Read this article to get information on the Chemical Composition of Chloroplasts!
By chemical analysis these consist of lipids and proteins basically. Lipids are mainly phospholipid components of the membranes and lamellae.
Proteins also contain enzymes which are fully present in mature plastids. Various components are present in plastids in the following percentage by dry weight:
Chloroplasts contain proteins, lipids, and chlorophyll, carotenoids, RNA and DNA. Starch granules or osmiophillic droplets are also present in it.
Proteins form a part of chloroplast membrane and lamellae, and are also present in the matrix in the form of enzymes.
Lipids are mainly phospholipids, fats, sterols and waxes found in the lamellae and wall of the plastid.
Chlorophyll is of two types-chlorophyll a and chlorophyll b. These are found in green plants and algae. Chlorophyll is an asymmetrical molecule having a hydrophyllic head made of four pyrrole rings bound to each other to form a porphyrin ring. This part of the molecule is similar to haemoglobin and cytochromes.
In chlorophyll, however, there is a Mg atom forming a complex with four rings (In animal pigments the Mg is replaced by Fe).Chlorophyll has a long hydrophobic chain attached to one of the rings. In chlorophyll b there is a —CHO group in place of the —CH3 group.
Chlorophyll absorbs light mainly in the red region of the spectrum which causes the green colour of green plants.
Chlorophyll molecules are disposed with in the thylakoid membrane in close association with integral proteins forming several complexes that have important functions in photosynthesis.
Carbohydrates are found in a very low percentage and in chloroplasts the common carbohydrates are starch and sugar phosphates.
DNA molecule in chloroplast was first of all reported by Ris and Plaut (1962) in Chlamydomonas (alga) and later it was reported in chloroplasts of other algae and higher plants. Chloroplast DNA resembles closely with bacterial DNA and differs from nuclear DNA in many respects.
RNA is found in ribosomes of plastids. RNAs are of two types—24S rRNA and 16S rRNA. In chloroplasts are also found aminoacyl-tRNAs, aminoacyl-tRNA synthetases and methionyl-tRNA.
Chlorophyll protein complexes:
The main chlorophyll complexes are —
1. Photosystem I or PS I complex:
It contains chlorophyll and a pigment called P 700 (because it absorbs light at 700 nm).
2. Photosystem II or PS II complex:
It contains chlorophyll and pigment P 680, P 700, and P 680 are reaction centres of the respective photosystems. PS I and PS II are photochemically active. PS I and PS II have the electron acceptors on the stromal side, while electron donors are inside toward intrathylakoid space.
3. Light-harvesting complex (LHCP):
In mature chloroplast it contains about half of the chlorophyll molecules. LHCP is photochemically inactive. It apparently transfers captured light energy to the photosystems particularly to PS II.
Besides these chlorophyll-protein complexes, there are other macromolecular complexes which lack chlorophyll. These are —
(a) Cytochrome f-b6 complex:
It contains cytochromes f and b6 and plastocyanine-a copper containing protein. This complex is involved in the electron-transport chain of the chloroplast.
(b) CFO or HFO:
It is a hydrophobic protein complex that functions in proton translocation and binds to С FI to form the ATPase complex. CF1 is a coupling factor found in chloroplasts. It is similar to FI particle of mitochondria.
It is involved in the photophosphorylation of ADP to ATP. CF1 particles are localized in the outer surface of stroma thylakoids membrane and not found in grana thylakoids membrane. CF1 particles are attached to the membrane by way of the CFO segment, which represents a proton channel made of hydrophobic proteins.