In this essay we will discuss about:- 1. Meaning of Plastids 2. Types of Plastids 3. Shape 4. Size 5. Chemical Composition 6. Structure 7. Functions 8. Autonomy.
- Essay on the Meaning of Plastids
- Essay on the Types of Plastids
- Essay on the Shape of Plastids
- Essay on the Size of Plastids
- Essay on the Chemical Composition of Plastids
- Essay on the Structure of Plastids
- Essay on the Functions of Plastids
- Essay on the Autonomy of Plastids
Essay # 1. Meaning of Plastids:
The term plastid was introduced by E. Haeckel in 1866. Plastids are semi-autonomous organelles having DNA and double membrane envelope which store or synthesise various types of organic compounds.
With the exception of some protistans, (e.g., Euglena, dinophyceae, diatoms) plastids are restricted to plants only. Plastids develop from colorless precursors called pro-plastids. Pro-plastids have the ability to divide and differentiate into various types of plastids.
Essay # 2. Types of Plastids:
Depending upon their colour, plastids are of three main types— leucoplasts, chromoplasts and chloroplasts.
(i) Leucoplasts (Gk. leucos- white, plastos- moulded).
They are colourless plastids which generally occur near the nucleus in non-green cells and possess internal lamellae. Grana and photosynthetic pigments are absent. Leucoplasts have variable size and form, e.g., rounded, oval, cylindrical, filamentous, etc.
There are three types of special leucoplasts:
They are the starch containing leucoplasts. An amyloplast is several times larger than the original size of leucoplast. It contains a simple or compound starch grain covered by a special protein sheath, e.g., Potato tuber, Rice, Wheat,
(b) Elaioplasts (Lipidoplasts, Oleoplasts):
The colourless plastids store fat, e.g., Tube Rose,
(c) Aleuroplasts, Proteoplasts or Proteinoplasts:
The plastids contain protein in the amorphous, crystalloid or crystallo- globoid state (e.g., aleurone cells of Maize grain, endosperm cells of Castor).
(ii) Chromoplasts (Gk. chroma- colour, plastos- moulded):
The plastids are yellow or reddish in colour because of the presence of carotenoid pigments. Chlorophylls are absent. Chromoplasts are formed either from leucoplasts or chloroplasts. Lamellae degenerate partially or completely during chromoplast formation. Change of colour from green to reddish during the ripening of Tomato and Chili is due to transformation of chloroplasts to chromoplasts.
The orange colour of Carrot roots is due to chromoplasts. The pigments are often found in crystallized state so that the shape of the plastids can be like needles, spindles or irregular,
(i) Chromoplasts provide colour to many flowers for attracting pollinating insects.
(ii)They provide bright red or orange colour to fruits for attracting animals for dispersal.
(iii)They are also the site of synthesis of membrane lipids.
(iii) Chloroplasts (Gk. chlorosgrass green, plastos-moulded).
They are greenish plastids which possess photosynthetic pigments, chlorophylls and carotenoids, and take part in the synthesis of food from inorganic raw materials in the presence of radiation energy. Chloroplasts of algae other than green ones are called chromatophores (e.g. rhodoplasts of red algae, phaeoplasts of brown algae).
The number of chloroplasts per cell of algae is usually fixed for a species. The minimum number of one chloroplast per cell is found in green alga Ulothrix and several species of Chlamydomonas.
However, different species of a 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. An inter nodal cell of Chara (an alga) has several hundred chloroplasts.
Essay # 3. Shape of Plastids:
In algae the chloroplasts have various shapes. They may be plate like (e.g., Ulothrix), cup-shaped (e.g., Chlamydomonas), ribbon-like (e.g., Spirogyra), polygonal or stellate (e.g., Zygnema) and reticulate (e.g., Oedogonium). The chloroplasts of higher plants are generally disc-shaped with oval or circular outline. Rarely, they may be lens-shaped, rounded or club-shaped.
Essay # 4. Size of Plastids:
Like shape, the size of the chloroplasts is different in different species. The discoid chloroplasts of higher plants are 4—10 pm in length and 2-4 pm in breadth.
The size is generally larger in case of polyploid cells as compared to diploid and haploid cells. Normally it is much smaller than that of the cell. However, in many algae the chloroplast may occupy almost the whole length of the cell, e.g., Spirogyra. The chloroplast of Spirogyra may reach a length of 1 mm.
Essay # 5. Chemical Composition of Plastids:
Protein- 50-60%. Lipids- 25-30%. Chlorophyll- 5-10%. Caro-tenoids (carotenes and xanthophylls)- 1—2%. DNA- upto 0.5%. RNA- 2-3%. Vitamins К and E, quinones, Mg, Fe, Co, Mn, P, etc.- in traces.
Essay # 6. Structure of Plastids:
A chloroplast has three parts— envelope, matrix and thylakoids. Pyrenoid and stigma are two additional structures present in the chloroplasts of some algae.
A chloroplast is covered by an envelope made up of two smooth membranes. Each membrane is about 90—100 A thick. It has trilaminar lipoprotein structure. The two membranes are separated by an inter-membrane space of 100-200 A width.
The outer membrane may be attached to endoplasmic reticulum. At places the inner membrane is connected to thylakoids. As in mitochondria, the outer membrane is more permeable than the inner membrane. The inner membrane has more of proteins including carrier proteins.
