Plastids – Definition, Types, and Functions

Plastids are double-membraned organelles found in the cells of plants and algae that perform essential biochemical functions, including photosynthesis, pigment synthesis and storage, and biosynthesis of important molecules such as fatty acids and amino acids. Plastids include chloroplasts, chromoplasts, leucoplasts, and several specialized subtypes. Like mitochondria, plastids have their own DNA and ribosomes and likely evolved from free-living prokaryotes through endosymbiosis. They are dynamic structures that convert from one type to another depending on developmental and environmental conditions.

Key Takeaways: Plastids

• Plastids are double-membrane organelles found in plant and algal cells, but absent in animal and fungal cells.
• They originate from proplastids in meristematic (dividing) cells.
• The main plastid types are chloroplasts, chromoplasts, and leucoplasts, which include subtypes like amyloplasts, elaioplasts, and proteinoplasts.
• Plastids are semiautonomous. They have their own circular DNA and ribosomes but depend on the nucleus for many proteins.
• They are inherited maternally in most plant species.
• Plastids often interconvert (e.g., chloroplasts can become chromoplasts during fruit ripening).
• Their endosymbiotic origin links them evolutionarily to cyanobacteria.
• Plastids and mitochondria share features such as double membranes and autonomous genomes, but differ in function and ancestry.

What Are Plastids?

Plastids are membrane-bound organelles unique to plant and algal cells. They perform vital roles in photosynthesis, storage, and biosynthesis. Their name derives from the Greek plastos (“formed” or “molded”), reflecting their diverse forms and functions.

All plastids share a similar basic structure:

• A double membrane envelope
• An internal stroma containing enzymes, DNA, and ribosomes
• Sometimes internal membrane systems, such as the thylakoids in chloroplasts

Plastids are semiautonomous, replicating by binary fission and containing their own DNA, though most plastid proteins are encoded by the nuclear genome and imported post-translationally.

Which Cells Have Plastids?

Plastics occur in:

• Plants (land plants and green algae, or Chloroplastida)
• Algae of various lineages, including:
  • Green algae (Chlorophyta) and red algae (Rhodophyta), both from primary endosymbiosis
  • Brown algae (Phaeophyceae), diatoms (Bacillariophyta), dinoflagellates, euglenids, and cryptophytes, which possess plastids derived from secondary or tertiary endosymbiosis
• Some protists, especially those descended from photosynthetic ancestors, such as:
  • Euglena (a euglenid with green plastids derived from a green alga)
  • Dinoflagellates, some of which have complex plastid histories involving serial replacements
  • Apicomplexan parasites (e.g., Plasmodium, Toxoplasma), which retain a non-photosynthetic plastid, the apicoplast, used for lipid and isoprenoid biosynthesis

Plastids are absent in:

• Animals
• Fungi
• Most heterotrophic protists, such as amoebas and ciliates, which rely on ingesting food rather than photosynthesis.

Plastids are essential for autotrophic metabolism, so heterotrophic organisms (animals, fungi) do not have them. Some parasitic or nonphotosynthetic plants retain nonfunctional plastids (such as apicoplasts in Apicomplexan parasites).

Types of Plastids

Plastids are categorized by their structure, pigment content, and function. They all originate from proplastids, which are undifferentiated precursors found in meristematic cells. The major types of plastids in plants and algae are proplastids, chloroplasts, chromoplasts, leucoplasts, and gerontoplasts.

1. Proplastids

• Appearance: Small, colorless, undifferentiated
• Function: Precursors to all plastid types
• Location: Actively dividing plant cells (e.g., meristems)

2. Chloroplasts

• Appearance: Green due to chlorophyll
• Function: Photosynthesis, starch synthesis, fatty acid and amino acid synthesis
• Structure: Thylakoid membranes forming grana and stroma lamellae
• Pigments: Chlorophylls a and b, carotenoids

3. Chromoplasts

• Appearance: Yellow, orange, or red due to carotenoid pigments
• Function: Pigment synthesis and storage, coloration of flowers and fruits
• Conversion: Often derived from chloroplasts during fruit ripening or leaf senescence

4. Leucoplasts (Colorless Plastids)

• Appearance: Colorless, non-pigmented
• Function: Storage and biosynthesis in non-photosynthetic tissues
• Subtypes:
  • Amyloplasts: Store starch (common in roots, tubers)
  • Elaioplasts: Store lipids (in seeds)
  • Proteinoplasts (Aleuroplasts): Store or crystallize proteins (in seeds, nuts)

