Plastids are double membre bounded cytoplasmic organneles found in all plant cells and in euglenoides.
They are easily observed under light microscope. E. Haeckel (1866) introduced the term plasmid.
On the basis of types of pigments they contain, Schimper (1883) classified them in three types:
(i) Leucoplasts- Colourless plastids
(ii) Chromoplasts – Coloured plastids (other than green)
(iii) Chloroplasts- Green plastids
All the three types of plastids can change one form into another. Further, all plastids have a common precursor called pro-plastid. The pro-plastids are colourless undifferentiated plastids found in meristematic cells.
They can divide and differentiate into any of the three types of plastids:
(i) Leucoplasts:
These are colourless, non-photosynthetic plastids found in those cells of plants which are not exposed to sunlight. They possess membranous lamellae that do not form thylakoids. They are the storage organneles and on the basis of stored food they are of three types.
Amyloplasts. They store starch, and found in underground stems (e.g. potato), cereals (e.g. rice, wheat) etc.
Elaioplasts (Lipidoplasts or oleoplasts). They store oils and found in the seeds of castor, mustard, coconut etc.
Aleuroplasts (Proteoplasts or proteinoplasts). They store proteins and found in seeds (maize)
(ii) Chromoplasts:
These are coloured plastids other than green. They are non-photosynthetic which synthesize and store carotenoid pigments. They provide colour to various parts of the plants which attract insects for pollination & dispersal of seeds. They also synthesize membrane lipids. During ripening of tomato and chili chloroplasts transformed into chromoplasts.
(iii) Chloroplasts (Green plastids):
The chloroplasts are green or chlorophyll containing plastids concerned with photosynthesis. The chloroplasts of algae, other than green ones (such as red and brown algae) are called chromatophores.
Number:
A leaf mesophyll cell may contain 40-50 chloroplasts; a square millimeter of leaf contains some 500,000. The number of chloroplasts per cell in algae is usually fixed for a specie.
The minimum number of one chloroplast per cell is found in green alga Ulotlirix arid several species of Chlamydomonas. However, different species of a genus may have different number of chloroplasts, for example—1 in Spirogyra indica. 16 in Spirogyra rectospora. The internodal cell of the green alga Chara possesses several hundred chloroplasts.
Shape and Orientation:
The shape of a chloroplast varies from species to species. It may be cup-shaped (e.g., Chlamydomonas), (e.g., Vaucheria), Girdle (e.g., Ulothrix), Stellate or Star-shaped (e.g., Zygnema), Reticulate or net-like (e.g., Cladophora, Oedogonium), Spiral or ribbon or scalariform (e.g., Spirogyra), ovoid or disc or spheroid in higher plants (Fig. 3.26).
The chloroplasts are usually found with their broad surfaces parallel to the cell wall. They can reorient in the cell under the influence of light. For example, gathering along the walls parallel with the leaf surface under low or medium light intensity. Under damaging, very high light intensity, they can orient themselves perpendicular to the leaf surface.
Size:
They are generally 4-10/µm in length and 2-4 n m in width. In many algae, the chloroplast may occupy almost the whole length of the cell, such as in green alga Spirogyra, where it may reach a length of 1 mm.
Ultra-structure:
A chloroplast has three types of membranes enclosing three types of compartments. The membranes are: outer membrane, inner membrane & a system of thylakoid membranes, while the compartments are: inter-membrane space, stroma & thylakoid space.
Each chloroplast is surrounded by chloroplast envelope which consists of outer & inner membranes. Both the membranes are separated by a fluid-filled inter-membrane space of 10-20 nm width. The outer membrane is freely permeable due to the presence of porin proteins, while the inner membrane is semipermeable. Sometimes extensions of outer membrane called stromules found to connect adjacent chloroplasts. The membranes of all plastids including chloroplast consists of entirely glycosylglycericles (=galactolipids and sulfolipids) rather than phospholipids.
The inner membrane encloses a fluid-filled space called stroma, which is analogous to the mitochondrial matrix. The stroma contains: thylakoids, various enzymes, protein synthetic machinery (i.e. 2-6 copies of circular DNAs, RNAs & 70S ribosomes), plastoglobuli, certain metallic ions (Fe, Mn, and Mg) starch grains etc. In green algae, proteinous pyrenoids present around which starch deposits in layers”.
The stroma contains a membrane system which consists of many flattened, fluid-filled sacs called thylakoids or lamellae. About 2-100 thylakoids are stacked like a pile of coins forming grana. In a typical chloroplast, as many as 40-60 grana may be present. Adjacent grana are interconnected by stroma lamella or frets. The C4 plants – maize, sugarcane- possess two type of chloroplasts (i.e. chloroplast dimorphism), agranal chloroplasts (inside bundle-sheath cells) & granal chloroplasts (inside mesophyll cells).
The thylakoid membrane system carry four protein assemblies i.e. Photosystem I (PS-I) Photosystem II (PS-ID), electrone transport system (ETS) consisting of cytochromes b and f & CF0-CF1 particles (ATP syntetase).
Photosystems are the light harvesting complexes (LHC) each made up of:
(1) An antenna complex of hundreds of photosynthetic pigments and
(2) A reaction centre consisting of one or more chlorophyll-molecules bound to a matrix of protein. PS-II is more abundant in the stacked membranes of the grana: PS-I is more abundant in non appressed thylakoid membranes (Fig. 3.28).
Functions of Chloroplasts:
1. Chloroplasts are the site of photosynthesis. The light reations occur in thalkoid membranes while dark reactions occur in stroma.
2. Chloroplasts convert radiant energy of sun light into chemical energy of sugars, hence called as biological transducers.
3. Chloroplasts store fat droplets in form of plastoglobuli which also contain vit-K and vit-E.
4. Synthesis of fatly acids in the chloroplast has also been reported in some plants (e.g., spinach)
5. Chloroplasts in some algae render photosensitivity because of the presence of stigma or eye spot.
6. They can be transformed into the chromoplasts which provide beautiful colours to many flowers and help in attracting insects, birds and animals for pollination and dispersal.
Endomembrane system:
It is a group of functionally coordinated membrane organelles that are suspended in the cytoplasm within a eukaryotic cell. It includes the nuclear envelope, the endoplasmic reticulum, the Golgi apparatus, lysosomes and vacuoles. The endomembrane system does not include the membranes of mitochondria, plastids, peroxisomes etc. whose functions are not coordinated.