Plastid
Membrane-bound organelles derived from endosymbiotic cyanobacteria.
Aslninat8 · CC BY-SA 4.0
Plastids are membrane-bound organelles found inside the cells of plants, algae, and certain other eukaryotes. They are thought to have originated as cyanobacteria that were taken up by a host cell and kept inside as endosymbionts. These organelles are where autotrophic eukaryotes manufacture and store pigments and other important chemical compounds.
Examples of plastids include chloroplasts, which carry out photosynthesis; chromoplasts, which make and store pigments; leucoplasts, which lack pigment and can sometimes change into other forms; and apicoplasts, which are non-photosynthetic plastids found in apicomplexan parasites and came from a secondary endosymbiosis event.
The first permanent primary endosymbiosis happened in the group Archaeplastida—which includes land plants, red algae, green algae, and glaucophytes—probably with a cyanobiont related to the genus *Gloeomargarita*. A later primary endosymbiosis occurred in the photosynthetic plastids of *Paulinella* amoeboids, involving cyanobacteria from the genera *Prochlorococcus* and *Synechococcus* (the "PS-clade"). Secondary and tertiary endosymbioses have also happened in many organisms, and some species can steal plastids from their food in a process called kleptoplasty.
Andreas Schimper first named, described, and clearly defined plastids. They contain a double-stranded DNA molecule that was long thought to be circular like prokaryotic chromosomes, though this may not always be the case. Plastids are sites for making and storing pigments and other compounds. Some contain biological pigments used in photosynthesis or that give a cell its color. In organisms that have lost their photosynthetic ability, plastids are still useful for making molecules like isoprenoids.
In land plants, plastids that contain chlorophyll—called chloroplasts—perform photosynthesis, using sunlight to create chemical energy, capturing carbon from the air, and releasing oxygen. Other plastids synthesize fatty acids and terpenes, which can be used for energy or to build other molecules. For instance, plastids in epidermal cells make components of the plant cuticle, including epicuticular wax, from palmitic acid that is synthesized in mesophyll chloroplasts. Plastids also store starches, fats, and proteins.
All plastids come from proplastids, which are found in meristematic regions. Proplastids and young chloroplasts usually divide by binary fission, but mature chloroplasts can also divide. Proplastids can differentiate into several forms depending on their function: chloroplasts (green, photosynthetic), etioplasts (precursors to chloroplasts), chromoplasts (colored, pigment-making), gerontoplasts (control dismantling of photosynthetic machinery during senescence), and leucoplasts (colorless, make monoterpenes). Leucoplasts can further specialize into aleuroplasts, amyloplasts (store starch and sense gravity for geotropism), elaioplasts (store fats), proteinoplasts (store and modify protein), or tannosomes (make tannins and polyphenols). Plastids can also change from one type to another.
Each plastid has multiple copies of its own genome, called a plastome. In a chloroplast, this is the chloroplast DNA. The number of genome copies per plastid varies—over 1,000 in rapidly dividing new cells with few plastids, down to 100 or fewer in mature cells with many plastids. A plastome typically encodes transfer RNAs, ribosomal RNAs, and some proteins involved in photosynthesis and plastid gene expression. However, these are only a small fraction of the total proteins needed to build and maintain a plastid; most plastid proteins are encoded by nuclear genes, and the expression of nuclear and plastid genes is co-regulated.
Many plastids, especially photosynthetic ones, have numerous internal membrane layers. Plastid DNA exists as protein-DNA complexes called plastid nucleoids, which are localized regions near the inner envelope membrane. Unlike the eukaryotic nucleus, a plastid nucleoid has no surrounding membrane. Each nucleoid may contain more than 10 copies of plastid DNA. In a proplastid, there is a single nucleoid near the center; in developing or differentiating plastids, many nucleoids are found at the periphery, bound to the inner envelope membrane. As proplastids turn into chloroplasts, or when plastids change from one type to another, the nucleoids change in shape, size, and location.
- field
- Cell biology, endosymbiosis
- known_for
- Photosynthesis, pigment storage, and plastid genome (plastome)
- first_described_by
- Andreas Schimper
Lore & Background
Plastids are derived from proplastids, which are present in the meristematic regions of plants. Proplastids may differentiate into chloroplasts, chromoplasts, leucoplasts, etioplasts, gerontoplasts, and other forms. Each plastid creates multiple copies of its own genome, or plastome, which encodes transfer RNAs, ribosomal RNAs, and proteins involved in photosynthesis and gene transcription. Nuclear genes encode the vast majority of plastid proteins, and their expression is co-regulated.
Reader's Guide
Plastids are fundamental to autotrophic eukaryotes, enabling photosynthesis and the synthesis of pigments, fatty acids, terpenes, and other molecules. Their origin as endosymbiotic cyanobacteria is supported by their double-stranded DNA, double membranes, and independent division. The primary endosymbiosis event in Archaeplastida about 1.5 billion years ago gave rise to land plants, red algae, green algae, and glaucophytes. Secondary and tertiary endosymbioses have occurred widely, and some organisms engage in kleptoplasty. Plastids in non-photosynthetic organisms, such as apicoplasts in Apicomplexa, retain biosynthetic functions. The plastid genome may be lost in some parasitic plants, though this is disputed.
Did You Know?
- Plastids are considered intracellular endosymbiotic cyanobacteria.
- A permanent primary endosymbiosis event occurred about 1.5 billion years ago in the Archaeplastida clade.
- Plastid DNA exists as protein-DNA complexes called plastid nucleoids, not surrounded by a nuclear membrane.
