Question

Describe the ultrastructure & function of plastid

Answer 1

Chloroplastid:

The most common of the plastids are the chloroplasts. Chloroplastids are the essential organelles of plant, responsible for synthesis of carbohydrates, utilizing solar energy. Chloro­phyll which is considered as the elixir of life is present along with proteins and lipids in the chloroplastids.

Chloroplasts may be spherical, ovoid or discoid in higher plants and stellate, cup shaped or spiral as in some algae. They are usu­ally 4-6 µm in diameter and 20 to 40 in number in each cell of higher plants, evenly distributed throughout the cytoplasm.

Three Dimensional Figure of a Chloroplast

Structure of Chloroplastid:

The chloroplast is bounded by two lipoprotein membranes, an outer and an inner membrane, with an inter-membrane space between them. The inner membrane encloses a matrix, the stroma which contains small cylindri­cal structures called grana. Most chloroplasts con­tain 10-100 grana

Each granum has a number of disc-shaped membranous sacs called grana lamellae or thylakoids (80-120Å across) piled one over the other. The grana are intercon­nected by a network of anastomosing tubules called inter-grana or stroma lamellae (Fig. 2.54). Single thylakoids, called stroma thylakoids, are also found in chloroplasts.

Electron dense bodies, osmophilic granules along with ribosomes (70S), circular DNA, RNA and soluble enzymes of

Submicroscopic Structure of the Plant Chloroplasts as seen in Cross Section

Fig. 2.54: Schematic diagram of the Submicroscopic Structure of the Plant Chloroplasts as seen in Cross Section

Calvin cycles are also present in the matrix of stroma. Chloroplasts thus have three different mem­branes, the outer, the inner and the thylakoid membrane. The thylakoid membrane consists of lipoprotein with greater amount of lipids which are galactolipids, sulpholipids, phospholipids.

The inner surface of thylakoid membrane is gra­nular in organization due to small spheroidal quantosomes (Park & Pon). The quantosomes are the photosynthetic units, and consist of two structurally distinct photosystems, PS I and PS II, containing about 250 chlorophyll molecules.

Each photosystem has antenna chlorophyll com­plexes and one reaction centre in which energy conversion takes place. In higher plants the pig­ments present are chlorophyll-a, chlorophyll-b, carotene and xanthophyll.

The two photosystems and the components of electron transport chain are asymmetrically distributed across the thylakoid membrane (Fig. 2.55). Electron acceptors of both PS I and PS II are on the outer (stroma) surface of the thylakoid membrane. Electron donors of PS I are on the inner (thylakoid space) .