Plastids: Characteristics, Types & Functions

• Plastid is the membrane-bounded organelle that is the container of the cell. It is the container of some pigments, which are the center of some cell activities (mainly the site of photosynthesis).
• They can move freely.
• Present in all plant cells but absent in bacteria and fungus.
• Present in the primitive cell that is exceptional, e.g. Euglena
• In prokaryotic cells, they are unorganized and randomly distributed but in eukaryotic cells, they are organized and membrane-bounded.

Based on structural complexity plastids are of four kinds:

1. Ribosome-like particles
2. Chromatophores
3. Cytoplasmic lamellae or lamellosome
4. Lamellate plastid or euplastid; stays in a eukaryotic cell, they contain grana in which the pigments are arranged properly. Euplastid is called grana plastid or chloroplast.

Types of plastids

A variety of plastids exist, and developmental and environmental factors determine their identity and abundance. Depending on the pigments, plastids are of different kinds:

1.Chloroplasts: photosynthetic plastids.

2.Amyloplasts: are starch-storing plastids;

3.Leucoplast: no pigment, contain starch and fatty acids, colorless).

4.Chromoplast: carotenoid pigment-accumulating plastids; colorful except green.

5.Proplastids: undifferentiated plastids that can differentiate into the different types of plastids.

6.Etioplasts: are chloroplast progenitors that form in darkness and accumulate chlorophyll precursors (prolamellar bodies (PLB)) that are ready for rapid differentiation upon illumination.

7.Elaioplasts: store lipids in lipid droplets known as plastoglobules.

8.Gerontoplasts: form during senescence, owing to resource recycling through the disassembly of the photosynthetic machinery and autophagy.

 

Chloroplast

The plastids that contain mostly chlorophyll pigment and are present in the eukaryotic cell, are true plastids.

Chlorophyll is absent in the border parenchymatous cell of maize leaves. 

Number

ØNumber of chloroplast per cell generally ranges from 20-40.

ØIn rare cases, 3-4 chloroplast/cell in some plant cell

ØIn Micromonas pusilla, single chloroplast/cell

Size and shape

Generally lens-shaped or biconcave.

Besides those, some are net-like, stellate, ribbon bell-like, spiral-shaped.

• Spiral shaped: Spirogyra
• Reticulate shaped: Oedogonium
• Stellate shaped (Star-shaped): Zygnema
• Cup shaped: Chlamydomonas

In the case of biconcave shape, diameter is 3-5 µm and thickness is 1-7 µm.

Shape and size may change.

Position and distribution

Chemical Composition

Chemical composition of chloroplast based on dry weight:

• Protein           : 50-69% (of which about 80% soluble)
• Lipid              : 21-34%
• Chlorophyll   : 5-8%
• Carotenoids   : 0.7-1.1%
• r-RNA            :  1-7.5%
• DNA               : 0.1-0.2%
• Carbohydrate : variable

Ultra Structure of chloroplast

The structure of chloroplast was studied by Steinomann and Wolken in 1952. 

According to the electron microscopic view, a chloroplast consists of the following components:

1. Membrane
2. Stroma
3. Grana or grana thylakoid
4. Stroma lamellae
5. Plastoglobulus
6. DNA
7. RNA
8. Ribosome

 Membrane

• ØDouble layer membrane also called semi-permeable membrane composed of lipoprotein.
• ØAverage breadth 50Å and thickness 40-60 Å.

Stroma

• The transparent, non-green, granular, homogenous jelly-like substances are called stroma/matrix. Many numbers of enzymes are present in the stroma.

Grana

• It is present in the stroma and is made up of many granum discs (40-60).
• The diameter of the granum disc ranges from 0.25-0.8 µm.
• Each granum disc has two layers. The distance between the two-layer is 65-70 Å.

In Euglena and Micromonas, a single granum disc is present in grana. The space of the granum disc is called the loculus. Probably, many chlorophyll molecules are present in that space.

A granum disc is double layered and has a covering membrane called thylakoid. Thylakoid is made up of lipid (45%) and protein (55%).

Granum discs are of three types:

1. with quantosome
2. with smaller particles
3. with uneven plane

 

Quantosome

The crystalline materials organized on the inner membrane of the granum disc are called quantosome. Probably these are the smallest unit of photosynthesis.

Pirk and Beggins (1964)

The volume of quantosome is 185×155×100 Å and each quantosome is consist of four subunits (diameter of each 80-90Å).

Each quantosome possesses several enzymes, in addition, it has:

1. 230-mole chlorophyll molecules (160-mole Chl-a, 70-mole Chl-b)
2. 48-mole carotenoids
3. 46-mole quinone
4. 116-mole phospholipid
5. 114-mole di-galactosyl di-glycerides
6. 48-mole sulpholipid

Plastoglobulus

Lipids can likely be exchanged between the plastoglobuli monolayer and the outer leaflet of the thylakoid membrane. Under conditions where the lipid: protein ratio in thylakoid membranes changes, the lipid pool in plastoglobuli can correct the altered lipid: protein ratio by either adding to or taking up lipids from thylakoid membranes.

Stroma lamellae

The tube which connects the neighboring two grana is called stroma lamellae (thickness 30Å).

Two stroma lamellae also connect to form a channel. Fret discovered this channel, that’s why the name of this channel is Fret channel.

 DNA

Circular DNA (10-20µm) is also present in the stroma.

Self-replicated within the chloroplast and able to transcribe into different RNAs.

1.  RNA: m-RNA, t-RNA, r-RNA present.
2.  Ribosome: 70s ribosome present.

Functions of Chloroplast

•  Their main function is to carry out the photosynthesis process involving the conversion of light energy into the energy of chemical bonds used for the synthesis of organic compounds and produce energy for plant growth, development, and defense.
• The Chloroplasts’ proteome consists of several thousand proteins that, besides photosynthesis, participate in the biosynthesis of fatty acids, amino acids, hormones, vitamins, nucleotides, and secondary metabolites.
•  Proteins that are essential for photosynthesis retains in the chloroplast genome.
• They also play a significant role in cytoplasmic inheritance.