Plastids

A plastid is defined as a membrane-bound organelle (part of an organism that carries out one or more functions) found in the cells of plants and related organisms. While plastids were discovered by Ernst Haeckel, who also coined their name, they were put into a better perspective and defined by A.F.W Schimper.

The primary plastids are estimated to have been formed about 1.5 billion years ago within eukaryotes. However, the given estimate of the establishment of plastids is a topic of debate among researchers.

Where are plastids located?

Plastids are found in plant cells. They are localised within the aerial parts of the plants. All types of plastids are generally the same structure, as they have to perform similar functions. They are located over the cytoplasm and are found in the meristematic tissues within the parenchyma cells of plants. 

Structure and composition

Plastids can be divided into four main groups, namely:

1. Outer membrane: Primary covering of the organelle.
2. Inner membrane: Secondary covering of the organelle.
3. Intermembrane space: The area between the outer and inner membrane.
4. Stroma: The aqueous fluid present within the plastid surrounding the other components. 

These are commonly present within all types of plastids and form an integral part of the structure of the same species.

 Some shapes of plastids

(i)  Discoidal or oval – Higher plants.

(ii) Girdle shaped – Ulothrix

(iii)    Spiral shaped – Spirogyra

(iv) Cup-shaped – Chlamydomonas

Types of plastids

While there are other classifications of plastids, we shall be looking at the three main types of plastids: chloroplasts, chromoplasts, and leucoplasts. 

Chloroplast

These plastids, as their name suggests, contain chlorophyll. They are green due to the pigments within them. These plastids are present in the mesophyll cells of the leaves. They facilitate photosynthesis and store the food within them for the nourishment of the plants. 

Chloroplast is made from proplastids, which can be considered as the origin of the plastids. Proplastids are underdeveloped plastids, and as they develop thylakoids, chlorophyll, and other such components, they eventually evolve into chloroplasts. These are double membrane-bound structures that consist of thylakoids, which together are known as grana. 

Thylakoids consist of a thylakoid membrane, which covers the thylakoid lumen. These components together carry out the process of photosynthesis. Like the nucleus, it has a double-stranded DNA with ribosomes of the chloroplast. Stroma is a granular transparent substance filled in the cavity of the chloroplast. It is also called a matrix. Grana are embedded in it. It contains enzymes of the Calvin cycle or dark reaction. Rubisco is the most abundant enzyme on the earth. It made 16% protein of the chloroplast. 

Chloroplasts have their genetic system & complete protein synthesis machinery (ds-DNA, RNA, 70 s type of ribosome enzymes. amino Acids), thus chloroplasts are called semi-autonomous organelle of the cell.  DNA of chloroplast is called a plastic dome.

Chromoplast

Chromoplasts are coloured plastids. Unlike chloroplast, their colour is not limited to green and consists of fat-soluble pigments like xanthophyll and carotenoids (water-soluble pigments are stored in the vacuoles). They facilitate photosynthesis and carry out the task of storing coloured pigments. Chromoplasts can be seen in varieties of colourful flowers, fruits, etc. They also serve another purpose. The colourful characteristics of the parts containing chromoplasts attract insects, which help in pollination.

Leucoplast

Leucoplasts are colourless in nature and mainly act as storage vessels for food. If chloroplasts are kept out of sunlight for a particular period, they will be unable to facilitate photosynthesis, and the green colour of the leaves will start fading away, turning the chloroplasts into leucoplasts. However, the cells will not die, as leucoplasts, regardless of colour, will carry out their function of nourishing the plant. Hence, they can be considered as nourishers under difficult conditions.

Leucoplasts are of three types, amyloplasts, elaioplasts, and aleuroplasts, which work as the storage modes for carbohydrates, fats, and proteins, respectively.

Functions

• Chlorophyll facilitates the conversion of light energy (sunlight) into chemical energy and thereby facilitates photosynthesis.
• Plastids are also required for the synthesis of food.
• Chromoplast is a plastid that consists of pigments that provide colour to the plant cells.
• Chromoplasts also act as an important contributor to pollination.
• Leucoplasts act as storage agents. As they are devoid of photosynthesis, they use the food that has been stored already.

While these are the main types of plastids, there are other types, such as protoplasts, etioplasts, etc, as well. These are either previously-formed plastids or ones that form as the product of actions between the main plastids.

Cells are the building block of every living thing, and their study has helped us understand the working of organisms as a whole. Plastids can be considered one of the most important components of a plant cell due to their functions. 

Conclusion

Plastids are the site of food production and storage in plants and algae. Plastids are the double-membraned cell organelles found in plant cells and some other organisms. There are many kinds of plastids. However, chloroplasts, chromoplasts, and leucoplasts are considered the most significant ones based on their pigmentation. Chromoplasts are the hued plastids, while leucoplasts are non-pigmented. Plastids that contain chlorophyll (green pigment) are known as chloroplasts.

Plastids have various functions, including the production of food through photosynthesis, storage of fats, proteins, and oils, and adding colour to different parts of the plants like flowers, fruits, and so on.