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Coming Full Circle: �From Cells to Ecosystems

by Dr. Evil

Lynn Margulis (1938-), currently a professor at the University of Massachusetts at Amherst

Endosymbiotic Theory

•  In 1970, Lynn Margulis (then known as Lynn Sagan, because she was married to astronomer Carl Sagan at the time) published Origin of Eukaryotic Cells.

•  The book took up some ideas from earlier biologists, but Margulis was able to back up those ideas with data from the new field of molecular biology.

Endosymbiotic Theory

•  Margulis proposed that: – Eukaryote mitochondria are descendants of free-living

bacteria that: •  entered an early eukaryotic cell •  took up residence •  developed a mutualistic symbiosis with the host

– Eukaryote chloroplasts are descendants of free-living photosynthetic bacteria that similarly formed an endosymbiosis (endo=inside) with host eukaryotes

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Lines of Evidence •  Mitochondria and chloroplasts have their own DNA

–  Sequences are more similar to bacterial DNA than to host DNA –  It's usually circular, like bacterial DNA—not like host DNA

•  Mitochondria and chloroplasts have their own ribosomes –  Although size varies, they're generally the same size as typical bacterial

ribosomes—host eukaryote ribosomes are larger •  Mitochondria and chloroplasts, if lost, can't be made anew by

the cell •  Mitochondria and chloroplasts can be killed by antibiotics that

attack bacteria

A map of the mitochondrial genome (set of all genes in an organism)—note that it’s circular

Another line of evidence: Mitochondria and chloroplasts are surrounded by a double membrane. . .

. . . which only makes sense if you assume that, at one time, mitochondria and chloroplast ancestors were engulfed by a host.

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Margulis points out that symbiosis is extremely common in nature. Symbiosis is a close relationship between two (or

sometimes more) unlike organisms that live in close proximity. One type of symbiosis is parasitism. . . as in these smut fungi or

huitlacoche growing on corn. . .

Parasitism is defined as one symbiotic “partner” benefiting while the other is harmed. For example, the giant

intestinal roundworm Ascaris takes nutrition from its human host, and can even block the intestines completely.

But in a type of symbiosis called mutualism, both “partners” benefit. Lichens, like these, are not plants—

they are fungi and algae growing together.

Many plants have fungi that grow inside their roots—the fungi get nutrients from the plant, and the plant gets

certain minerals from the fungus. Such fungi are called mycorrhizae; in the picture below, the hyphae are stained

darker blue, and are actually inside the plant cells.

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95% of all plant species depend on mycorrhizae—they will grow poorly without mycorrhizae, even if given

enough water and fertilizer. These days you can even buy mycorrizal spores to

add to your garden or potted plants. They grow much better with thriving mycorrhizae. (I’ve tried this myself—it works!)

We see mutualism in the animal kingdom as well. Convoluta roscoffensis, the “green flatworm”, has no mouth and never eats

—it lives on the protists that grow inside its tissues. This giant clam relies on photosynthetic protists growing in its tissues to survive.

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An extremely important mutualistic symbiosis is that between many flowering plants and pollinators—animals that transfer

pollen (analogous to sperm) from one plant to another, making it possible for the plant to produce seeds and reproduce.

This honeybee feeds on nectar (a sweet nutritious liquid) and pollen from flowers. She transfers pollen from one flower to another as she

visits them, enabling the plants to produce seeds and fruits.

Honeybee pollination is necessary for about 30% of the human food supply (many fruits, nuts, and vegetables, plus animal feed crops like alfalfa and clover) and adds about $15 billion to the

value of US crops. . . so humans are mutualistically dependent on both plants and bees.

Bees aren’t the only pollinators! Bats pollinate many plants (including economically important ones like mangoes, guavas, vanilla orchids, and agave). So do hummingbirds, butterflies,

moths, and even flies.

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Finally, commensalism refers to a relationship in which one partner benefits and the other is neither helped nor harmed. These cattle

egrets follow cows and eat insects that the cows have stirred up. The cows (probably) are neither benefited nor harmed.

QUIZ 10�

•  1. Protein production, or ______, takes place on tiny structures called _____.

•  2. The “adaptor” that carries information from the nucleus to where proteins are made is ____, made by the process of _____.

•  3. The monomers that make up a protein are _____, and there are ___ different ones that are used in protein synthesis.

QUIZ 10�

•  4. A cell that makes lots of proteins would have a lot of the structure ________ inside its cytoplasm. It would also have several “packaging and shipping” structures called ________ ________.

•  5. A cell that requires lots of energy would have lots of organelles called _________. These organelles came into existence by ________________.