The ground substance of a chloroplast is known as matrix or stroma. It is semifluid colloidal complex that is made of 50% soluble proteins. The remaining is DNA, RNA, ribosomes, plastoglobuli and enzymes. Chloroplast or cpt DNA is naked, circular or occasionally linear.
A chloroplast may have several copies of it. 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 the chloroplast is able to synthesize most of the enzymes required by it. The important enzymes present in chloroplast are those that take part in synthesis of photosynthetic pigments, photolysis of water, photophosphorylation, dark assimilation of CO2, synthesis and degradation of starch, synthesis of lipids, etc.
Plastoglobuli are lipid droplets of 10-500 nm diameters. They may contain some enzymes, vitamin К and quinones.
The chloroplast matrix of higher plants may store starch temporarily, as starch grains. It is known as assimilation starch. In green algae (e.g., Spirogyra, Ulothrix), the chloroplasts possess special starch storing structures called pyrenoids.
Thylakoids (Menke, 1961):
They are membrane lined flattened sacs which run throughout the stroma or matrix of the chloroplast. Since, they take part in photosynthesis, they are also called photosynthetic thylakoids. Thylakoids are thus the structural elements of the chloroplast. They generally run parallel but may show interconnections. Thylakoids may also be attached to the inner membrane of chloroplast envelope.
In the chloroplasts 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 bundle sheath and algal chloroplasts. The latter are, therefore, agranal.
Because of the presence of grana, thylakoids are differentiated into two— granal thylakoids and stroma or interregnal thylakoids. A granum is attached to only a few stroma or nongranal thylakoids, though it is made up of upto 100 thylakoids. It is, therefore, believed that the thylakoids get folded and bifurcated in the region of grana.
Thylakoid membranes possess photosynthetic pigments and coupling factors. Coupling factors are involved in ATP synthesis. Photosynthetic pigments include chlorophyll a, chlorophyll b, carotenes and xanthophylls.
They occur in specific groups called photosystems (previously quantasomes). There are two photosystems, I and II. Photosystem II occurs in appressed parts of granal thylakoids while photosystem I am found in stromal thylakoids and nonappressed parts of granal thylakoids.
Essay # 7. Functions of Plastids:
Chloroplasts are the centers of photosynthesis or formation of organic compounds from inorganic raw materials. The organic substances, thus synthesised, not only provide body building material to autotrophic plants themselves but also to all heterotrophic plants as well as animals.
2. Energy Transduction:
Chloroplasts are able to trap sun energy and change it into chemical energy. The chemical energy is used by all living organisms to perform their life activities.
3. Consumption of Carbon Dioxide:
Chloroplasts pick up carbon dioxide and use the same in photosynthesis. This keeps the percentage of this gas balanced in the atmosphere as carbon dioxide is being constantly added to it through combustion and respiration.
4. Liberation of Oxygen:
Chloroplasts liberate oxygen which is passed into the atmosphere. This keeps the balance of oxygen constant in the atmosphere, as oxygen is being consumed in respiration and combustion.
5. Storage of Starch:
They store starch either temporarily (in higher plants) or permanently (in several algae).
Chloroplasts of some algae provide photosensitivity because of the presence of stigma or eye spot.
7. Reducing Power:
The reducing power produced during light reaction (NADPH) is used in the reduction of nitrate and synthesis of amino acids.
8. Synthesis of Fatty Acids:
Murphy and Leech (1978) have reported the synthesis of fatty acids in Spinach chloroplasts.
9. Storage of Lipids:
Chloroplasts store fat in the form of plastoglobuli.
10. Formation of Chromoplasts:
They can be changed into the chromoplasts to provide colour to many flowers and fruits for attracting animals.
Essay # 8. Autonomy of Plastids:
Though chloroplasts are under the overall control of the nucleus of the cell, they possess a great degree of functional autonomy:
(i) A chloroplast have its own DNA. The DNA is naked. It can show both replication and transcription (or produce RNA).
(ii) The plastid manufactures some of its own proteins, enzymes and other biochemical because of the presence of 70 S ribosomes which can help translate the coded information contained in mRNAs transcribed over chloroplast DNA.
(iii) New chloroplasts arise either from division of pre-existing ones or the division of their precursors known as pro-plastids.
Sphaerosomes or Oleosomes:
Sphaerosomes (= spherosomes) are small cell organelles bounded by single membrane which take part in storage and synthesis of lipid. They were discovered by Pemer in 1953. Sphaerosomes are small spherical and refractile vesicles which are 0.5-1.0 pm in diameter.
They arise from endoplasmic reticulum and are surrounded by a single but half unit membrane with phospholipid monolayer having polar heads towards the cytosol and hydorphobic tails towards the inner side. The membrane is stabilized by proteins called oleosins (Buchanan et al, 2000). 98% of a sphaerosome is lipid. Proteins constitute the remaining 2%.
Some proteins are probably enzymatic and take part in the synthesis of lipids. Because of the presence of lipids, sphaerosomes can be seen under light microscope after staining the cells with Sudan dyes and osmium tetra oxide.
Sphaerosomes occur abundantly in the endosperm cells of oil seeds. Sphaerosomes of some tissues (e.g., tobacco endosperm, maize root tip) contain hydrolytic enzymes. Therefore, they are considered to have lysosomic activity.