# Plastids – Definition, Types, Structure, Functions, and Development

> Learn what plastids are, how chloroplasts, chromoplasts and leucoplasts differ, their structure, functions, development, DNA, and origin in plant cells.

Canonical URL: https://biologynotesonline.com/plastids-types-structure-functions/
Author: Sourav Pan
Last updated: September 9, 2026

![Plastids – Definition, Types, Structure, Functions, and Development](https://biologynotesonline.com/wp-content/uploads/2023/11/Plastids-Definition-Types-Structure-Functions-1-scaled-1.jpg)

Plastids are a group of membrane-bound cell organelles that are found in plant and algal cells. These are present in different forms and perform different functions inside the cell. Not every plastid carries out photosynthesis. 

Chloroplast is a photosynthetic form of plastid, while other forms are mainly involved in storage, accumulation of pigments and different biosynthetic reactions. Plastids possess their own genetic material (plastid DNA) and ribosomes. 

They are also formed from the pre-existing plastids by division. Due to these characteristics, plastids are referred to as semi-autonomous organelles. It is not completely independent from the cell. Most of the proteins needed for plastid development and functioning are encoded by the nuclear genome and then transported into the plastid, making their functioning dependent on the cell.

## Common Characteristics of Plastids

- Plastids are membrane-bound cell organelles that are found in plants and algae. They are covered by two membranes (outer and inner membrane).

- These organelles are present in different forms. All plastids are not photosynthetic, and their structure and function may differ according to cell type and developmental condition.

- Plastids develop from small undifferentiated plastids called proplastids. These are mainly present in dividing cells of meristematic tissues.

- They possess their own genetic material (plastid DNA) and ribosomes. Some transcription and protein synthesis can be carried out within the plastid.

- Plastids are referred to as semi-autonomous organelles. Most of the proteins required for their development and functioning, however, are encoded by nuclear genes and then transported into the plastid.

- Different plastids may contain different pigments. Chloroplasts contain photosynthetic pigments, while several non-green plastids lack these pigments.

- One form of plastid can change into another during development. This conversion depends on tissue and environmental conditions.

- They perform different functions inside the cell including photosynthesis, storage and synthesis of different cellular compounds.

- New plastids are formed by division of the pre-existing plastids. This helps in maintaining plastids during growth and cell division.

## Plastid vs Chloroplast

Plastid is a broad group of organelles found in plant and algal cells, whereas chloroplast is one type of plastid mainly involved in photosynthesis. All chloroplasts are plastids, but all plastids are not chloroplasts.

PlastidChloroplastPlastid is a broad group of organelles found in plant and algal cells.Chloroplast is one photosynthetic type of plastid.Plastids are present in different forms such as chloroplasts, chromoplasts and leucoplasts.Chloroplast represents the green plastid mainly associated with photosynthesis.All plastids are not photosynthetic. Some are involved in storage, pigments and other metabolic functions.It contains the photosynthetic machinery and carries out photosynthesis.Pigments may be present or absent depending on the type of plastid.Chlorophyll is present along with other photosynthetic pigments.Internal membrane organization differs among different plastids.It possesses a well-developed thylakoid membrane system, commonly arranged into grana.Plastids possess their own genetic system and arise from pre-existing plastids.Chloroplast also contains its own genetic system and divides from pre-existing chloroplasts/plastids.

## Types of Plastids

Plastids are generally divided into chloroplasts, chromoplasts and leucoplasts based mainly on pigmentation and function. This is the familiar classification used for plastids. Apart from these, proplastids, etioplasts and gerontoplasts occur during different stages of plastid development, and several specialized forms are also known. Thus, plastids are not kept under one fixed number of types in all classifications.

![Classification of plastids showing chloroplasts, chromoplasts, leucoplasts and their storage forms, together with proplastids, etioplasts and gerontoplasts.](https://biologynotesonline.com/wp-content/uploads/2024/04/Types-and-Classification-of-Plastids-in-Plant-Cells-1024x768.webp)Classification of plastids showing chloroplasts, chromoplasts, leucoplasts and their storage forms, together with proplastids, etioplasts and gerontoplasts.

