Plastids: Types, Structure and Function (With Diagram)

In this article we will discuss about:- 1. Types of Plastids in Cell 2. Structure of Chloroplastid 3. Origin 4. Function.

Types of Plastids in Cell:

Plastids may be coloured or colour­less and are of three types. The leucoplasts are the colourless plastids principally serving the purpose of storage.

On the basis of nature of storage compound, leucoplastids are amyloplasts (starch), elaioplasts (oil) or aleuroplasts (protein). The green plastids or chloroplastids are needed for photosynthesis. Plastids also contain coloured pigments termed chromo plastids and control colour of petals and other plant parts.

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.

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 (Fig. 2.53).

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

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) surface.

The components of electron transport chain are distributed in three segments:

(i) Water to PS II (P680): Mn-protein;

(ii) PS II (P680) to PS I (P700): quinone, plastoquinone, cytochrome be, cytochrome f, plastocyanin; and

(iii) PS I (P700) to NADP+: iron-sulphur pro­tein, ferredoxin, NADP⁺.

The thylakoid membrane contains all the enzymatic components required for photosyn­thesis. Interaction between chlorophyll, electron carriers, coupling factors and other components takes place within the thylakoid membrane. Thus the thylakoid membrane is a specialized struc­ture that plays a key role in the capture of light and electron transport.

Origin of Chloroplastid:

Chloroplasts originate from pro-plastids which are small spheres with double mem­branes. The inner membrane invaginates to form vesicles in the presence of sunlight. The vesicles become transformed into larger discs. At certain regions these discs pile closely into bundles of thylakoids to form grana (Fig. 2.56). Plastids can also originate from pre-existing plastids by divi­sion and budding.

Function of Chloroplastid:

Chloroplasts are the centres of synthesis and metabolism of carbohydrates. During photosynthesis, carbon dioxide and water are converted into organic substances (sugars, polysaccharides, fats and amino acids) in the presence of light. Photosynthesis consists of a light reaction and a dark reaction, occurs in grana and stroma respectively.

During the light reaction, light energy is converted to chemical energy in thylakoid. This process involves photo­lysis of water, transfer of electron, photophosphorylation and reduction of NADP. During the dark reaction, glucose is synthesized through Calvin cycle in stroma.

This process involves utilization of NADPH and ATP to reduce CO₂ to a primary product which eventually becomes a carbohydrate. Plastidial genes (plastom) control some hereditary characters e.g., plastid inheri­tance in four O’ Clock.

Semi-autonomy: Chloroplasts exhibit a cer­tain degree of functional autonomy:

(a) Undergo multiplication by division;

(b) Contain genetic information showing cyto­plasmic inheritance (plastid inheritance in four O’clock);

(c) Presence of cp DNA (circular);

(d) Presence of plastidial ribosomes (70S);

(e) Synthesize mRNA; and

(f) Evidence of pro­tein synthesis.

According to symbiotic hypothesis, chloro­plasts may have originated from a symbiotic rela­tionship between an autotrophic microorganism (photosynthetic bacteria) and a heterotrophic host cell.