Células, Endosimbiosis [Homenaje a Lynn Margoulis] y Viajes – Pinturas de Ana María Vacas – I

 

 

 

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Las dendritas (del griego δένδρος, déndros, «árbol») son prolongaciones protoplasmáticas ramificadas, bastante cortas de la neurona, dedicadas principalmente a la recepción de estímulos y, secundariamente, a la alimentación celular. Son terminales de las células nerviosas (neuronas) y sirven como receptores de impulsos nerviosos provenientes desde un axón perteneciente a otra neurona. Su principal función es recibir los impulsos de otras neuronas y enviarlas hasta el cuerpo (soma) de la neurona.

Las dendritas nacen como prolongaciones numerosas y ramificadas desde el cuerpo celular. Sin embargo en las neuronas sensitivas espinales se interpone un largo axón entre las dendritas y el pericarion. A lo largo de las dendritas existen las espinas dendríticas, pequeñas prolongaciones citoplasmáticas, que son sitios de sinapsis. El citoplasma de las dendritas contiene mitocondrias, vesículas membranosas, microtúbulos y neurofilamentos.

Poseen quimiorreceptores capaces de reaccionar con los neurotransmisores enviados desde las vesículas sinápticas de la neurona presináptica siendo fundamentales para la correcta transmisión de los impulsos quimioeléctricos a través de la vía nerviosa.

 

 

[Neuron Synapse]

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Endosymbiosis

It’s good to be friendly with your neighbors, right? Individuals and communities do better if they help each other out. Cooperation isn’t just important for humans; without a bit of interaction with neighbors, life as we know it would not exist.

The earliest living neighbors on our planet were all single-celled creatures. Some of the neighboring single-cells joined and began living together as one organism, one inside the other. This partnership was so successful that it led to the evolution of many of the life forms on our planet, including humans.

How did the eukaryotes become so complicated? And where did these battery-like organelles come from?

We think we know part of the answer. Eukaryotic cells may have evolved when multiple cells joined together into one. They began to live in what we call symbiotic relationships. The theory that explains how this could have happened is called endosymbiotic theory. An endosymbiont is one organism that lives inside of another one. All eukaryotic cells, like your own, are creatures that are made up of the parts of other creatures.

The mitochondrion and the chloroplast are both organelles that were once free-living cells. They were prokaryotes that ended up inside of other cells (host cells). They may have joined the other cell by being eaten (a process called phagocytosis), or perhaps they were parasites of that host cell.

Rather than being digested by or killing the host cell, the inner cell survived and together they thrived. It’s kind of like a landlord and a tenant. The host cell provides a comfortable, safe place to live and the organelle pays rent by making energy that the host cell can use. This happened a long time ago, and over time the organelle and the host cell have evolved together. Now one could not exist without the other. Today they function as a single organism, but we can still find evidence of the free-living past of the organelles if we look closely.

As early as 1883, botanist Andreas Schimper was looking at the plastid organelles of plant cells using a microscope. He watched the plastids divide and noticed something odd. The process looked very similar to the way some free-living bacteria divided.

During the 1950s and 60s, scientists found that both mitochondria and plastids inside plant cells had their own DNA. It was different from the rest of the plant cell DNA. When scientists looked closer at the genes in the mitochondrial and plastid DNA, they found that the genes were more like those from prokaryotes. This tells us that organelles are more closely related to prokaryotes.

The green chloroplasts in this cell are now a critical part of plant cells, but they evolved from an entirely different organism than the plant cell. The chloroplast is thought to have evolved from a cyanobacterial cell that managed to survive the cell’s defenses.

We know that multiple membranes surround the organelles too. If we look at the molecules of those membranes, they look like the membranes that surround modern day free-living prokaryotes.

So, organelles have their own DNA, and their genes are very similar to the genes of modern-day prokaryotes. They have membranes that look like those of prokaryotes, and they also seem to divide and replicate in similar ways. If a eukaryotic cell loses an organelle, it cannot remake it. Each eukaryote cell has to inherit at least one copy of an organelle from its parent cell if it is to live. That means that the genetic information needed to make the organelles is not found in the DNA of the eukaryotic cell. All of this evidence supports the theory that the organelles came from outside the eukaryotic cell. We think it tells us that they were once free-living prokaryotes.

Eukaryotic cells have many structures not found in prokaryotic cells.

A scientist named Lynn Margulis put all of this information together and published it in 1967. Her paper is called “On the origin of mitosing cells”. Mitosing cells are eukaryotes. Today scientists know her paper is very important, but it took many years before they accepted her theory.

But our story of the evolution of eukaryotic cells is far from complete. We haven’t talked at all about the other structures that we can find in eukaryotic cells but not in prokaryotic cells, and how they evolved. These include the nucleus, Golgi apparatus, endoplasmic reticulum, lysosomes, and cytoskeleton.

Where did they come from? The truth is we are still not sure. They could have evolved over time within the eukaryotic cells. Or, they could also be the result of other ancient endosymbiotic events. How they evolved is a problem that still needs to be solved.

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[Texto extraído de: https://askabiologist.asu.edu/explore/cells-living-in-cells

Para continuar leyendo y aprendiendo: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4571569/]

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