5. Etioplasts

• Appearance: Pale yellow with a crystalline prolamellar body
• Function: Precursors of chloroplasts in dark-grown (etiolated) plants
• Conversion: Transform into chloroplasts upon light exposure

6. Gerontoplasts

• Appearance: Degenerated chloroplasts
• Function: Found in senescing leaves, recycling pigments and macromolecules

7. Apicoplasts (in Apicomplexans)

• Appearance: Non-photosynthetic, relic plastid
• Function: Fatty acid and isoprenoid synthesis in certain parasites (e.g., Plasmodium)
• Significance: Evidence of plastid endosymbiotic origin in diverse lineages

Summary of Types and Functions of Plastids

Additional Types of Plastids in Algae and Protists

While plant plastids are mainly chloroplasts, chromoplasts, leucoplasts, and their derivatives, algae and protists possess a broader range of plastid forms that differ in pigment composition, membrane organization, and evolutionary origin.

1. Rhodoplasts (Red Algal Plastids)

• Color and pigments: Red due to phycoerythrin and phycocyanin (phycobiliproteins), along with chlorophyll a.
• Structure: Thylakoids arranged in pairs; no grana; surrounded by two membranes.
• Functions: Photosynthesis in deeper or dimly lit marine environments where red wavelengths penetrate poorly; efficient light capture.
• Example: Found in red algae (Rhodophyta).

2. Phaeoplasts (Brown Algal Plastids)

• Color and pigments: Brown or olive-green from fucoxanthin, a carotenoid that masks chlorophyll a and c.
• Structure: Four bounding membranes (due to secondary endosymbiosis with a red alga), with thylakoids grouped in triplets.
• Functions: Photosynthesis; synthesis and storage of laminarin and mannitol.
• Example: Found in brown algae (Phaeophyceae) and diatoms (Bacillariophyta).

3. Cyanoplasts

• Color and pigments: Blue-green, containing chlorophyll a and phycobiliproteins.
• Origin: Derived directly from cyanobacteria through primary endosymbiosis.
• Example: Found in Glaucophyte algae, which are thought to retain the most ancestral plastids, complete with a vestigial peptidoglycan wall.

4. Cryptoplasts

• Color and pigments: Golden or brownish-green, containing chlorophylls a and c, plus unique accessory pigments.
• Structure: Four membranes; one contains nucleomorph remnants from a red algal endosymbiont.
• Example: Found in cryptophyte algae.

5. Euglenid Plastids (Euglenoplasts)

• Color and pigments: Bright green with chlorophylls a and b (like green algae).
• Structure: Three membranes, acquired through secondary endosymbiosis with a green alga.
• Functions: Photosynthesis, paramylon carbohydrate storage.
• Example: Euglena gracilis.

6. Dinoflagellate Plastids

• Color and pigments: Variable (golden-brown, green, or even red) depending on origin and pigments (chlorophylls a, c, peridinin, or fucoxanthin).
• Structure: Highly diverse; some species have tertiary plastids from engulfing other algae.
• Special Note: Some dinoflagellates can lose their plastids entirely and acquire new ones through a phenomenon known as serial endosymbiosis.

Plastid Development and Interconversion

Plastids are dynamic organelles capable of differentiation and redifferentiation:

• Proplastid → Chloroplast: Triggered by light in young leaves.
• Chloroplast → Chromoplast: Occurs during fruit ripening (e.g., green tomato to red).
• Chloroplast → Gerontoplast: In senescing tissues.
• Proplastid → Leucoplast: In non-photosynthetic tissues like roots.

This flexibility helps plants adapt plastid function to developmental stage and environmental cues.

Inheritance of Plastids

Plastids exhibit non-Mendelian inheritance. In most flowering plants, plastids are maternally inherited through the egg cytoplasm. However, some species show paternal or biparental inheritance.

Because plastids contain their own DNA (cpDNA for chloroplasts), mutations can lead to variegation (green and white leaf patches) depending on plastid distribution during cell division.

Evolution and Origin of Plastids

Plastids probably arose from a single endosymbiotic event in which a eukaryotic ancestor engulfed a cyanobacterium about 1.5 billion years ago. Evidence includes:

• Circular DNA similar to cyanobacterial genomes
• 70S ribosomes (prokaryotic type)
• Double membranes (from engulfment)
• Binary fission reproduction

Later, these plastid-bearing eukaryotes were engulfed by other eukaryotes in secondary and tertiary endosymbioses, spreading plastids across diverse protist groups such as euglenids, alveolates, and stramenopiles. This explains why plastid structure, pigment composition, and membrane number differ among algal lineages.