The Chemical Architecture of Strand Orientation
Every single strand of nucleic acid possesses an inherent chemical polarity defined by the numbering of carbon atoms within its pentose sugar ring. At one terminus sits the fifth carbon, which typically bears a phosphate group; at the opposite terminus rests the third carbon, carrying an unmodified hydroxyl substituent. This asymmetry is not merely a naming convention—it is the structural foundation upon which all downstream molecular biology depends. When two strands come together to form a double helix, they must run in opposite orientations. This antiparallel arrangement is what allows complementary bases to pair correctly, and without it, neither replication nor transcription of genetic information would be possible. The convention of labeling these ends as five-prime and three-prime stems directly from the carbon numbering in the ribose or deoxyribose ring, giving molecular biologists a universal language for describing where any given sequence sits along a strand. Whether discussing the location of a gene, a protein-binding site, or a regulatory element, the five-prime-to-three-prime framework provides the coordinate system for all positional language in nucleic acid biology.
The Unidirectional Rule of Polymerization
In living cells, the construction of new nucleic acid strands follows a single, non-negotiable direction: from the five-prime end toward the three-prime end. This constraint arises from the biochemistry of the polymerase enzymes that carry out the task. These enzymes harness the energy released when nucleoside triphosphate bonds are broken, and they use that energy to forge a phosphodiester bond between the incoming nucleoside monophosphate and the three-prime hydroxyl group already present on the growing chain. No known in-vivo mechanism reverses this order. This directional constraint shapes how scientists describe the layout of genetic information. Elements positioned toward the five-prime end are called upstream; those toward the three-prime end are downstream. It also means that directionality and the concept of sense are related but distinct ideas. When a ribosome reads messenger RNA to build a protein, it scans from the five-prime end, and the resulting polypeptide chain grows from its N-terminus toward its C-terminus. In bacteria, mitochondria, and plastids, the first amino acid incorporated is N-formylmethionine rather than plain methionine, a small but notable variation in an otherwise universal reading process.
The Five-Prime End: Capping, Ligation, and Regulatory Architecture
The five-prime terminus of a nucleic acid strand is the carbon-five position of the terminal sugar ring, and it carries outsized functional importance. When a phosphate group is attached there, it becomes the chemical handle that allows two nucleotides to be covalently joined—a process called ligation, in which the five-prime phosphate links to the three-prime hydroxyl of a neighboring nucleotide to create a phosphodiester bond. Molecular biologists exploit this chemistry: by stripping the five-prime phosphate with a phosphatase enzyme, they can prevent a plasmid vector from self-ligating during cloning experiments. In eukaryotic gene expression, the five-prime end of newly made messenger RNA receives a protective cap—a methylated guanosine nucleotide attached through an unusual five-prime-to-five-prime triphosphate linkage. This cap shields the transcript from exonucleases and stabilizes it during translation. The five-prime flanking region upstream of the coding sequence houses the promoter and often enhancers, while the five-prime untranslated region, stretching from the cap site to just before the AUG start codon, contains the ribosome binding site and Kozak sequence, both of which govern how efficiently translation initiates.
The Three-Prime End: Termination, Polyadenylation, and the Sanger Legacy
The three-prime terminus, named for the hydroxyl group on the third carbon of the terminal sugar ring, serves as the active growth point for every new nucleic acid strand. Because polymerases require that free three-prime hydroxyl to attach the next incoming nucleotide, removing it effectively halts chain elongation. Molecular biologists turned this vulnerability into one of the most important tools in genetics: the Sanger dideoxy chain-termination method. By introducing dideoxyribonucleotides that lack the three-prime hydroxyl, researchers can deliberately stop replication at specific points, revealing the precise order of bases in a DNA molecule. At the three-prime end of mature messenger RNA, a tail of fifty to two hundred and fifty adenosine residues is added through polyadenylation. This poly-A tail acts as a timer, influencing how long the transcript survives in the cell and consequently how much protein it ultimately yields. The three-prime untranslated region, spanning from the stop codon to the poly-A tail, carries no protein-coding information yet can modulate both translation efficiency and mRNA stability. The three-prime flanking DNA, once assumed to be silent, is now known to be transcribed and then trimmed away during processing of the primary transcript.
Gallery
Frequently Asked Questions
What is a Plastid?
A plastid is a membrane-enclosed organelle that lives inside the cells of plants, algae, and a few other eukaryotic organisms. It acts as the cell's dedicated workshop and storage depot for pigments and other essential chemical compounds.
What are Plastid's powers or role?
Plastids drive photosynthesis, produce and store pigments, and house the plastome—the organelle's own small genome. Together these functions let autotrophic eukaryotes build their own energy-rich molecules from light and carbon dioxide.
Where did Plastid come from?
Plastids trace their origin to free-living cyanobacteria that were engulfed by an ancestral eukaryotic cell in a single endosymbiotic event. Over billions of years those bacteria lost independence and became permanently embedded as intracellular organelles.
Who first described Plastid?
The Austrian botanist Andreas Schimper is credited with first formally describing plastids. His early observations laid the conceptual groundwork that later cell biologists built on to understand these organelles.
Why is Plastid important in cell & molecular biology?
Plastids are central to the field because they power photosynthesis, sustain most terrestrial food webs, and carry their own genome (the plastome), making them a premier model for studying endosymbiosis, organelle genetics, and pigment biochemistry.
More in Cell & Molecular Biology 1-16
Spotted an error? Know more?
This is a living reference — every entry is fact-audited, and reader corrections feed straight into our audit queue. Suggest an edit · See this site's audit record