Imagine if you're a regular teenager and, one day, you decide to invite a friend to come and live inside your house. Sounds unusual, right? Well, that's what happens in endosymbiosis! Here, one organism (the endosymbiont) lives inside another organism (the host). This is kind of like your friend (the endosymbiont) living inside your home (the host).

Just like you'd open the door to let your friend inside, the host cell uses a process called endocytosis to welcome the endosymbiont. Imagine it like this: endocytosis is the process where the cell forms a tiny balloon from its outer surface, called a vesicle, to capture and bring the endosymbiont inside. It's like using a drone to pick up your friend and bring them into your house!

In certain cases, this act of "inviting a friend over" turns out to be beneficial for both parties, just like you and your friend benefit from each other's company. This mutual benefit is the basis for a mutualistic endosymbiosis relationship. You share your home and resources, and your friend helps you with tasks around the house.

Our cellular ancestors, prokaryotes, which lacked a true nucleus and other structures, lived in an oxygen-less world. It's like living in a house with no electricity. Then, some smaller prokaryotes, which had figured out how to use oxygen (like your tech-savvy friend who figured out how to generate electricity), were invited into the larger prokaryotes' homes via endocytosis. Instead of eating these smaller prokaryotes, the larger ones let them live and provide energy.