Chloroplasts are structures, or organelles, in plant cells. They are the energy centers of the cell, as mitochondria are in the cells of animals. It is here that the plant uses chlorophyll to harness the sun’s energy and convert carbon dioxide and water into sugar and oxygen.
The chloroplast uses these molecules to build organic compounds. They also synthesize fatty acids and many amino acids. A most important byproduct of photosynthesis is the oxygen we breathe. Without chloroplasts, animals could not live.
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The first person to describe chloroplasts was Hugo von Mohl (above) in 1837. He called them “chlorophyll grains.” The name “chloroplast” was given by Eduard Strasburger (below) in 1884.
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Chloroplasts have two membranes enclosing the stroma, a gel-like fluid. The thylakoid structures (where photosynthesis takes place), ribosomes and nucleoid (DNA rings) float in the stroma.
Chloroplasts have their own DNA, separate from that of the cell that contains them.
The plant cell cannot make a new chloroplast. New cells inherit their chloroplasts from their parent cells through cell division.
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Unicellular algae have one chloroplast per cell, while wheat may have 100 per cell. Most chloroplasts are green, though there are red ones too! “One square millimeter of leaf tissue can contain half a million chloroplasts.”
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Chloroplasts are very mobile. They rearrange themselves to take the best advantage of available light. If light is low, they spread out in a sheet to catch as much light as possible (below, on right). If light is intense, they turn sidewise or shelter behind each other to protect themselves from too much exposure (below, on left).
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All plant cells help protect the plant from pathogens, as plants don’t have specialized immune cells. Chloroplasts play a key role. They are sensors. If they detect stress, they produce molecules that serve as defense signals.
They also damage their own photosynthetic system to produce “reactive oxygen species” which trigger the cell’s defense systems!
Pathogens often target the chloroplast precisely because it is so important in protecting the cell.
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The chloroplast is essential not only to plants but to all animals that get their energy from plants, and to all life that breathes the oxygen that is a byproduct of photosynthesis.
The molecule above is probably the most abundant protein on the planet. It’s the enzyme that fixes carbon dioxide and turns it into sugar.
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All chloroplasts except for those in one genus of freshwater amoeba trace their ancestry back to one event, from 600 million to 2 billion years ago, when a eukaryote ( a cell with a nucleus containing DNA) engulfed a cyanobacterium or blue-green alga.
The host cell didn’t eat its guest, which, by providing food for its host through photosynthesis, was “allowed” to stay and became a chloroplast.
This arrangement is called “endosymbiosis, “a cell living inside another cell with a mutual benefit for both.”
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“Only plant cells have chloroplasts.”
That’s what I was taught 60 years ago. Wrong!!
“The fact that plant chloroplasts can survive and function in animal cells is now well documented.” https://www.degruyter.com/document/doi/10.1515/9783111385068-067/pdf
It’s not just 60 year old textbooks that haven’t caught up with modern science. You’ll find the same misinformation on many current websites such as this one: http://www.biology4kids.com/files/cell_chloroplast.html
Enter The Lowly Sea Slug!
Some sea slugs in the group Sacoglossa ingest chloroplasts from the algae they eat and they store the chloroplasts for months, years, or (in one species) for the rest of their lives. “One algae-rich binge in its youth” allows this sea slug to thrive without another meal for the rest of its life!
(Weirder still: “All of them are hermaphrodites, and they mate by colliding head-to-head and mutually inseminating each other with penises that unspool from beneath their right eye.”)
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I was taught that life was either plant or animal. We’re now told that living organisms fall into 5 or 6 or 7 or 8 “kingdoms.”
And now I learn that animals can have chloroplasts. So what’s a plant and what’s an animal?
Where are those good old binaries?!
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Image Credits
https://en.wikipedia.org/wiki/Chloroplast: images of Bryum capillare, chloroplast, leaf cross section, chloroplast mobility, RuBisCO and endosymbiosis; cited [“one square millimeter …”]
https://en.wikipedia.org/wiki/File:Plant_cell_structure-en.svg: image of plant cell
https://geneee.org/hugo/von+mohl?lang=en: image Hugo von Mohl
https://biology.hi7.co/prophase-56ce2df1ed17d.html: image Eduard Strasburger
https://www.lookandlearn.com/history-images/YCH005359-001/The-Lookout: Winslow Homer’s “The Lookout”
No copyright infringement intended.
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Articles on Chloroplasts in Animals
Stefano, George B., Christopher Snyder and Richard Kream. “Mitochondria, Chloroplasts in Animal and Plant Cells: Significance of Conformational Matching.” Medical science monitor : international medical journal of experimental and clinical research vol. 21 (17 July, 2015): 2073-8. doi:10.12659/MSM.894758.
https://https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4517925/
Trench, Robert K., Richard W. Greene, and Barbara G. Bystrom. “Chloroplasts as Functional Organelles in Animal Tissues.” The Journal of Cell Biology vol. 42 (1969): 404-417. https://rupress.org/jcb/article-pdf/42/2/404/1384887/404.pdf
Trench, R. K. “Uptake, retention and function of chloroplasts in animal cells.” In Endosymbiosis and Cell Biology, edited by Hainfried Schenck and Werner Schwemmlerk 703-728. Berlin: De Gruyter, 1980. https://doi.org/10.1515/9783111385068-067 (cited on Slide 10)
Wu, Katherine J. “These Sea Slugs Break A Cardinal Rule of Animal Life.” The Atlantic, September 28, 2021. https://www.theatlantic.com/science/archive/2021/09/solar-powered-sea-slug-chloroplasts/620227/ (quotes and image on Slide 11)
Long, James. “Blurring the Line Between Plants and Animals.” https://askabiologist.asu.edu/plosable/blurring-line-between-plants-and-animals
http://www.biology4kids.com/files/cell_chloroplast.html: an example of incorrect information!
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