Plastids: Definition, Function, Types Of Plastids

Plastids are found in the cells of plants and algae that are responsible for manufacturing and storing food. These are double-membrane organelles that contain pigments helpful in Photosynthesis and also govern the change in the colours of the cells. The plastid is a semi-autonomous, self-reproducing organelle. Plastids are remnants of a photosynthetic organism that was engulfed by the host, although not all are now photosynthetic. Plastid genomes encode genes for rRNAs, tRNAs and between about 28 and 150 proteins. Plastids can be categorized into 4 main groups: chloroplasts, cyanelles, apicoplasts and non-photosynthetic.

In this article, we will provide you with all the necessary plant cell information regarding Plastids including what are plastids, types, etc. Read on to know everything about Plastids.

What are Plastids?

Plastids are the largest cell organelle in plants. These are responsible for photosynthesis and act as storage devices for starch and help in the synthesis of various molecules like fatty acids and terpene.

Types of Plastids and Functions of Plastids

Plastids are further divided into 3 types that have different functions and some have biological pigments as well.

1. Leucoplasts
2. Chromoplasts
3. Chloroplasts

Leucoplasts

These colourless Plastids possess internal lamellae and do not contain photosynthetic and grana pigments. These are found near cells that are not exposed to sunlight like underground stems, seeds, rhizomes, roots and are used to store food materials. These can be subdivided into 3 categories:

i) Proteinoplast or Aleuroplast – Responsible for storing proteins.
ii) Amyloplast – Store and synthesize carbohydrates.
iii) Elaioplast or Lipidoplast or Oleoplast – These store oils and lipids.

Chromoplasts

Coloured Plastids found in petals and fruits having fat-soluble carotenoid pigments are Chromoplasts. These are non-green in colour and give part of plant colours like red, yellow, orange. The various colours imparted help in attracting pollinators.

If you have ever seen a non riped tomato or chill, it is green in colour. These turn red after ripening due to the replacement of chlorophyll molecules in chloroplasts (lycopene in tomatoes and capsanthin in chillies) and that is how chloroplasts are changed to chromoplast.

Chloroplasts

These are the sites where food is synthesised after Photosynthesis. Chloroplasts have chlorophyll and carotenoid pigments that are responsible for food generation using sunlight. These are double membraned organelle with a biconvex shape.

Etioplasts are a type of Chloroplasts that are not exposed to light. These are usually found in flowering plants that grow in dark and if a plant is kept away from sunlight its Chloroplasts get converted into Etioplasts. These can be found in indoor plants and Cactus kept in dark.

Sample Problems & MCQs On Plastids

Let us now look at some questions that will help you in preparing for the exams:

Q1. Which of the following species lack plastids?
(a) Fungi and animals
(b) Plants and animals
(c) Bacterium, animals and fungi
(d) None of the above

Q2. Which of the following is not a colourless plastid?
(a) Amyloplast
(b) Elaioplast
(c) Rhodoplast
(d) Proteinoplast

Q3. What is a common feature of plastids and mitochondria?

Q4. Cell organelles found only in plants are:
(a) Ribosomes
(b) Mitochondria
(c) Golgi complex
(d) Plastids

Q5. Fret channels are associated with _________.
(a) Two lamellae of a granum
(b) Two grana of a chloroplast
(c) Two quantasomes
(d) Two plastids of a cell

Q6. The type of plastids helping in pollination is
(a) Leucoplasts
(b) Chloroplasts
(c) Chromoplasts
(d) All of the above

Frequently Asked Questions

Here are some of the frequently asked questions on the topic:

Q. Why are plastids important?
Ans. These are important as they perform various functions in plants like – food storage, attracting pollinating agents, etc. They are helpful in Photosynthesis.

Q. Is Plastid and Chloroplast the same?
Ans. Chloroplasts are a part of Plastids. Although Plastids have the same genome as Chloroplasts, they differ in function and structure.

Q. What are the 3 types of Plastids?
Ans. The 3 types of Plastids are:
1. Chloroplasts
2. Leucoplasts
3. Chromoplasts.

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