Chloroplast
In 1837, German botanist Hugo von Mohl published a definitive description of chloroplasts as discrete green bodies within plant cells. He called them Chlorophyllkörnen, which translates to grain of chlorophyll. This marked the first time scientists identified these structures as distinct organelles rather than just general green matter. The term chloroplast itself emerged later through linguistic evolution. In 1883, Andreas Franz Wilhelm Schimper named these bodies chloroplastids. Two years after that, Eduard Strasburger adopted the simpler term chloroplasts in 1884. The word combines two Greek roots: chloros meaning green and plastes meaning the one who forms. These etymological choices reflected early observations of the organelle's color and its role in creating plant life.
Chloroplasts evolved from free-living cyanobacteria billions of years ago through a process called endosymbiosis. Approximately two billion years ago, an early eukaryotic cell engulfed a cyanobacterium either as food or as an internal parasite. Instead of being digested, this bacterium escaped the phagocytic vacuole and persisted inside the host cell. This event created a mutual benefit where the cyanobacterium provided sugar from photosynthesis while receiving protection and nutrients. Over time, many genes were lost or transferred to the host nucleus. Some cyanobacterial proteins were then synthesized by the host cell and imported back into what became the chloroplast. This origin was first suggested by Russian biologist Konstantin Mereschkowski in 1905 after Andreas Franz Wilhelm Schimper observed similarities between chloroplasts and cyanobacteria in 1883. With one exception involving the amoeboid Paulinella chromatophora, all chloroplasts trace back to this single ancient event around two billion years ago.
All primary chloroplasts belong to four distinct lineages: glaucophyte, rhodophyte, chloroplastida, and the unique Paulinella chromatophora lineage. Glaucophytes represent the smallest group with only 25 described species that diverged before red and green lineages split. These early-diverging organisms retain peptidoglycan walls between their inner and outer membranes, features otherwise found only in bacteria. Their thylakoids form concentric unstacked arrangements surrounding carboxysomes containing RuBisCO enzymes. Rhodophyta or red algae constitute a large diverse lineage where chloroplasts called rhodoplasts contain phycobilin pigments organized into phycobilisomes on thylakoid membranes. Red phycoerythrin pigment gives many red algae their distinctive color while allowing them to capture sunlight efficiently in deep water. Chloroplastida includes both green algae and land plants under the Viridiplantae classification. Green chloroplasts differ by having lost phycobilisomes and containing chlorophyll b instead of just chlorophyll a. They also lost the peptidoglycan wall leaving an intermembrane space between double membranes.
Many other organisms acquired chloroplasts through secondary endosymbiosis involving engulfment of primary algae. Secondary plastids typically have three or four membranes including original cyanobacterial layers plus additional host-derived membranes. Some organisms like Cryptomonas and chlorarachniophytes retain remnants of the engulfed alga's nucleus as nucleomorph structures located between second and third membranes. Dinoflagellates such as Karlodinium and Karenia obtained chloroplasts by engulfing organisms with secondary plastids creating tertiary plastids. These complex events involved multiple rounds of symbiotic relationships where one eukaryote consumed another that already contained chloroplasts. The CASH lineage comprising cryptomonads, alveolates, stramenopiles and haptophytes represents major diversification from red algal derived chloroplasts. Some dinoflagellate lineages replaced their original peridinin-type chloroplasts with green algal derived versions while others maintain both types simultaneously. Diatom-derived chloroplasts found in genera like Kryptoperidinium and Durinskia show how entire cells can be engulfed yet still function inside host cytoplasm. These complex evolutionary pathways demonstrate how chloroplasts spread across diverse organism groups through repeated endosymbiotic events rather than single origins.