Comparison: Plastids vs. Mitochondria

Plastids and mitochondria are both semiautonomous organelles essential for cellular metabolism, energy conversion, and biosynthesis. Despite performing opposite roles, with plastids primarily capturing and storing energy from sunlight, and mitochondria releasing energy through respiration, they share many structural and genetic similarities. Both originated through endosymbiosis with prokaryotic ancestors, possess double membranes, contain circular DNA, and divide independently of the nucleus. However, they differ in origin, specific functions, internal structure, and distribution among organisms. Comparing these two organelles highlights the parallel strategies by which eukaryotic cells manage energy and sustain life.

History of Plastid Discovery and Study

The study of plastids began in the 19th century, when improvements in microscopy first revealed green structures within plant cells. In 1837, German botanist Matthias Schleiden described these structures as chlorophyll grains, which were later recognized as chloroplasts. Over the following decades, botanists observed similar but non-green bodies in other tissues, leading to the broader concept of “chromatophores” or “plastidules” to encompass various pigment-containing and colorless forms.

By the late 1800s, scientists realized that these organelles were not mere inclusions but living, self-replicating entities within the cell. The term “plastid” (from the Greek plastos, meaning “formed”) was introduced by Ernst Haeckel to describe this family of morphologically variable organelles. In the early 20th century, cytologists such as Andreas Schimper and Eduard Strasburger showed that plastids divide independently and are transmitted from one cell generation to the next, establishing their continuity and hereditary nature.

With the advent of electron microscopy in the mid-20th century, researchers discovered the intricate internal membranes of chloroplasts (thylakoids and grana), clarifying their role in photosynthesis. The discovery of plastid DNA and ribosomes in the 1960s provided key evidence that plastids are semiautonomous organelles, supporting the endosymbiotic theory proposed by Lynn Margulis in the late 1960s. Molecular sequencing later confirmed that plastids evolved from cyanobacteria, transforming our understanding of eukaryotic evolution.

Today, plastid research spans genomics, biochemistry, and cell biology, illuminating their diverse roles not only in plants and algae but also in complex protist lineages derived from secondary and tertiary endosymbioses.

Common Misconceptions

• “All plastids perform photosynthesis.”
  Only chloroplasts are photosynthetic. Others specialize in storage or pigmentation.
• “Plastids are only found in green tissues.”
  Colorless plastids (leucoplasts) exist in roots, seeds, and tubers.
• “Plastids cannot change from one type to another.”
  Plastids can interconvert depending on environmental and developmental cues.
• “Plastid inheritance follows Mendelian genetics.”
  Plastids are inherited cytoplasmically, often maternally.

FAQs About Plastids

1. Are plastids and chloroplasts the same thing?
Not exactly. Chloroplasts are one type of plastid specialized for photosynthesis. Plastids also include chromoplasts, leucoplasts, and other forms.

2. Do all plant cells contain chloroplasts?
No. Non-photosynthetic tissues (like roots) have leucoplasts instead.

3. Do plastids reproduce?
Yes. They divide by binary fission, independently of the cell cycle, though regulated by nuclear genes.

4. Do plastids have DNA?
Yes. Plastid DNA is circular and encodes some, but not all, of the organelle’s proteins.

5. How do plastids support plant life?
They enable photosynthesis, pigment production, and storage of starch, oils, and proteins, which are key to plant survival and reproduction.

References

• Birky, C.W. (2001). “The inheritance of genes in mitochondria and chloroplasts: laws, mechanisms, and models”. Annual Review of Genetics. 35: 125–48. doi:10.1146/annurev.genet.35.102401.090231
• Gould, S.B.; Waller, R.F.; McFadden, G.I. (2008). “Plastid evolution”. Annual Review of Plant Biology. 59 (1): 491–517. doi:10.1146/annurev.arplant.59.032607.092915
• Kumar, R.A.; Oldenburg, D.J.; Bendich, A.J. (2014). “Changes in DNA damage, molecular integrity, and copy number for plastid DNA and mitochondrial DNA during maize development”. Journal of Experimental Botany. 65 (22): 6425–39. doi:10.1093/jxb/eru359
• Vries, Jan de; Gould, Sven B. (2018). “The monoplastidic bottleneck in algae and plant evolution”. Journal of Cell Science. 131 (2): jcs203414. doi:10.1242/jcs.203414
• Wise, Robert R. (2006). “The Diversity of Plastid Form and Function”. The Structure and Function of Plastids. Advances in Photosynthesis and Respiration. Springer. doi:10.1007/978-1-4020-4061-0_1. ISBN 978-1-4020-4060-3.