Plastid typePigmentationTypical locationDistinguishing featurePrincipal roleChloroplastGreen (chlorophyll present)Leaves and other green tissuesThylakoid membrane system, commonly arranged into granaPhotosynthesisChromoplastYellow, orange or redFlowers, fruits, seeds and some roots or tubersLarge accumulation of carotenoidsPigment synthesis and accumulationLeucoplastColourlessMainly non-photosynthetic tissuesNo obvious photosynthetic pigmentsStorage and different metabolic functionsProplastidUsually colourlessMeristematic and reproductive tissuesSmall and relatively undifferentiatedPrecursor of specialized plastidsEtioplastNon-greenDark-grown photosynthetic tissuesCharacteristic prolamellar bodyIntermediate stage before chloroplast formationGerontoplastChlorophyll gradually lostSenescing tissuesDisorganization and breakdown of thylakoidsDismantling and recycling of chloroplast components

### 1. Chloroplasts

[Chloroplasts](https://biologynotesonline.com/chloroplast/) are green plastids that are mainly found in leaves and other photosynthetic parts of plants. They contain chlorophyll and carry out photosynthesis. Inside the chloroplast, a well-developed thylakoid membrane system is present, which is commonly arranged into stacks called grana in plant chloroplasts.

### 2. Chromoplasts

Chromoplasts are coloured plastids containing high amounts of carotenoid pigments. These are commonly present in flowers, fruits, seeds and also in some roots or tubers, giving yellow, orange or red colour to the tissues. Colour produced by these plastids can help in attraction of pollinators and seed dispersers, and chromoplast development also takes place during ripening of many fruits.

### 3. Leucoplasts

Leucoplasts are non-pigmented plastids mainly associated with non-photosynthetic tissues. They perform storage as well as different metabolic functions. Based on the material accumulated in them, leucoplasts are commonly described as amyloplasts, elaioplasts and proteinoplasts.

#### a. Amyloplasts

Amyloplasts are mainly involved in synthesis and storage of starch. They are common in storage tissues such as roots, tubers and seeds. In the columella cells of root tips, starch-filled amyloplasts act as statoliths and their movement helps in perception of gravity during root gravitropism.

#### b. Elaioplasts

Elaioplasts are plastids associated with accumulation of lipids. They also take part in lipid metabolism, including synthesis of long-chain fatty acids in some seeds, and certain elaioplasts contain large amounts of sterol and wax esters.

#### c. Proteinoplasts

Proteinoplasts contain large amounts of proteins, usually accumulated in protein bodies or sometimes as protein crystals. Protein storage is an important feature, but these plastids may have other specialized activities as well. For example, protein bodies containing heme proteins with strong oxidase activity have been reported in tobacco.

### 4. Developmental Plastid Forms

Some plastids are better considered according to their developmental stage rather than only their colour. Proplastids, etioplasts and gerontoplasts are important examples. These forms can change as the tissue develops or environmental condition changes.

#### a. Proplastids

Proplastids are small and relatively undifferentiated plastids. They are mainly found in meristematic tissues, reproductive cells and dormant seeds. During development, these plastids can differentiate into other specialized plastid forms according to the tissue and environmental signals.

#### b. Etioplasts

Etioplasts are formed in tissues that normally develop chloroplasts but are grown in darkness. A characteristic structure called the prolamellar body (PLB) is present inside them. After exposure to light, etioplasts rapidly change into chloroplasts during the process of de-etiolation.

#### c. Gerontoplasts

Gerontoplasts are formed from chloroplasts during senescence of plant tissues. In this process, chlorophyll is degraded and the organized thylakoid system is gradually dismantled. Different carbon and nitrogen-containing compounds from the chloroplast are also broken down and recycled during this stage.

Some other specialized plastids such as iridoplasts, xyloplasts and phenyloplasts have also been described. These show that plastid diversity is much greater than the traditional three-type classification.

## General Structure of Plastids

Plastids are membrane-bound cell organelles having their own internal structural organization. The general structure is made up of a double membrane envelope, stroma, plastid DNA and ribosomes. Internal membranes and different storage inclusions can also be present depending upon the type of plastid.

![Comparison of plastid structure showing the common double envelope, stroma, DNA and ribosomes alongside chloroplast, chromoplast and storage-plastid internal organization.](https://biologynotesonline.com/wp-content/uploads/2024/04/General-Structure-and-Internal-Organization-of-Plastids-1024x576.webp)Comparison of plastid structure showing the common double envelope, stroma, DNA and ribosomes alongside chloroplast, chromoplast and storage-plastid internal organization.

- The plastid is surrounded by two membranes i.e. outer membrane and inner membrane. These together form the plastid envelope.

- Inside the inner membrane is a protein-rich matrix called stroma. It contains different enzymes and components required for plastid metabolism.

