer coat contains porin proteins. Chloroplast membranes contain glycosyl glycerides instead of phospholipids. It gives specific shape to chloroplast and protects it from external injury.
- Stroma: The transparent, granular and semi-fluid material inside the chloroplast is called stroma. It is a colloidal and hydrophilic liquid. The stroma contains about 200 DNA, RNA and circular DNA or plasmids. It contains protein plastogaebuli, osmophilic granules, sugar, oil droplets, vitamins, mineral salts etc. The stroma contains glucose-forming enzymes and a variety of rubisco enzymes. C3 and C4 cycles occur in the stroma.
- Thylakoid: The sac-shaped three-dimensional structure in the stroma is called thylakoid. It is called the structural and functional unit of chloroplast. Its width is 100-300 Å. Thylakoid membranes contain photosystem-I, photosystem-II, various types of electron carriers, ATP-ase enzymes etc. Inside the thylakoid cells are chlorophyll-a, chlorophyll-b, carotenoids, xanthophylls and enzymes. These elements together look like crystalline grains. Then they are called quantosomes.
- Granum: 10-100 thylakoids arranged in a row forming a stack is called granum. Some quantosomes are present on the inner surface of the granum cycle membrane. Each chloroplast contains 40-60 grana. Each granule is 0.3-1.70 microns in size.
- Stroma lamellae: Two adjacent thylakoids are connected by ducts called stroma lamellae or intergranular frets. Stroma lamellae contain chlorophyll.
- ATP Synthase: The round or oval object in the thylakoid membrane is called ATP synthase. It contains enzymes for making ATP. It makes ATP for cells.
- Photosynthetic Unit: Photosynthetic units are present on the thylakoids. It is called photosynthetic unit. Each photosynthetic unit contains 300-400 molecules of chlorophyll-a, chlorophyll-b, carotene and xanthophyll. It contains various enzymes, phospholipids, sulfolipids, metal ions, quinones, etc. Photosynthetic units absorb light energy and cause photophosphorylation.
- DNA: Chloroplasts contain short and circular DNA. DNA has its own characteristics. This is called chloroplastic DNA. Each chloroplast contains 200 cpDNA. Without cpDNA, photosynthesis cannot occur in chloroplasts.
The chloroplast genome consists of 120–160 kb. It consists of inverted duplicate repeats. It contains the coding sequence of 120 types of proteins.
- Ribosomes: Chloroplasts contain 70S ribosomes. It synthesizes proteins.
- Unidentified granules: Chloroplasts contain some unidentified granules. Nothing is known about their nature and function yet.
Chemical Structure of Chloroplast
- Protein: About 35-55% of the dry weight of chloroplasts is protein. Of this, insoluble protein is 80% and soluble protein is 20%.
- Lipids: 10-20% of the dry weight of chloroplasts are lipids. Among them, phospholipids are the most.
- Dyes: It contains 6-12% dyes. Among them 75% chlorophyll-a, 25% chlorophyll-b, carotene, xanthophyll etc.
- Nucleic acid: Chloroplast contains 5% nucleic acid. Nucleic acids are DNA and RNA.
- Mineral Salt: It contains 3% mineral salt. Mineral salts are high in magnesium. Besides, ion and copper are worth mentioning.
- Carbohydrates: Chloroplasts do not contain specific amounts of carbohydrates. It is always changing. Starch is its simple carbohydrate.
Importance of Chloroplast
- Energy conversion: Chloroplast is the only converter of solar energy in the living world. Chloroplasts convert solar energy into chemical energy.
- Food production: Chloroplast is the only green organelle present in the plant body. It produces sugary food in the process of photosynthesis. As a result, the food needs of the living world are met.
- Protein production: 70S ribosomes are present in chloroplasts. It synthesizes proteins. So plants that have more chloroplasts produce more protein.
- Nucleic acid production: Nucleic acid is the most essential element of the organism. Chloroplasts play an important role in making nucleic acids. Nucleic acid acts as the container and carrier of heredity in organisms.
- Cytoplasmic heredity: Cytoplasmic heredity is a hot topic in modern research. Chloroplastic DNA of cells plays an important role in cytoplasmic inheritance. Heredity holds its own characteristics.
- Photophosphorylation: Chloroplast produces ATP by combining ADP and Pi in the presence of sunlight. This process is called photophosphorylation. ATP stores and supplies energy.
- Photorespiration: Photorespiration or light respiration takes place in the chloroplasts of plants. In this process, oxygen is taken in and carbon dioxide is released. Photolysis is a harmful process.
- Photolysis: Photolysis occurs in chloroplasts. In this process, H2O is broken down to produce O2.
- Carbon dioxide fixation: Carbon dioxide fixation occurs in the chloroplast quantum. This phenomenon occurs in the light neutral phase of photosynthesis.
- Stored material: Enzymes, co-enzymes, DNA, RNA, sugars, proteins, fats etc. are stored in the matrix of chloroplasts.
- Production of phosphoglyceric acid: It uses absorbed energy to produce 3-carbon phosphoglyceric acid from 6-carbon sugars.
Role of chloroplasts in food production
Chloroplast is the only green organelle present in the plant body. It produces sugary food (glucose) in the process of photosynthesis.
- Ribulose 1, 5 bisphosphate reacts with atmospheric CO2 to form 3-phosphoglyceric acid.
- 3-phosphoglyceric acid is then converted to 1,3-bisphosphoglyceric acid.
- 1, 3 Bisphosphoglyceric acid produces 3-phosphoglyceraldehyde and dihydroxyacetone phosphate.
- Fructose 1, 6 bisphosphate is produced from 3-phosphoglyceraldehyde.
- Fructose 1,6 bisphosphate is converted to fructose 6-phosphate.
- Glucose 6-phosphate is produced from fructose 6-phosphate
- Glucose/food is produced from glucose 6-phosphate.
Ecological importance of chloroplasts
- Chloroplasts of green plants produce O2 during photosynthesis. Plants and animals take in O2 and release CO2 in the process of respiration. As a result, the atmosphere of the environment is purified.
- Green plants release water vapor in the process of respiration. This water vapor condenses and forms precipitation.
- Plants provide shade and keep the environment cool. It makes the living environment happy.
- Green gardens and meadows create a unique environment.
- Green plants increase soil fertility.
- High chloroplast content in plants increases fruit and crop production. As a result, the food demand is met. In this orderly condition prevails in the living environment.
- Green grass is used as animal feed. Cattle consuming green grass have higher milk and meat production capacity.
- Green algae produce large amounts of oxygen in the water of ponds, reservoirs, rivers, seas and oceans. It keeps the water environment clean.
- Green chlorella is used to keep the bathing environment clean. Chlorella algae produce oxygen.
- Green forests meet demand for wood and fuel. Necessary furniture is made from wood.