The term plastid was introduced by E. Haeckel in 1886. Plastids are semi-autonomous organelles having DNA and double membrane envelope which store or synthesise various types of organic compounds. Plastids develop from colourless precursors called proplastids. The plastids are large cytoplasmic organelles found in eukaryotic cells. They are absent in all prokaryotes and eukaryotic animal except Euglena and Volvox. On the basis of colour, plastids are of three main types:-
i) Leucoplasts
ii) Chromoplasts
iii) Chloroplasts
i) Leucoplasts:- They are the colourless plastids and do not contain any pigments. They usually perform the function of storage of reserve food material. There are three types of spherical leucoplasts. a) Amyloplasts. They are the starch contaibg leucoplasts. An amyloplast is several times larger than the original size of leucoplast, e.g. potato tuber, rice, wheat. b) Elaioplasts. The colourless plastids store fat, e.g. Tube rose. c) Aleuroplasts, Proteoplasts or Proteinoplasts. The plastids contain protein in the amorphous or crystalloid state, e.g. endosperm cells of Castor.
ii) Chromoplast:- the plastids are yellow or reddish in colour because of the presence of carotenoid pigment. Chlorophylls are absent. Chromoplasts are formed either from leucoplasts or chloroplasts. Change of colour from green to reddish during the ripening of tomato and chilli is due to transformation of chloroplasts to chromoplasts. The orange colour of carrot roots are due to chromoplasts. Some important functions related to chromoplasts are;
a) Chromoplasts provide colours to many flowers for attracting pollinating agents.
b) They provide bright red or orange colour to fruits for attracting animals for dispersal.
c) They are also the site of synthesis of membrane lipids.
iii) Choroplasts:- They are greenish plastids which posses photosynthetic pigments, chlorophylls and carotenoids, and take part in the synthesis of food from the inorganic raw material in the presence of sun light.
The number of chloroplasts per cell of algae is usually fixed for a species. However different species of different genus may have different number of chloroplasts, e.g. 1 in Spirogyra indica and 16 in S. rectospora. A photosynthetic leaf chlorenchyma cell has 20-40 chloroplasts.
Usually chlororplasts are spherical, oval or discoidal in shape. They are cup-shaped in chlamydomonas and ribbon-shaped in in spirogyra.
Ultrastructure:- A chloroplast has three parts- envelop, matrix and thylakoids.
Chloroplast Envelop:- A chloroplast envelop is made up of two smooth membranes. The outer membrane is more permeable than inner membrane. the outer membrane may be attached to endoplasmic reticulum. At places the inner membrane is connected to thylakoids.
Matrix:- The ground substance of chloroplast is known as matrix or stroma. It is semifluid colloidal complex that is made up of 50% of soluble proteins.The remaining is DNA, RNA, ribosomes and enzymes. Chloroplast DNA is naked, circular or occasionally linear. DNA makes the chloroplast genetically autonomous because it can both replicate and transcribe to form RNA. Chloroplast ribosomes are 70 S. They resemble the ribosomes of prokaryotes. With the help of ribosomes chloroplast is able to synthesize most of the enzymes required by it.
Thylakoids :- They are membrane lined flattened sacs which run through out the stroma or matrix of the chloroplast. Since, they take part in photosynthesis. Thylakoids are thus the structural elements of the chloroplast. In the chloroplast of higher plants, thylakoids are stacked at places to form grana. 40-60 grana may occur in a chloroplast. Each granum has 2-100 thylakoids. Grana are absent in algal chloroplast. The latter are, therefore, agranal.
Because of the presence of grana, thylakoids are differentiated into two- granal thylakoids and stroma thylakoids. thylakoids other than grana present in matrix or stroma are called stroma thylakoids.
Functions:-
i) Photosynthesis:- Chloroplasts are the centres of photosynthesis and formation of organic compounds from inorganic raw material.
ii) Energy Transduction:- Chloroplasts are able to trap sun energy and change it into chemical energy.
iii) Consumption of carbon Dioxide:- Chloroplasts pick up carbon dioxide and use the same in photosynthesis.
iv) Liberation of Oxygen:- Chloroplasts liberate oxygen which is passed into the atmosphere. This keeps the balance of oxygen constant in the atmosphere.
v) Storage of starch:- They store starch either temporarily or permanently.
vi) Photosenstivity:- Chloroplast of some algae provide photosenstivity because of the presence of stigma or eye spot.
vii) Storage of Lipids:- Chloroplast store fat in the form of plastoglobuli.
viii) Formation of chromoplast:- They can be change into the chromoplasts to provide colour to many flowers and fruits.