- Plastids contain their own genetic material (plastid DNA). The DNA is not enclosed within a nucleus, but is arranged in regions called nucleoids in the stroma.

- 70S ribosomes and different RNAs are also present. These are involved in synthesis of some proteins within the plastid.

- The internal membrane system is different among the plastid types. In chloroplasts, thylakoid membranes are highly developed and commonly arranged into stacks called grana. Such developed photosynthetic membranes are absent in several other plastids.

- The plastid envelope is involved in movement of proteins, ions and metabolites between the plastid and cytoplasm. Many proteins formed under the control of nuclear genes are transported into the plastid through this envelope.

- Different inclusions may also be present inside the plastid. Starch grains and lipid-containing plastoglobules are some of the important examples, and their amount differs with plastid type and function.

- Thin tubular extensions may arise from the plastid surface. These extensions are called stromules. Their occurrence is variable in different plastids and cells.

### Internal Organization Varies by Plastid Type

The internal organization of plastids is not the same in all types. Chloroplasts have a highly developed photosynthetic membrane system, while chromoplasts develop different carotenoid-containing structures. In leucoplast-derived forms, storage inclusions become more prominent according to the material stored.

- Chloroplasts- A well-developed thylakoid membrane system is present inside chloroplasts. The thylakoids are commonly arranged into stacks called grana, surrounded by stroma. Chlorophyll and other photosynthetic pigments are associated with these membranes.

- Chromoplasts- Their internal structure changes mainly for accumulation of carotenoids. These pigments may be deposited in globules (plastoglobules), crystals, membranes, fibrils or tubular structures. One chromoplast may also contain more than one of these forms.

- Amyloplasts- Large starch grains occupy much of the internal region. These are storage plastids, and the internal membrane system is much less developed as compared to chloroplasts.

- Elaioplasts- Lipid-rich structures are the major internal feature. They commonly contain a large number of plastoglobules, which are associated with storage and metabolism of oils and other lipids.

- Proteinoplasts- Protein-rich inclusions are accumulated inside these plastids. The proteins may occur as large bodies and sometimes as crystalloid-like structures.

## Plastid Development and Interconversion

Plastids develop from small undifferentiated plastids and later form different specialized types. The type which is formed depends on the tissue, developmental stage and environmental condition. One mature plastid may also change into another form. This is referred to as plastid interconversion.

![Developmental pathway showing proplastids forming chloroplasts, etioplasts and leucoplasts, with chloroplast-to-chromoplast and chloroplast-to-gerontoplast transitions.](https://biologynotesonline.com/wp-content/uploads/2024/04/Plastid-Development-and-Interconversion-Pathway-1024x768.webp)Developmental pathway showing proplastids forming chloroplasts, etioplasts and leucoplasts, with chloroplast-to-chromoplast and chloroplast-to-gerontoplast transitions.

The process is as follows-

Step 1- Proplastid stage

Proplastids are small undifferentiated plastids, mainly present in meristematic and reproductive tissues. These act as the precursor of other plastids. Chloroplasts, leucoplasts, chromoplasts and other plastid forms can be developed from them depending upon the tissue and conditions.

Step 2- Development of chloroplast

In green tissues, when light is available, the proplastid develops into a chloroplast. Chlorophyll is formed during this process. The internal membrane system also develops and thylakoids are organized, forming the photosynthetic structure of chloroplast.

Step 3- Etioplast formation

If the developing photosynthetic tissue is kept in darkness, normal chloroplast formation does not take place. Instead, etioplast is formed. It contains a characteristic structure called prolamellar body (PLB).

On exposure to light, the etioplast changes into chloroplast. During this process, thylakoid membranes are developed.

Step 4- Development of leucoplast

In the non-photosynthetic tissues, proplastids can develop into colourless leucoplasts. These plastids mainly perform storage and different metabolic functions. Depending upon their function, different forms are produced i.e. amyloplasts, elaioplasts and proteinoplasts.

Step 5- Chromoplast formation

Chromoplasts may be developed from proplastids or leucoplasts. In many ripening fruits, chloroplast is changed into chromoplast. During this process, chlorophyll is broken down and carotenoid pigments are accumulated. Thylakoid membranes are also reduced, together with reorganization of the plastid structure.

Step 6- Gerontoplast formation

During senescence of green tissues, chloroplasts are converted into gerontoplasts. Chlorophyll is degraded. The thylakoid membrane system gradually breaks down and different chloroplast components are degraded for recycling of nutrients.

