Plastid Structure,Functions and Types

Plastid
The term plastid is coined by Haeckel.
Schimper named the plastids as the chloroplast.
It is double membrane organelles
Present only in the plant cell.
Cyanobacteria contain a similar structure called Chromospheres.
Plastids are derived from Proplastids that are a non-pigmented precursor.
Angiosperm plastids originated from the mother while gymnosperm plastids originated from the father.
There are three types of plastids
(A) Leucoplast –
(Gr. Leucos- White; Plastos – Moulded)
These are present in plant away from the sunlight.
Ther are colorless plastid that found in dark portion of plants.
They store food materials. They have lamellae. They are three types –

  1. Amyloplast-  store starch found in potato tuber, wheat, rice
  2. Elaioplast-  Also known as Oleosomes store fat, endosperms of seed
  3. Aleuroplast – Also known as Proteinoplasts store protein endosperms of legume seed.

(B) Chromoplast –
colorful contains xanthophyll and carotene (Red-orange in color). They do not have lamellae and chlorophyll so not photosynthetic. Change of color of chili or tomato green to red is due to the transformation of chloroplast into Chromoplast that contains lycopene pigment.
(C) Chloroplast –
Term “Chloroplast” was proposed by Schimper.
Most algae cells like Chlamydomonas have one chloroplast and higher plants have 20-40 chloroplast.
Green plastid that contains chlorophyll, xanthophyll, and carotene.
Present in the mesophyll of leave cell.
The shape of Chloroplast: –
Cup-shaped – Chlamydomonas, Chlorella.
Spiral ribbon like – Spirogyra,
Girdle shape – Ulothrix,
Star-shaped – Zygnema,
Discoid – Vaucheria,
Reticulate – Oedogonium, Cladophora.
Structure of Chloroplast:-

  1. Outer Membrane– Made up of phospholipids. Contains porins.
  2. Inter Mediate Space – called periplastidial space. Highly impermeable substances moving through transporters.
  3. Inner Membrane – Made up of phospholipids. Inner and outer membrane thickness is 25-70 Ǻ.
  4. Stroma – Ground substance of chloroplast is called matrix or stroma. Consist bundle of the membrane of lamellae called thylakoids.

Thylakoids: –  Membrane lined flattened sacs.  Thylakoids are structural elements of the chloroplast. They generally run parallel and may show interconnections.
Thylakoids inner membrane contains quantasome.
Thylakoids are arranged in stacks bundles called grana (singular granum).
Lamellae connecting grana are called Fretz channels or stromal lamellae o stroma thylakoid.
Each granum is made up of 3 layers – two outer protein layers and one phospholipid layer in between.  Middle layer contains chlorophyll, carotenoid pigments, and phospholipid.  Porphyrin part of chlorophyll pigment remains towards protein and alcohol part is towards lipid.
Chloroplast DNA:-
The matrix contains RNA, circular DNA and 70s ribosomes, starch granules and enzymes for Calvin cycle.
DNA of chloroplast is naked and is called plastidome. It is bigger than mitochondrial DNA.
When the plants are kept in dark the chloroplast form colorless Etioplasts that reform chloroplast when exposed to light.

 Function:-

  1. Leucoplast helps in-store starch, protein, and oil.
  2. The chloroplast synthesize carbohydrates through photosynthesis
  3. The balance of O2 & CO2 in nature.
  4. Conversion of Chromoplast takes place on fruit ripening.

Quantasomes:- this is the smallest structural unit of photosynthesis. These are Membrane-bound particles.
Quantasomes are made up of lipids and proteins that include photosynthetic pigments and redox carriers. term quantasome is given by Park and Biggins. Quantasome is not a participant in photoreduction reactions, but, after isolation, shows a Ca++-dependent ATPase activity. The Particle is separable from carboxydimutase, ATP-ADP exchange, and Mg++ dependent ATPase activities by gradient centrifugation. The particle reconstitutes the dark stage of photophosphorylation in uncoupled chloroplast membrane preparations. (BY STEPHEN H. WELL AND EVANGELOS N. MOUDRIANAKIS)

Chlorophyll:-
Chlorophyll is Mg containing pigment. They have four pyrrole ring that is hydrophilic bound to each other by Mg.
There are following types of chlorophyll found in cells.

  1. Chl-a: – C55H72O5N4Mg – universal found in all photosynthetic eukaryotic cell.
  2. Chl-b: – C55H70O6N4Mg- mostly in higher plants. Yellowish green in color
  3. Chl-c1: – C35H32O5N4Mg – found in algae (Bacillariophyceae and Phaeophyceae)
  4. Chl-c2: – C35H28O5N4Mg – found in algae
  5. Chl-d: – C54H70O6N4Mg – found in Rhodophyceae and Cyanobacteria
  6. Chl-e: – C55H70O6N4Mg – found in Cyanobacteria and Xanthophyceae

Reference
Wikipedia
BSER books
NCERT books
Molecular Biology of the Cell-Alberts
Cell and Molecular Biology Concepts and Experiments.Gerald karp
Cell Biology, Genetics, Molecular Biology, Evolution and Ecology by Verma, Agarwal

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