Chloroplasts are plant organelles that conduct photosynthesis. this allows plants to obtain energy from carbon dioxide, water, and sunlight. humans have evolved to obtain energy from food ingested. no animal has chloroplasts, that is an organelle specific to plant life. what a different world it’d be if humans could eat from internal chloroplasts…
Chloroplasts evolved from free living cyanobacteria that were engulfed by a bigger cell. Rather than the bigger cell digesting the smaller (as happens in species like ameba) or the smaller cell replicating out of control eventually killing the larger cell (as parasites like the one that causes malaria do within our own cells), the two cells developed a symbiotic relationship where the chloroplast turned sunlight into energy for both itself and the bigger cell, and the bigger cell (which is the ancestor of all plants living today) protected the chloroplast and provided it with nutrients. We don’t have chloroplasts because the single celled life forms that were our ancestors never formed that kind of symbiotic relationship but instead got better and better at eating other living things to get energy and nutrients.
Because I’m the annoying guy who always has to point out the exception to every rule in biology (jaksongitr said “no animal has chloroplasts”):
There is a sea slug called Elysia chlorotica which is an animal, just like us, yet does have chloroplasts and gets energy from photosynthesis like a plant. When the sea slugs are young, they eat algae (simple plants that live in the ocean), but instead of completely digesting them, the sea slug steals the algae’s chloroplasts and insert them into its own cells. After that it can get energy by simply lying around and absorbing sunlight. [citation 1 below]
Isn’t that amazing?
Chloroplasts are the energy powerhouses of plant cells, while the mitochondria are the energy powerhouses of animal cells. There are many papers and much research that has gone into comparing the mitochondria and the chloroplast. They both have their own DNA separate from the cell and both have an inner and outer cell wall and produce their own proteins as well. Thus, its not necessarily that humans don’t have chloroplasts; they have an equivalent organelle.
Chloroplasts and mitochondria are both endosymbiotes, but they don’t fulfill equivalent roles. Plant cells contain both mitochondria and chloroplasts, while animal cells contain only mitochondria.
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