Chloroplasts: Notes on the Ultrastructure of Chloroplasts!
The chloroplast which is a widely occurring plastid of green plants may be filamentous, saucer-shaped, spheroid, ovoid, discoid or club – shaped. It is vesicular, having a colourless centre.
It’s size varies from 2 to 3µ in thickness and 5 to 10µ long (Hall et al., 1974). In higher plants there are usually 20 to 40 per cell.
However, in many algae there may be only one or a few chloroplasts, and these may be star-shaped or a spiral ribbon or may have other exotic forms.
Ultrastructure of Chloroplasts:
Structurally each plastid consists of two parts:
(1) outer limiting membrane, and
(2) inner matrix or stroma.
1. Limiting membrane:
This is a double membrane and each membrane is trilaminar and lipoproteinous and lacks chlorophyll and cytochromes. All molecular interchanges occur between the cytoplasm and chloroplast through this limiting membrane. The outer membrane is smooth and the inner membrane is much folded into the chloroplast stroma.
The inner membrane is highly selectively permeable to molecules. Each membrane is of about 40-60 A in thickness. Intervening space between the two membranes is called the periplastidial space and is 25-75 A.
In Chlamydomonas, this is continuous with endoplasm. It serves as a semipermeable membrane reticulum and as the boundary of the plastids. Their chemical structure is essentially like that of reticulum.
2. Stroma (matrix):
Plastids are filled with a watery, proteinaceous substance called the matrix or stroma. It contains about 50% of the chloroplast proteins and most of these are of the soluble type.
It has ribosomes and also DNA. It also contains starch granules and osmiophilic droplets. During inactivity of the plastids the osmiophilic droplets increase, in number.
In the stroma are embedded disc-like flattened structures made of double membranes. These discs are called lamellae (thylakoids). The outer surface of the thylakoid is in contact with the stroma and its inner surface encloses an intrathylakoid space. Thylakoids may be stacked like a pile of coins, forming the grana or they may be unstacked called stroma thylakoids.
Thus, a granum consists of a series of membrane discs packed back to back, like a stack of coins. However, each disc is interconnected at an angle to all other discs in a granum by tubules called frets. By branching, a fret connects a disc to each of the other discs in turn.
Thylakoids provide a large membrane area to hold the photosynthetic pigments and enzymes. Thylakoids containing chlorophyll (photic apparatus) for photosynthesis, permit separation of the light reactions that occur there from the dark reactions in the chloroplast stroma that fix CO2 to synthesize sugars, starch, fatty acids and some proteins.
ATP and reduced coenzymes diffuse from thylakoids where they are formed into the stroma, where they are used as energy source and reductant respectively, for the fixation of carbon dioxide.
Maintenance of grana and frets in a chloroplast requires the presence of inorganic salts. If the salts are replaced by organic buffer, the granum opens up to form an extensive membrane system. Replacement of the ions leads to spontaneous reformation of grana and frets.
Grana vary in size from 0.3 to 1.7 µm in various species. In spinach chloroplasts, they are about 0.6 µm in diameter.
Membrane lamellae of the grana contain protein, and lipid layers, and in between these two is present the chlorophyll layer (Hodge, 1959).
The chloroplasts may contain 40-60 grana in their stroma. Each granum of the chloroplasts of higher plant cells contains 10 to 100 disc-like, superimposed membranous compartments called the thylakoids.
Recently Park and Pon (1968) have discovered some smaller paracrystalline spheroid bodies inside the membranes of thylakoids of the grana called quantasomes. These are roughly rectangular (18.0 nm × 15.5 nm × 10.0 nm). Each quantasome is composed of four subunits (Park and Beggins, 1964).
The quantasomes are composed of about equal amounts of lipid and protein, and contain chlorophyll, carotenoids and other components of the photosynthetic apparatus. Quantasomes are plates or elipsoids measuring about 200 A in diameter and 100 A thick, arranged in a regular, lateral array. These quantasomes are believed to be the units of photosynthesis and each quantasome comprises about 300 molecules of chlorophyll, necessary amount for photosynthesis.