Chloroplast DNA was first identified biochemically in 1959 and confirmed via electron microscopy in 1962. Most chloroplast genomes combine into single large circular molecules typically ranging from 120,000 to 170,000 base pairs long. While physical DNA molecules take various linear and branching forms inside cells, new chloroplasts may contain up to 100 genome copies before decreasing to about 15-20 as organelles age. Many chloroplast genomes feature two inverted repeats separating long single copy sections from short single copy regions varying from 4,000 to 25,000 base pairs each. Over time many parts of the original cyanobacterial genome containing over 3,000 genes transferred to the nuclear genome leaving contemporary chloroplasts with only approximately 100 genes. In land plants some 11-14% of nuclear DNA traces back to chloroplast sources corresponding to roughly 4,500 protein-coding genes. About 95% of proteins found in chloroplasts are now encoded by nuclear genes requiring coordinated synthesis between nucleus and organelle. Recent research shows that parts of retrograde signaling networks regulating gene expression emerged already in algal progenitors before land plant evolution.
In land plants chloroplasts generally measure 3-10 micrometers in diameter and 1-3 micrometers thick forming lens-shaped structures. Corn seedling chloroplasts reach volumes around 20 cubic micrometers while algae display greater diversity including net-shaped Oedogonium cup-shaped Chlamydomonas ribbon-like Spirogyra spirals and star-shaped Zygnema forms. All chloroplasts contain at least three membrane systems: outer chloroplast membrane inner chloroplast membrane and thylakoid system. Inside these membranes lies stroma semi-gel-like fluid making up much of chloroplast volume where thylakoids float. Glaucophyte algae uniquely maintain peptidoglycan layers between inner and outer membranes called muroplasts from Latin mura meaning wall. Other chloroplasts typically have intermembrane spaces about 10-20 nanometers thick though some mosses lycophytes and ferns retain peptidoglycan precursors. Stromules or stroma-containing tubules sometimes protrude into cytoplasm increasing surface area for cross-membrane transport. Plastoglobuli spherical lipid bubbles measuring 45-60 nanometers across become more common during oxidative stress or aging transitions to gerontoplasts.
Photosynthesis divides into light reactions occurring on thylakoid membranes and dark reactions known as the Calvin cycle taking place in stroma. Light reactions split water molecules releasing oxygen while storing energy in ATP and NADPH carriers. Hydrogen ions pumped into thylakoid space create concentration gradients allowing ATP synthase to generate adenosine triphosphate as protons flow back into stroma. The Calvin cycle fixes carbon dioxide into sugar molecules using RuBisCO enzymes abundant throughout plant chloroplasts but concentrated specifically in bundle sheath cells of C4 plants. Mesophyll cell chloroplasts lack RuBisCO performing only light reactions producing four-carbon compounds transported to bundle sheath where they release carbon for sugar synthesis. Bundle sheath chloroplasts contain large starch grains and free-floating thylakoids enabling cyclic electron flow without generating oxygen that would disrupt RuBisCO activity. Guard cell chloroplasts differ from typical epidermal cells by containing well-developed structures involved in both photosynthesis and plant immune responses through reactive oxygen species production.
Common questions
When did Hugo von Mohl publish the first description of chloroplasts?
Hugo von Mohl published a definitive description of chloroplasts in 1837. He identified these structures as discrete green bodies within plant cells and called them Chlorophyllkörnen.
Who named chloroplasts and when was the term established?
Eduard Strasburger adopted the simpler term chloroplasts in 1884. Andreas Franz Wilhelm Schimper had previously named these bodies chloroplastids in 1883 before Strasburger simplified the name two years later.
How many base pairs are typically found in chloroplast genomes?
Most chloroplast genomes combine into single large circular molecules ranging from 120,000 to 170,000 base pairs long. These genomes often feature inverted repeats separating long single copy sections from short single copy regions varying from 4,000 to 25,000 base pairs each.
What is the size range for chloroplasts in land plants?
Chloroplasts in land plants generally measure 3-10 micrometers in diameter and 1-3 micrometers thick. They form lens-shaped structures that vary in volume depending on the specific organism type.
When did scientists first identify chloroplast DNA biochemically?
Scientists first identified chloroplast DNA biochemically in 1959. This discovery was confirmed via electron microscopy in 1962.