The Structure of Plastids | Chloroplast Structure and Function

Structure and function of plastids along with its origin, discovery and types are explained with the help of diagram below.

  • Why the leaves are green?
  • Why the colour of petals are so bright?
  • Why seeds colour are colourless?
The answer to these questions is "Plastids".

What are plastids? 

The plastid word comes from the Greek word “plastós” meaning "formed" or "molded". It is a double membrane-bounded organelle present in the cells of the plant and photosynthetic protists such as algae. It was first discovered by Ernst Haeckel and first clearly explained by A. F. W. Schimper.

Plastids often contain pigments which help in photosynthesis and different types of pigments give different colors to the cell. The important chemical substances such as starch, lipids food are produced and stored in the plastids.

Video lesson


How plastids are evolved?

Plastids are considered to be originated about 2 billion years ago as a result of the endosymbiotic relationship. Cyanobacteria-like organisms developed photosynthesis and formed an endosymbiotic relationship with a eukaryotic host and transformed into plastids producing an autotrophic line of nucleus-containing cells that were driven by light.

Types of plastids

In the meristematic region of the plant, proplastids are present, which are the key precursor of plastid formation. They are divided into different types based on functions performed in the cell. The three main types of plastids are:

Chloroplasts: In 1837, Hugo von Mohl was the first person to provides explanation of chloroplast as grain of chlorophyll. These grains of chlorophyll present in plant cells were named as chloroplast in 1883 by Andreas Franz Wilhelm Schimper. They are involved in the process of photosynthesis. They contain chlorophyll pigment which not only helps in photosynthesis but also provided green color to plants.

Leucoplasts: They are colorless plastids and present in roots, seeds, or bulbs. They are involved in the storage of proteins, starch, and lipids. They are further subdivided into different categories based on storing substances.

  • Amyloplasts stores starch storage and also helps in sensing gravity
  • Proteinoplasts stores protein and also modifies them
  • Elaioplasts stores fat
  • Tannosomes producing and storing phenolic compounds such as tannins

Chromoplasts: They contain bright color pigments and mostly present in the cells of petals of flowers and fruits to attract birds and insects and help in the pollination and dispersal of fruits.

Structure of Plastids

Let’s take the example of chloroplast to understand the structure of plastids.

Outer chloroplast membrane

The chloroplast is a double membrane-bounded organelle. The outer membrane is semi-porous allowing the passage of small molecules in and out of the chloroplast.

Inner chloroplast membrane

The inner chloroplast membrane gives rise to membranous sacs called thylakoids. It also controls the movement of materials.

Thylakoid and grana

The thylakoid membranes contain a colored pigment called chlorophyll. Thylakoids are stacked to formed grana, in which each granum contains about 10-20 thylakoids. It is the site for the light reaction of photosynthesis and converts light energy into chemical energy.

Stroma

The fluid inside the inner membrane and outside the thylakoid system is called the stroma. Grana, chloroplast DNA and ribosomes float in the stroma. It is the site for the Calvin cycle in which carbon dioxide is converted into Glyceraldehyde 3-phosphate.

Chloroplast DNA

It also contains its own DNA known as ctDNA and encodes plastids proteins to maintain the structure and function of plastids.
Chloroplast also contains ribosomes which is the site of protein synthesis.

structure of plastids, origin, types and functions

Function of plastids

The main functions of plastids are:

  • They allow the plant to prepare its food through the photosynthesis process.
  • They convert light energy into chemical energy.
  • They produce ATP (Adenosine triphosphate) which is energy currency.
  • They store important chemical substances such as starch, proteins, and lipids.
  • They give bright colors to plant and attracts birds and animals. In turn, helps in pollination and seed dispersal.

Inheritance of plastids

Plastids are tranferred from parent plants to daughter plants through their germ cells. Gymnosperms receive plastids from pollen grains, while angiosperms inherit plastids from female gametes.

Some questions and answers

1. What are the main types of plastids?
A. The plastids are divided into three main types based on function they perform. The three main types of plastids are chloroplast, leucoplast and Chromoplast.

2. What are the main functions of plastids?
A. Plastids are involved in photosynthesis process and store important chemical ssubstances suc as starch and lipids.

3. Are plastids present in animal cells?
A. No, plastids are only present in plant and algal cells and are totally absent in animal cells.

4. Where plastids are found in plant cell?
A. Plastids can be found in leaves, fruits and flowers, where they perform various functions.

5. Is plastid a double membrane organelle?
A. Yes, plastids are double membrane organelles.

6. Why the color of leaves is green?
A. The green color of leaf is due to the presence of chloroplast. Chloroplast contains special pigment called chlorophyll, which reflects green color and thus the leaf appears green in color.

7. Which type of plastid is responsible for the photosynthesis?
A. chloroplast is the site for the process of photosynthesis. It contain chlorophyll which is responsible for the capturing of sunlight. In chloroplast, light energy is converted into food energy.

8. From where does chloroplast comes?
A. According to endosymbiotic theory, Cyanobacteria-like organisms were engulfed by eukaryotic host and formed endosymbiotic relationship. These cyanobacteria-like organisms have the ability to perform photosynthesis and eventually after billions of years converted into chloroplast.

9. In which part of chloroplast Calvin cycle takes place?
A. Stroma is the site for the Calvin cycle in which carbon dioxide is converted into Glyceraldehyde 3-phosphate.

10. Where chlorophyll is present in chloroplast?
A. Chlorophyll is present in the thylakoid membranes of chloroplast. It is the site for the light reaction of photosynthesis and converts light energy into chemical energy.

11. From where does the chloroplast DNA comes from?
A. Chloroplast DNA is thought to be the DNA of engulfed cyanobacteria like organism.

12. What does word plastid means?
A. The plastid word comes from the Greek word “plastós” meaning "formed" or "molded".

References

  • Laetsch, W. M., & Stetler, D. A. (1965). Chloroplast structure and function in cultured tobacco tissue. American Journal of Botany, 52(8), 798-804.
  • Albertsson, P. Å. (1995). The structure and function of the chloroplast photosynthetic membrane—a model for the domain organization. Photosynthesis Research, 46(1), 141-149.
  • Wise, R. R. (2007). The diversity of plastid form and function. In The structure and function of plastids (pp. 3-26). Springer, Dordrecht.

Post a Comment

0 Comments