Chloroplast is the most common plastid which contains pigments for photosynthesis. It was first observed by Antony Von Leeuwenhoek in 1679 A.D. and termed as chloroplast by A.F.W Schimper in 1883 A.D. It is bounded by two-unit membranes, an outer and an inner lipoproteinous membrane with an inter membrane space between them.
Shape, Size & Number of Chloroplast in Plant Cell
It is most common type of plastid found in all photosynthetic cells of plants, algae and some bacterias. It's shape differs from one photosynthetic cell to other. Different variation of shapes may be for accomodating and fiting inside cells to maximize cell functioning. Its size also varies from species to species. It measures about 2-3 micrometer in thickness and 5-10 micrometer in diameter.
Some common shapes of chloroplast found in different plant cells are:
- Filamentous (e.g. spirogyra)
- Ovoid (e.g. maize)
- Discoid (e.g. higher plants, number:20-40 per cell)
- Spiral ribbon like (e.g. spirogyra, number:1-16/cell)
- Cup shaped (e.g. Chlamydomonas, number:1/cell)
- Girdle/band shaped (e.g. Ulothrix, number:1/cell)
- Star shaped (e.g. zygonema)
- Reticulate/net shaped (e.g. oedogonium) etc.
Ultrastructure of Chloroplast
Microscopic structure of chloroplast consists of granum and matrix (ground substance) called stroma. Stroma are site for dark reaction. It contains important enzyme "Rubisco" about 16% (essential for carbon dioxide fixation) as well as protein, starch, enzymes etc. It also contains naked DNA, 70s ribosome, mRNA, tRNA and minerals like Mg++, Mn++, Fe++ etc.
Granum are flat, disk shaped and sac like structure (plural-grana). Each grana is connected to each other by stroma lamella or intergranal lamella. A single chloroplast contains about 50-100 grana. Each unit of granum is called thylakoid. Thylakoid contains small granular structure called quantasome. Quantasome contain photosynthetic pigments. Each quantasome contains 280 photosynthetic pigments. Out of 280 pigments, 230 are chlorophyll pigments and 50-carotenoid pigments.
It consists of chlorophyll, carotenoid and phycobilin pigments. Chlorophyll trap sunlight for photosynthesis. There are of different types chlorophyll-a, b, c, d. Chlorophyll has tadpole shape structure with tetrapyrrole porphyrin hydrophobic head and phytol hydrophobic tail. It has a magnesium ion in bivalent state (Mg++) present in middle of head region. Chlorophyll and carotenoid are common pigments, they are also called as “Green Blood”.
Chlorophyll-a is universal pigment found in all photosynthetic plants except photosynthetic bacteria. It is antenna(major) pigment with molecular formula C55H72O5N4Mg and molecular weight 893. It is bluish green in color and absorb energy from wavelength of violet-blue and orange-red. It has methyl (-CH3) functional group at 3rd carbon of 2nd pyrrole ring.
Chlorophyll-b is also found in almost all higher plants except some groups of algae. It is accessory pigment with molecular formula C55H70O6N4Mg and molecular weight 907. Its color is green and absorb energy from blue-green wavelength. It has aldehyde group(-CHO) at 3rd carbon of 2nd pyrrole ring.
Chlorophyll-c: Molecular formula C35H32O5N4Mg
Chlorophyll-d: Molecular formula C54H70O6N4Mg
Carotenoids are light absorbing pigment as well as protective in nature which protects chlorophyll from photo oxidation and damages. It is of two types-carotenes and xanthophyll. It have molecular formula C40H56 and orange in color. It is responsible for appearance of orange color of carrot, sweet potato and yellowish color of milk, fat, butter. It does not contain oxygen molecule.
Whereas xanthophylls are yellow in color with molecular formula C40H56O2. It contains oxygen molecule. The yellowish color of yolk, fat, skin is due to presence of xanthophyll.
Phycobilin are water soluble photosynthetic pigment due to presence of water-soluble protein "phycobiliproteins". It is found in red green algae, blue green algae. It contains chromophores which make it colored.
It is of different types:
Phycocyanobilin- blue phycobilin
Phycoerythrobilin- red phycobilin
Phycourobilin- orange phycobilin
Phycoviolobilin- magenta phycobilin
Function of Chloroplast
- The most fundamental and important function of chloroplast is photosynthesis that's why it is also called as kitchen of the cell.
- It helps in production of oxygen molecules by the process of photosynthesis.
- It has enzyme “RuBiSCo” which is required for fixation of CO2 in carboxylation from where glucose is formed.
- It also stores starch.
- Leaves are green due to pigment-chlorophyll present in chloroplast.
- It contains important pigments like carotene, xanthophyll, phycobilin.
Some FAQs Questions on Chloroplast
1. Is chloroplast found in all living cells?
Ans: Chlorophyll is found in all photosynthetic cells and eukaryotic algae except non green fungi.
2. What is the function of stroma in the cell?
Ans: Stroma contains all the enzymes of dark reaction and forms the site for dark reaction during photosynthesis.
3. What is the function of grana in the cell?
Ans: Grana contains all the enzymes of light reaction and forms the site for light reaction during photosynthesis.
4. Do all chloroplast are green?
Ans: Chloroplast are gren due to pigment chlorophyll. But photosynthetic red and blue-green algae contains red and blue pigments called as phycoerythrin and phycocyanin respectively in addition to chlorophyll.