Step 7- Interconversion of mature plastids

Mature plastids are not always present in one fixed form. One type can change into another under suitable conditions. For example, starch-containing amyloplasts of potato tubers can develop into chloroplasts after exposure to light. The conversion, however, varies with the plant species, tissue and developmental condition.

## Origin and Semi-Autonomous Nature

Plastids are considered to be originated from [cyanobacteria](https://biologynotesonline.com/cyanobacteria/) by endosymbiosis. During evolution, the cyanobacterium became a permanent part of the host cell and some bacterial characters are still retained in plastids. They contain their own DNA, ribosomes and division machinery, but are not completely independent. Therefore, plastids are referred to as semi-autonomous organelles.

![Diagram showing cyanobacterial endosymbiosis, gene transfer to the nucleus, retained plastid DNA and ribosomes, and import of nuclear-encoded proteins into modern plastids.](https://biologynotesonline.com/wp-content/uploads/2024/04/Endosymbiotic-Origin-and-Semi-Autonomous-Nature-of-Plastids-1024x576.webp)Diagram showing cyanobacterial endosymbiosis, gene transfer to the nucleus, retained plastid DNA and ribosomes, and import of nuclear-encoded proteins into modern plastids.

### Endosymbiotic Origin of Plastids

The origin of plastids is explained by the [endosymbiotic theory](https://biologynotesonline.com/endosymbiotic-theory-origin-of-the-eukaryotic-cell/). According to this, an ancestral eukaryotic cell engulfed a cyanobacterium. Instead of digestion, the cyanobacterium remained inside the cell and later developed into the primary plastid.

During this process, a large number of genes were lost or transferred to the nuclear genome of host cell. Some genetic machinery remained inside the plastid.

Some of the important evidence for cyanobacterial origin are-

- Plastids possess their own DNA. Its genetic system shows relationship with cyanobacteria.

- Bacterial-type ribosomes are present inside the plastid.

- Plastids are formed by division of pre-existing plastids. The division machinery also contains bacterial-related components such as FtsZ.

- Their genetic system shows a close evolutionary relationship with cyanobacteria.

Primary plastids gave rise to the plastids of green plants, red algae and glaucophytes. In some algae, secondary and tertiary endosymbiosis occurred later during evolution.

### Why Plastids Are Semi-Autonomous

Plastids contain some machinery of their own. Plastid DNA and ribosomes are present, and some genes can be transcribed and translated within the plastid. New plastids also arise from the pre-existing plastids.

However, it is not completely autonomous. Most proteins needed for plastid structure, development and different metabolic functions are encoded by nuclear genes. These proteins are formed outside the plastid and then transported into it.

Own machinery of plastidsCellular dependencePlastids possess their own DNA.Many ancestral genes have been lost or transferred to the nucleus.Their own ribosomes and RNAs are present.Most of the plastid proteins are encoded by nuclear genes.Some transcription and protein synthesis takes place inside the plastid.These nuclear-encoded proteins are synthesized outside and transported into the plastid.Plastids divide from pre-existing plastids.Their development and division also depend on nuclear-encoded proteins.

## Functions of Plastids

Plastids perform different functions in plant cells. The function mainly depends upon the type of plastid and the tissue in which it is present. The following are some of the important functions of plastids-

- Photosynthesis- Chloroplasts are the main site of [photosynthesis](https://biologynotesonline.com/photosynthesis/). During this process, light energy is used for formation of organic compounds with the release of oxygen.

- Starch storage- Synthesis and storage of starch takes place in plastids. Amyloplasts perform this function mainly in roots, tubers, seeds and other storage parts.

- Lipid synthesis- Plastids also synthesize fatty acids and different lipids. In several non-photosynthetic plastids, it is an important metabolic function.

- Amino acid synthesis- Several amino acids are formed by different metabolic reactions inside plastids.

- Pigment formation- Different pigments are formed as well as accumulated in plastids. Chlorophyll is present in chloroplasts. Chromoplasts, on the other hand, accumulate carotenoid pigments.

- Nitrogen and sulfur assimilation- Inorganic forms of nitrogen and sulfur are processed inside plastids and converted into forms which can be used by the cell.

- Formation of other compounds- Some plant hormones, vitamins and secondary metabolites are also synthesized with the involvement of plastids.

- Cellular response- Plastids are involved in responses to light, stress and developmental conditions. Their metabolic condition can also affect communication with other parts of the cell.

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