The chloroplast which is the seat of photysynthesis in cells has a covering of two membranes with an inter membrane space. These membranes are smooth and there are no perforations or partticles. The membranes are differentially permeable (Mudrak 1969). A section of the chloroplast reveals an intricate system of membranes enclosed in a granular matrix. These mem­branes are called lamellae and the surrounding matrix – the stroma.

In a sectional view, the lamellae can be seen packed and these stacks are called thylakoids. In the chloroplasts of higher plants, the thylakoids themselves form highly copmact bundles called grana. Some thylakoids of granum extend into the stroma and maintain contact with other grana. These are called – Stroma thylakoids or stroma lamellae or intergrana.

The chloroplasts of algae lack the granum arrangement and in cyanobacteria (blue green algae), the thylakoids lie naked in cytoplasm without any enve­lope. In such instances, pigments are uniformly distributed on or in the lamel­lae.

The stroma has in addition to the lamellae – granules (globuli), lipid drop­lets, starch grains and vesicles.


Structure of the lamellae:

All the photosynthetic pigments are concentrated in the chloroplast lamellae. The lamellae as well as the outer membranes are composed of lipoprotein sub units. Each sub unit according to Weir and Benson has a protein core surrounded by a lipid sheath. The chloroplast membranes have a single layer of sub units, whereas in thylakoids they are double due to the appressing of two membranes. The appressed areas are called partitions.

These are hydrophobic in nature. The space between the two membranes of a thylakoid (fret membrane) enclose a space called fret channel. The space between two thylakoids is called loculus while the ends of disc shaped thylakoids are called margins.

These areas (loculus, fret chan­nel) are hydrophilic. The chlorophyll molecules are concentrated in the fret channels i.e., in the space between the two membranes of thylakoids. The chlorophyll molecules may be entirely in the space or the heads may be in the fret channel while the tails are buried in the sub units. Occassionally the entire chlorophyll molecule may be found in the fret membrane. Other pig­ments like cytochromes, carotenoids etc. are also found in the fret mem­brane.