i) Leucoplasts
ii) Chromoplasts
iii) Chloroplasts
i) Leucoplasts:- They are the colourless plastids and do not contain any pigments. They usually perform the function of storage of reserve food material. There are three types of spherical leucoplasts. a) Amyloplasts. They are the starch contaibg leucoplasts. An amyloplast is several times larger than the original size of leucoplast, e.g. potato tuber, rice, wheat. b) Elaioplasts. The colourless plastids store fat, e.g. Tube rose. c) Aleuroplasts, Proteoplasts or Proteinoplasts. The plastids contain protein in the amorphous or crystalloid state, e.g. endosperm cells of Castor.
ii) Chromoplast:- the plastids are yellow or reddish in colour because of the presence of carotenoid pigment. Chlorophylls are absent. Chromoplasts are formed either from leucoplasts or chloroplasts. Change of colour from green to reddish during the ripening of tomato and chilli is due to transformation of chloroplasts to chromoplasts. The orange colour of carrot roots are due to chromoplasts. Some important functions related to chromoplasts are;
a) Chromoplasts provide colours to many flowers for attracting pollinating agents.
b) They provide bright red or orange colour to fruits for attracting animals for dispersal.
c) They are also the site of synthesis of membrane lipids.
iii) Choroplasts:- They are greenish plastids which posses photosynthetic pigments, chlorophylls and carotenoids, and take part in the synthesis of food from the inorganic raw material in the presence of sun light.
The number of chloroplasts per cell of algae is usually fixed for a species. However different species of different genus may have different number of chloroplasts, e.g. 1 in Spirogyra indica and 16 in S. rectospora. A photosynthetic leaf chlorenchyma cell has 20-40 chloroplasts.
Usually chlororplasts are spherical, oval or discoidal in shape. They are cup-shaped in chlamydomonas and ribbon-shaped in in spirogyra.
Ultrastructure:- A chloroplast has three parts- envelop, matrix and thylakoids.
Chloroplast Envelop:- A chloroplast envelop is made up of two smooth membranes. The outer membrane is more permeable than inner membrane. the outer membrane may be attached to endoplasmic reticulum. At places the inner membrane is connected to thylakoids.
Matrix:- The ground substance of chloroplast is known as matrix or stroma. It is semifluid colloidal complex that is made up of 50% of soluble proteins.The remaining is DNA, RNA, ribosomes and enzymes. Chloroplast DNA is naked, circular or occasionally linear. DNA makes the chloroplast genetically autonomous because it can both replicate and transcribe to form RNA. Chloroplast ribosomes are 70 S. They resemble the ribosomes of prokaryotes. With the help of ribosomes chloroplast is able to synthesize most of the enzymes required by it.
Thylakoids :- They are membrane lined flattened sacs which run through out the stroma or matrix of the chloroplast. Since, they take part in photosynthesis. Thylakoids are thus the structural elements of the chloroplast. In the chloroplast of higher plants, thylakoids are stacked at places to form grana. 40-60 grana may occur in a chloroplast. Each granum has 2-100 thylakoids. Grana are absent in algal chloroplast. The latter are, therefore, agranal.
Because of the presence of grana, thylakoids are differentiated into two- granal thylakoids and stroma thylakoids. thylakoids other than grana present in matrix or stroma are called stroma thylakoids.
Thylakoids membranes posses photosynthetic pigments and coupling factor ( involved in ATP synthesis). There are twp photosystems, I and II present on the thylakoid membranes. Photosystem II occurs in appressed parts of granal thylakoids while photosystem I is found in stromal thylakoids and nonappressed parts of granal thylakoids.
Functions:-
i) Photosynthesis:- Chloroplasts are the centres of photosynthesis and formation of organic compounds from inorganic raw material.
ii) Energy Transduction:- Chloroplasts are able to trap sun energy and change it into chemical energy.
iii) Consumption of carbon Dioxide:- Chloroplasts pick up carbon dioxide and use the same in photosynthesis.
iv) Liberation of Oxygen:- Chloroplasts liberate oxygen which is passed into the atmosphere. This keeps the balance of oxygen constant in the atmosphere.
v) Storage of starch:- They store starch either temporarily or permanently.
vi) Photosenstivity:- Chloroplast of some algae provide photosenstivity because of the presence of stigma or eye spot.
vii) Storage of Lipids:- Chloroplast store fat in the form of plastoglobuli.
viii) Formation of chromoplast:- They can be change into the chromoplasts to provide colour to many flowers and fruits.
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